Chemical and Sensory Effects of Microorganisms on Grape Musts And Wine

¹ Department of Viticulture and Enology, University of California, Davis.

Techniques were developed for the infection and decay of grapes by monofungus cultures. Blends of up to 40% juice from decayed Thompson Seedless grapes and the balance from uninfected fruit were fermented by standard enological procedures. Botrytis cinerea, Rhizopus arrhizus, Aspergillus niger and A. flavus had little or no effect on the volatile acidity of either the musts or wines. However, volatile acidity of musts and wines was raised when grapes were decayed by combinations of Acetobacter roseus and Saccharomyces cereviseae with A. niger, R. arrhizus or B. cinerea; but very little by combinations which included A. flavus. The fixed total acid of the musts and wines was raised only by A. niger, which also lowered the pH of the wines. Wines which did not ferment to dryness were those with appreciable volatile acidity. There were no marked differences in the amount of residual reducing sugar. There was an inverse relation between alcohol and the volatile acid content of the wines. As determined by the Folin-Denis method, wines from grapes decayed by A. flavus were much higher in tannin-like materials than wines from unaffected grapes or from grapes decayed by the other three fungi. These wines had an unusual yellow-pink color.