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Review

A Review on Stems Composition and Their Impact on Wine Quality

1
Agroscope, Route de Duillier 50, 1260 Nyon, Switzerland
2
Changins, Viticulture and Enology, HES-SO University of Applied Sciences and Arts Western Switzerland, Route de Duillier 50, 1260 Nyon, Switzerland
*
Author to whom correspondence should be addressed.
Molecules 2021, 26(5), 1240; https://doi.org/10.3390/molecules26051240
Received: 12 January 2021 / Revised: 19 February 2021 / Accepted: 22 February 2021 / Published: 25 February 2021
(This article belongs to the Special Issue Wine Chemistry: The Key behind Wine Quality)

Abstract

:
Often blamed for bringing green aromas and astringency to wines, the use of stems is also empirically known to improve the aromatic complexity and freshness of some wines. Although applied in different wine-growing regions, stems use remains mainly experimental at a cellar level. Few studies have specifically focused on the compounds extracted from stems during fermentation and maceration and their potential impact on the must and wine matrices. We identified current knowledge on stem chemical composition and inventoried the compounds likely to be released during maceration to consider their theoretical impact. In addition, we investigated existing studies that examined the impact of either single stems or whole clusters on the wine quality. Many parameters influence stems’ effect on the wine, especially grape variety, stem state, how stems are incorporated, when they are added, and contact duration. Other rarely considered factors may also have an impact, including vintage and ripening conditions, which could affect the lignification of the stem.

1. Introduction

For white winemaking, stems are generally kept during pressing because they allow for better juice extraction yields. Given the short contact time, compounds are extracted from the stems in relatively low levels. In red winemaking, the maceration phase—where color is extracted from the grape skin and tannins from the grape seeds—occurs before pressing. Originally, stems were kept during this phase, but destemming practices appeared at the end of the 19th century, improving wine quality by reducing excessive astringency and negative strong green tastes from the stems [1]. Initially used in cellars with high production capacity, the first destemming machines quickly trivialized this practice; today, this technique is systematic for winegrowers in most wine-producing countries [2]. However, in some regions, using whole clusters of grapes is a matter of tradition. The stem is considered a natural additive that, if well mastered, brings complexity, freshness, and phenolic structure to the wine and facilitates chemical stability during aging [3] (e.g., the Pinot Noir in Burgundy, the Cabernet Franc in the Loire Valley, or the Gamay in the Beaujolais, Kakhethian wines from Georgia, etc.). In recent years, winemakers in Europe and other countries, such as Australia and South Africa, have shown an interest in using stems, and several technical articles mention the advantages of this practice [4,5].
These winemaking techniques are not used for all grape varieties, nor for all vintages. Since these techniques have very little research behind them, they are generally passed along by word of mouth. Therefore, it is difficult to know which stem conditions will lead to improved or deteriorated wine quality.
Stem composition has often been analyzed to value winemaking by-products and has been relatively well studied. Many compounds of interest can be found in stems’ overall composition. Their richness in polyphenolic compounds makes them very interesting for the food and medicine industries, in relation to their antioxidant potential. In some studies, units used to express stem extract composition is very specific and makes it impossible to compare results. Therefore, such data are not presented in this article [6,7,8,9,10]. This review gathered information from the literature on stem chemical composition to examine how these compounds contribute to variations in aroma and taste when stems are included during winemaking. Although stems are only used for red wines, we also examined data on the chemical composition of white grape variety stems. We then compiled the main results observed when whole clusters of grapes or single stems were incorporated into the winemaking process, from a technological, chemical, and sensorial perspective.

2. Grape Stems

2.1. Morphology

The stem is the skeleton of the grape cluster or bunch. The longest part, the rachis (main axis), is branched with peduncles, and a pedicel attaches each grape berry to the stem (Figure 1).
The stem’s final size is reached around veraison [1]. For each grape variety, the number, length, and distance between two ramifications varies. Along with other morphological criteria, these components determine the compactness of the bunch [11]. The stem accounts for 3 to 7% of the total bunch weight, depending on the grape variety, number of grapes on the bunch, and its sanitary state [1,2,12].

2.2. General Composition

This part of the review aims to summarize the main compounds found in grape stems. An estimation of their quantification based on the available data is shown in Figure 2. This composition is close to the one described by Foulonneau et al. which is similar to that of the vine’s leaves and tendrils [2]. For each type of compound, available data were summarized. It should be noticed that the comparison of published data is difficult, as their proportions can be impacted by different factors, such as grape variety, vintage, maturation state, as well as differences in extraction techniques and units.

2.2.1. Water

As the stem’s main component, water accounts for 55 to 80% of stem weight [2,3,13,14]. In 1976, Rice et al. measured the moisture of fresh grape stems from ten grape varieties, five reds (Concord, Ives, Baco noir, Red hybrids, and Cascade) and five whites (Aurore, Concord CP, Delaware, Niagara, and Catawba) planted in New York state, USA [14]. The values ranged between 68.4 and 79.1% of stem fresh weight (FW). No significant differences were found between red and white grapes and variability was imputed to the grape variety. In 2010, Gonzalez-Centeno et al. studied the overall stem composition of ten other grape varieties, six reds (Cabernet Sauvignon, Callet, Manto Negro, Merlot, Syrah, and Tempranillo) and four whites (Chardonnay, Macabeu, Parellada, and Premsal Blanc) planted on Mallorca Island, Spain, and found similar values, ranging from 55 to 80% of FW [13]. Of the grape varieties studied, white grape varieties appeared to have significantly higher moisture content (71.7 g/100 g FW) than red varieties (62.5 g/100 g FW). Stem water content appeared to depend on the grape variety. However, none of these studies considered stem maturity, which could have a major influence on the values.

2.2.2. Cellulose and Hemicellulose

In stems, as in classical vegetable biomass [15], cellulose is the most abundant biopolymer followed by hemicelluloses (mannans, xyloglucans, xylans) [13,16,17,18,19]. Cellulose content values range from 12 to 38% dry matter (DM) (Table 1). The observed large variability might relate to differences in analytical procedures (extraction, analyses, and calculation) [17] and/or variability between grape varieties [20].

2.2.3. Lignin

Lignin content ranges from 13 to 47% of DM (Table 1), with many studies reporting on the variability and providing different explanations, such as analytical method [17,18], grape variety [16,18], or stem maturity [21]. Indeed, studies have used different measurement and calculation methods to evaluate the lignin content, with some including acid soluble and insoluble lignin [16,17] and others including only acid-insoluble residues as the amount of lignin [18]. These method variations can lead either to an over- or under-estimation of total lignin content. The stem’s ripening speed depends mainly on the grape variety and climatic conditions [22]. Full lignification often occurs beyond berry maturity [1]. Indeed, the maturity stage of the stem at harvest will affect its composition. To our knowledge, stem maturity has not been considered in previous studies. It would be interesting to evaluate this maturity to better understand stem composition evolution during maturation.

2.2.4. Proteins

Stem protein content ranges from 5 to 11% DM (2–3% of fresh weight) with a mean of 7% [13,18,21,23] (Table 1). The obtained values do not consider whether variations are related to grape variety or only to the biological variability of the raw material induced either by stem maturity or the extraction process (drying, crushing, etc.). These values are consistent because stems are not vine storage organs. Notably, different studies mention the presence of resistant proteins, referring to proteins bound with lignin, which are difficult to access, suggesting that the protein level could be underestimated [21,23]. Total protein quantification is, therefore, complex.

2.2.5. Ashes

As with protein content, reported ash content is relatively consistent across different studies, with a mean value of 6.9% DM, regardless of the grape variety or origin (Table 1). Prozil et al. used inductive coupled plasma (ICP) to analyze detailed metal cation composition and identified potassium as the main mineral element of grape stems (K: 0.9%, Ca: 0.15%, Mg: 0.02%, Zn: 0.01% and Na < 0.01% of total ash content) [18].

2.2.6. Acids

Stem acidic composition has been estimated by measuring stem extracts’ total acidity using a reaction with Bromothymol blue, with values ranging from 13.5 to 15.0 g/kg FW, or approximatively 1 to 2% of the total stem weight [2]. No information regarding further analysis of acid types was found in the literature.

2.2.7. Sugars

Stems have a low sugar content [2,21]. According to Gonzalez-Centeno et al. soluble sugar content, determined as glucose, according to the Haas colorimetric method (which uses anthrone as the reactive and measures the absorbance at 620 nm), ranges between 1.8 and 3.7 g/100 g stem FW [13]. Sugar concentration variability is related to grape variety rather than color. Similar values were found in other studies: 1.70% for Manto Negro [23], 1.04% for Premsal Blanc [21], with soluble sugar content lower than 10 g/kg of stem FW [1]. Therefore, stems do not represent a significant sugar input for fermentation compared to grape berries (sugar content 14.9 g/100 g FW) [13].
The main components of grape stems and their respective concentration, as described in the literature, are summarized in Table 1.

2.3. Polyphenolic Composition

Phenolic compounds are widely present in the plant kingdom, and red grape varieties contain a high concentration of these compounds, especially in grape solid parts, skin, and seeds. Studies have also reported their presence in vine-shoots [25,26,27]. Stem extract analysis found that stems are rich in polyphenolic compounds [28,29], with intermediate concentrations between the higher concentrations in grape seeds and the lower concentrations in grape skins [30].

2.3.1. Total Phenolic Content

The Folin–Ciocalteu method is a common technique for estimating total polyphenolic content in a vegetal fraction. Gallic acid is used as the standard and results are reported in gallic acid equivalent (GAE). Table 2 presents data from the literature on white and red grape varieties. To allow for comparison between the reported results, the unit was standardized (mg GAE/100 g DM). Grape stems show a wide range of total polyphenolic content. In white grape varieties, total polyphenolic values range from 400 to 22,900 mg GAE/100 g DM and in red grape varieties, similar values were found, ranging between 348 to 38,400 mg GAE/100 g DM. Variability in total polyphenol content can be attributed to many factors. Grape variety is one of these factors, as shows the important difference between the content measured in Asyrtiko (1248 and 1115 mg) and Athiri (400 and 480 mg) grape varieties by Anastasiadi et al. in two consecutive years [29], or in Chardonnay (4764 mg), and Pemsal blanc (9002 mg) by González-Centeno et al. [31]. Furthermore, Anastasiadi et al. and several other studies have also observed an important effect of vintage [29,32,33]. The importance of vineyard localization on the polyphenolic content was highlighted by the study of Spatafora et al. and Gouvinhas et al. demonstrated the effect of altitude [33,34].
In addition to the effect related to the grape and its growing conditions, several authors highlighted the impact of the extraction method on total polyphenolic content in the same grape variety. Makris et al. obtained values ranging from 3120 to 7468 mg GAE/100 g DM for Roditis grape variety, merely by changing the composition of the extraction solution [35,36,37]. Jimenez-Moreno et al. obtained similar results on Mazuelo stems, with values ranging from 1276 to 51,045 mg GAE/L/100 g of DM. They highlighted the fact that ethanol concentration was the most determinant parameter among temperature, solid/solvent ratio and ethanol content [38]. Interactions between these three different extraction parameters were also found. The size of the stem particles used for extraction also had an impact on the total polyphenol content, where smaller particles increased the exchange surface, allowing better extraction [39].
Further investigation to develop a standardized extraction method could help to compare results, including factors linked to either the grapes (e.g., grape variety [31,32,37,40,41], vintage conditions [29], vineyard localization [31,42], stem maturity [43]) or to differing extraction parameters [36,37] (e.g., solvent, duration, stem particle size, temperature).

2.3.2. Non-Flavonoid Compounds

The total phenolic content comprises an important variety of molecules containing phenol rings in their chemical structure. Polyphenolic compounds are divided into two major categories, flavonoids and non-flavonoids. The presence of both types of molecules has been studied in grape stem extracts, with detailed molecular compositions reported in the literature.
Non-flavonoid molecules include phenolic acids, stilbenes, and hydrolysable tannins (Figure 3). To our knowledge, no hydrolysable tannin content has been reported in the literature.

