The Formation of Succinic Acid during Alcoholic Fermentation

Abstract

During alcoholic fermentation by yeast there is an increase in total acidity and a change in the constituent organic acid composition of the fermented medium. The formation of acetic acid, widely referred to as volatile acidity, is generally recognized as a normal by-product of alcoholic fermentation. In bacteria-free fermentations, the amount of acetic acid formed does not exceed 0.03 to 0.04 g per 100 ml. By far the largest increase in acidity during fermentation is brought about by the formation of nonvolatile organic acids, and it generally varies between 0.1 to 0.4 g per 100 ml but averages about 0.2 g per 100 ml. Three strains of Saccharomyces cerevisiae widely used in California for commercial wine fermentations were studied using dormant yeast, cake or proliferating yeast slurry. The net increase of nonvolatile organic acidity was not influenced by the yeast strain. The highest rate of increase of nonvolatile organic acidity took place during the early stages of fermentation; after the fermenting medium reached 4 to 5% alcohol, the rate of increase leveled off gradually. The net increase in nonvolatile acidity was not significantly influenced by the initial pH of the fermenting medium. However, the percentage increase in nonvolatile acidity was highest with media of low initial total acidity. With most grape juice fermentations, qualitative changes in the nonvolatile acid composition of the resulting wine are obscured by the over-all decrease in total acidity brought about by the precipitation of potassium acid tartrate. Significant amounts of organic acids, not present in the grape, are formed during fermentation and perhaps influence wine quality. At least two new organic acids are formed during fermentation, which constitute the bulk of the nonvolatile organic acidity increase. Of these, about 90% is due to one acid and about 10% to the second. On the basis of paper chromatagraphy, radiochromatagraphy, infrared absorption spectroscopy, melting point, and other traditional methods of organic chemistry, it was determined that the predominant nonvolatile organic acid formed during fermentation is succinic acid (90%). Lactic acid appears to be the other acid formed in significant amounts (10%).