3.1. Color Parameters of Wine
shows the evolution of the main colorimetric parameters over aging time. In general, all the samples followed a similar chromatic evolution, independent of the treatment carried out.
CI of the wines increased during the first two months, mainly in the micro-oxygenated wines and in the wines aged in barrels. From that moment, this parameter decreased in all the treatments, maintaining the initial differences until the end of the aging process (Figure 2
a). These results coincide with those obtained by other authors [16
]. Furthermore, a higher CI in micro-oxygenated wines was also observed by other authors [18
], which could be due to an increase in the blue color by the contribution of pigments with ethyl bridges. As noted by these authors, a pronounced increase in the blue color percentage in micro-oxygenated wines was observed (Figure 2
In the same way as CI, the percentage of red color increased in all wines until two months, with a notable decrease observed from that moment (Figure 2
d). Del Álamo et al. [20
] also observed a loss of CI, especially in the red component, from the third month of aging with fragments and barrels.
The percentage of yellow color in wines followed a trend similar to the tonality (data not shown), appreciating a decrease during the first two months, and then a significant increase until the end of the period studied (Figure 2
b). Pérez-Prieto et al. [21
] attributed the increase in yellow tones to the extraction of color compounds from the oak wood during aging. Our results also coincided with those of Cadahía et al. [22
], who observed a decrease in the percentage of red color and an increase in the percentage of yellow color and tonality, while these authors did not observe significant variations in the CI in wines aged for 12 months in oak barrels.
A decrease in TPI was observed over the aging period (Figure 2
c). This parameter, at 4 and 12 months of aging, was higher in barrels compared to the other aging systems, but at 24 months, the highest TPI value corresponded to wine with staves and micro-oxygenation. In the case of total anthocyanins, the decrease was more pronounced in micro-oxygenated wines during the first 12 months, although these differences disappeared at 24 months (Figure 2
e). Tavares et al. [23
] also observed a decrease in anthocyanins during wine contact with chips, probably due to anthocyanin condensation and polymerization reactions, and the precipitation of these compounds during wine aging.
3.2. Volatile Compounds from Oak Wood in Wine
shows the evolution of furanic compounds, benzoic aldehydes, and oak lactones during the period of aging. Furanic compounds, with the exception of furfuryl alcohol, are formed during wood toasting through degradation of carbohydrates [24
]. Wines in contact with staves and aged in barrels had a much higher concentration of these compounds than those treated with chips (Figure 3
a–c). Moreover, their evolution was similar, with their content increasing during the first 6 months and then decreasing sharply, until practically disappearing in the case of furanic aldehydes.
Towey and Waterhouse [25
] also observed that the concentration of these compounds decreased significantly after 7 months of barrel aging. Garde-Cerdán and Ancín-Azpilicueta [26
] also found a decrease of furfural and 5-methylfurfural from the sixth month of aging in new barrels, as well as 5-hydroxymethylfurfural from the ninth month. Other authors [27
] also observed a maximum extraction in furfural at 6 months of barrel aging and a decrease from that moment. The degradation of these compounds can be due to their reduction to the corresponding alcohols by biological mechanisms [28
], although they can also participate in other reactions that contribute to the decrease of their concentrations in free form in wines (such as the formation of 2-furanmethanethiol [29
] or of color adducts with the wine catechin [30
]). For a short time of aging, extraction of these compounds from the wood is greater than their degradation so that they are accumulated in wine. However, when the aging time increases, degradation reactions may exceed the extraction, so that their concentration tends to decrease [24
On the other hand, in wines treated with oak chips, hardly any extraction of furfural or 5-methylfurfural was observed, while the content of 5-hydroxymethylfurfural increased during the first month of contact with the wood and remained practically constant until 6 months, at which point it disappeared. Similar results were obtained by Fernández de Simón et al. [33
], who observed the highest amount of furanic aldehydes in wines treated with chips at 30 days, with the concentration of these also being lower than those treated with staves.
The concentration of furfuryl alcohol also reached higher values in wines treated with staves and aged in barrels (Figure 3
d). This compound originates from the microbiological reduction of furfural, even after the alcoholic and malolactic fermentations have been completed [34
], and its concentration depends on factors that affect enzymatic reactions, such as pH, temperature, or residual microbiological activity [35
Benzoic aldehydes (vanillin and syringaldehyde) are formed during the thermal degradation of lignin. These compounds achieved their maximum concentration during the first month in wines with oak chips, and between 2 and 4 months in the case of wines treated with staves, which obtained the highest concentrations throughout the process studied (Figure 3
e–f). In both cases, the concentration decreased from 6 months, probably due to the microbiological reduction to the corresponding alcohols [31
]. On the other hand, the wines aged in barrels presented a lower content of these compounds than the wines in contact with staves, increasing until 12 months of aging, and decreasing later during the bottle-aging period. Coinciding with our results, different authors [24
] found that the concentration of benzoic aldehydes was at maximum after 10–12 months of barrel aging. Like furanic aldehydes, vanillin accumulates in wine during short aging times, since initially the extraction is high due to the different concentration between wine and wood [26
]. However, when the aging time is prolonged, it can be transformed into vanillic alcohol, so that the vanillin concentration can decrease.