Phenolic Acids

Different phenolic acids have been identified in grape stem extracts: caftaric acid [28,36,37,39,40,44,47,48], coutaric acid [28,40], gallic acid [19,28,29,34,38,39,48,49], coumaric acid [19,28,29,41], caffeic acid [29,41], syringic acid [19,28,29], ferulic acid [29], protocatechuic acid [48], trans-cinnamic acid [48], and other unidentified hydrocynnamic acids [44,47]. Many of these acids have also been identified in other grape parts, such as pulp, skin or seeds, or in other wine by-products, such as pomace [28,40,50].
Some authors have quantified these different phenolic acids using HPLC techniques. Results for white and red grape varieties are shown in Table 3. When the units differed, the values were standardized by converting to mg/kg DM. Comparing these values across studies remains difficult because extraction protocols differed according to the study and not all papers quantified phenolic acids. However, the major phenolic acids in stems appear to be caftaric acid and gallic acid. Caftaric acid was found in a concentration between 5.1–12,820 mg/kg DM in white grape varieties and 12.5–1500 mg/kg DM in red varieties. Gallic acid reached concentrations of 30–469 mg/kg DM in white varieties and 6.5–300 mg/kg DM in red varieties. As for the total phenolic content, individual phenolic acid concentrations are influenced by the grape variety [28,29,44], geographical origin [34], and the vintage [29]. The study of Gouvinhas et al. shows that the concentration of phenolic acids, such as caftaric and hydroxycinnamic acids, are highly correlated to the altitude and the vintage [33]. A strong thermal and water stress, related to the lower altitude, increases the synthesis of phenolic compounds in the plant and consequently in the stem. The effect of water stress is well demonstrated in the study of Alonso et al. where, in the no irrigated variants of Tempranillo and Syrah stems, the concentration of major phenolic acids, namely caftaric and coutaric acids, are significantly higher than in the irrigated variants [28]. In wine, these compounds have no odor, nor aroma. However, they can be precursors to volatile phenols, molecules that can induce defects in wines. Their extraction from stems is not necessarily interesting for winemaking.

Stilbenes

Trans-resveratrol and ε-viniferin are the two main stilbenes identified in grape stem extracts [29,33,34,36,38,41,44,47,48,49,51]. Values found in the literature are summarized in Table 4. Trans-resveratrol values ranged from 31 to 393 mg/kg DM and ε-viniferin, value range from 1.91 to 900 mg/kg DM. According to several authors, the differences between the values are mainly due to the different cultivars, geographical regions, and vintages [29,33,52]. As mentioned in Piñiero et al.’s study, the extraction protocol has a great impact on the extraction yields, especially the ethanol concentration and the sample-solvent ratio [52]. On the other hand, extraction duration (between 15 and 35 min) did not have a significant impact. Bavaresco et al. studied the transfer of stilbenoid compounds in wine in extraction conditions similar to wine (11% (v/v) ethanol and 250 ppm (v/v) methanol) and found only trans-resveratrol in the extract. The values ranged from 6.0 to 17.8 mg/kg DM, meaning that the level of stilbenes potentially extractable during maceration would be lower than the values found in the extracts presented in Table 4. The presence of other stilbenoid compounds, such as piceatannol, was also mentioned in the literature [41,52]. Finally, some studies reported that no stilbenes were found in the stem extracts [37,44]. This could either be related to the extraction method or to grape stem composition variability.

2.3.3. Flavonoid Compounds

Flavonoid compounds share the same basic structure formed by two aromatic rings linked by three carbons: C6-C3-C6. This group of molecules includes flavonols, flavanols, flavanonols, flavones, flavanones (intense yellow pigments), and anthocyanins (red or blue pigments). Flavan-3-ols form oligomers and polymers, called proanthocyanidins or condensed tannins. Their different structures are presented in Figure 3. The most common flavonoid compounds in grapes and wines are flavonols, flavanols, anthocyanidins, and their derivatives.
Only one recent study tentatively identified a flavone in stem extracts, chrysoeriol malonyl-apiosyl-glucoside [36]. To our knowledge, no other flavones or flavanones content has been reported in the literature.

Flavonols

The different flavonols identified in grape stem extracts are quercetin 3-O-glucuronide [34,36,37,40,44,47,49], quercetin 3-O-glucoside [29,34,40,41], kaempferol 3-O-glucoside [36,40,44,47], myricetin 3-O-glucoside [40], myricetin 3-O-glucuronide [40], quercetin 3-O-rutinoside [37,39,44,47,48,49], quercetin 3-O-galactoside [29], quercetin 3-O-rhamnoside [29], kaempferol [29], quercetin [29], isorhamnetin-3-O-(6-O-feruloyl)-glucoside [44,47], and kaempferol-3-O-rutinoside [44,47].
Different authors have reported concentration values of these compounds for various white and red varieties (Table 5). Quercetin derivatives were reported to be the main flavonols followed by kaempferol derivatives. Quercetin-3-O-glucuronide, quercetin-3-O-rutinoside and quercetin-3-O-galactoside appeared to be the most abundant flavonols in grape stem extracts, depending on the extraction solvent used for sample preparation. The solubility in water of flavonol derivatives increases in the following order: rhamnoside < glucoside < galactoside < glucuronide < rutinoside [54]. Using only water for the extraction Kosinska–Cagnazzo et al. found only quercetin-3-O-rutinoside in the extracts, and the quantity varies with the size of the stems [39] when Barros et al. and Leal et al. with 50 and 70% of methanol in water extracted meanly quercetin-3-O-glucuronide [44,47]. The addition of organic solvent allows for the extraction of more apolares substances, such as kaempferol derivatives. However the high amount of organic compounds in the extraction mixture, as the 90% acetonitrile used by Anastasiadi et al. could conduct to the loss of the water-soluble derivatives [29]. This demonstrates the importance of the extraction conditions on the profile and the quantity of polyphenols measured in stems.

Flavanonols

Astilbin [36,37,40,49] and engeletin [40] are the two main flavanonols identified in grape stem extracts. Only Souquet et al. quantified astilbin (35 mg/kg of stems) and found traces of engeletin in the stem extracts [40]. Dihydroquercetin, also called taxifolin, is the flavanonol mainly identified in grapes and wine, and was not found in grape stem extracts.

Flavan-3-ols and Proanthocyanidins

The profile of flavan-3-ols and proanthocyanidins was measured in the stem extracts using HPLC-DAD or HPLC-MS techniques. Information about molecular ion and the typical fragments are summarized in Table 6.
Among flavanols monomers, many studies reported the presence of catechin and epicatechin in grape stem extracts. Epicatechin gallate was found in two studies [29,47]. To our knowledge, no epigallocatechin was identified in grape stem extracts as a monomer unit.
Proanthocyanidin dimers and trimers were identified in stem extracts using the HPLC-MS technique: dimers B1, B2, B3, B4, B1-3-O-gallate, B2-3-O-gallate, B3-3-O-gallate, and trimers T2 and C1.
The three main compounds found in the stem extracts are catechin and the dimers B1 and B3 (Table 7). The proportion of all compounds seems to depend on the grape variety and the vintage, but also on the study. The choice of extraction conditions and analytical method essentially influences the profile reported by the different authors. This can be the reason why Alonso et al. did not find catechin in all extracts [28], when others found catechin content ranging from 50 to 7640 mg/kg DM. The epicatechin content is low compared to catechin content. Barros et al. reported the sum of catechin and epicatechin in the extracts, with values ranging from 22 to 32 mg/g DM, depending on the grape variety [47]. In general, the concentrations of dimers B1 and B3 were found in the same magnitude as catechin, from 133 to 1958 mg/kg DM, and from 41 to 993 mg/kg DM, respectively.
Procyanidin B1 has been reported as the main oligomer in skins [58,59,60], whereas procyanidin B2 [60,61,62] is the main oligomer in seeds. Therefore, the phenolic composition of stem extracts is likely to be closer to grape skins.
Proanthocyanidins or condensed tannins are present in plants in different degrees of polymerization. When this degree is higher than three, these compounds cannot be quantified by actual HPLC-MS methods. Total proanthocyanidin content can be estimated by several methods. The Bate–Smith reaction is most commonly used and is based on the ability of condensed tannins to be depolymerized under acidic conditions. This chemical depolymerization, followed by auto-oxidation, generates anthocyanidins, hence they are also called “proanthocyanidins” [63]. The concentration of the resulting colored molecules can be measured by spectrophotometry to estimate the quantity of monomers included in the condensed tannins. Other techniques use a reaction between the nucleophile site of the tannin and an aldehyde, such as vanillin or DMACA, to produce a colored product where the measured intensity increases with the quantity of tannins, but decreases with the polymerization degree of the tannins, as only the terminal monomer is reactive. The DMACA method is based on the reaction between catechin and 4-dimethylaminocinnamaldehyde, resulting in the formation of a blue complex that absorbs red light (around 640nm). In the vanillin assay, vanillin is protonated in an acidic solution and reacts specifically with the flavan-3-ols, dihydrochalcones, and proanthocyanidins, producing a red-colored compound where the concentration is measured by spectrophotometry at a wavelength between 500 and 550 nm. In this case, catechin is often used as a standard. Methylcellulose precipitation method allows proanthocyanidic polymers to be selectively precipitated with methylcellulose (MC), with which they form insoluble complexes. The MC plays the same role here as the salivary proteins in tasting. Ammonium sulfate (NH4)2SO4 in the reaction medium increases its polarity, thus promoting complex insolubilization and precipitation. For the protein precipitation method, a known amount of protein (BSA) binds to the tannin in the sample, forming a protein–tannin complex that precipitates. Then, precipitate is washed by a ferric chloride solution, which forms a colored complex, the absorbance of which can be read on a spectrophotometer at 510 nm. The amount of color is proportional to the amount of tannins in the stem extract [64].
Values obtained for different grape varieties are presented in Table 8, classified according to the analysis method. As expected, different methods produced sensibly different results. Values are variable, even using the same analysis method, and these differences could not be linked to the grape color. Moreover, for the same grape variety (Premsal blanc), values found in two different studies were significantly different: 79.0 mg/g DM in Llobera et al. and 181.4 mg/g DM in Gonzalez-Centeno et al. [21,31]. Makris et al. showed that the extraction method can modify the measured total proanthocyanidin value for the same grape variety by a factor of 5 [35].
Based on the values obtained by the Bate–Smith method (expressed in mg/g DM), proanthocyanidins appear to be the most abundant type of polyphenols in stem extracts. The dimer concentrations shown in Table 7 (expressed in mg/kg DM) represent only a small proportion of the proanthocyanidin content. The high values of total proanthocyanidin content suggest an abundance of polymerised forms, which is confirmed by the results of the mean degree of polymerization (mDP), which was found to be higher than 4.6 for all studied grape varieties (Table 9).
Thiolysis or phloroglucinolysis are used to analyze the condensed tannin composition. These reactions are depolymerization methods that cut the polymers into subunits. Only the extension unit forms adducts with the reactive, allowing for differentiating them from terminal units. The different monomers can be separately quantified by HPLC and the mean degree of polymerization can be determined. Results reported in the literature are presented in Table 9. The experimental values of the mean degree of polymerization (mDP) range from 4.6 to 10.2. The general composition shows that epicatechin is the main unit of the polymerized proanthocyanidins; it is also mainly found in extension units, whereas catechin is mostly found in terminal units. Merlot and Chardonnay were studied in two different papers, with different analysis methods, and the mDP were slightly different; higher for Souquet et al. than for Gonzalez-Centeno et al. [31,40]. This difference could be explained by vintage conditions, vine location, and analytical techniques. Sensitive difference can be observed between the two methods; EcG and EgC were found in higher concentrations using the thiolysis method than using phloroglucinolysis.

Anthocyanins

Anthocyanins are mainly located in grape skins [2,30]. However, recent studies analyzing different grape varieties identified some anthocyanin compounds in grape stem extracts: malvidin-3-O-glucoside [47,48], malvidin-3-O-(6-O-caffeoyl)-glucoside [47], malvidin-3-O-galactoside [48], and malvidin-3-O-rutinoside [47]. Total anthocyanin content of the stem extracts ranged from 0.06 to 1.4 mg/g of DM, and these compounds were not detected in some varieties. The concentration in anthocyanins was low compared to other flavonoid contents.

