This work represents a more comprehensive understanding of how and why commercial wine yeast respond at the transcriptional and metabolic level during fermentation of Icewine juice, and how these responses contribute to increased acetic acid and decreased ethyl acetate production.

1. The effect of hormones on (32)P incorporation into various RNA fractions in germinating pear embryos was studied by fractionation on methylated albumin-kieselguhr columns. Abscisic acid inhibited labelling of soluble RNA, DNA-RNA hybrid and light-ribosomal RNA fractions with (32)P and this effect was reversed by both kinetin and gibberellic acid. 2. Kinetin reversed the inhibition by abscisic acid of (32)P incorporation into total ribosomal RNA and appeared to promote labelling of heavy-ribosomal RNA. Gibberellic acid was more active than kinetin in reversing the inhibition by abscisic acid of labelling of the DNA-RNA hybrid fraction with (32)P, but in contrast with kinetin appeared to increase further the inhibition by abscisic acid of labelling of total ribosomal RNA. 3. The percentage of radioactivity in various RNA fractions showed marked variation in response to hormones. 4. The pattern of labelling of RNA in pear embryos during reversal of inhibition by abscisic acid with a combination of kinetin and gibberellic acid was similar to that after cold-treatment of dormant pear embryos. This is suggestive of hormonal interplay in dormancy release by cold-treatment in pear embryos.

Acetic acid is undesired in Icewine. It is unclear whether its production by fermenting yeast is linked to the nicotinamide adenine dinucleotide (NAD + /NADH) system or the nicotinamide adenine dinucleotide phosphate (NADP + /NADPH) system. To answer this question, the redox status of yeast cytosolic NAD(H) and NADP(H) were analyzed along with yeast metabolites to determine how redox status differs under Icewine versus table wine fermentation. Icewine juice and dilute Icewine juice were inoculated with commercial wine yeast Saccharomyces cerevisiae K1-V1116. Acetic acid was 14.3-fold higher in Icewine fermentation than the dilute juice condition. The ratio of NAD + to total NAD(H) was 24-fold higher in cells in Icewine fermentation than the ratio from the dilute juice condition. Conversely, the ratio of NADP + to total NADP(H) from the dilute fermentation was 2.9-fold higher than that in the Icewine condition. These results support the hypothesis that in Icewine, increased NAD + triggered the catalysis of NAD + -dependent aldehyde dehydrogenase(s) (Aldp(s)), which led to the elevated level of acetic acid in Icewine, whereas, in the dilute condition, NADP + triggered NADP + -dependent Aldp(s), resulting in a lower level of acetic acid. This work, for the first time, analyzed the yeast cytosolic redox status and its correlation to acetic acid production, providing a more comprehensive understanding of the mechanism of acetic acid production in Icewine.

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