Means and standard errors (SE) of berry weight (W), total soluble solids (TSS), real acidity (pH), malic acid (MA), tartaric acid (TcA), and yeast assimilable nitrogen (YAN) (2014).
Viticultural and biotechnological strategies are two approaches to deal with higher must sugar levels at harvest time. A wide range of factors could significantly affect sugar accumulation in the grape such as choice of vineyard site, soil composition, irrigation strategy, rootstock, and grape cultivar selection as well as grape yield. In this sense, approaches to canopy management are continually evolving in response to changes in other vineyard management practices; some of these could contribute to reduce soluble sugars on grape berries at harvest time. On the other hand, among possible biotechnological strategies, one of the most relevant is the control of the fermentative process by using selected yeast strains. In this chapter, we will show how some viticultural practices have influenced the accumulation of soluble sugars and other enological parameters in grape berries at harvest time. We will also report how a careful yeast selection and the implementation of different fermentation strategies can also contribute to reduce ethanol content in wines.
- total soluble solids
The current demand by consumers toward well-structured, full body wines has driven the requirement for late harvests. These practices ensure an optimal phenolic maturity, which entails very mature grapes with high level of sugars [1, 2]. Additionally, the timing of harvest is probably the single most important viticultural decision taken each season. “Critical ripening period” and “physiological maturity” are phrases used by winemakers that appear frequently in conjunction with wine grape harvests, on winery websites, and in wine press reviews of vintages, winegrowing regions, and wines . Thus, the properties of the grapes at harvest set limits on the quality of the wine potentially produced . Grape is a nonclimacteric fruit and does not ripen further after harvest, so harvesting at the proper stage of maturity is essential for optimal grape quality in terms of soluble solids, berry weight, titratable acidity, and overall sensory characteristic. This is a very important period that influences grape composition and determines varietal characteristics .
There are several measurable parameters in grapes that relate in some way to quality factors. One of these is some measure of sugar concentration, which usually is accomplished by estimating the amount of dissolved compounds in the juice . The ripening of grape berries is accompanied by a massive accumulation of soluble sugars, and by the synthesis and accumulation of a wide range of phenolic compounds and aroma precursors. All of these processes play major roles in the quality of the berries and wine. Sugars accumulate in the vacuoles of flesh (mesocarp) cells, which account for 65–91% of the fresh weight in a mature berry . Most of those soluble sugars are two hexoses easily metabolized by yeasts and bacteria, glucose, and fructose, which decrease the perception of sourness, bitterness, and astringency, enhancing the “mouthfeel”, “body”, or “balance” of wines . From veraison, and throughout ripening, the berries accumulate roughly equal amounts of glucose and fructose . However, while glucose and fructose concentration increases in the grape berry during ripening, there are multiple biochemical processes affecting the concentration of grape-derived compounds, which may, positively or negatively, influence wine composition and sensory properties . Thus, determining grape harvest date for commercial winemaking usually involves a delicate balance, minimizing potential negative characters and maximizing positive flavor and phenolic substances, while avoiding excessive sugar concentration .
Although ethanol is very important for wine quality (most aroma volatiles are more soluble in ethanol than in water), wine’s aroma is declined with increasing ethanol content . Additionally, higher sugar levels at harvest produce not only higher alcohol content on wines, but also alter the content of yeast-derived metabolites . Thus, one of the major issues of higher alcohol content in wines is its effect on the sensory properties of the wine, in such a way that relatively small changes in alcohol content could have a great influence on how the wines are perceived. Another major concern has to do with market trends due to the leading critics around the world, whose ratings have a strong effect on sales. Accordingly, because of the significance of viticulture and the winemaking socioeconomic sector in Europe and other areas of the world, it is important for wineries to consider market demands when adjusting alcohol levels in wines derived from their vineyards.
On average, wines have gradually increased in alcohol content and pH in recent years and winemakers are concerned about the problem. Moreover, climate change may increase this tendency. Changes in rainfall distribution and average temperatures will probable affect vine and grape physiology, and impact wine composition and quality . Under a future warmer climate, higher temperatures may inhibit the formation of anthocyanin, increasing volatilization of aroma compounds  and total soluble solids, suggesting a decrease in wine quality. Hence, high alcohol levels in wines should receive more prominent attention to improve the technologies for reducing alcohol content of wines by conserving organoleptic balance, flavor, and high quality. The strategies to achieve moderate alcohol levels fit mainly into four basic groups as viticultural, prefermentation, fermentation, and postfermentation strategies . Prefermentation and fermentation applications can be include under the name of biotechnological strategies.