2.3.4. Impact of Polyphenolic Composition

Polyphenolic compounds have been widely studied and, apart from their influence on wine color and structure, they can influence different parameters, such as astringency or antioxidant activity.

Astringency

Astringency produces a contraction of the buccal mucosa when salivary proteins form complexes with tannins. Salivary amylase reacts strongly with astringent compounds and causes the mouth dryness sensation.
The influence of grape stem extracts’ polyphenolic composition on astringency has been studied using ovalbumin as a precipitation agent and tannic acid as a standard. Ovalbumin mimics the salivary proteins and quantifies astringency related to the precipitation of polyphenolic compounds and saliva. The results showed that stem extract astringency increases with maceration time and remains stable after 4–5 days [43]. Three maturation stages were studied, and ripening appeared to increase proanthocyanidin extraction during maceration and decrease the astringency of the extracts of all cluster parts. One hypothesis for the decreased astringency was a decrease in the mDP of proanthocyanidin extracted during stem ripening [43]. The relation between quantity, mDP, percent galloylation, and percent trihydroxylated units of proanthocyanidin and astringency has been studied in many wines and seed extracts [43,50,65,66]. In red wines, weaker astringency was found for lower mDP. In addition, the mDP is an average of the degree of polymerization and does not give clear indications of the proportion of polymeric and oligomeric proanthocyanidin content. Li et al. showed that polymeric polyphenols react more strongly with salivary proteins than oligomeric ones, inducing a higher sensation of astringency. During the ripening of the stems, the proportion of oligomeric forms may increase and could explain the decrease in astringency. The variation in proanthocyanidins, according to grape variety and stem ripening stage, appears to have a great influence on sensorial perception, especially regarding astringency. It would be interesting to study this parameter further when stems are kept during winemaking.

Antioxidant Activity

The antioxidant potential of polyphenolic compounds can be measured by different methods [67]. Studies have used different measurement techniques to characterize the antioxidant potential of stem extracts, such as 2,2-azinobis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS), 1,1-diphenyl-2-picrylhydrazine (DPPH), ferric reducing antioxidant power (FRAP), cupric reducing antioxidant capacity (CUPRAC) [31], oxygen radical absorbance capacity (ORAC) [31], and superoxide radical scavenging activity (O2·-) [47]. ABTS, FRAP, and DPPH appeared to be the most used methods. Antioxidant capacity measured by DPPH can be expressed in different ways: either as the quantity of antioxidant necessary to decrease the concentration of initial DPPH or using a reference such as trolox. Summarizing the available data and comparing the values is not straightforward. Therefore, only the ABTS and FRAP values found in the literature are presented in Table 10.
As mentioned by Gonzalez-Centeno et al. it is difficult to compare the values reported in the literature because there is no standardized method to characterize the antioxidant potential; extracts are obtained using different techniques and the results are expressed in different units [31]. According to the literature, ABTS and FRAP results usually show good positive correlation [31,68,69,70,71,72]. Despite the difficulties of cross-study comparisons, all studies reported that stems can be a good source of antioxidant compounds.

2.4. Aromatic Composition

The use of stems during winemaking has been reported to bring vegetal and green aromas to the wine. Different studies focused on the aromatic compound found in stem extracts. In 1997, Hashizume et al. listed eight different green odorant compounds detected in grape stems from Cabernet Sauvignon and Chardonnay grape varieties: hexanal, (E)-2- hexanal, (Z)-1,5-octandien-3-one, 2-methoxy-3-isopropylpyrazine, 2-methoxy-3-isobutylpyrazine, dodecanal, (E,Z)-2,6-nonadienal, and an unknown compound (Table 11) [73]. These compounds were the same in both grape varieties. For Cabernet Sauvignon, four other aromas (a cooked vegetable-like odorant, a burned bamboo-like odorant, a sweaty unpleasant odorant, and a floral aroma) were also found during the extract analysis but were not analyzed because the study focused on vegetal aromas. Quantifying each compound showed that (Z)-1,5-octandien-3-one was the main green odorant compound from stems. These extracts were also compared to leaf, berry, and skin + seed extracts and stems appeared to contain the highest proportion of methoxypyrazine. Roujou de Boubée et al. focused on 2-methoxy-3-isobutylpyrazine (IBMP) in the Cabernet Sauvignon grape variety to determine the localization of this aromatic compound within the grape cluster, and found that stems were richer in IBMP, confirming the results of the Hashizume research team [73,74]. They also studied IBMP location during ripening and showed that it decreases in stems and seeds but increases in skins. Matarese et al. studied the entire fraction of volatile compounds of ground stems and other grape plant parts and reported that geraniol and geranic acid were the two main monoterpenes of the stem volatile fractions [75]. Other compounds, such as linalool and nerol, were also identified but in smaller proportions.
Ruiz-Moreno et al. performed a GC-olfactometry and a GC-MS on Syrah stem extracts and found more than 80 odorant zones (OZ) [76]. Among them, eight OZ were found to be predominant and GC-MS identified the responsible molecules (Table 11). This study specified that stem extracts have a similar composition to that of wine in terms of aromatic compounds and should have a quantitative rather than qualitative effect if added to the wine.
In 2016, a study of Cabernet Sauvignon stems found a large amount of 1,8-cineole in the stems compared to the grape berries, and that this quantity substantially decreased during ripening [77]. Larger amounts of 2-methoxy-3-isopropylpyrazine (IPMP) and IBMP were found in the stems than in the grape berries. Again, these amounts decreased with ripening. Finally, this study identified methyl salicylate, which is reported to have a fresh and minty aroma and has higher levels in stems than berries (250 times higher).
Stems appear to be a rich source of valuable compounds, including polyphenols. Considering waste in wine production, their availability is high at harvest time and, as reported in the literature, grape stems may provide a cheap source of these compounds of interest. Recent studies have examined stem extracts for human health applications [44,48,78]. Although the destemming technique is widespread, some winemakers keep the stems during the winemaking process.

3. The Use of Stems during Winemaking

As explained in the introduction, in most cases of white winemaking, stems are kept for pressing and removed with the pomace. Because stems act as drains, keeping them during pressing induces better juice extraction. According to the literature, it also limits the presence of the thermo-unstable proteins responsible for protein breakdown [1]. For red winemaking, the impact of keeping the stems during fermentation and maceration appears to be more empirical. Understanding which elements are transferred and what impact they have on the wine is essential for advising winegrowers on this practice.
When stems are included, the whole cluster addition is the most common technique used. However, to precisely study the impact of stems, researchers have often added the stems back in the tank after destemming [3,12,30,43,57,73,79,80,81,82,83,84]. Different proportions of stems and whole clusters [81,83,84,85], different maceration time [30,43,79], and different stem pretreatments [3] have been used. Some studies also tested the use of stem powder [39] or stem extracts [76,86] as an oenological additive compound.

3.1. Impact on the Winemaking Process

Adding stems or using whole clusters can increase the must volume by 30%, which has a technological impact on the vatting and the maceration phases. In addition, a higher pressuring capacity is required [1].
To our knowledge, the impact of stems on alcoholic and malolactic fermentations (MLF) is barely described in the literature. Comparing destemmed and full-clustered musts has shown that must containing stems start fermenting faster, resulting in a wine with fewer residual sugars [1]. This may result from the structural configuration of stems, which allows higher incorporation of oxygen in the must, encouraging yeast proliferation. Moreover, the presence of such structures acting as temperature buffers could reduce temperature variations and hence prevent stuck fermentations. These effects are different depending on the volume of whole clusters or stems added. A recent study showed that 20% of whole clusters or 3% of stem weight in the vat did not influence either the temperature or the alcoholic fermentation kinetics [84]. For both fermentations, further studies are needed to better understand the impact of stems on microbial activity and kinetics.

3.2. Impact on the Main Wine Compounds

Stems release compounds, such as must [43,57,80,81], in a matrix, which then interact with the other grape-extracted compounds (from berries and seeds) and provoke a change in the overall balance of the final wine. Among the different types of compounds found in wines, some are widely influenced by the presence of stems, including ashes, acids, alcohols, and phenolic compounds.

3.2.1. pH and Acid Composition

Summarized data on the pH and acidic composition of wine made with and without stem addition are presented in Table 12.
Hashizume et al. found an increase in pH for Pinot Noir and Muscat Bailey A musts when incorporating the stems back in the vats [3]. This phenomenon was also observed by Pascual et al. on Cabernet Sauvignon musts and more recently by Casassa et al. during Pinot Noir winemaking using either whole clusters, raw or dried stems [12,83,84].
However, pH increases are not always significant. These differences in terms of variations may be linked to the high buffering capacity of wine matrices over acido-basic balance that mainly depends on the grape variety [50]. The impact of stem contact duration has also been studied. For Castelao musts, Spranger et al. reported that pH did not show a significant increase after seven days, but showed a significant impact after 21 days of contact [79]. Therefore, contact duration seems to influence pH variation.
According to the acid composition, titrable acidity was shown to be significantly lower than the control samples for Cabernet Sauvignon [12] and Pinot Noir wines going through stem contact [84]. However, this finding seems to depend on how the stems are incorporated in the Casassa et al. latest study; the use of whole clusters did not show a significant decrease in titrable acidity [83].
Changes in acid composition appear to be responsible for these pH variations in wine. More specifically, tartaric acid, which is the most abundant acid found in wines and musts, seems to be affected by the addition of stems, but less by the use of whole clusters (Table 12). After seven days of contact, its concentration was lowered by 4% for Castelao wines [79], 9% for Muscat Bailey A [3] and 10% for Pinot Noir wines [3]. With longer stem contact duration, the decrease in tartaric acid was greater (from 4% after seven days to 7% after 21 days for Castelao wines [79]). This loss could result from precipitation mechanisms of tartaric acid. As noted earlier, stems are rich in mineral compounds, especially potassium, so their interaction with tartaric acid is a possible explanation [12].
Other acid concentrations might also be affected by stem contact as a result of molecular interactions. For instance, lower concentrations of succinic acid were reported for Muscat Bailey A (−16%), whereas phosphoric acid concentration was increased by 38% [3]. Differences in lactic and malic acid concentrations mainly resulted from the winemaking process and whether the MLF was performed. Lastly, Casassa et al. highlighted the fact that the use of the whole cluster may increase the volatile acidity of the wine potentially due to undesirable bacterial growth taking place in the air spaces within the whole clusters [83].

3.2.2. Ashes

Only a few authors have studied the impact of stems on the mineral composition of wines. However, both Hashizume et al. and Sun et al. found significant increases in the concentration of several mineral ions [3,80]. The related values are summarized in Table 13.
Both authors found that stem addition increases potassium (K), phosphorus (P), and calcium (Ca) concentrations. This reinforces the hypothesis of tartaric acid precipitation inducing a pH increase.
Some variations were recorded depending on the grape varieties, especially for magnesium (Mg), sodium (Na), copper (Cu), and zinc (Zn), indicating that the impact of stems could also depend on the grape variety.

3.2.3. Ethanol Content

Several authors related the addition of stems to a lower wine ethanol content [12,80,84] (Table 14). Hashizume et al. attributed this decrease to a dilution phenomenon [3]. Indeed, stems have a high water content (see Section 2.2.1. Water), which could be transferred to the wine during maceration. Pascual et al. presented similar conclusions and added that the stem surface could also capture ethanol molecules by adsorption [12].
Nevertheless, lower ethanol values are not always clearly observed. While comparing the impact of different technologies on the wine profile, Spranger et al. did not observe any real change when adding stems to the fermenting wines [79]. The impact of stems on alcohol content is not yet well understood; it might be interesting to compare these results with the moisture content of stems.

3.3. Impact on Polyphenolic Composition

3.3.1. Total Phenolic Compounds

Two methods were used to analyze the total phenolic fraction of wines: Folin Index (FI) and Total Polyphenol Index (TPI). The FI is based on the Folin–Ciocalteu method. TPI uses the typical properties of the benzenic structures found in phenolic compounds, which can absorb at 280 nm when measured by spectrometry. Even though this measure is not very accurate for quantification, it gives a good indication of the phenolic content in wines. Several types of phenolic compounds contribute to this index, such as anthocyanins and tannins, as well as a small fraction of non-phenolic compounds [50]. Data available in the literature are summarized in Table 15. For most of the grape varieties, the total phenolic content increased when stems were included during winemaking. Castelao is the only grape variety for which no significant difference was found. The magnitude of the variation seems to be correlated both to varietal differences and maceration duration.

