Advances in the Quality Improvement of Fruit Wines: A Review

: Fruit wines have gained great interest in recent years due to the increasingly diverse demands of consumers for different fruit wines with different colors, flavors, and nutritional values. Some fruits such as blueberry and strawberry are perishable and have a short shelf life. The production of fruit wine reduces fruit losses after harvest and enhances fruit utilization. The production of fruit wine with premium quality is determined by both intrinsic (i.e., genetic background) and extrinsic factors (e.g., yeast and fermentation protocol). This article provides an updated overview on the strategies and technologies aiming to improve the quality of fruit wines. Recent progress in improving fruit wine quality by variety selection, post-harvest treatments, yeast selection, fermentation protocols, fermentation conditions, and aging technologies has been comprehensively reviewed.


Introduction
There has long been a focus on fruits due to their high levels of nutritive contents and bioactive compounds, as well as good taste and pleasant flavors [1,2].However, some fruits such as blueberry, strawberry, plum, and peach are perishable and have a short shelf life, which leads to relatively large losses varying from 10 to 30% of the production volume [3].In order to reduce the post-harvest loss of fruits and improve their availability in preserved form, fruit wines are made by the fermentation of a large variety of fruits other than grape, such as blueberry, bilberry, cherry, peach, apple, plum, and mango.In addition, the increasing demands of consumers for diverse alcoholic beverages or wines also contribute to the development of fruit wine.
The demand and consumption of fruit wines has increased rapidly in recent years, attracting attention to improve their quality.The overall quality of fruit wine (e.g., sensory characteristics and nutritional values) depends on a variety of factors, mainly including the quality of raw fruit material (i.e., freshly harvested fruits), post-harvest treatments, fermentation conditions, vinification procedures, and aging processes [4][5][6][7][8].The quality of raw fruit material is subjected to the genetic background of the fruit, environmental conditions, and cultural practices during fruit growth and development [1].
Recent studies have provided enhancements in the improvement of fruit wine quality, especially by simultaneous inoculation (SIM) or sequential inoculation (SEQ) of mixed cultures of non-Saccharomyces yeast and Saccharomyces yeast [7,[9][10][11][12], followed by yeast selection [13][14][15], fermentation condition optimization [6,16,17], raw fruit material selection [4,18], post-harvest treatment [5,18], and wine aging technology [8].This article systematically summarized the recent advances in improving fruit wine quality and provided general guidelines for selecting high-quality raw fruit materials, proper post-harvest treatments and yeasts, optimal inoculation protocols and fermentation conditions, and appropriate wine aging technologies to improve fruit wine quality.