Viticultural and biotechnological strategies are two approaches to deal with higher must sugar levels at harvest time. The former involves practices as partial defoliation in vineyards, which has as main objectives increasing sunlight and ventilation for the fruit, aiming to improve color and maturity in red grapes, and helping to reduce fungal diseases, which should result in better wine quality . A wide range of factors could significantly affect sugar accumulation in the grape such as choice of vineyard site, soil composition and vine nutrition, irrigation strategy, rootstock, and grape cultivar selection as well as grape yield . In this sense, approaches to canopy management are continually evolving in response to changes in other vineyard management practices; some of these could contribute to reduce soluble sugars on grape berries at harvest time. On the other hand, a review among putative biotechnological-based strategies has been carried out, mainly related to the use of yeast strains in wine elaboration. Between all approaches one of the most relevant is the amendment of the fermentative process by using selected yeast strains and making changes in the way to proceed. A procedure consisting in the use of mixed yeasts inoculum was development and wines with up to one degree less alcohol strength were obtained. This chapter attempts to show how different viticultural and biotechnological strategies impact on the potential alcohol concentration in wines.
2. Managing the time of grape ripening
There is an increasing interest in using a number of plant growth regulators (PGRs) to manipulate berry composition for the benefit of the wine industries. PGRs that control the coordination of berry ripening and act to coordinate global changes in gene expression during crucial events of plant development could become ideal targets for altering ripening in a global manner . Research on the role of auxins as PGRs in grape berry development to manipulate the timing of the onset of ripening, harvest date, and berry composition [18, 19] has showed lower total soluble solids levels in those grapes treated with auxins at harvest time. Since extending the time before harvest increases sugar concentration, which in turn leads to wines with elevated ethanol concentration , it could be advisable the use of auxins to delay grape maturity. The mechanism by which auxins delay ripening is unknown, but auxin treatments maintain the berry in the preveraison state, as judged by a delay in the physical and biochemical changes normally associated with ripening. These include a delay in the accumulation of sugars and anthocyanins, and also a delayed decrease in acidity and chlorophyll .
Figure 1 reports differences in maturity of
Means and standard errors of all evaluated parameters arranged by treatment are shown in Figure 1. According to ANOVAs there were significant effects (
In the same year (2015), another study with NAA was performed in Villafranca de Duero (Spain). The trial established two parcels within two
With the data set obtained in both experiments with NAA (cvs. Tempranillo, Cabernet Sauvignon, and Syrah), the relationship between YAN, MA, and TSS levels in order to assess the intercorrelations among these must quality parameters was studied (Figure 3). From the data in Figure 3, it is apparent a possible linear relation between YAN and MA levels. Furthermore, levels of MA are positively correlated with levels of YAN. On the other hand, the findings do not indicate an apparent pattern in case of TSS with any of the other parameters.
Although the mechanisms that control the ripening of the nonclimacteric grape berry are poorly understood , the results of this study indicate the ability of NAA to decrease TSS at harvest time. Although the lower levels of TSS in treated berries may be mainly due to a delay in sugar accumulation, these data suggest that auxin treatments may be useful in controlling high must sugar levels at harvest time.
3. Effects of Mg2+ foliar fertilization on berry sugar content
It must be recognized that grapevine nutrition remains an important part of managing a vineyard since it impacts on berry development and, finally, wine quality is derived to a large degree from berry composition. Some grape growers avoid any fertilizer for fear of overstimulating growth, whereas in other cases vineyard blocks might be fertilized when only specific areas of the block require fertilizer. Therefore, it is important that growers have a sound basis for determining the fertilizer needs of their vines . Elsewhere, since the general relationship between vine nutritional status (in both nutrient macro- and microelements) and grape composition is obscure, further efforts are necessary to acquire greater knowledge in this topic. This is an important knowledge gap because these elements should necessarily influence grape juice quality and, therefore, the vinification process.
It is recognized that plants need K+ for the formation of sugars and starches, for protein synthesis, and for cell division. Additionally, K+ also neutralizes organic acids, regulates the activities of other mineral nutrients in plants, activates certain enzymes, and helps adjust water relationships (Hewitt, cited by ), but free potassium ions are released when the grape cell membranes are broken during grape processing, and form crystals with tartrate, which drop grape juice and wine acidity . On the basis of the antagonistic interaction between levels of K+ and Mg2+ reported by several authors at the root-soil interface [23, 24], another study was performed during 2014 vintage in the PDO area of Ribera del Duero, Spain. The impact of Mg2+ supply on berry chemistry attributes from this trial is shown below.
