EPR investigation of direct oxyradical scavenging and metal chelation activities of chitosan as an alternative to sulfites in enology

Exposure to dissolved oxygen during the winemaking process adversely affects the sensory properties of wines such as loss of colour, aromatic defects and increase of astringency. For decades the management of juice browning in enology has been managed by adding sulphur dioxide as a powerful antioxidant and antimicrobial agent. However, since sulfites are growingly rejected by consumers because of their chemical nature and known allergenic properties, developing biologically relevant alternatives is an expanding field in enology. Recently chitosan, an abundant biopolymer obtained by deacetylation of chitin from animal or vegetal sources, has been recommended as alternative fining agent in wines, showing antimicrobial, biodegradable and non-allergenic properties. Chitin derivatives also demonstrated metal chelating effects (which slower oxidation in wines), interacting with grape polyphenols such as caftaric acid (whose enzymatic oxidation results in excessive browning [1]), and inhibiting (+)-catechin oxidation in model white wines [2]. Using electron paramagnetic resonance (EPR) techniques involving nitrones spin traps and standard oxyradical generating systems, this study presents for the first time complementary facets of the antioxidant action of chitosan, including direct scavenging of hydroxyl radical (a highly oxidizing species formed during wine oxidation) and chelation of metals (iron and copper). Experiments were conducted in aqueous medium, wine matrix, and commercial white wines, and different chitosans were tested at concentrations (0.12 g/L) allowed by enology regulations. Altogether, EPR data support the preferential use of chitosan as a reliable alternative for the reduction of sulphur dioxide to maintain wine freshness.