The ability of wine yeast to consume fructose

Introduction Stuck fermentations have been the subject of numerous studies, and seve­ ral have determined the factors re ­ sponsible for this fermentation prob­ lem. Research has shown how certain fermentation conditions, such as nutri­ tional deficiencies, high initial levels of sugar, and the presence of inhibiting compounds, can lead to fermentation problems. The results of this type of research are helping winemakers lo wer the risk of stuck fermentations significantly. Under oenological conditions, the main sugars fermentable by Saccharomyces cerevisiae are glucose and fructose. Both of these hexoses are ge ­ nerally present in musts in equivalent quantities, but the proportions may vary in some musts. S. cerevisiae pre­ fers to consume glucose, which ex ­ plains why, when fermentations be ­ come stuck, the remaining sugar is mainly fructose. The frequency of stuck fermentations showing residual fruc­ tose raises the question of the ability of yeast to consume this hexose. The kinetics of sugar utilization by S. cerevisiae during fermentation is largely driven by sugar transport, and glucose is typically consumed at a faster rate than fructose. In sluggish fermenta­ tions, the maximal rate of fermentation is reduced after most of the glucose is consumed, and fermentation can become stuck with a significant con­ centration of fructose remaining. According to the literature, the level of residual glucose in stuck wines is 10 times lower than the fructose concen­ tration. According to Gafner and Schûtz (1996), it is possible to predict stuck fermentation when the glucose/ fructose ratio (GFR) is under 0.1. During alcoholic fermentation, sug­ ars are consumed mainly during the stationary phase. During this phase, the available nitrogen gradually becomes less available, and since it is an essential nutrient involved in the transport of sugars into the cell via protein synthesis, this partially explains why both the yeast metabolism and the fermentation activity (Salmon, 1996) slow down. The alcohol level also gradually increases, becoming toxic to the yeast cell, and the use of fructose is even more compromised. At the molecular level, research has confirmed the genes coding for the hexose transporters in yeast. Under oenological conditions, several genes are involved in sugar transport, which is regulated by a large, multi­gene family called HXT. There are 20 HXT genes. Hxt1 and Hxt7 are the main transporters. Hxt2, Hxt6 and Hxt7 are high­affinity carriers, whereas Hxt1 and Hxt3 are low­affinity carriers. Sev­ eral other Hxt carriers have intermedi­ ate affinity. Both the high­ and low­ affinity carriers have greater affinity for glucose than fructose, which may affect the rate of utilization of those hexoses. Hexose concentrations in the medium will influence the expression of individual HXT genes (Perez et al., 2005; Guillaume et al., 2007). It has been shown that Hxt3 has the highest capacity to support fermentation (Luyten et al., 2002) and very recent studies have also identified that this gene is indeed responsible for the capacity for consuming fructose among certain yeasts (Guillaume et al., 2007). They also showed that a mutation on an allele of the Hxt3 gene was respon­ sible for improving the performance of wine yeast by utilizing fructose during fermentation and in cases of stuck fer­ mentation. It is now established that variations exist in the capacity of yeast to con­ sume fructose. The objective of this study was to evaluate the fermentation performance of selected yeasts under oenological conditions, paying particu­ lar attention to their capacity to con­ sume fructose. A method was devel­ oped to measure the “fructophilic index,” which would help determine the ability of a particular yeast to con­ sume fructose.