Formation of higher alcohols by wine yeasts,and relationship to taste thresholds

The amounts of n-propanol, iso-butanol and iso-amyl plus active amyl alcohol produced during fermentation of grape juice were found to vary considerably according to the yeast used. The yeasts studied included 11 wine yeasts and one brewery yeast, all belonging to the genus Saccharomyces, and 4 yeasts belonging to genera which sometimes cause wine spoilage. Experiments were carried out in the laboratory and confirmed on pilot-plant scale. The average production of n-propanol in juices from 4 grape varieties varied from 13 to 106 ppm depending on the wine yeast used. The corresponding variations in the production of iso-butanol and isoamyl plus active amyl alcohol were 9 to 37 ppm and 115 to 262 ppm respectively,. Juices from different varieties' of grapes (Vitis vinifera) differed somewhat in the amounts of higher alcohols formed, irrespective of the yeast strain used. An increase in temperature of fermentation from 15° to 25° with four yeasts produced an average of 24% more iso-amyl plus active alcohol and 39% more iso-butanol, and 17% less n-propanol. A yeast/temperature interaction occurred. An increase in pH from 3.0 to 4.2 with four yeasts produced 28% more iso-amyl plus active amyl alcohol, 85% more iso-butanol, and 11% more n-propanol. A yeast /pH interaction occurred. Spoilage yeasts examined also formed higher alcohols and the amounts were not related to sugar consumed during the fermentation. Taste thresholds of the higher alcohols, with the exception of active amyl alcohol, were measured in dry white wine with 7 tasters. Iso-amyl alcohol thresholds ranged from 100 to 900 ppm (mean 300 ppm) and iso-butanol and n-propanol both exceeded 500 ppm. The average threshold of iso-amyl alcohol in distilled water was 4 ppm. For certain tasters strains of yeast can produce sufficient differences in iso-amyl alcohol to be detectable organoleptically, but differences in iso-butanol and n-propanol are not large enough to be detectable.