Grape must contains equal amounts of glucose and fructose but glucose ferments faster than fructose when using Saccharomyces cerevisiae since this yeast genus is significantly glucophilic. During alcoholic fermentation, the ratio of fructose to glucose increases progressively, fructose becoming the dominant sugar at later stages. The limited ability of most yeast strains to utilize fructose is a major cause of sluggish and then arrested fermentation, in particular when combined with nutritional imbalance in the must, (Schutz and Gafner, 1993; Berthels et al, 2004).
Hence, the ability of wine yeast to ferment fructose is of critical importance to ensure must fermentation to dryness. Although variations have been claimed in the ability of Saccharomyces strains to ferment fructose, the molecular reasons for such differences were not thoroughly investigated and the underlying mechanisms were not known.
The commercial wine yeast Fermichamp® (strain INRA 67J) is successfully used worldwide to restart stuck and sluggish fermentations. The reliability of this strain in restarting fermentation has been tested in comparison with other strains on the same application (Fernandez et al. 2005). Fermichamp displayed the greater capacity to ferment residual sugars. The strain has a very high ethanol tolerance (above 16 %) but this alone does not explain the results, hence the ability of the strain to ferment fructose was investigated.
Hexose transport is known to be an important step in sugar metabolism; it determines the rate of sugar utilization by the yeast. Saccharomyces cerevisiae has 18 genes, which encode for hexose transporters, but only five of them (Figure 3) were found to play a significant role in wine fermentation (Luyten et al, 2002; Perez et al., 2004). In the later stages of wine fermentation, HXT3 is the most active hexose transporter.
Genetic studies conducted on the gene encoding for the HXT3 hexose transporter of Fermichamp® revealed that this gene contained several mutations when compared to the HXT3 genes of other yeasts. Further studies confirmed that Fermichamp® ’s enhanced “fructose utilization phenotype” is dependent on the expression of this mutated HXT3 gene, this mutation coding for a hexose transport protein with enhanced affinity for fructose. This study showed for the first time that the fructose utilization pattern may depend on the nature of the hexose transporter present in a wine yeast cell wall.
For best results in restarting stuck fermentations we recommend the product synergy of Extraferm® yeast hulls (for must detoxification) and Fermichamp® wine yeast.
Figure 1: Hexose transport during wine fermentation. Low affinity carriers HXT1 and HXT3 are thought to play a major role in sugar (glucose and fructose) uptake.
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