In contrast, the concentra- tion of acetaldehyde increased as D increased for cultures of both strains. Acetaldehyde is produced by S. cerevisiae metabolism during alcoholic fermentation; in this process, acetaldehyde forms as an intermediate product of pyruvate metabolism by the effect of glycolytic enzymes in the yeast. In addition, acetaldehyde is a precursor of ethanol, acetate and diacetyl (2, 3-butanedione).