Aims: To determine acetic acid, acetaldehyde and glycerol production by wine yeast throughout Icewine fermentation. The expression of yeast cytosolic aldehyde dehydrogenases (ALD3 and ALD6) and glycerol-3-phosphate dehydrogenase (GPD1) were followed to relate metabolites in the wines to expression patterns of these genes.
Methods and results: Icewine juice (38.8 degrees Brix, 401 +/- 7 g l(-1) sugar), diluted Icewine juice (21.3 degrees Brix, 211 +/- 7 g l(-1) sugar) and the diluted juice with sugar levels equal to the original Icewine juice (35.6 degrees Brix, 402 +/- 6 g l(-1) sugar) were fermented in triplicate using the commercial wine yeast K1-V1116. Acetic acid production increased 7.1-fold and glycerol production increased 1.8-fold in the Icewine fermentation over that found in the diluted juice fermentation. ALD3 showed a 6.2-fold induction and GPD1 showed a 2.5-fold induction during Icewine vs the diluted fermentation. ALD3 was not glucose repressed when additional sugar was added to diluted juice, but was upregulated 7.0-fold.
Conclusions: The NAD+-dependant aldehyde dehydrogenase encoded by ALD3 appears to contribute to acetic acid production during Icewine fermentation. Expression of GPD1 was upregulated in high sugar fermentations and reflects the elevated levels of glycerol. Solutes in Icewine juice in addition to sugar contribute to the yeast metabolic response.
Significance and impact of the study: This work represents the first descriptive analysis of the fermentation of Canadian Icewine, the expression patterns of yeast genes involved in metabolite production, and their impact on Icewine quality. A role for ALD3 in acetic acid production during Icewine fermentation was found.