Selection of Raw Material and Post-Harvest Treatment to Improve Fruit Wine Quality
The quality of raw fruit material depends on various factors and their interactions [19].Firstly, the genetic intrinsic factor implies that some varieties naturally produce higher volatile compounds, polyphenols, or other compounds than others.Additionally, extrinsic factors such as environmental conditions, cultivation management, and post-harvest treatments also have an impact on the quality of fresh fruits.
Fruit variety affects not only the physical properties and chemical compositions of fresh fruits but also plays an important role in the sensory perception of the resultant fruit wine.In grape wine, aroma compounds are traditionally divided into three types: varietal, fermentation, and aging aroma [20].The varietal aroma compounds refer to the aroma compounds originating from the grape and are the major contributor to the overall aroma of wine [21].Wines made by different varieties usually have a specific varietal aroma [22].Additionally, red wines made from different varieties have different tastes.For instance, 'Merlot' red wine is very soft, while Cabernet Sauvignon red wine is powerful and sharp [23].However, the impact of fruit varieties on fruit wine quality has not been extensively investigated.Recent studies mainly compared the compositions and content of targeted compounds among different varieties of raw fruit material [24][25][26], but only a few investigated the impact of fruit varieties on the quality of their corresponding fruit wines.Specifically, Yuan et al. identified the volatile markers for distinguishing 'O'Neal' and 'Misty' blueberry wines using a non-targeted volatile metabolomic approach [4] and tracked the changes in volatiles in the two blueberry wines during alcoholic fermentation [27].Wang et al. further combined volatomic analysis and sensory assessment to illuminate the aroma characteristics of nine types of blueberry wines derived from different blueberry cultivars [18].The content and composition of non-anthocyanin phenolics were investigated in white bilberry wine and blue bilberry wine, suggesting that non-pigmented bilberries could be potential candidates for white bilberry wine production [28].In plum wine, Čačanska lepotica wine had the highest contents of total phenols, total anthocyanins, and total flavan-3-ols, as well as the highest color intensity, and the strongest antiradical activity compared to Čačanska rana and Požegača wines [29].In mango wine, Reddy et al. suggested that mango varieties Banginapalli, Banglora, and Alphonso were the most suitable ones for fruit wine production among the selected six varieties in South India [30].Based on organoleptic properties, the mango wines made from Banginapalli and Alphonso varieties had better sensory characteristics (e.g., flavor, taste, and mouth feel) [31].Wang et al. found a lower degradation of bioactive melatonin in mulberry red wine made by 'Hongguo2' during fermentation than in mulberry white wine made by 'Baiyuwang', which may be attributed to the higher antioxidative phenolics in black mulberry cultivar compared to the white cultivar [32].There are a wide variety of cultivated and wild fruit varieties available today [33], and future research should focus more on the selection of appropriate fruit varieties for fruit wine production.
Environmental conditions such as temperature, light, water, soil, and microbial populations originating from the native environment surrounding the vineyard significantly affect grape growth and development, and subsequently the quality of wine [34,35].Cultivation management such as leaf removal, training systems, vine spacing, cluster thinning, and shoot density also affect the aroma compounds and phenolic compounds in wine [36][37][38].However, to the best of our knowledge, there is little research investigating the effects of environmental conditions and cultivation management on the quality of fruit wine.
The effect of post-harvest treatments on fruit wine quality has rarely been investigated.Current knowledge suggests that berry sorting and partial dehydration after harvest affect the quality of blueberry wines.After berry sorting, 'Misty' berry wine made with smaller blueberries had a more intense fruity and floral aroma [18].Moderate postharvest dehydration (20-30% weight loss) increased the contents of total terpenes, benzeneacetaldehyde and phenylethyl alcohol, ethyl butanoate, methyl salicylate, 1-hexanol, and γ-nonalactone in blueberries and corresponding wines, leading to the enhancement of floral, fruity, and sweet notes of blueberry wines [5].

Effects of Fermentation on Fruit Wine Quality
Fermentation is a crucial step in fruit wine production.Alcohol is converted from sugar during this process and diverse metabolites (e.g., esters and higher alcohols) determining fruit wine quality are also formed.A variety of factors including yeasts (i.e., Saccharomyces and non-Saccharomyces) [14,39,40], inoculation protocols of mixed culture fermentation (i.e., combination of mixed cultures, inoculation modality, and inoculum ratio) [7,9,10], and fermentation conditions (i.e., temperature and pH) [6,16] have been reported to affect the physico-chemical and sensory quality of fruit wines.

