Comprehensive study of the dynamic interaction between SO and acetaldehyde during alcoholic fermentation.

In this work, we focused on the effect of the initial content of SO in synthetic grape juice on yeast metabolism linked to the production of acetaldehyde. Lengthening of the lag phase duration was observed with an increase in the initial SO content. Nevertheless, an interesting finding was a threshold value of an initial SO content of 30 mg L in the juice led to equilibrium between intracellular SO diffusion and SO production from the sulfate pool by yeast. The ratios of free and bound acetaldehydes were measured during fermentation, and the maximum accumulation of free acetaldehyde was observed when SO concentration equilibrium between diffusion and production was reached in the fermenting juice. Moreover, it was observed that SO addition resulted in significant changes in the synthesis of aroma compounds. Production of volatile molecules related to sulfur metabolism (methionol) was changed. But, more surprisingly, synthesis of some volatile carbon compounds (diacetyl, isoamyl alcohol, isobutyl alcohol, phenyl ethanol and their corresponding esters) was also altered because of major disruptions in the NADPH/NADP redox equilibrium. Finally, we demonstrated that acetaldehyde bound to SO could not be metabolized by the yeast during the time course of fermentation and that only free acetaldehyde could impact metabolism.