Abstract
SummaryAlcohol is fundamental to the character of wine, yet too much can put a wine off‐balance. A wine is regarded to be well balanced if its alcoholic strength, acidity, sweetness, fruitiness and tannin structure complement each other so that no single component dominates on the palate. Balancing a wine's positive fruit flavours with the optimal absolute and relative concentration of alcohol can be surprisingly difficult. Over the past three decades, consumers have increasingly demanded wine with richer and riper fruit flavour profiles. In response, grape and wine producers have extended harvest times to increase grape maturity and enhance the degree of fruit flavours and colour intensity. However, a higher degree of grape maturity results in increased grape sugar concentration, which in turn results in wines with elevated alcohol concentration. On average, the alcohol strength of red wines from many warm wine‐producing regions globally rose by about 2% (v/v) during this period. Notwithstanding that many of these ‘full‐bodied, fruit‐forward’ wines are well balanced and sought after, there is also a significant consumer market segment that seeks lighter styles with less ethanol‐derived ‘hotness’ on the palate. Consumer‐focussed wine producers are developing and implementing several strategies in the vineyard and winery to reduce the alcohol concentration in wines produced from well‐ripened grapes. In this context, Saccharomyces cerevisiae wine yeasts have proven to be a pivotal strategy to reduce ethanol formation during the fermentation of grape musts with high sugar content (> 240 g l−1). One of the approaches has been to develop ‘low‐alcohol’ yeast strains which work by redirecting their carbon metabolism away from ethanol production to other metabolites, such as glycerol. This article reviews the current challenges of producing glycerol at the expense of ethanol. It also casts new light on yeast strain development programmes which, bolstered by synthetic genomics, could potentially overcome these challenges.
Highlights
In spontaneously fermenting grape must that is not seeded with a high-density inoculum of S. cerevisiae, there is a sequential succession of nonSaccharomyces species of Candida, Cryptococcus, Hanseniaspora (Kloeckera), Metschnikowia, Pichia and Rhodotorula (Jolly et al, 2014)
The contribution of these yeasts’ metabolites to wine flavour depends on how active they are during the initial phases of fermentation, and this in turn depends on how well, and for how long, they can cope with the high osmotic pressure, equimolar mixture of glucose and fructose, high sulfite concentration, suboptimal growth temperature, decreasing nutrients as well as increased alcohol concentrations and anaerobic conditions
Sugar fermentation in S. cerevisiae is a redox neutral process influenced by the NAD+/NADH balance
Summary
Alcohol is fundamental to the character of wine, yet too much can put a wine off-balance. Since detailed knowledge of yeast’s fermentative metabolism – alongside the development of modern vineyard practices, winemaking equipment and packaging material, as well as ever-changing consumer preferences – placed the global wine industry on a never-ending cyclical journey of today’s innovation is tomorrow’s tradition across the entire from-grapes-to-glass value-chain (Fig. 1) One such consumer-driven innovation that has become a tradition over the past three decades is the extension of the time before grapes are harvested in dry, warm wine-producing regions of the world. In terms of overall wine quality, balance between alcohol strength, acidity, tannin, sweetness and fruit flavour intensity is extremely important These three drivers and a growing market demand are calling for a reduction of alcohol concentration in wines, preferably without compromising wine flavour, consumer acceptance or increasing the cost of production (Varela et al, 2015). This article appraises the fourth winemaking strategy, which is aimed at microbiological practices and yeast strain development programmes
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