3.3.2. Non-Flavonoid Compounds

Even if some interesting non-flavonoid compounds were found in the grape stem extracts, their transfer and presence in wines has not been thoroughly investigated. Pascual et al. examined hydroxycinammic acid derivatives in Cabernet sauvignon wines, and Benitez et al. in Palomino fino [12,85]. These compounds were measured using reversed-phase HPLC, diode array detector, electrospray ionization, and tandem mass spectrometry systems (HPLC-DAD-ESI-MS). Overall, the tested wines were not significantly affected by stem contact in terms of phenolic acid content. Caftaric and gallic acids that were the main phenolic acids found in grape stem extracts does not seem to be significantly transferred to wine (Table 16).
Although the concentration of stilbene and stilbenoid compounds has been extensively studied for the antioxidant properties in the stem extracts, to our knowledge, their transfer from the stem to the wine has not been studied. However, in their study, Bavaresco et al. mimicked alcoholic fermentation, using an hydroalcoholic solution (11% (v/v) ethanol and 250 ppm (v/v) methanol) as an extraction solvent, in order to quantify the content of potentially extractable stilbenes [51]. Their results showed that only trans-resveratrol was extracted. For this experiment, the ethanol content remained constant during the extraction. It would be interesting to carry out the same study with an increasing concentration of ethanol and also in fermenting must in order to valid the transfer of these compounds to the wine.

3.3.3. Flavonoid Compounds

To our knowledge, the impact of stem contact on flavones or flavanones content has not been reported in the literature. Pascual et al. [12] studied the impact of stem contact on the flavonol content of Cabernet Sauvignon wine. Their results showed a significant decrease in total flavonols, mainly resulting from aglycones, and they suggested that stems might absorb these compounds. Further study is needed to confirm this hypothesis. Apart from this study, no further information was found in the literature.

Flavan-3-ols and Proanthocyanidins

Total proanthocyanidin content results are shown in Table 17 and individual flavan-3-ol and proanthocyanidin composition results are shown in Table 18. The total proanthocyanidin content seems to significantly increase when either stems or whole clusters are kept during maceration, regardless of the grape variety. According to Casassa et al.’s latest study on Pinot Noir wines, the increase seems to be more or less correlated to the amount of stems in the vat, whether fresh or dry [83]. This observation is also valid for Suriano et al.’s results on Primitivo wines [81].
According to the flavan-3-ol monomeric and polymeric composition, the intensity of the variations differed across studies. It is hard to draw conclusions from these values because stem proportions added for alcoholic maceration and maceration duration varied. However, there was a tendency for higher concentrations of catechins, epicatechin, dimer B1 and B3 in nearly every study. For dimers B2 and B4, the concentration variation did not seem to depend on either the grape variety or the maceration duration.
Suriano et al. reported a clear increase in the concentration of catechin, epicatechin, epicatechin gallate, and procyanidins B1 and B3 that evolved to the quantity of full clusters present in the wines [81]. Conversely, gallocatechin and procyanidin T2 (not shown here) concentrations decreased when stems were added, which could be linked to adsorption by stem bodies or interactions with wine molecules.
Little information is available regarding the impact of stems on proanthocyanidin mDP [12,79,80]. Available results did not show significant difference induced by stem contact (data not shown).

Anthocyanins

According to several authors, stem contact during maceration decreased total anthocyanin concentration [12,30,79,80,81] (Table 19). Spectrophotometric measurements to determine anthocyanin concentration are possible thanks to chemical methods based on anthocyanin color properties. pH variation methods, such as the Puissant–Leon method are based on matrix acidification by HCl that cause a change in anthocyanin color [50,87]; the SO2-bleaching method is based on the discoloration of anthocyanins in the presence of sulfur dioxide [88]. These measurement techniques are only partially accurate, because they only quantify the sum of free anthocyanin and the part of the combined anthocyanins fraction that is sensitive to sulfur-dioxide bleaching [50]. However, among the wines tested with the same method, anthocyanin concentration decreases seemed proportional with the stem contact maceration duration, with Malbec wines studied by Ribéreau-Gayon and Mihlé the only exception; anthocyanin content decreased with stem addition but not proportionally to the contact duration [30]. When whole grape clusters were used, the concentration in anthocyanin was even lower and tended to decrease with an increasing proportion of full clusters [79,80,81]. Although molecular interactions between compounds from the stems and the musts could explain part of this anthocyanin loss, Ribéreau-Gayon and Mihlé rejected this hypothesis, because the addition of stem extract did not affect the anthocyanin concentration [30]. Instead, the authors explained this loss by the adsorption phenomenon provoked by the stem bodies on the anthocyanin molecules, similar to the explanation given by other authors [81]. However, this finding was not found in Casassa et al.’s latest study [83]. The anthocyanin content of wine made of 50% and 100% of whole cluster was not significantly different from the control wine (fully destemmed). The authors highlighted that the vintage conditions can have more of an impact on the anthocyanin content than the winemaking process; an additional argument shows that it would be relevant to evaluate and take into consideration the maturity of the stems.
Some authors studied the individual anthocyanin composition by HPLC analysis (Table 20). Anthocyanin 3-monoglucosides, which are the major anthocyanins in the tested wines, were the most affected by stem addition, where the malvidin-3-O-glucoside counted as more than 50% of the fraction. Studies led by Spranger et al. showed a 19.6% decrease in its concentration after seven days of stem contact, and this decrease was even more important (30.2%) with extended stem contact (21 days) [79]. Similar results were reported by Sun et al. and Suriano et al. who found decreases of about 17 to 18% [80,81]. A decrease in anthocyanin 3-monoglucosides was also reported by Pascual et al. but to a lesser extent (6.0%) [12]. Furthermore, the proportion of stems added had a smaller impact on the anthocyanin 3-monoglucoside concentration than the stem contact duration.
Other types of anthocyanins, combined with specific molecules, were affected by stems, including p-coumarylated anthocyanins that showed decreasing patterns depending on stem contact duration [12,81]. However, the quantity of stems in contact with the must did not significantly lower their concentration. Decreases in acetylated anthocyanins did not always reach significance.

3.4. Impact on the Wine Sensorial Characteristics and Aging Potential

3.4.1. Color

Wine color measurements were mainly performed using color intensity calculation, summation of the absorbance at 420, 520, and 620 nm, and the hue ratio between absorbance at 420 and 520 nm. The results are shown in Table 21. Using stems during winemaking tended to decrease color intensity and increase hue, giving the wine a more reddish color, but this result was not always observed. In some cases, adding stems decreased the hue [30,81]. CIELAB measurements were also performed in some studies but results were not significantly different when stems were included during winemaking [79,80] (data not shown). Different explanations were offered for the color changes: dilution linked to stem water released in the must, pH modification allowing the transformation of anthocyanins into uncolored compounds, and possible adsorption of anthocyanin content by the stems [3,12,30]. The impact of stems on color intensity was more important for short maceration and tended to have no significant impact on long duration maceration [30].
Other work reported increased color intensity despite lower anthocyanin content [81]; this was explained as stems bringing more oxygen to the must and promoting condensation between anthocyanins–tannins–acetaldehyde, which is important for color stability.

3.4.2. Aroma and Volatile Compounds

Spranger et al. and Benitez et al. studied the impact of various proportions of stems on the volatile fraction of wines [79,85]. With Castelao wines, 1-hexanol (grass odor) and ethyl decanoate (waxy odor) were the two compounds affected after 21 days of maceration with stems. Among all the other compounds identified in both studies, no significant effect of stem quantity was found on the concentration of molecules.
Among the 14 volatile compounds identified by Casassa et al. the concentration of β-damascenone, a nor-isoprenoid described as a fruity aroma enhancer, was higher when whole clusters where used during winemaking [84]. However, sensory analysis did not confirm these results because wines were described as less fruity. No green taste was identified in the wines, although compounds that give a green taste, such as 1-hexanol, isobutanol, or hexanoic acid, differed in concentration when stems were added. In Casassa et al. latest study, wines made of 100% whole clusters of grapes showed higher levels of ethyl cinnamate and benzaldehyde (spice and almond-like odor) and those in which dried stems were added exhibited higher levels of esters (potential fruity and floral odors) [83]. Sensory analysis confirmed these differences: 100% whole cluster wine had higher vegetal, cooked fruit flavors and spicy notes and wines made with dried stems had more herbal and fruity odors. Compounds known to bring a vegetal note to wine, such as methoxypyrazines, were not examined in this study.
IBMP is known to be easily extracted during pressing and maceration [74]. Because its concentration is high in stems, stem contact processes could increase IBMP levels in must and wine. Consistent with this hypothesis, Hashizume and Samuta identified methoxypyrazine compounds in wines from Cabernet Sauvignon and Chardonnay varieties fermented with and without stems [73]. Compared to wines made from fully destemmed bunches, stem contact wines had significantly higher concentrations of 2-methoxy-3-isopropylpyrazine (IPMP), 2-methoxy-3-sec-butylpyrazine (SBMP), and IBMP. Similar patterns were found in a more recent study on Sauvignon Blanc wines [82]. The results are presented in Table 22.
More precisely, IBMP was the only compound detected without stem addition at concentrations greater than 1 ng/L, suggesting that IPMP and SBMP were introduced by the stem contact [73,82].
Although values were the same order of magnitude, differences in concentration reported in the literature could be linked to winemaking practices involving various stem quantities in contact with the must. Moreover, the number of wounds provoked during grape harvest, the lignification state of stems, and potential varietal effects could be responsible for these variations. Further work is needed to investigate the implication of each factor in methoxypyrazine concentration in stem-contact wines.

3.4.3. Taste

Studies that performed sensory analysis of wines made using either stems or whole clusters reported similar conclusions regarding bitterness and astringency. Pascual et al. reported that the stem significantly increased astringency and tended to increase the bitterness of Cabernet Sauvignon wines [12]. More recently, Casassa et al. reported similar results for Pinot Noir wines, for both fresh or dried stems [83,84]. In these two studies, the degree of stem lignification was not considered. It might be interesting to examine whether the sensory impact is different when the stems are lignified.
On the other hand, in the conclusions of their latest study, Casassa et al. wrote about the impact of stems on the “freshness” of the wines [83]. This effect is relatively well known from an empirical point of view. In this study, the wines were noted as fresher, although no chemical compound could explain it. As mentioned before, methyl salicylate has been identified to bring a fresh and minty aroma [77]. It would be interesting to dose this compound in wines made with stems in order to see if it could explain the increased “freshness”. It seems that the stems have a simultaneous action on several factors, so the result is an increase in complexity and freshness of the wines. More studies seem to be needed to understand the complex effect of stems on wine quality.