Yeasts
The selection of yeasts is vital to obtaining grape wines and fruit wines with distinctive yet pleasant flavors [41,42].Commercial Saccharomyces cerevisiae yeasts have been widely used in the production of blueberry [5,33], plum [6,43], pineapple [44], and strawberry [45] wines, achieving controllability and reproducibility during the fermentation process and predictability in the sensory quality of the fermented beverages.The Saccharomyces cerevisiae strain can also produce a mixed fruit wine from pawpaw, banana, and watermelon, which was acceptable by consumers [46].Although Saccharomyces cerevisiae is the most common yeast species used in fruit wine fermentation, few studies investigated the impact of different Saccharomyces cerevisiae yeasts on the fruit wine quality.Lin et al. compared the influence of four commercial Saccharomyces cerevisiae on volatile profiles of pineapple wine and proposed that strains D254 and BV818 could be used for making intense pineapple wine and imparting characteristic aromas (e.g., 4-hydroxy-2,5-dimethylfuran-3-one, limonene, and ethyl 3-methylthiopropionate), respectively [44].
Despite the advantages of using Saccharomyces cerevisiae, grape wines produced by pure Saccharomyces cerevisiae lack the complexity of flavor when compared to those produced by spontaneous fermentation, during which non-Saccharomyces yeasts play an important role [41,47].Non-Saccharomyces yeasts generally have low fermentability and weak alcohol tolerance [48].Therefore, these yeasts were initially considered as undesired or spoilage strains during wine fermentation [41].In recent years, non-Saccharomyces yeasts have gained attention due to their ability to enhance the flavor and improve the quality of wine [9,49].Here, the effects of pure non-Saccharomyces yeasts inoculation on fruit wine have been reviewed, when compared to Saccharomyces yeasts (Table 1), mainly focusing on oenological characteristics (e.g., ethanol content, pH, titratable acidity) and volatile compounds.Generally, in contrast to Saccharomyces yeasts, pure inoculation of non-Saccharomyces yeasts leads to a lower content of ethanol in fruit wines such as blueberry [9], bilberry [50], peach [10], lychee [51], as well as apple, pear, and kiwifruit wines [40], compared to inoculation with Saccharomyces yeasts.The lower ethanol could be attributed to the different metabolic flux distribution during fermentation between non-Saccharomyces yeasts and Saccharomyces yeasts, and the lower tolerance of non-Saccharomyces yeasts to ethanol [52].Lower ethanol levels were also observed in cherry wines made by Torulaspora delbrueckii and Metschnikowia pulcherrima when compared to the cherry wines made by Saccharomyces yeasts, but they were not statistically significant [53].In addition to ethanol, the pH and titratable acidity also play a crucial role in the properties, quality, and microbiological stability of wine [54].Sun et al. reported a lower pH in cherry wine inoculated with pure T. delbrueckii compared to that with Saccharomyces yeasts [53].Likewise, lower pH was also observed in apple wine inoculated with Pichia kluyveri than that with Saccharomyces yeasts [40].Fruit wine like blueberry wine fermented with industrial Saccharomyces yeasts usually has an excessively sour taste caused by high contents of residual organic acids, negatively affecting the flavor and quality of blueberry wine [55].Fermentation with non-Saccharomyces yeasts could potentially solve this problem.Wang et al. found that titratable acidity was decreased in blueberry wine using pure T. delbrueckii when compared to Saccharomyces yeast inoculation [9], which could attribute to the decreased malic acid and citric acid contents [56].A Pichia fermentans yeast also had a strong ability to degrade citric acid during blueberry wine fermentation [55].Likewise, Hanseniaspora uvarum and M. pulcherrima reduced the titratable acidity in peach wine, mainly attributing to the reduced malic acid [10].
Regarding volatile compounds, increasing evidence demonstrated the positive contributions of non-Saccharomyces yeasts on the improvement of aroma complexity and sensory perception of fruit wines.Padilla et al. have concluded which non-Saccharomyces yeast species produced higher or lower volatile compounds in wine [41].For instance, M. pulcherrima, Candida zemplinina, and Lachancea thermotolerans were described as higher producers of higher alcohols.M. pulcherrima still seems to be a strong producer of higher alcohols in peach wine [10] and bilberry wine [49], in accordance with its fermentation characteristics in wine [33].Nevertheless, pure M. pulcherrima fermentation led to a decrease in higher alcohols in mango wine [57], indicating the impact of fruit matrices on higher alcohols during fermentation.A commercial strain of T. delbrueckii Zymaflore Alpha was found to increase total anthocyanins, total flavonoids, and total phenols in blueberry wine; however, three ethyl esters (ethyl 3-methylbutanoate, ethyl hexanoate, and ethyl octanoate) with fruity notes were simultaneously decreased [9], indicating pure T. delbrueckii fermentation in blueberry wine may not be the best choice.In contrast, four ethyl esters of ethyl L (−)-lactate, ethyl palmitate, ethyl 2-furoate, and ethyl caprate increased in peach wine inoculated with T. delbrueckii Zymaflore Alpha [10].The different effects of the same commercial T. delbrueckii strain on ethyl esters in blueberry wine and peach wine might be attributed to the different chemical compositions of blueberry and peach fruits.
Additionally, some studies evaluated and compared the effects of different non-Saccharomyces yeasts on the quality of fruit wine, which improved the current understanding of the potential application of non-Saccharomyces yeasts in fruit wine production.Through comparing eight non-Saccharomyces yeasts, Liu et al. found that H. uvarum inoculation led to the highest levels of phenolic acids and flavan-3-ols in bilberry wine, and fermentations with Saccharomycodes ludwigii, T. delbrueckii, and M. pulcherrima resulted in higher levels of myricetin-3-O-glucoside and syringetin-3-O-glucoside but lower levels of most phenolic acids [15].A recent study also compared ten non-Saccharomyces yeasts from Daqu in blueberry wine and found that Wickerhamomyces anomalus yeast showed good fermentation ability and the ability to convert anthocyanins and vinylphenols into more stable vinylphenol pyranoanthocyanins, and produced no H2S in the meantime.This might be a potential non-Saccharomyces yeast for the production of fruit wines with premium quality [13].

