# Biotechnological Approaches to Lowering the Ethanol Yield during Wine Fermentation
## 1 Introduction  
Commercial wines have been experiencing a steady increase in alcohol levels since the 1980s. On average, the gain in alcohol has been about 1% (\(p/v\)) every ten years, currently reaching a total increase of 3-4% (\(v/v\)). There are two main reasons behind this trend. From one side, global climate warming affects, in different ways, the different physiological processes involved in grape berry ripening. Both sugar accumulation and phenolic and aromatic maturity develop faster, but the impact is greater for sugars. This imbalance prevents early harvesting from being an adequate solution to this problem, as it would lead to organoleptic defects associated with unripe berries. On the other hand, the consumer demand for tasty, structured, and full-bodied wines has pushed the market towards winemaking styles that require the harvest of very ripe grapes. As a result, quality grapes today tend to be richer in sugar content, leading to the observed increase in wine alcohol levels when all this sugar is fermented by yeasts [1]. Excess alcohol in wines is becoming a problem for the industry from several points of view. Wine balance is a delicate equilibrium between sweetness, acidity, bitterness, and aroma compounds, with alcohol as a vehicle for the perception of the latter. Ethanol surplus can lead to a sensory imbalance by promoting bitterness and masking fruit perceptions, and increasing the perception of sweetness, astringency and hotness. Besides this, sticking to moderate alcohol consumption becomes more difficult, and can therefore lead to increased risks for consumer health and road safety. Taxation based on alcohol levels in several importing countries is an additional incentive to look for wines with lowerethanol contents. Taken together, all of these factors might contribute to discourage wine consumption. Furthermore, from a bioprocess point of view, high levels of ethanol could lead to sluggish or stuck fermentation processes, with significant economic consequences. In recent years, there has been a collective effort by researchers, engineers, and wine-makers to develop approaches to limit the ethanol content of wines [2]. This community is targeting almost every stage of the production cycle, including, among other examples, grapevine clonal selection, vineyard management, winemaking practices adapted to unripe grapes, the use of yeast strains with a lower ethanol yield (often recombinant) or metabolic inhibitors, and partial dealcoholisation by physical means. Unfortunately, straightforward solutions, such as early harvesting or post-fermentation treatments, often have a negative impact on the wine quality (e.g., a green character and altered aromatic profile). Apart from sweet wines, most wines on the market are dry wines, which should contain very low levels of residual sugar. Solutions that result in less alcohol but an increase in residual sugars (i.e., stopping fermentation before completion) will drastically change the wine style, and will not be considered in this review. Here, we will present biotechnological solutions related to the fermentation process, mainly involving the selection and improvement of wine yeast strains for reduced ethanol yield, but also the use of some enzymes or metabolic inhibitors. In this context, a pivotal concept is alcohol yield, which is the amount of ethanol produced per unit of sugar consumed. Most microbiological approaches to the reduction of the alcohol level in wines will target the ethanol yield of _S. cerevisiae_ or alternative wine yeast species. Associated with the lowering of the alcohol yield is the concept of alternative carbon sinks (carbon-containing metabolic end products other than ethanol). Under standard fermentation conditions, about 60% of the hexose carbon consumed by _S. cerevisiae_ ends up in the form of ethanol, 30% goes to carbon dioxide, and the remainder goes to biomass and less abundant metabolites.