3.4.4. Wine Aging Potential and Stability

The impact of stem use on Tinta Miuda wine aging was studied by Sun and Spranger, who found that anthocyanin content was significantly affected after two years of bottle aging [89]. The anthocyanin content at bottling was higher for wines made without stems. It then decreased and reached a level where no significant difference could be detected between the two winemaking techniques. Individual anthocyanins were affected differently, where decreased delphinidin-3-O-glucoside, cyanidin-3-O-glucoside, petunidin-3-O-glucoside, peonidin-3-O-glucoside, and palvidin-6′′-O-acetylglucoside concentrations were not significantly affected by the presence of stems. Conversely, when stems where used, palvidin-3-O-glucoside and palvidine-3-O-glucoside-pyruvic acid adducts decreased more, whereas decreases in palvidine-6′′-O-p-coumarylglucoside and peonidine-6′′-O-p-coumarylglucoside were less important. The flavan-3-ols and the proanthocyanidins suggested that procyanidin B2, B3, B4, and C1 decreases were significantly lower when stems were used. Total oligomeric and polymeric proanthocyanidin contents showed lower decreases when stems were used. These results suggest that using stems during winemaking allows for a similar stability of anthocyanins as the control wine, but provides better stability of flavan-3-ols and proanthocyanidins. After two years of aging, mDP was significantly lowered but no significant differences were found between the stem and non-stem contact wine; the percentage of galloylation was not affected by the winemaking process or aging time; the total polyphenolic content stayed stable, suggesting that polyphenolic compounds were converted to other phenolic forms. No significant differences were found in wine color intensity, but the color of stem contact wine appeared more yellowish orange than the one of the non-stem contact wine.
Suriano et al. reported similar results regarding the anthocyanins content in Primitivo wines [81]. Fully destemmed grape wines showed higher anthocyanin content after 12 months of aging. For 25 to 50% of non-destemmed grape wines, color intensity increased, suggesting condensation phenomena between anthocyanins and tannins. No difference was found in the color shade after 12 months of aging, suggesting that using the whole harvest with stems could improve the color stability of wines during aging.
In Casassa et al.’s study, the Pinot Noir wines showed similar aging behavior after three and 12 months, regardless of the winemaking technique (added stems or 20% whole cluster) [84]; the anthocyanin content decreased, polymeric pigment levels increased, and tannin contents were stable. Same results were obtained for wine obtained using 50%, 100% whole cluster or dried stem [83]. Few studies have examined the impact of using stems on the aging potential. Excluding the rearrangement of anthocyanins classically observed in red wines, few conclusions can be drawn from these works. Although the antioxidant activity of stem extracts has been widely studied, the parallel with winemaking has not yet been sufficiently examined. If adding stems represents a source of antioxidants, it could be a way to reduce SO2. Two articles on this topic were found in the literature [76,86]. Ruiz-Moreno et al. showed positive results using stem extracts as a SO2 alternative in model solution for both antioxidant and antimicrobial action. In the other study, performed on red wines, Esparza et al. highlighted that the use of stem extracts could be a promising strategy to reduce SO2 in wines, but it still needs some optimization. In addition, a recent study on the use of grape stem extracts for protein precipitation showed that, in a model wine solution, these extracts could represent a good agent to remove unstable proteins [39]. Among the different grape varieties tested, Chasselas stem extracts, rich in polyphenols, showed the best results. Although used almost exclusively in red wines, it would be interesting to investigate the influence of stems on the protein stability of white wines.
Table 23 summarizes the main effects of the use of stems or whole clusters on wines from an oenological point of view.

4. Conclusions

Analysis of the available research has allowed us to highlight the main compounds that compose stems. Although they do not seem to contain any new specific compounds, the transfer of certain molecules such as metal ions, phenolic compounds, or even some aromatic compounds, may induce changes in equilibrium, and thus could explain the increase in aromatic complexity in some cases. Stems’ high phenolic compound content could make them good candidates for antioxidants and stabilizers. Stem composition was mainly studied to evaluate their potential use as a source of compounds of interest, particularly phenolic compounds and stilbenes, for other sectors, such as pharmacy and health. Consequently, stem extracts are often obtained through extraction procedures that involve treating the stems upstream (freezing, grounding, etc.) using strong organic solvents to produce good yields. This does not represent the extraction phenomena that could take place during the winemaking process. It would be interesting to approach the extraction procedures in a similar way to alcoholic fermentation and maceration processes. This would identify which stem components have a real impact on the must and the wine matrices.
For winemaking trials with stems, the variability of grape varieties and limited knowledge regarding stem maturity makes it difficult to compare the different studies. However, several points emerged, such as decreased alcohol content, increased pH, and decreased anthocyanins content. Very few studies investigated the impact of stems on the stability and aging potential of wines. It would be interesting to look further into the phenolic compounds present in the stem extracts and their antioxidant capacity.
In using whole bunches of grapes or only stems, it is important to consider the general state of the stems. Very few studies focused on stem maturity or the degree of lignification, which varies according to the grape variety, the terroirs, and the vintage. Using stems is not systematic for a winegrower; it depends on the conditions of the vintage. Therefore, it is appropriate to consider the state of the stems to acquire new knowledge and facilitate a better understanding of this winemaking technique.

Author Contributions

Conceptualization, B.B., L.Z., M.C. and M.B.; writing—original draft preparation, M.C., Á.D.-N. and M.B.; writing—review and editing, M.B. and Á.D.-N.; supervision, G.B., F.L. and B.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors would like to thank the viticulture and the wine technology and analysis teams for their collaboration. The authors also wish to acknowledge and thank Alain Handley for his translation and editorial assistance.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

ABTS2,2-azinobis(3-ethylbenzthiazoline-6-sulphonic acid)
B1 (2, 3, 4 etc.)Procyanidin dimer B1 (2, 3, 4 etc.)
CAECatechin equivalent
CatCatechin
CUPRACCupric reducing antioxidant capacity
CyECyanidin equivalent
DMDry matter
DMACA4-(N,N′-dimethylamino)cinnamaldehyde
DPPH1,1-diphenyl-2-picrylhydrazine
EcEpicatechin
EcGEpicatechin gallate
EgcEpigallocatechin
EgcGEpigallocatechin gallate
ESIElectron spray I
FIFolin Index
FRAPFerric reducing antioxidant power
FWFresh weight
GAEGallic acid equivalent
GcGallocatechin
GC—MSGas Chromatography—mass spectrometry
HPLCHigh pressure liquid chromatography
IBMP2-methoxy-3-isobutylpyrazine
ICPInductive coupled plasma
IPMP2-methoxy-3-isopropylpyrazine
LCMSLiquid chromatography—mass spectrometry
MCMethyl cellulose
mDPMean degree of polymerization
ORACOxygen radical absorbance capacity
OZOdorant zone
SBMP2-methoxy-3-sec-butylpyrazine
TPITotal polyphenolic index

