Effect of Grape Maturation on Soluble Protein Characteristics of Gewürztraminer and White Riesling Juice and Wine

Abstract

Soluble protein in juice and wine increased with increasing grape maturity for both White Riesling (WR) and Gewürztraminer (GZ). It tended to decrease during fermentation. Protein retention in fermenting must was greatest at pH 3.2. Regardless of maturity, GZ wines had six protein fractions in common which ranged in MW from 11 000 to 69 500 daltons. The GZ wine fermented from the most ripe grapes (24.3% soluble solids) had an additional MW fraction at 41 200. Dependent upon grape maturity, WR wines had from four to seven protein fractions which ranged in molecular weight from 12 400 to 88 100 daltons. GZ wines contained from eight to ten protein fractions which ranged in isoelectric point from 8.5 to 3.1. WR wines contained six to nine protein fractions which ranged in isoelectric point from 9.20 to 3.05. Wines from both cultivars had protein fractions with isoelectric point at or below the wine pH. Residual soluble protein was found in all heat stabilized wines. The Coomassie Brilliant Blue G-250 assay, used in conjunction with bentonite fining trials, provided a suitable means for determining the maximum amount of bentonite needed for removing the maximum amount of soluble protein.