In wines treated with oak chips, the maximum extraction of the two isomers of β-methyl-γ-octalactone occurred during the first month of contact with the wood. In the case of staves, cis
-β-methyl-γ-octalactone was extracted during the 6 months of contact time, while the trans
isomer was extracted up to 12 months (Figure 3
g–h). Other authors [39
] also observed that the accumulation of cis
oak lactones increased with the aging time of wines in barrels. Meanwhile, wines aged in barrels showed a higher content of these compounds than those macerated with fragments, with these two isomers increasing during the 12 months of contact with the wood, and then decreasing during bottle-aging. The decrease in the concentration of these compounds could be due to the wine undergoing different chemical transformations [42
The evolution of volatile phenols during the aging time is shown in Figure 4
. These compounds are formed from the thermal degradation of the lignin at a high temperature. In general, a greater extraction of these compounds was observed in the wines in contact with the staves, mainly in the case of 4-methylguaiacol, trans
-isoeugenol, and syringol.
Guaiacol concentration increased throughout the process, obtaining higher values in wines treated with staves and in those aged in barrels, reaching its maximum concentration at 12 months of aging, at which point its content remained practically constant (Figure 4
a). These results coincide with those obtained by Garde-Cerdán et al. [38
]. The extraction kinetics of 4-methylguaiacol showed major differences between treatments, finding the highest concentration in wines in contact with staves at 6 months, while in wines aged with chips and in oak barrels, it was achieved during the first month of contact (Figure 4
b). Pérez-Prieto et al. [43
] observed that 4-methylguaiacol reached its maximum concentration at 3 months, while the guaiacol extraction continued for up to a maximum of 9 months of aging in barrels. Although the staves had medium toasting, their appearance indicated that they had undergone a stronger toasting than the chips, probably because the toasting system applied was different. This fact could explain the higher content of guaiacol and 4-methylguaiacol in wines in contact with staves, since they are compounds that are formed at high temperatures of toasting [26
As can be seen in Figure 4
c,d, the wines aged in barrels sharply increased the levels of ethylphenols (4-ethylguaiacol and 4-ethylphenol) during their aging in bottles. These compounds can be extracted from wood in very low concentrations, but mainly they are formed during the aging of wines by microbiological transformations of cinnamic acids carried out by Brettanomyces
yeast contaminants [44
]. The concentrations of ethylphenols found in wines aged in barrels at the end of the process exceeded those considered harmful to the wine aroma, 140 and 620 μg/L for 4-etilguaiacol and 4-ethylphenol, respectively [45
]. This fact could be due to contamination of the wines aged in barrels during the bottling process.
The concentration of 4-vinylguaiacol tended to decrease progressively up to 12 months, when the concentration in wines with chips and staves remained practically constant (Figure 4
e). However, in wines aged in barrels, its concentration decreased sharply, probably as a consequence of its reduction to 4-ethylguaiacol by the effect of contamination by Brettanomyces
. The 4-Vinylphenol content decreased slightly or remained constant in all the wines up to 12 months. In the case of wines aged in barrels, its concentration remained practically constant from this moment, and yet, in wines in contact with chips and staves, its content increased until 24 months (Figure 4
The concentration of phenol increased until 12 months in all treatments (Figure 4
g). From that moment, its concentration continued to increase in wines in contact with staves, while its level decreased slightly in wines aged with chips and more sharply in wines aged in oak barrels. Garde-Cerdán et al. [38
] found the highest concentration of this compound at 10 months of aging in barrels and they also observed a decrease in its content from that moment.
On the other hand, the maximum extraction of eugenol was reached in wines during the first month of contact with chips, and after 6 months in the wines macerated with staves. In addition, eugenol was extracted constantly during the contact time of wines with the oak barrels, thus obtaining a maximum concentration at 12 months, and its concentration was much higher than that obtained in wines in contact with fragments (Figure 4
h). Similar results regarding the increase in barrels of this compound were observed by other authors [25
-Isoeugenol reached its maximum concentration in the first month of contact in wines aged in the barrel and with chips, and at 4–6 months in those treated with staves, which was much higher than in the other wines. In all the wines, a decrease in the concentration of this compound was observed after 6 months, disappearing in wines aged in oak barrels and with chips (Figure 4
i). Finally, syringol presented a similar evolution in all the treatments, reaching the maximum concentration at 4 months, which was higher in wines treated with staves throughout the process (Figure 4
j). Different results were obtained by Fernández de Simón et al. [33
], who observed an increase in the concentrations of trans
-isoeugenol and syringol until the end of aging in wines treated with alternative oak products.