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Figure 1. Bunch of grapes and stem morphology.
Figure 1. Bunch of grapes and stem morphology.
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Figure 2. General composition of grape stems.
Figure 2. General composition of grape stems.
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Figure 3. Non-flavonoid and flavonoid polyphenolic compound of grapes and wine.
Figure 3. Non-flavonoid and flavonoid polyphenolic compound of grapes and wine.
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Table 1. Main chemical components of stems (values expressed in % DM).
Table 1. Main chemical components of stems (values expressed in % DM).
Grape VarietyCelluloseHemicelluloseLigninProteinsAsh
Pinot Noir [17] (Bellucci method)24.65 7.66
Pinot Noir [17] (Sluiter et al. method)25.313.9547.29 7.66
Pinot Noir [17] (Goering-Van Soest method)37.8814.9332.98 7.66
Red grapes [19] 34.614.5
Vitis vinifera L. [18]30.32117.46.17.0
Cabernet Sauvignon [13]23.011.612.8 to 22.65.810.8
Callet [13]23.313.18.37.1
Manto Negro [13]23.113.76.76.9
Merlot [13]27.112.75.711.2
Syrah [13]35.017.26.84.8
Tempranillo [13]19.610.24.910.0
Chardonnay [13]26.111.87.78.6
Macabeu [13]25.013.66.65.5
Parellada [13]26.314.011.26.4
Premsal Blanc [13]22.29.89.25.9
Manto Negro [23] 31.67.295.48
Premsal Blanc [21] 22.915.126.94
Alsacian white grape variety [16]36.324.539.6 3.9
Mix of Bonarda and Barbera [20]12.1925.732.35 6.11
Albariňo [24]29.9535.3322.94
Table 2. Total polyphenol content of stems of white and red grape varieties (mg GAE/100g DM).
Table 2. Total polyphenol content of stems of white and red grape varieties (mg GAE/100g DM).
Grape VarietyVintageTotal Polyphenol Concentration
White Grape Varieties
Aidani [29]20091072.6
Aidani [29]2010722
Asyrtiko [29]20091248
Asyrtiko [29]20101114.6
Athiri [29]2009399.9
Athiri [29]2010480.8
Chardonnay [31]20094764
Chasselas [39]2015300 to 4300 c
Fernao Pires [44]201711,015
French Colombard [32]19872430 a
French Colombard [32]19881980 a
Macabeu [31]20097809
Malvasia Fina [44]201712,309
Moscatel (Sanfins du Douro) [33]20173235
Moscatel (Sanfins du Douro) [33]20188305
Moscatel (Penajóia) [33]20177802
Moscatel (Penajóia) [33]201810,349
Moscatel (Medrões) [33]20173793
Moscatel (Medrões) [33]20188832
Moscatel [44]201710,871
Parellada [31]20098924
Premsal blanc [31]20099002
Premsal blanc [21]-8730
Premsal blanc [45]-17,200 to 22,900 d
Rabigato [44]20179471
Roditis [35]-3120 to 7468 d
Semillon [32]19871950 a
Semillon [32]19881690 a
Viosinho [44]20179699
Red Grape Varieties
Cabernet [39]20151200 to 2000 c
Cabernet Sauvignon [31]20097076
Cabernet Sauvignon [28]20002500 b
Cabernet Sauvignon [34]2009348.0
Callet [31]200911,525
Carnelian [32]19872170 a
Carnelian [32]19881850 a
Frappato [34]2009998.5
Mandilaria [29]20091057
Mandilaria [29]20101434.3
Manto Negro [31]20098470
Manto Negro [23]-11,600
Manto Negro [45]-29,400 to 38,400 d
Mavrotragano [29]20091011.1
Mavrotragano [29]2010557.9
Mazuelo [38]20161276 to 5104 b,d
Merlot [31]20094704
Merlot [39]2015900 to 2900 c
Nerello Mascalese (Lingualossa) [34]20092179.8
Nerello Mascalese (Milo) [34]20094000.1
Nerello Mascalese (Santa Venerina) [34]20091241.7
Nero d’Avola [34]20092632.9
Ruby Cabernet [32]19871950 a
Ruby Cabernet [32]19881730 a
Sousao [46]-3135 a
Syrah [31]20099642
Syrah [28]20002500 to 5000 b,e
Syrah [39]2015200 to 2250 c
Tempranillo [42]-4679
Tempranillo [31]20097622
Tempranillo [28]20001250 to 3750 b,e
Voidomato [29]2009840.2
Voidomato [29]2010610
a Unit mg GAE/100 g FM. b Unit mg GAE/L/100 g DM. c According to the size of the stem parts during extraction. d According to the extraction method. e According to the irrigation of the vine.
Table 3. Quantification of phenolic acids in white and red grape varieties (mg/kg DM).
Table 3. Quantification of phenolic acids in white and red grape varieties (mg/kg DM).
Grape VarietyVintageCaftaric AcidCoutaric AcidCoumaric AcidGallic AcidCaffeic AcidSyringic AcidFerulic Acid
White Grape Varieties
Aidani [29]200971.6 0.91711.43.51.1
Aidani [29]2010136 0.11050.1n.d.n.d.
Arinto [48]2018220 40
Asyrtiko [29]200969.6 1.1469n.d.1.8n.d.
Asyrtiko [29]2010146 0.74540.4n.d.n.d.
Athiri [29]20095.1 0.61220.50.4n.d.
Athiri [29]20106.1 0.71460.50.50.2
Chasselas [39]20151500 to 3600 b 30 to 250 b
Fernao Pires [47]-100
Fernao Pires [44]20171710
Fernao Pires [48]2018680 50
Malvasia Fina [44]201712,820
Moscatel (Sanfins du Douro) [33]2017135
Moscatel (Sanfins du Douro) [33]2018342
Moscatel (Penajóia) [33]2017480
Moscatel (Penajóia) [33]2018485
Moscatel (Medrões) [33]2017203
Moscatel (Medrões) [33]2018338
Moscatel [44]20175010
Rabigato [47]-250
Rabigato [44]20172280
Viosinho [47]-200
Viosinho [44]20171510
Red Grape Varieties
Amarela [47]-300
Cabernet Sauvignon (irrigated) [28]200020.311.78.16.9 14.7
Cabernet Sauvignon (non irrigated) [28]200030.82.71.4n.d. 11.4
Cabernet Sauvignon [39]2015900 to 1500 b 80 to 260 b
Castelao [48]2018200 n.d.
Mandilaria [29]200957.9 0.92865.31.40.7
Mandilaria [29]201041.1 1.570.41.3n.d.0.6
Mavrotragano [29]2009166 41829.21.21.5
Mavrotragano [29]201078.4 1.1902.80.3n.d.
Mazuelo [38]2016 43 to 310 c
Merlot [40]-40 a4.5 a
Merlot [39]2015800 to 1400 b 80 to 300 b
Nerello Mascalese (Milo) [34]2009 87.2
Nerello Mascalese (Lingualossa) [34]2009 71.9
Nero d’Avola [34]2009 49.8
Sousao [47]-900
Syrah (irrigated) [28]200012.5n.d.n.d.7.6 n.d.
Syrah (non irrigated) [28]200095.6389n.d. n.d.
Syrah [39]2015200 to 1600 b 10 to 150 b
Syrah [48]2018400 40
Tempranillo (irrigated) [28]200031.516.3n.d.n.d. n.d.
Tempranillo (non irrigated) [28]200066.932.80.9n.d. n.d.
Tinta Barroca [47]-1100
Tinta Roriz [48]2018430 n.d.
Touriga Nacional [47]-500
Touriga Nacional [48]2018980 n.d.
Voidomato [29]2009274 21958.62.4n.d.
Voidomato [29]201053.9 0.72783.51.4n.d.
a Unit mg/kg FM. b According to the size of the stem parts during extraction. c According to the extraction method.
Table 4. Quantification of trans-resveratrol and ε-viniferin in white and red grape varieties (mg/kg DM).
Table 4. Quantification of trans-resveratrol and ε-viniferin in white and red grape varieties (mg/kg DM).
Grape VarietyVintage trans-Resveratrolε-Viniferin
White Grape Varieties
Aidani [29]200974167
Aidani [29]2010124174
Arinto [48]20188070
Asyrtiko [29]2010178253
Asyrtiko [29]200987.6223
Athiri [29]200996415
Athiri [29]2010115499
Castealo [48]20187070
Chardonnay [52]2010n.d.60.6
Chardonnay [52]201242.225.7
Fernao Pires [53]2013 1.91
Fernao Pires [44]2017 900
Fernao Pires [48]20189080
Gewürztraminer [51]-393 a69 a
Malvasia Fina [44]2017 170
Moscatel [44]2017 760
Moscatel (Medrões) [33]2017 97
Moscatel (Medrões) [33]2018 122
Moscatel (Penajóia) [33]2017 75
Moscatel (Penajóia) [33]2018 96
Moscatel (Sanfins du Douro) [33]2017 23
Moscatel (Sanfins du Douro) [33]2018 39
Moscato [51]-100 a56 a
Palomino fino [52]2010traces24.7
Palomino fino [52]2012n.d.14.3
Pinot Gris [51]-159 a34 a
Rabigato [53]2013 29.9
Rabigato [44]2017 310
Sauvignon [51]-95 a171 a
Sauvignon blanc [52]2010n.d.147.1
Tocai friulano [51]-82 a35 a
Vijiriega [52]2010traces48
Viosinho [53]2013 26.1
Viosinho [44]2017 830
Red Grape Varieties
Cabernet franc [51]-238 a138 a
Cabernet Sauvignon [52]2012n.d.17.6
Granacha [52]2010traces29.4
Mandilaria [29]2009266476
Mandilaria [29]2010176282
Marzemino [51]-31 an.d.
Mavrotragano [29]200987.6258
Mavrotragano [29]201096.5235
Mazuelo [38]201621 to 162 b91 to 310 b
Merlot [51]-38 a54 a
Merlot [52]2012n.d.30.1
Nerello Mascalese (Lingualossa) [34]2009158.85176.13
Nerello Mascalese (Milo) [34]2009102.63114.75
Nero d’Avola [34]2009111.0725.80
Petit Verdot [52]2012n.d.20.5
Sousao [53]2013 24.8
Syrah [48]20186070
Syrah [52]2010122.571.1
Syrah (treatment A) [52]2010135.452
Syrah (treatment A) [52]20116441.7
Syrah (treatment B) [52]2011139.165.1
Tempranillo 1 [52]201079.860.5
Tempranillo 2 [52]201087.880.6
Tempranillo [52]2012n.d.28.3
Tinta Amarela [53]2013 2.2
Tinta Baroca [53]2013 10.8
Tinta Roriz [48]20187070
Tintilla de Rota [52]2010118.991.6
Tintilla de Rota [52]2012traces39.2
Touriga Nacional [53]2013 11.4
Touriga Nacional [48]2018140.00110
Vitis silvestris 1 [52]201049.959
Vitis silvestris 2 [52]2010traces38.7
Vitis silvestris 3 [52]20103374.8
Voidomato [29]2010174414
Voidomato [29]200992.9217
a Unit mg/kg FM. b According to the extraction method.
Table 5. Flavonol content quantified in different grape stem extracts (mg/kg DM).
Table 5. Flavonol content quantified in different grape stem extracts (mg/kg DM).
Grape VarietyVintageQuercetin-3-O-GalactosideQuercetin-3-O-GlucosideQuercetin-3-O-RhamnosideQuercetin-3-O-GlucuronideQuercetin-3-O-RutinosideQuercetinKaempferolKaempferol-3-O-RutinosideKaempferol 3-O-Glucoside
White Grape Varieties
Aidani [29]200987.257.717.3 9.40.5
Aidani [29]201019771.519.3 7.31
Arinto [48]2018 150
Asyrtiko [29]200919365.14.6 211.3
Asyrtiko [29]201030513724.1 5.60.8
Athiri [29]200914250.915.8 7.71.4
Athiri [29]201017061.119 9.21.6
Chasselas [39]2015 600 to 3000 b
Fernao Pires [47]- 40070.0 6050
Fernao Pires [44]2017 40,270140 14040
Fernao Pires [48]2018 440
Malvasia Fina [47]2017 73,790190 40020
Moscatel (Sanfins du Douro) [33]2017 211 22 519
Moscatel (Sanfins du Douro) [33]2018 285 117 821
Moscatel (Penajóia) [33]2017 387 45 1029
Moscatel (Penajóia) [33]2018 445 187 1230
Moscatel (Medrões) [33]2017 374 36 727
Moscatel (Medrões) [33]2018 423 104 1029
Moscatel [44]2017 29,270230 14040
Viosinho [44]2017 34,63050 10040
Viosinho [47] 80080.0 3075
Red Grape Varieties
Amarela [47]- 60050.0 5533
Cabernet Sauvignon [39]2015 500 to 800 b
Castelao [48]2018 240
Mandilaria [29]200912754.16.7 12.74.4
Mandilaria [29]20102431304.2 10.30.7
Mavrotragano [29]200922386.517.5 20.7
Mavrotragano [29]201014970.18.4 9.51.8
Mazuelo [38]2016 96 to 485 c 8 to 38 c
Merlot [39]2015 200 to 1000 b
Merlot [40]- 18.0 a 200.0 a traces a
Nerello Mascalese (Milo) [34]2009 36.4 70.7
Nerello Mascalese (Lingualossa) [34]2009 152.9 229.5
Nero d’Avola [34]2009 65.7 161.3
Rabigato [47]- 350.0 50.0 2525
Rabigato [44]2017 37,560150 4020
Sousao [47]- 1380120.0 7525
Syrah [39]2015 50 to 600 b
Syrah [48]2018 410
Tinta Barroca [47] 140120 15030
Tinta Roriz [48]2018 370
Touriga Nacional [47] 70025 8020
Touriga Nacional [48]2018 440
Voidomato [29]200920565.515.3 13.7n.d.
Voidomato [29]201012661.423.8 19.62.3
a Unit mg/kg FM. b According to the size of the stem parts during extraction. c According to the extraction method.
Table 6. Identification of flavan-3-ols and proanthocyanidins in grape stem extracts (ESI).
Table 6. Identification of flavan-3-ols and proanthocyanidins in grape stem extracts (ESI).
Compound[M-H]− (m/z)[M-H]+ (m/z)MS2 (m/z)
(+)-catechin [41]289 245; 205; 179; 203; 227; 165; 161
(+)-catechin [37,41] 291123; 139; 165; 273; 151; 147; 249
(−)-epicatechin [29,40,41,47,55] 291245; 205; 179; 203; 231; 271; 161
(−)-epicatechin [41]289 123; 139; 165; 151; 273; 147; 231
(epi)catechin gallate [29,47,55]441 331; 289; 169
Catechin gallate [41]441 289; 395; 169; 331; 245; 193; 405
Procyanidin dimer A [47]575 573; 477; 441
B1 Ec-(4β→8)-Cat [41]577 425; 407; 289; 451; 245; 287