Inoculation Protocols of Mixed Culture Fermentation
Although non-Saccharomyces yeasts have positive effects on fruit wine quality, such as increasing the complexity of aroma, reducing the anthocyanin adsorption, and benefiting the formation of some stable pigments like pyranoanthocyanins and proanthocyanins [9,13], these yeasts are usually characterized by a weaker fermentation ability when compared to Saccharomyces cerevisiae.Therefore, mixed fermentations of non-Saccharomyces yeast and Saccharomyces yeast have been proposed to retain the positive effects and reduce the negative impacts of non-Saccharomyces yeasts on wine quality.Previous studies found that the interaction between non-Saccharomyces yeast and Saccharomyces yeast during alcoholic fermentation is not only species-specific but also strain-specific [59].In addition, the inoculation modality (i.e., SIM or SEQ) and inoculum ratio can significantly affect the quality of fruit wine [7,9,56].On the basis of these findings, a suitable inoculation protocol with the optimal combination of mixed cultures, inoculation modality, and inoculum ratio of mixed cultures is necessary for mixed fermentation.Here, the effects of mixed fermentation of one non-Saccharomyces yeast and one Saccharomyces yeast on fruit wine quality have been reviewed, when compared to the Saccharomyces single yeast fermentation (Table 2).The improvement of 'fruity', 'floral', and 'global aroma' notes is of great interest in fruit wines.Terpenes and norisoprenoids with low olfactory perception thresholds are responsible for fruity and floral notes in wine [60,61].Additionally, some ethyl esters of organic acids and straight-chain fatty acids, as well as acetates of higher alcohols, also contribute to the fruity aroma of wine [62].
The increase in desirable esters is another target of mixed fermentation of non-Saccharomyces yeast and Saccharomyces yeast.Generally, the combinations of H. uvarum/S.cerevisiae, H. opuntiae/S.cerevisiae, M. pulcherrima/S.cerevisiae, Schizosaccharomyces pombe/S.cerevisiae, and T. delbrueckii/S.cerevisiae with proper inoculation modality and ratio were found to increase the fruity esters in fruit wines.Specifically, higher total esters were observed after fermentation with both 10:1SIM and 10:1SEQ of H. uvarum/S.cerevisiae in plum wine [12].Likewise, Hu et al. found that fermentation with 10:1SEQ of H. opuntiae/S.cerevisiae and H. uvarum/S.cerevisiae resulted in higher ethyl acetate, isoamyl acetate, and phenethyl acetate in citrus wine [65].In peach wine, 10:1SEQ of H. uvarum/S.cerevisiae, M. pulcherrima/S.cerevisiae, L. thermotolerans/S.cerevisiae, and T. delbrueckii/S.cerevisiae increased the concentration of ethyl acetate, with the highest concentration of 51.16 mg/L by H. uvarum/S.cerevisiae [10].Ethyl acetate shows favorable effects on wine aroma at concentrations below 80 mg/L [66], while it imparts spoilage characteristics to wine at levels of 150-200 mg/L [67].Regarding M. pulcherrima/S.cerevisiae, a previous study in plum wine showed that 10:1/1:1/1:10SEQ of M. pulcherrima/S.cerevisiae increased the levels of total esters and several individual esters (i.e., hexyl acetate, isoamyl acetate, phenethyl acetate, and ethyl caproate), which contribute to the fruity odor [7].The authors also found that 10:1/1:1SIM of the same mixed cultures increased the contents of hexyl acetate, isoamyl acetate, and ethyl caproate, while 1:10SIM increased the level of ethyl acetate in plum wine.In cherry wine, 10:1EQ of M. pulcherrima/S.cerevisiae was observed to increase the levels of ethyl butyrate, ethyl 3-methylbutanoate, ethyl hexanoate, ethyl hex-3-enoate, methyl octanoate, ethyl octanoate, ethyl decanoate [53].In bilberry wine, 1:1SIM of S. pombe/S.cerevisiae led to higher ethyl hexanoate content, and 1:1SEQ of S. pombe/S.cerevisiae resulted in higher total ester, ethyl acetate, ethyl hexanoate, and ethyl heptanoate contents [50].The effects of T. delbrueckii/S.cerevisiae on esters were inconsistent among different studies.A ratio of 10:1SEQ of T. delbrueckii/S.cerevisiae might be suitable for increasing ester contents, as it resulted in higher ethyl acetate, isoamyl acetate, and phenethyl acetate contents in citrus wine [65], higher ethyl acetate content in peach wine [10], and higher ethyl butyrate, ethyl 3-methylbutanoate, ethyl hexanoate, ethyl hex-3-enoate, and ethyl octanoate contents in cherry wine [16,53].In bilberry wine, 1:1SEQ of T. delbrueckii/S.cerevisiae also led to higher levels of phenethyl acetate and ethyl dodecanoate [50], whereas blueberry wine fermented with 1:1SIM/SEQ of T. delbrueckii/S.cerevisiae had lower levels of two fruity esters ethyl hexanoate and ethyl octanoate [9].These results suggested that the ester production by T. delbrueckii/S.cerevisiae could be good candidates of mixed cultures for improving ester production during fruit wine fermentation, but the effects of these mixed cultures are highly dependent on the inoculation ratio and fruit matrices.
Recent studies also suggested that inoculation of more than one non-Saccharomyces species in combination with Saccharomyces cerevisiae might be a new strategy to improve wine quality [41,68].The concentrations of ethyl esters of fatty acids were higher in wine fermented by a combination of C. zemplinina, H. uvarum, and S. cerevisiae compared to that fermented by pure S. cerevisiae, C. zemplinina/S.cerevisiae, or H. uvarum/S.Cerevisiae [69].Moreover, Zhang et al. suggested that the triple mixed cultures of T. delbrueckii, H. vineae, and S. cerevisiae could be an option for making up the species shortages and further improving the overall quality of 'Petit Manseng' wine [68].However, knowledge on the effects of triple mixed cultures with more than one non-Saccharomyces species and one Saccharomyces species on fruit wine quality is still limited.Recently, the effects of triple mixed cultures of S. cerevisiae, Dekkera bruxellensis, and W. anomalus, and quadruple mixed cultures of S. cerevisiae, D. bruxellensis, W. anomalus, and M. pulcherrima, as well as quadruple mixed cultures of S. cerevisiae, D. bruxellensis, W. anomalus, and Metschnikowia sinensis, on the quality of apple wines were evaluated [11].The highest contents of volatile compounds were found in apple wines fermented by mixed cultures of M. pulcherrima/W.anomalus/D.bruxellensis/S.cerevisiae.

