The Linkage of Yeast Metabolites, Produced Under Hyperosmotic Stress, to Cellular Cofactor Systems During Icewine Fermentation.

Abstract

Icewine is a dessert wine of critical importance to the Canadian wine industry. The Icewine grapes are frozen on the vine, creating ice crystals, and subsequently concentrating the solutes in the juice. Icewine juice places yeast under increased osmotic stress, resulting in altered metabolism. This includes increased glycerol production, an internal osmolyte, and higher acetic acid production as they are linked to the cytosolic NAD+ and NADP+ cofactor systems. The yeast glycerol transporter Stl1p allows for glycerol uptake, lowering the production of glycerol and therefore acetic acid. Here we compare two Saccharomyces cerevisiae wine yeast strains, K1-V1116 wild type and K1-V1116 Δstl1, with Saccharomyces uvarum CN1, and relate the differences in metabolite production to the cofactor systems. To that end, starter cultures of each strain were established Icewine juice with samples collected at fixed intervals and assayed for acetic acid, glycerol, ethanol, acetaldehyde, sugar, and the NAD+/NADH and NADP+/NADPH cofactor systems. K1-V1116 wild-type, K1-V1116 Δstl1 knockout, and CN1 showed different kinetics of glycerol and acetic acid production. Although glycerol production per unit time did not vary among the three yeast strains, per unit sugar consumed, K1V1116 Δstl1 produced the most glycerol followed by CN1 and then K1-V1116. K1-V1116 Δstl1 was found to produce the highest amount of acetic acid as a function of sugar consumed compared to the wildtype. CN1 produced the lowest amount of acetic acid as a function of sugar despite producing higher glycerol than the K1 V1116 wild-type. While there was no statistical difference in the NAD(H) redox system ratios between the three yeast to account for the differences in glycerol and acetic acid production, S. uvarum CN1 showed statistically lower amounts of oxidized NADP+ to total NADP(H) compared to both of the S. cerevisiae K1 strains. These findings provide further insight about yeast metabolism under hyperosmotic stress.