B1 Ec-(4β→8)-Cat [41] 579427; 409; 291; 301; 247; 289; 287
B2 Ec-(4β→8)-Ec [41]577 425; 407; 287; 289; 451; 559; 299
Procyanidin dimer B [47]577 559; 425; 407; 287
Procyanidin dimer [36]577 289; 425; 407; 451; 559
(epi)gallocatechin-(epi)catechin dimer [47]593 575; 531; 425; 423; 273
Galloylated flavanol dimer (epi)catechin-(epi)catechin gallate [37,49] 731579; 291; 139
Procyanidin dimer gallate729 711; 577; 559; 451; 407; 289
Procyanidin dimer gallate [36]729 577; 559; 451; 407; 425; 289
Prodelphinidine gallate [36]745 593; 405; 575
Flavanol dimer [37] 579601
Flavanol trimer [49] 867579; 427
Procyanidin trimer [36]865 695; 577; 739; 451
Prodelphinidin trimer [36]881 695; 577; 755; 407; 303
Procyanidin trimer Gallate [36]1017 729; 407
Procyanidin tetramer [36]1153 865
Procyanidin tetramer [36]1169 881; 999; 1043; 729
Procyanidin pentamer [M-H]2− [36]720 635; 577; 521; 407
Table 7. Quantification of different flavan-3-ols and proanthocyanidins in grape stem extracts.
Table 7. Quantification of different flavan-3-ols and proanthocyanidins in grape stem extracts.
Grape VarietyVintageCatEcEcGB1B2B3B4
Unit: mg/g FM
Castelao Frances [56]19981.30.7 3.50.40.21
Merlot [40]-60traces
Tinta Miuda [57]199664.42.2 128.23.427.13.1
Touriga Francesa [56]199820.5 5.81.21.20.4
Viosinho [56]19981.50.6 1.20.40.10.1
Unit: mg/kg DM
Aidani [29]200969951.677.0 48.8383
Aidani [29]20107375834.2 36215
Arinto [48]201866030
Asyrtiko [29]2009108918.259.1 36.2454
Asyrtiko [29]2010185827.986.0 165646
Athiri [29]2009385n.d.53.9 55.2161
Athiri [29]201046212.364.7 66.2193
Cabernet Sauvignon [39]2015500 to 800
Cabernet Sauvignon [31]200949331 56421120n.d.
Cabernet Sauvignon (irrigated) [28]2000n.d.7.6
Cabernet Sauvignon (non-irrigated) [28]2000368.8n.d.
Callet [31]200945316 45420156n.d.
Castelao [48]201844040
Chardonnay [31]200931412 2551556n.d.
Chasselas [39]2015600 to 3000
Fernao Pires [48]20181270170
Macabeu [31]2009930.5 1331145n.d.
Mandilaria [29]2009126170.9108.0 96.6482
Mandilaria [29]2010169194.671.3 46.2993
Manto Negro [31]2009122 06 2461141n.d.
Mavrotragano [29]2009107779.8130.0 108587
Mavrotragano [29]2010102764.488.0 44.3243
Mazuelo [38]2016225 to 710
Merlot [31]200957524 86822132n.d.
Merlot [39]2015200 to 1000
Nerello Mascalese (Milo) [34]20093611.0 1370.2
Nerello Mascalese (Lingualossa) [34]20092066.3 793.3
Nero d’Avola [34]20091562.7 1771.4
Parellada [31]2009133958 187748222n.d.
Premsal blanc [31]200974040 121840104n.d.
Syrah [31]2009114624 1320traces208
Syrah [39]201550 to 600
Syrah [48]20181330110
Syrah (irrigated) [28]2000n.d.24.1
Syrah (non-irrigated) [28]2000n.d.n.d.
Tempranillo [31]20091269111 195894232n.d.
Tempranillo [42]-7640
Tempranillo (irrigated) [28]2000674.4338.5
Tempranillo (non-irrigated) [28]2000280.8n.d.
Tinta Roriz [48]2018162090
Touriga Nacional [48]20182030180
Voidomato [29]200979518995.3 n.d.349
Voidomato [29]2010712n.d.64.9 n.d.138
Cat = catechin; Ec = epicatechin; EcG = epicatechin gallate; B1, 2, 3 and 4 = proanthocyanidins dimers.
Table 8. Total proanthocyanidin content of grape stem extracts (mg/g DM).
Table 8. Total proanthocyanidin content of grape stem extracts (mg/g DM).
Grape VarietyTotal Proanthocyanidin
Method: Bate-Smith Reaction
Cabernet Sauvignon [31]124.9
Callet [31]202.3
Chardonnay [31]79.1
Macabeu [31]108.8
Manto Negro [31]165.3
Merlot [31]84.0
Parellada [31]165.2
Premsal blanc [31]181.4
Syrah [31]161.4
Tempranillo [31]147.3
Manto Negro [23]103
Premsal blanc [21]79.0
Roditis [35]55.5 to 255.7 b,d
Method: LCMS/MS Quantification
Amarela [47]39
Fernao Pires [47]35
Rabigato [47]27
Sousao [47]45
Tinta Barroca [47]45
Touriga Nacional [47]37
Viosinho [47]27
Method: Vanillin Assay
Castelao Frances [56]53.7 a
Manto Negro [45]between 217 and 270 c
Premsal blanc [45]between 126 and 162 c
Tinta Miuda [57]2.2 a
Touriga Francesa [56]52.8 a
Viosinho [56]37.8 a
Methyl Cellulose Precipitation
Tempranillo [42]24.29
a Unit mg/g FM. b mg CyE/100g DM. c mg CAE/g DM. d According to the extraction method.
Table 9. Mean degree of polymerization (mDP) and structural composition of stem polymeric proanthocyanidins.
Table 9. Mean degree of polymerization (mDP) and structural composition of stem polymeric proanthocyanidins.
Grape Variety mDPGeneral CompositionTerminal UnitsExtension Units
% Cat% Ec% EcG% EgC% Cat% Ec% EcG% EgC% Cat% Ec% EcG% EgC
Cabernet sauvignon [31]Phloroglucinolysis Method5.925741.0 97tr3 1189
Callet [31]4.729701.0 8974 1288
Chardonnay [31]4.628711.0 8965 1189
Macabeu [31]6.224751.0 83116 1387
Manto Negro [31]5.826731.0 97tr3 1189
Merlot [31]6.025750.0 97tr3 1090
Parellada [31]5.027721.0 9523 1090
Premsal blanc [31]8.525741.0 95tr4 1684
Syrah [31]6.122771.0 97tr3 793
Tempranillo [31]6.920791.0 95tr5 892
Stems Vitis vinifera sp. [55]Thiolysis Method516.855.317.110.584.211.34.5n.d.6.562.319.112.2
Commercial stem powder [55]6.623.759.38.08.9100.0n.d.n.d.n.d.13.567.39.110.1
Chardonnay [40]9.11469.415.70.8
Clairette [40]7.717.368.413.40.9
Merlot [40]9.214.467.715.62.48.61.80.6 5.865.815.02.4
Négrette [40]10.211.761.721.15.4
Pinot [40]8.215.365.118.11.5
Tannat [40]8.71365.519.81.7
mDP = Mean degree of polymerization; Cat = catechin; Ec = epicatechin; EcG = epicatechin gallate; Egc = epigallocatechin.
Table 10. Antioxidant activity of grape stem extracts (ABTS and FRAP methods).
Table 10. Antioxidant activity of grape stem extracts (ABTS and FRAP methods).
Grape Variety ABTSFRAP
Unit: mg Trolox/g DM
Arinto [48]87.687.6
Cabernet Sauvignon [31]168.9114.8
Callet [31]253.2170.1
Castelao [48]115.1140.2
Chardonnay [31]99.765.4
Fernao Pires [44]150.2
Fernao Pires [48]172.7247.8
Macabeu [31]131.785.5
Malvasia fina [44]275.3
Manto Negro [31]198.2134.6
Merlot [31]109.876.6
Moscatel [44]300.3
Parellada [31]223.4159.1
Premsal blanc [31]218.5169.1
Rabigato [44]250.3
Syrah [31]203.1155.3
Syrah [48]147.7212.7
Tempranillo [31]186.8127.4
Tinta Roriz [48]175.2235.3
Touriga Nacional [48]210.2257.8
Viosinho [44]200.2
Unit: mM Trolox/100 g DM
Amarela [47]5737
Fernao Pires [47]3125
Mazuelo [38]8 to 30 a4 to16 a
Moscatel (Sanfins du Douro) 2017 [33]3833
Moscatel (Sanfins du Douro) 2018 [33]6774
Moscatel (Penajóia) 2017 [33]7384
Moscatel (Penajóia) 2018 [33]7385
Moscatel (Medrões) 2017 [33]4141
Moscatel (Medrões) 2018 [33]6975
Rabigato [47]3220
Sousao [47]7046
Tinta Barroca [47]5940
Touriga Nacional [47]5030
Viosinho [47]4024
a According to the extraction method.
Table 11. Identification of aromatic compounds in grape stem extracts.
Table 11. Identification of aromatic compounds in grape stem extracts.
Grape VarietyCompoundRetention IndexOdor Description
DB-WAXUltra-1DB-5
Cabernet Sauvignon and Chardonnay [73]hexanal1099800 green
(E)-2- hexanal1200844 green
(Z)-1,5-octandien-3-one1346963 geranium-like, metallic green
2-methoxy-3-isopropylpyrazine13941092 grassy, earthy
unknown1484- cucumber-like
2-methoxy-3-isobutylpyrazine15001211 herbaceous, earthy
(E,Z)-2,6-nonadienal15611150 cucumber-like
dodecanal17371402 citrus skin -like
Syrah [76]3-isobutyl-2-methoxypyrazine1530 1184green pepper
γ-octalactone1877 1276sweet, almond, coconut
trans-4,5-epoxy-E-2-decenal2011 1381metallic
furaneol (2.5-dimethyl-4-3(2H)-furanone)2036 1073caramel, red fruit jam aroma
eugenol (4-allyl-2-methoxyphenol)2171 1365clove
sotolon (3-hydroxy-4.5-dimethylfuran-2(5H)-one2198 1105curry
vanillin1564 1407vanilla
Table 12. pH and acidic composition of wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Table 12. pH and acidic composition of wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Grape VarietyModalityMaceration Time (Days)pHTitrable Acidity (g/L)Total Acidity (g/L Tartaric Acid)Volatile Acidity (g/L Acetic Acid)Tartaric Acid (g/L)Malic Acid (g/L)Lactic Acid (g/L)Acetic Acid (mg/L)Phosphoric Acid (mg/L)Citric Acid (mg/L)Succinic Acid (g/L)Shikimic Acid (mg/L)
Cabernet sauvignon [3]no stem73.78 1.446b4.2200.322161795a571a1.576
stem addition3.85 1.308a4.3030.304165948b633b1.486
Cabernet sauvignon [12]no stem153.23a5.5b
stem addition3.45b4.7a
Castelao [79]no stem73.19 7.250.482.7b1.7000.0
stem addition3.22 7.150.472.6a1.7000.0
no stem213.19a 7.250.482.7b1.7000.0a
stem addition3.28b 6.550.492.5a0.9000.300b
Merlot [3]no stem73.54
stem addition3.54
Muscat bailey A [3]no stem73.83a 1.105b3.7870.343276669a7291.228b
stem addition3.95b 1.011a3.7900.382247926b7671.032a
Pinot Noir [3]no stem addition73.60a
stem addition3.69b
Pinot Noir 2014 [84]no stem103.74
stem addition3.74
Pinot Noir 2015 [84]no stem103.66bc4.03b 0.90b 0.7601.31
stem addition3.71c3.86a 0.94b 0.6601.30
20% whole cluster3.63ab3.96a 0.85a 0.8101.31
Pinot Noir 2016 [83]no stem addition103.36b6.4b 0.77b 0.050.86ab
50% whole cluster3.55a6.8a 0.81b 0.050.81ab
100% whole cluster3.53a6.8a 1.11a 0.060.79b
dried stems3.52a6.5ab 0.85b 0.060.87a
Pinot Noir 2017 [83]no stem addition103.31c7.1 0.79b 0.040.58b
50% whole cluster3.42bc7.2 0.95ab 0.040.66ab
100% whole cluster3.60a7.2 1.12a 0.040.76a
dried stems3.51ab7.1 0.91ab 0.060.72a
Primitivo [81]no stem103.91 6.150.751.601.730.04 1.141.2818.4a
25% whole cluster3.84 6.380.751.532.40.05 0.961.2323.7b
50% whole cluster3.90 6.30.66a1.691.70.06 0.971.3322.4b
Tinta Miuda [80]no stem63.0 8.60.8
stem addition3.0 8.50.7
Table 13. Mineral composition of wines made with and without stem addition (mg/L) (for each study, different letters indicate significant statistical differences).
Table 13. Mineral composition of wines made with and without stem addition (mg/L) (for each study, different letters indicate significant statistical differences).
Grape VarietyModalityKPCaMgNaMnFeCuZn
Cabernet Sauvignon [3]no stem addition1454a198a62a714.40.65.80.5a0.3
stem addition a1927b277b69b734.80.85.00.9b 0.3
Muscat Bailey A [3]no stem addition2046a208a74a55a4.70.85.10.90.5a
stem addition a2476b389b90b60b4.70.97.00.80.8b
Tinta Miuda [80]no stem addition1065.8a 79.2a88.0a12.4a 2.40.1
stem addition b1088.6b104.0b96.0b20.0b2.00.1
Stems were left in contact with the must for: a 7 days; b 6 days.
Table 14. Ethanol content in wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Table 14. Ethanol content in wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Grape VarietyModality Maceration time (Days)Alcohol (% v/v)
Cabernet sauvignon [12]no stem addition15≈13b
stem addition≈12.6a
Castelao [79]no stem addition713.3
stem addition13.3
no stem addition2113.3
stem addition13.2
Pinot Noir 2014 [84]no stem addition713.03b
stem addition 12.75a
Pinot Noir 2015 [84]no stem addition1015.16b
stem addition 15.03b
20% whole cluster 14.31a
Pinot Noir 2016 [83]no stem addition1013.07
50% whole cluster13.24
100% whole cluster13.02
dried stems13.48
Pinot Noir 2017 [83]no stem addition1014.54ab
50% whole cluster13.90b
100% whole cluster14.24ab
dried stems14.68a
Primitivo [81]no stem addition1019.67b
25% whole cluster19.38c
50% whole cluster20.05a
Tinta Miuda [80]no stem addition68.4b
stem addition7.7a
Table 15. Total polyphenol compounds in wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Table 15. Total polyphenol compounds in wine made with and without stem addition (for each study, different letters indicate significant statistical differences).
