The fundamental role of pH in the anthocyanins chemical behavior and in their extractability during winemaking

Abstract

The chemical behavior of anthocyanins is considerably affected even by slight pH variations with impor-tant implications for the winemaking as well as for the wine conservation. Considering that this is a cen-tral issue to the enological sector, we decided to better investigate the influence of pH on the anthocyanin chemistry.

Initially, by chromatographic and advanced NMR techniques the chemical behavior of malvi-din- 3-O-glucoside was studied in wine-like solutions with pH values ranging from 3 to 4. First, the already composite aqueous equilibrium of malvidin-3-O-glucoside turned out to be even more complex than so far thought, as a new hydration product of the anthocyanin was detected and characterized in solution.

More importantly on account of its technological implications, the anthocyanin solubility appeared to decrease remarkably as the pH value of the wine-like solutions increased. A dramatic drop in terms of anthocyanin solubility was observed at pH 3.32, where the measured molecule concentration was reduced to almost 25% the expected one. Also, at such pH level the anthocyanin self-association appeared significantly affected.

In more detail,the flavylium ion self-association predominant at lower pH levels was altered and found to co-occur with a preferential co-pigmentation involving flavylium ion species and the trans-chalcone form of malvidin-3-O-glucoside. At higher pH values, this latter association was the only one detected in so-lution. In the light of these results, we set up an experimental protocol with the purpose of analyzing the pH influence on the anthocyanin chemistry and extractability in real wines produced by varying their pH levels during the maceration-fermentation phases.

Preliminary chemical analysis of such wines provided data consistent with those obtained in wine-like solutions. Indeed, the extraction of malvidin-3-O-gluco-side and that of anthocyanins in general was more contained as the pH value increased.

Additionally, other molecules of enological interest, including catechins, cinnamates, syringic acid and terpenoids, turned out to be affected by the wine pH. These analytical data highlight the fundamental role of pH during the wine-making and the importance of regulating its level to obtain wines with the desired polyphenolic structure.

Boulton. American journal of enology and viticulture, 52(2), 67-87 (2001). Forino, M., Gambuti, A., Luciano, P., Moio, L. J. Agric. Food Chem. (2019) doi:10.1021/acs.jafc.8b05895