Effect of Reductive Pressing on the Concentration of Reduced Glutathione and Phenols in the Musts of Four Italian Cultivars

Abstract

This research investigated the effect of pressing under air or nitrogen on the color and concentration of reduced glutathione (GSH), hydroxycinnamoyl tartaric acids (HCTA) and polyphenols in the musts of four Italian white grape cultivars: Arneis, Cortese, Moscato bianco, and Manzoni bianco. Grapes from different locations were pressed using a prototype press (500 kg capacity) under the same operating conditions as a commercial winery. The musts of the four cultivars were significantly different from one another. The trend of the measured GSH concentration in the musts was: Moscato bianco < Cortese < Manzoni bianco < Arneis, without any significant interactions between cultivar and year. The intake of oxygen during pressing in air caused large decreases in GSH concentration, yet its complete exhaustion was only observed in a few trials. When present, the residual GSH was proportional to the values observed in the respective trials pressed under nitrogen. The contact with air during pressing also lowered the amounts of total polyphenols and catechins. The loss of catechins did not depend on the composition of HCTA, GSH, and HCTA/GSH in musts, while color browning was influenced by the HCTA/GSH molar ratio. The color intensity of musts (A₄₂₀) increased exponentially with the decrease in GSH: A₄₂₀ values higher than 0.2 AU were observed for GSH concentrations less than 8 μM.