Biological Assay of Nitrogen Content of Grape Juice and Prediction of Sluggish Fermentations

Abstract

A bioassay based upon final culture absorbance was developed and utilized to estimate nitrogen content of grape musts. Amino acid analyses of grape musts were run for comparison to the values obtained from the bioassay. In Chenin blanc and French Colombard musts, there was a good correlation between nitrogen content as measured by biomass yield with that determined from amino acid analysis. The nitrogen content of Grenache musts showed a positive correlation with cell yield in the bioassay, but was not as strongly correlated as in the other two varieties. The amino acids generally recognized as good yeast nitrogen sources were more highly correlated with biomass yield than were poorer nitrogen sources. Ammonium ion supplementation of nutrient-deficient Chenin blanc juices either stimulated fermentation rate or had no effect, depending upon the juice. Ammonium utilization was found to be dependent upon the pH and ethanol concentration of the medium. Fermentation of nitrogen deficient musts was found to be stimulated by a variety of growth factors in addition to nitrogen.