The cultivar chosen,
4. Effects of leaf removal and lateral shoot removal on berry sugar content
One of the most important and commonly applied summer canopy management operations in viticulture is the removal of leaves  and shoots in the fruit zone. Both practices are performed on grapevines to increase air circulation, light exposure, penetration of fungicide sprays, as well as decrease disease incidence. In general, exposing fruit to the sun will increase fruit temperature along with the enzymatic activities therein. Consequently, when compared to shaded fruit, exposed fruit will normally contain higher soluble solids . Nevertheless, it should be noted that these actions on the vine canopy microclimate, which basically depends on the amount and distribution of leaf area in space and its interaction with above-ground climate , will have different effects on harvest quality according to the time and the shoot position when they were carried out. Most canopy microclimate components are of different values than those around the canopy, due to attenuation by the canopy. The degree of shading within grapevine canopies can be altered by three principal means: by varying the shoot number, the vine vigor, and/or the training system employed . At the same time, a number of viticultural practices in wine grape improvement programs have been a topic of discussion in the scientific community in order to improve grape quality at harvest: optimum balance in vine pruning, shoot thinning, leaf and lateral shoot removal, early cluster thinning, late cluster thinning, shoot positioning, and tipping or irrigation scheduling.
On the basis of the above, a research was performed during 2014 vintage in the PDO area of Ribera del Duero, Spain. The cultivar chosen was
5. Biotechnological approaches to reduce the alcohol content in wine
Currently, several different technological strategies are available in order to reduce the ethanol content in final wines. Yeasts are the main microorganisms involved in the ethanol production from grapes and wine production, and accordingly, some of these strategies are based in a different management of wine yeasts, including the isolation of strains with a lower ability to produce ethanol.
Natural screening might be the first attempt to obtain lower ethanol-producing strains. However, this approach is unlikely to succeed because different aspects of the biochemistry, physiology, and genetics of
Microorganisms, and particularly yeast, have a huge ability to adapt rapidly to different environmental conditions. This property has been used in recent years to modify the natural properties of yeasts by conducting adaptive laboratory evolution (ALE) experiments . This approach mimics the natural evolution, by environmental or metabolic constraints, with the main purpose of obtaining improve yeast strains for several biotechnological applications and, of course, in winemaking processes [37–40]. A recent ALE study, by using KCl as osmotic and salt stress agent during 450 generations, achieved a wine with 0.6% (v/v) less ethanol in pilot scale fermentation when it was compared to the previous ancient strain. Besides that, the use of intrastrain hybrids by breeding techniques (a non-GMO technique) has proven the reduction of the alcoholic strength to 1.3% (v/v) .
An alternative approach to modify the final alcohol content of wines is related to the performance of modified fermentation procedures. Although
There are significant differences in sugar metabolism between some of these species and
|Young vineyard||Middle-age vineyard||Old vineyard|
|Fermentation phase||Yeast analyzed||Identification||Yeast analyzed||Identification||Yeast analyzed||Identification|
Although high levels of ethanol content in final wines is a worldwide issue, as mentioned earlier, in Spain this problem is still more pronounce in the Denomination of Origin (DO) Toro (Toro, Zamora, Spain), whose wines easily reach and exceed 15–16° alcohol content. For this reason a biotechnological-based approach was developed with the final aim to reduce their ethanol levels. During 2013 vintage, the population of indigenous strains associated to a winery belonged to DO Toro was characterized. Spontaneous fermentations were carried out on natural grape juice (“
The analysis of final wines were performed by HPLC using an Agilent 1200 series (Agilent Technologies, Santa Clara, CA, USA) chromatograph equipped with a HyperREZ XP Carbohydrate H+ column (8 μm particle size, 300 × 7.7 mm) and a HyperREZ XP carbohydrate H+ Guard pre-column (Thermo Scientific, Waltham, MA, USA), maintained at 50°C. Samples were filtered using 0.45 μm cellulose acetate filters (Costar, Washington, DC, USA) prior to analysis. A refraction index detector (RID) (positive polarity) at a flow rate of 0.8 ml/min with 4 mmol/l H2SO4 as mobile phase (injection volume 25 μl) was used to detect glycerol and ethanol. One-way analysis of variance was carried out to determine the influence of the “yeast used” factor on ethanol and glycerol content. The results are shown in Table 3. Coinoculation methodology decreased the alcohol level in final wines from 0.55 to 0.62% (v/v) when a
|Compound||Control (Sc-1)||Sc-1 + ||Sc-1 + |
|12.70 (0.01)a||12.08 (0.06)b,c||11.90 (0.03)c||12.15 (0.09)b||11.87 (0.15)c|
|4.04 (0.01)a||3.93 (0.15)a||5.57 (0.29)b,c||4.41 (0.16)a,b||6.23 (1.26)c|
Recently, a novel study addressed the same issue with a similar experimental design. In fact, Morales et al.  used a
Therefore, the implementation at the industrial level of strategies to lower the ethanol content of wine, owing to breakdown of sugars by non-
Taken together the findings showed in this chapter, it has become evident that there are several potential efficient practices to overcome high must sugar levels at harvest time. Most favorable results were obtained by using plant growth regulators (auxins) and yeast selection. The generalizability of these results could be subject to certain limitations. Thus, in the case of auxins the cultivar behaves as an important factor to be taken into consideration, whereas in the case of yeast selection, more research is required to determine the efficacy of implementation at the industrial level.
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