3.3. Low Molecular Weight Phenols
Benzoic acids are constituents of wood, and its content in wine increased as a result of contact with it (Figure 5
). These results are in agreement with those observed by other authors [46
Gallic and ellagic acids are very important compounds due to their strong antioxidant activity, even at very low concentrations [48
]. The concentration found for both compounds was slightly higher in wines treated with chips in most of the process (Figure 5
a,f), probably due to its greater contact surface and a less intense toasting than in the staves, since gallic acid is a compound which is sensitive to thermal degradation. Similar results were obtained by Alañón et al. [46
] when comparing chestnut wood chips and barrels.
Protocatechuic acid concentration increased more in wines in contact with staves and in wines aged in barrels than in those treated with chips at almost every moment (Figure 5
b). This may be because this compound is generated during the toasting process, as a consequence of the thermal degradation of lignin [49
], and as has been noted previously the toasting seemed more intense in the staves than in the chips.
Higher concentrations of p-hydroxybenzoic and vanillic acids were detected in micro-oxygenated wines (Figure 5
c,d), as well as syringic acid in wines treated with staves, compared to those aged in the barrel and in contact with chips (Figure 5
e). The content of these three compounds increased in the wines during the entire study period.
Cinnamic acids were detected in the wines in their free forms (caffeic, coumaric, and ferulic acids) and in their respective tartaric esters (caftaric, coutaric, and fertaric acids). The concentration of these acids in wine depends on grape variety and winemaking technique, but they are not found in oak wood. The evolution of their content over time is shown in Figure 6
Coumaric acid concentration in wines may decrease in the presence of the enzyme cinnamate decarboxylase, by transformation into 4-vinylphenol, which can be transformed into 4-ethylphenol in the presence of a vinylphenol reductase. In the same way, ferulic acid can decrease by decarboxylation, transforming into 4-vinylguaiacol, and in the presence of the enzyme vinylphenol reductase can form 4-ethylguaiacol [45
Caffeic, coumaric, and ferulic acids presented a more or less stable concentration up to 12 months, with a similar evolution for all treatments (Figure 6
a,c,e). It is important to note the decrease of these acids at 12 months in wines aged in barrels, which could be explained by the decarboxylation reactions described in the previous paragraph. The decrease in coumaric and ferulic acids at 12 months coincided with a significant increase in 4-ethylphenol and 4-ethylguaiacol (Figure 4
c,d), which as explained above, could be due to contamination of the wine by Brettanomyces
at the time of bottling. On the other hand, Cadahía et al. [50
] justified the variations of the concentration of caffeic acid due to their involvement in esterification reactions to give caftaric acid, as well as in the co-pigmentation reactions that allow the color of anthocyanins to stabilize.
There was no clear relationship between the content of the free cinnamic acids and their esterified forms. While the free form concentrations remained relatively constant or increased slightly, their corresponding esters had a heterogeneous evolution. Caftaric and coutaric acids diminished or maintained their concentration constantly during the first months, whereas the fertaric acid content remained practically stable. In these compounds there was a notable increase from 12 to 24 months (Figure 6
b,d,f). Tartaric esters (caftaric and coutaric acids) decrease during aging because they are very reactive compounds and participate in oxidation processes. The only differences between treatments were observed in the caftaric acid, which had a slightly higher concentration in wines aged in oak barrels (Figure 6
b), probably because this type of container encourages the esterification processes [41
The evolution of flavanols and flavonols in wines can be seen in Figure 7
. As can be observed, the concentrations of catechin and epicatechin decreased slightly until 12 months, producing a greater decrease in the case of the epicatechin from 12 to 24 months (Figure 7
a,b). These results coincide with those obtained by other authors [18
] who also obtained a decrease in these phenols over the aging period. This is related to their participation in the oxidative processes, polymerization, and condensation reactions with other compounds, favored in barrels by the continuous diffusion of oxygen [52
]. On the other hand, Del Barrio-Galán et al. [53
] found lower concentrations of flavanols (catechin and epicatechin) in wines and in model solutions treated with chips than in control wines, corroborating the theory that these compounds can be adsorbed on the surface of the wood. Modifications of the flavanols content between treatments during aging could be due to the reactivity of these compounds. Thus, their concentration may decrease due to oxidation and polymerization reactions and may increase due to the hydrolysis of higher oligomers [54
Quercetin concentration was higher in wines without micro-oxygenation than in the micro-oxygenates and those aged in barrels, over the whole time studied (Figure 7
c). The values of this compound decreased in wines aged in oak barrels and micro-oxygenated wines during the first two months, remaining practically constant during the rest of the process. Likewise, in wines without micro-oxygenation, its concentration remained more or less constant throughout the time studied. Cejudo-Bastante et al. [55
] also found lower concentrations in wines aged with chips and micro-oxygenated, compared to those treated with chips but without micro-oxygenation. Castellari et al. [56
] also observed a decrease of this compound in micro-oxygenated wines with respect to the control one.