Fermentation Conditions
In addition to the yeasts and inoculation protocols, fermentation conditions such as temperature, pH, SO 2 level, and aeration are also important factors that affect yeast growth, duration, fermentation rate, and the subsequent quality of fruit wine [70,71].
Low-temperature fermentation inhibited yeast growth and sugar consumption but was able to maintain the varietal aroma from the fruit [72,73], while high-temperature fermentation can lead to the loss of key aroma compounds such as isobutyl acetate and isopentylacetate [74].Therefore, fruit wines are commonly suggested to be fermented at a moderate temperature between 20 and 25 • C for optimal aromatic and sensory characteristics [6,16,74].Specifically, apple wine fermented at 20 • C had the highest levels of the most key aroma compounds (e.g., isobutylalcohol and isopentylalcohol) and the highest consumer acceptance, in comparison to those fermented at 17  C can result in the best sensory quality of cherry wine, which was partially attributed to the altered yeast-yeast interactions by temperature [16].Furthermore, two cultivars of mulberry wines fermented at 25 • C showed higher levels of bioactive melatonin than those fermented at 16 • C [32].To obtain a mango wine with a satisfactory production of ethanol, increased glycerol, and minimized volatile acidity, the fermentation conditions of temperature 22.5 • C, pH 3.8, and inoculum size 11.9% were recommended as optimal conditions [71].A further study evaluated the effects of temperature, pH, SO 2 , and aeration on the microorganisms and sensory quality of mango wine, and suggested that temperature 25 • C, pH 5, 100 ppm of SO 2 , and must with initial oxygen were optimum for a better quality of mango wine [70].Sun et al. unraveled that the amount of added SO 2 was related to the contents of reducing sugars, soluble solids, ethanol, and volatile aroma compounds in strawberry wine, and suggested that the addition of 60-80 mg/L of SO 2 at the beginning of fermentation was able to improve strawberry wine quality [17].

