Redox Processes in Grapes Processing

Introduction

The processing of the grapes themselves involves redox processes, especially in the case of damaged and contaminated grapes, when the contact of oxygen in the air with the grape juice is facilitated (Briedis et al., 2003). The substances, which form redox couples in grapes, must and wine, are numerous - polyphenols/quinones, GSH / GSSG, endiol / diketones, acetoin/diacetyl, anthocyanins / reduced anthocyanins, dioximaleic acid / dioxitartaric acid, etc., and their influence on the product depends on their share in the general transformations during the life of the wine and become responsible for their self-life (Zironi et al., 2010). In oxidative-reducing chemical transformations, an essential role belongs to the pairs of transition metals Fe³⁺/Fe²⁺, and Cu²⁺/Cu⁺, naturally present in wines, but also in many enzymes (until their inactivation by ethanol and elimination by technological processes). The presence of phenolic substances in grapes, must, and wine largely determines the physicochemical properties and quality of the finished product (Tarko et al., 2020). In seeds, their oxidative transformations are catalyzed by oxidase enzymes, present in healthy grapes. Monophenols are converted to ortho-diphenols by oxidation in the presence of tyrosinase (monophenoloxidase), while di- and triphenols are oxidized to quinones in the presence of polyphenol oxidase (PPO). This ferment is quite specific to ortho-diphenols, which are oxidized to ortho-quinones, compounds highly reactive (Sun et al., 2011). They form brown polymers in the must, which affect the color of white wines. Grapes affected by Botrytis cinerea contain a much more active yeast produced by gray rot, which oxidizes a much wider spectrum of substances in must and wine. All of these oxidative enzymes contain active sites of Cu²⁺ ions (de Simone et al., 2020).

Oxidations are favored by high temperatures, high pH, and oxygen content (Scutaru et al., 2022). At the stage of processing white grapes, the inactivation of these enzymes is crucial, which is achieved by the addition of grapes or must the antioxidants, SO₂, and ascorbic acid. Monitoring redox processes during alcoholic fermentation by oxygen management is essential for the resulting quality of dry wines. Oxidations in the wine production process led to a decrease in varietal aromas (Kanavouras et al., 2019). At the same time, the reducing conditions are responsible for possible undesirable aromas of disulfides and hydrogen sulfide (Bekker et al., 2016). In particular, ensuring the reduced environment in bottled white wines affects the quality of white wines, in which, after several months, atypical, reduced aromas can be attested (Sturza et al., 2021). Ensuring strongly reducing bottling conditions by applying inert gases and sealing, contributes, in the long run, to achieving reactions with the participation of varietal thiols.

The quality of wines can be disastrously affected by the phenomenon of redox photoactivation of wine bottled in transparent vessels, exposed for a certain period (a few hours being sufficient) to natural or artificial light (Celotti et al., 2022). As a result, the redox state is modified, which causes the modeling of the organoleptic properties of wines and causes the olfactory defect called “light taste” (sunlight flavor, light-struck gout). It is accompanied by the smell of boiled cabbage, onions, damp wool, and rubber. At the same time, the redox balance of wines is disturbed by the simultaneous formation of peroxides, glyoxylates and acrolein. In the production of red wines, the redox processes are no less important, although the impact is less significant (Márquez et al., 2019).