Kinetics of browning onset in white wines: influence of principal redox-active polyphenols and impact on the reducing capacity

The browning capacity of white wines was studied, employing an accelerated test and samples produced and stored under identical conditions. Browning was approached from a kinetic point of view and efforts were focussed on the investigation of plausible correlations with major redox-active polyphenols, including substances with an o-diphenol feature, such as gallic acid, caftaric acid, 2-S-glutathionylcaftaric acid (GRP), caffeic acid, catechin, and epicatechin. Over a period of ten days, browning development was shown to obey zero-order kinetics from the third day of the treatment, and browning rate constants (k) varied from 15.3 to 74.5 × 10−3 day−1. Regression analysis between k values and concentration of individual phenolics provided strong evidence that epicatechin is the principal browning agent (r2 = 0.8033, P < 0.01). Furthermore, the monitoring of the reducing power (PR), throughout treatments, indicated that increases in browning are accompanied by a commensurate decline in the reducing ability, raising concerns about the impact of browning reactions on the in vitro antioxidant properties of white wines.