Quantification and characterization of cell wall polysaccharides released by non-Saccharomyces yeast strains during alcoholic fermentation.

In order to improve knowledge about the oenological characteristics of non-Saccharomyces yeast strains, and to reconsider their contribution to wine quality, we studied the release of polysaccharides by 13 non-Saccharomyces strains of different species (three wine yeasts, six grape yeasts, and three spoilage yeasts) during alcoholic fermentation in synthetic must. Three Saccharomyces cerevisiae strains were included for comparison. All of the non-Saccharomyces strains released polysaccharides into fermentation medium; the amount released depended on the yeast species, the number of cells formed and their physiological conditions. Normalizing the quantity of macromolecules released to the cell biomass revealed that most non-Saccharomyces strains produced a greater quantity of polysaccharides compared to S. cerevisiae strains after 7 and 14days of fermentation. This capacity was particularly expressed in the studied wine spoilage yeasts (Saccharomycodes ludwigii, Zygosaccharomyces bailii, and Brettanomyces bruxellensis). Chemical characterization of exocellular polysaccharides produced by non-Saccharomyces yeasts revealed them to essentially be mannoproteins with high mannose contents, ranging from 93% for S'codes. ludwigii to 73-74% for Pichia anomala and Starmerella bombicola. Protein contents varied from 9% for P. anomala to 29% for Z. bailii. These compositions were very similar to those of the S. cerevisiae strains, and to the chemical composition of the cell wall mannoproteins of different yeast species. The presence of galactose, in addition to mannose and glucose, in the exocellular polysaccharides released by Schizosaccharomyces pombe, confirmed the parietal nature of the polysaccharides released by non-Saccharomyces yeasts; only this species has a galactomannan located in the outer layer of the cell wall.

Effect of yeast strain and fermentation conditions on the release of cell wall polysaccharides.

To improve our understanding of the factors involved in polysaccharide release during alcoholic fermentation, we investigated three Saccharomyces cerevisiae strains in fermentation trials conducted at two temperatures (25 degrees C and 32 degrees C) and three sugar concentrations (20%, 23.5%, and 27%), with or without supplementation of grape juice with diammonium phosphate (DAP) or microcrystalline cellulose. In two yeast strains, the release of cell wall polysaccharides increased significantly with an increase in fermentation temperature and sugar concentration of the grape juice; the polysaccharide release was greater in stressed conditions, in which the cells were less viable and less metabolically active. In the third strain, the average amount of polysaccharides released into the medium decreased significantly at 32 degrees C with 27% sugar, and increased in grape juice supplemented with DAP. Thus, this strain released more polysaccharides when conditions were nearer to optimal and the yeast cells were more viable and metabolically active. Our results suggest that the yeast strains released cell wall polysaccharides via different mechanisms, and that the cell wall integrity pathway may account for some of the differences in polysaccharide release among the strains.

Release of cell wall polysaccharides from Saccharomyces cerevisiae thermosensitive autolytic mutants during alcoholic fermentation.

In order to increase the release of cell wall polysaccharides during alcoholic fermentation, a wine strain of Saccharomyces cerevisiae was subjected to UV mutagenesis to obtain thermosensitive autolytic mutants affected in cell wall integrity. Five mutants and the parental strain were utilized in fermentation trials conducted at 28, 32 and 34 degrees C. Results showed that at all temperatures the mutant strains released into the medium a higher polysaccharide quantity than the parental strain. In particular, at 28 degrees C there was a doubling of these macromolecules. At the end of alcoholic fermentation, all strains showed at 28 degrees C elevated and similar levels of viable cells; at 32 degrees C this parameter remained high for mutant strains ts16 and ts39 and the parental strain; at 34 degrees C all strains underwent a drop in cell viability, which was less intense in the case of strain ts16. As a relationship between cell viability and the quantity of polysaccharides released by the yeast strain was not found, it can be assumed that the mutation led to cells with a less stable wall and thus an easier release of macromolecules into the medium.