Effects of Wine Aging Technologies on Fruit Wine Quality
The main drawback for some fruit (e.g., blueberry and plum) wine making is destitute anthocyanin contents and/or unstable anthocyanins in the corresponding fruit wines.
The degradation and absorption of these anthocyanins during aging directly threaten the organoleptic quality and dramatically shorten the shelf life of fruit wine.In the wine aging process, physical methods, such as ultrasonic waves, gamma rays, electric fields, nanogold photocatalysis, and high pressure have been proven to greatly reduce the aging time and improve the wine quality [75].In blueberry wine, low-frequency power ultrasound treatment with specific treatment time and cycles (i.e., 180 W, 20 min, and 2 cycles) has been reported to improve color characteristics and reduce chromatic aberration of blueberry wine, which were attributed to unattenuated anthocyanins protected from the ultrasound treatment [76].The improved color characteristic of the L* value in blueberry wine was also obtained by high-power pulsed microwave with low frequencies (50 and 100 Hz), and an increased maturity of blueberry wine body along with a shortened aging time were achieved at the same time [77].Ultrasound treatments at 28 and 40 kHz improved the color performance (i.e., a*, b*, and C* values) and intensity of aged plum wine [8].In addition, Cao et al. found that high hydrostatic pressure greatly affected alcohol and ester contents, as well as increased phenolic compounds in red raspberry wine [78].The ultrasonicated mulberry wine was found to possess more antiradical properties than manosonicated and pressurized mulberry wines, as the different effects of these non-thermal processing methods on antioxidant compounds such as flavonols and anthocyanins [79].

Prospects
Fermentation plays an important role in determining the quality of fruit wine due to the complex biochemical reactions and formation of quality-related compounds during the process.Increasing evidence showed the benefits of mixed culture fermentation of non-Saccharomyces and Saccharomyces to improve the quality of fruit wine; however, the combination of mixed cultures, inoculation modality, and inoculum ratio still need further optimization according to the chemical composition and nutritional characteristics of raw fruit material, as well as the preference of winemakers and consumers.The actual performance of mixed inoculation strategies must be evaluated in fermenters at both pilot scale and industrial scale before they are applicable for commercial use.It is worth noting that mixed fermentation is characterized by complex and largely unknown interactions between non-Saccharomyces and Saccharomyces yeasts [80], which might result in unpredictable, uncontrollable, and unreproducible fruit wine quality.Omics approaches (e.g., genomic, proteomic, and metabolomic) and molecular tools could be promising tools to solve this problem [81].
Additionally, knowledge on the effects of the genetic background of raw fruit material, growing environmental conditions, cultivation management, post-harvest treatments, and wine aging technologies on fruit wine quality is rather limited at this stage, which also restricts the strategies for the quality improvement of fruit wines and processing of perishable fruits.It remains necessary to investigate the effects of these factors on the chemical components and sensory quality of fruit wines, and this will provide the winemakers with fundamental knowledge on fruit wine quality and therefore allow winemakers to manipulate and improve fruit wine quality through various strategies and technologies.

Table 1 .
Effects of pure non-Saccharomyces yeasts on fruit wine quality when compared to Saccharomyces yeasts.

Table 2 .
Mixed cultures of yeasts designed to improve the quality of fruit wines when compared to pure Saccharomyces yeast fermentation.
• C, 23 • C, and 26 • C [74].Likewise, in plum wine, the highest sensory evaluation score was obtained during fermentation at 20 • C when compared to those at 16 • C, 18 • C, 22 • C, and 24 • C [6].Sun et al. found that sequential fermentation of T. delbrueckii/S.cerevisiae at 25 • C rather than 20 • C and 30