Grape VarietyModalityMaceration Time (days)Total Phenolic Compounds (FI)
(mg GAE/L)
Total Polyphenol Index (TPI)
Cabernet sauvignon [12] no stem addition15 42.0a
stem addition 48.2b
Cabernet sauvignon [3]no stem addition71769a
stem addition2160b
Castelao [79]no stem addition7 46.2
stem addition 50.0
no stem addition7 46.2
stem addition21 49.0
Merlot [3]no stem addition71483a
stem addition1923b
Muscat bailey A [3]no stem addition71334a
stem addition1671b
Pinot Noir [3]no stem addition71013
stem addition1100
Primitivo [81]no stem addition102685a
25% whole cluster3127b
50% whole cluster3164b
Tinta Miuda [80]no stem addition6 26.47a
stem addition 32.19b
Table 16. Impact of stems on phenolic acids composition of the wine (mg/L) (for each study, different letters indicate significant statistical differences).
Table 16. Impact of stems on phenolic acids composition of the wine (mg/L) (for each study, different letters indicate significant statistical differences).
Grape VarietyModalityMaceration Time (Days)Gallic AcidSyringic AcidCaftaric Acid2-S-Gluthationyl Caftaric Acidtrans p-Coutaric Acidcis p-Couratic AcidFertaric AcidCaffeic AcidTrans p-Coumaric AcidFerulic Acid
Palomino fino [85]100% whole cluster92.401.44a36.987.389.083.490.655.09a0.380.32
75% whole cluster910.471.7640.95a10.129.49b3.620.652.82b0.41b0.62
50% whole cluster96.081.2037.17b8.369.24a3.49a0.644.270.27b0.35
25% whole cluster93.291.82b38.578.5310.14.27b0.865.000.56a0.41
Cabernet sauvignon [12]no stem addition15 18.38 1.030.620.452.12
stem addition 20.24 1.030.72 0.502.20
Table 17. Impact of stems during winemaking on total proanthocyanidin content (for each study, different letters indicate significant statistical differences).
Table 17. Impact of stems during winemaking on total proanthocyanidin content (for each study, different letters indicate significant statistical differences).
Grape VarietyVintage Modality Maceration Time (days)Total Proanthocyanidin (mg/L)
Method: Bate-Smith Reaction
Cabernet [30]1966no stem addition41700
stem addition2100
Cabernet [30]1966no stem addition204200
stem addition4500
Cabernet [30]1967no stem additionn.d.1700
stem addition2000
Malbec [30]1966no stem addition42400
stem addition3500
no stem addition83200
stem addition3900
no stem addition143500
stem addition4500
no stem addition303700
stem addition4700
Merlot [30]1966no stem addition82500
stem addition4000
Method: Vanillin Assay
Primitivo [81]2012no stem addition101744a
25% whole cluster2180b
50% whole cluster2275b
Method: Precipitation Methods
Cabernet sauvignon 1 [12]2013no stem addition15403a
stem addition778b
Pinot Noir 2 [84]2014no stem addition10370
20% whole cluster350
Pinot Noir 2 [84]2015no stem addition10540b
stem addition 860c
20% whole cluster440a
Pinot Noir 2 [83]2016no stem addition10100a
50% whole cluster210b
100% whole cluster320c
dried stems325c
Pinot Noir 2 [83]2017no stem addition10112a
50% whole cluster175a
100% whole cluster270c
dried stems275c
1 Methyl cellulose precipitation. 2 Protein precipitation (mg/L catechin equivalent (CE).
Table 18. Impact of stems during winemaking on flavan-3-ol monomeric and polymeric composition.
Table 18. Impact of stems during winemaking on flavan-3-ol monomeric and polymeric composition.
Grape VarietyVintageModality Maceration Time (Days)CatEcGcEcGEgcEgcGB1B2B3B4
Cabernet sauvignon [12]2013stem addition15++0++ ++
Castelao [79]2000stem addition7+0 +000
stem addition21++++ 0000
Primitivo [81]201225% whole cluster10++0+++++++
50% whole cluster10++++−−+++++++++++
Tinta Miuda [57]1996stem addition21++++ 0000
Tinta Miuda [80]1998stem addition6+++ +++0+++0
Cat = catechin; Ec = epicatechin; Gc = gallocatechin; EcG = epicatechin gallate; Egc = Epigallocatechin; EgcG = Epigallocatechine gallate; B1, 2, 3 and 4 = proanthocyanidins dimers; % of variation: 0–50 (+/−); 50–100 (++/−−); 100–250 (+++/−−−); 250–500 (++++/−−−−); >500(+++++/−−−−−).
Table 19. Impact of stems during winemaking on total anthocyanin content (for each study, different letters indicate significant statistical differences).
Table 19. Impact of stems during winemaking on total anthocyanin content (for each study, different letters indicate significant statistical differences).
Grape VarietyVintageModalityMaceration Time (Days)Total Anthocyanin (mg/L)
Method: pH Variation—HCl
Primitivo [81]2012no stem addition10401 1 a
25% whole cluster374 1 b
50% whole cluster368 1 b
Cabernet sauvignon [12]2013no stem addition15474.5 3 b
stem addition426.4 3 a
Method: SO2 Bleaching
Cabernet [30]1966no stem addition4800 2
stem addition690 2
Cabernet [30]1966no stem addition20800 2
stem addition690 2
Cabernet [30]1967no stem additionn.d.710 2
stem addition700 2
Castelao [79]2000no stem addition7283 3
stem addition261 3
no stem addition21283 3 b
stem addition221 3 a
Malbec [30]1966no stem addition4630 2
stem addition570 2
no stem addition8610 2
stem addition500 2
no stem addition14600 2
stem addition540 2
no stem addition30390 2
stem addition320 2
Merlot [30]1966no stem addition85402
stem addition580 2
Pinot Noir [84]2014no stem addition10270 3
20% whole cluster260 3
Pinot Noir [84]2015no stem addition10250 3
stem addition270 3
no stem addition10250 3
20% whole cluster280 3
Pinot Noir [83]2016no stem addition10251 3
50% whole cluster250 3
100% whole cluster251 3
dried stems251 3
Pinot Noir [83]2017no stem addition10150 3
50% whole cluster140 3
100% whole cluster135 3
dried stems150 3
Tinta Miuda [80]1998no stem addition6148.77 3 b
stem addition129.72 3 a
1 mg/L malvidin chloride. 2 mg/L unspecified reference. 3 mg/L malvidin-3-O-glucoside.
Table 20. Impact of stems during winemaking on individual anthocyanin content (mg/L) (for each study, different letters indicate significant statistical differences).
Table 20. Impact of stems during winemaking on individual anthocyanin content (mg/L) (for each study, different letters indicate significant statistical differences).
Grape VarietyVintageModalityMaceration Time (Days)Delphinidin-3-O-GCyanidin-3-O-GPetunidin-3-O-GPeonidin-3-O-GMalvindin-3-O-GAcetylated AnthocyaninsP-Coumarylated Anthocyanins
Reference Standard: Malvidin Chloride
Primitivo [81]2012no stem addition105.67a0.77a15.31a8.72a181.37a19.6b26.44a
25% whole cluster 4.42b0.66b12.6b7.51b150.50b21.65a22.98b
50% whole cluster 4.3b0.54c12.25b7.31b149.21b18.27b22.36b
Reference Standard: Malvidin-3-O-Glucoside
Cabernet sauvignon [12]2013no stem addition15 27.211.6b
stem addition 27.08.9a
Castelao [79]2000no stem addition710.8b1.6b13.5b19.1b115.9bn.d.16.6b
stem addition9.1a1.4a10.9a15.6a91.9an.d.15.1a
no stem addition2110.8b1.6b13.5b19.1b115.9bn.d.16.6
stem addition8.1a1.2a9.3a13.2a80.2an.d.12.3
Pinot Noir [84]2014no stem addition104n.d.928168
20% whole cluster4n.d.928160
Pinot Noir [84]2015no stem addition104382496
stem addition4382596
20% whole cluster43825112
Tinta Miuda [80]1998no stem addition65.73b1.89b6.59b17.51b62.96b12.7715.2
stem addition5.19a1.86a5.93a13.94a51.52a11.8113.15
Table 21. Impact of stems on color intensity and hue of wine (for each study, different letters indicate significant statistical differences).
Table 21. Impact of stems on color intensity and hue of wine (for each study, different letters indicate significant statistical differences).
Grape VarietyVintageModalityMaceration Time (Days)Color Intensity A420 + A520 + A620Hue A420/A520
Cabernet [30]1966no stem addition41.510.42
stem addition1.110.55
Cabernet [30]1966no stem addition201.390.55
stem addition1.240.43
Cabernet [30]1967no stem additionnd1.350.41
stem addition1.210.51
Cabernet sauvignon [3]1996no stem addition7 0.505a
stem addition 0.592b
Cabernet sauvignon [12]2013no stem addition159.7b0.529a
stem addition8.1a0.583b
Malbec [30]1966no stem addition41.520.52
stem addition1.220.56
no stem addition81.620.56
stem addition1.220.57
no stem addition141.360.51
stem addition1.330.59
no stem addition301.2b0.67
stem addition1.190.67
Merlot [30]1966no stem addition81.410.55
stem addition1.190.54
Merlot [3]1996no stem addition7 0.630a
stem addition 0.717b
Muscat Bailey A [3]1996no stem addition7 0.758a
stem addition 0.866b
Pinot Noir [3]1996no stem addition7 1.020a
stem addition 1.133b
Pinot Noir [84]2014no stem addition100.39
20% whole cluster0.36
Pinot Noir [84]2015no stem addition100.6
stem addition0.64
no stem addition100.6
20% whole cluster0.56
Pinot Noir [83]2016no stem addition100.5
50% whole cluster0.57
100% whole cluster0.59
dried stems0.65
Pinot Noir [83]2017no stem addition100.58
50% whole cluster0.52
100% whole cluster0.57
dried stems0.6
Primitivo [81]2012no stem addition1011.97a0.72b
25% whole cluster15.9b0.67a
no stem addition1011.97a0.72b
50% whole cluster15.2b0.70a
Table 22. Impact of stems on methoxypyrazine composition in wine samples.
Table 22. Impact of stems on methoxypyrazine composition in wine samples.
Grape VarietyModalityMethoxypyrazine Compounds
IPMP (ng/L)SBMP (ng/L)IBMP (ng/L)
Cabernet Sauvignon [73]No stem additionn.d.n.d.25.3
Stem addition2.72.833.8
Chardonnay [73]No stem additionn.d.n.d.11.6
Stem addition2.52.018.0
Sauvignon Blanc [82]No stem addition0.85n.d.4.8
Stem addition3.60.814.1
n.d.: not detected; IPMP = 2-methoxy-3-isopropylpyrazine; SBMP = 2-methoxy-3-sec-butylpyrazine; IBMP = 2-methoxy-3-isobutylpyrazine.
Table 23. Main effect of the use of stems or whole clusters on wines from an oenological point of view.
Table 23. Main effect of the use of stems or whole clusters on wines from an oenological point of view.
ParameterVariation Percentage of Change Compared to Fully Destemmed WinesReferences
pH(+)1 to 9[3,12,79,83,84]
0 [3,79,80,81,84]
Titrable acitidy(−)2 to 15[12,83,84]
0 [79,83,84]
Volatile acidity(+)4 to 44[83,84]
(−)6 to 12[81,84]
0 [79,83,84]
Potassium (K)(+)2 to 33[3,80]
Ethanol content(+)1 to 3[81,83]
(−)1 to 8[12,80,81,83,84]
0 [79,83]
Total polyphenolic content(+)14 to 30[3,12,80,81]
0 [3,79]
Total proanthocyanidin content(+)7 to 225[12,30,81,84]
(−)19[83,84]
0 [84]
Total anthocyanin content(+)7 to 12[30,84]
(−)1 to 22[12,30,79,80,81,83,84]
0 [81,83]
Color Intensity(+)7 to 33[81]
(−)1 to 26[12,30]
0 [83,84]
Color hue(+)10 to 17[3,12]
(−)3 to 7[81]
0 [30]
Aroma and volatile compounds(+)1-Hexanol, IPMP, SBMP, IBMP[73,79,82,84,85]
Taste(+)astringency and bitterness[12,83,84]
(+)complexity and freshness[83]
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MDPI and ACS Style

Blackford, M.; Comby, M.; Zeng, L.; Dienes-Nagy, Á.; Bourdin, G.; Lorenzini, F.; Bach, B. A Review on Stems Composition and Their Impact on Wine Quality. Molecules 2021, 26, 1240. https://doi.org/10.3390/molecules26051240

AMA Style

Blackford M, Comby M, Zeng L, Dienes-Nagy Á, Bourdin G, Lorenzini F, Bach B. A Review on Stems Composition and Their Impact on Wine Quality. Molecules. 2021; 26(5):1240. https://doi.org/10.3390/molecules26051240

Chicago/Turabian Style

Blackford, Marie, Montaine Comby, Liming Zeng, Ágnes Dienes-Nagy, Gilles Bourdin, Fabrice Lorenzini, and Benoit Bach. 2021. "A Review on Stems Composition and Their Impact on Wine Quality" Molecules 26, no. 5: 1240. https://doi.org/10.3390/molecules26051240

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