Rutin concentration decreased significantly during the first month in all the wines, decreasing later in the micro-oxygenated wines and wines in barrels, and remaining practically constant in the non-micro-oxygenated wines during the rest of the time studied (Figure 7
d). Fernández de Simón et al. [51
] also observed a decrease in some flavonols in wines aged for 21 months in oak barrels of different origins. This decrease could be due to the fact that flavonols can react with anthocyanins in co-pigmentation reactions [57
shows the evolution of the concentration of trans
-resveratrol and its glycoside, trans
-piceid. The concentration of trans
-piceid was hardly modified throughout the process, with no important differences being observed between the treatments (Figure 8
a). Alañón et al. [46
] did not find differences in the stilbene concentration between wines aged in chestnut wood barrels and wines in contact with chips.
Finally, the concentration of trans
-resveratrol decreased in all treatments, more in micro-oxygenated wines and in those aged in oak barrels compared to non-micro-oxygenated wines (Figure 8
b). Barrera-García et al. [59
] estimated an average rate of decrease of trans
-resveratrol of 50% in a model wine in the presence of oak wood, partly due to adsorption mechanisms on the surface of oak. This coincides with our results when considering micro-oxygenated wines and wines aged in barrels. The decline of stilbenes during aging was also observed by other authors [41
3.4. Sensorial Analysis
shows the sensory evaluation of wines during the aging period considered. After 2 months, the most highly valued wines (lowest score) were those treated with staves, followed by those aged in barrel, while wines with chips scored worse. However, at 4, 6, and 12 months, the wines aged in barrels obtained better scores than those treated with fragments, and of these, the wines with staves were better scored than those with chips in most cases.
Cano-López et al. [61
] obtained the best sensory results in wines with fragments in the form of cubes compared to those aged in barrels, after 3 and 6 months of contact. For this reason, they affirmed that the use of oak chips could be a good choice for short aging wines.
At 24 months, only the sensory analysis of the micro-oxygenated wines and those aged in barrels was carried out. At this time the valuation changed, wines with chips obtaining the best score, followed by wines aged in barrels, and finally those treated with staves. The organoleptic deterioration of wine aged in barrels could be due to its contamination by Brettanomyces during bottling. In addition, in wines with staves a high volatile acidity was detected in the organoleptic analysis (data not shown).
shows the sensory profiles of the wines at the different moments studied. Fruity and varietal notes were hardly appreciated in the wines at any of the moments. Up to 6 months, notes related to wood were perceived more intensely in wines treated with staves (toasted, almond, caramel, vanilla, smoked), while at 12 months they were perceived equally in wines aged in barrels and treated with staves. This greater intensity of tertiary aromas in the wines with staves could possibly have been due to a more intense toasting of these, since these aromas come from compounds generated during the wood toasting. Casassa et al. [62
] also found an increase in notes related to wood (toast, clove, vanilla, etc.) as the degree of toasting of the chips increased.
With respect to the gustatory phase, wines with chips were perceived as less structured, persistent, astringent, and with a lesser retronasal aroma after 2 months. However, at 4 months they were the ones considered best in this phase. Wines which were micro-oxygenated and in contact with staves obtained the best evaluations at 6 and 12 months, with scores at this time similar to those for wines aged in barrels.
Sensory evaluation of wines at 24 months could be considered invalid due to the strong impact that ethylphenols had on wines aged in barrels and the high volatile acidity in wines treated with staves, which did not allow an adequate evaluation of the rest of the attributes (data not shown).
Regarding triangular tests, the results obtained in the sensory assessment at 6 and 12 months are shown in Table 1
. At 6 months, wines added with fragments were clearly discriminated (with a level of significance of 99.9%) from those aged in barrels, independently of the size of the fragments. However, an adequate level of significance was not obtained for their discrimination at 12 months. When comparing the size of the fragments (chips vs. staves), it was possible to differentiate between the treatments at both moments (6 and 12 months) with a level of significance of 95% and 99% respectively.
As regards preferences, at 6 months, the preferred wines were those aged in barrels (72.2%), while at 12 months they did not opt for any of the two types of aging. In relation to the size of the fragments, the staves were preferred at 6 months (57.1%), and both treatments were evaluated equally at 12 months.