Efficacy of fungoid chitosans from Aspergillus niger and Agaricus bisporus in controlling the oxidative browning of model white wines

Chitosan (KT) is a polymer obtained from the partial deacetylation of chitin (poly b(1→4)-2-acetamido-2-deoxy-d-glucose) (Fig. 1), a widely diffused homopolysaccharide extracted from natural sources such as the exoskeleton of arthropods or the cell wall of yeasts and fungi (Dutta, Dutta, & Tripathi, 2004; Rinaudo, 2006; Struszczyk, 2002a). Due to their physical-chemical and biological properties, such as biocompatibility, metal chelation, film-forming capabilities, antioxidant and fungistatic behavior, chitin and chitosan have been the focus of interest in a wide range of industrial and biomedical applications in recent decades (Dutta et al., 2004; Gamage & Shahidi, 2007; Jayakumar, Menon, Manzoor, Nair, & Tamura, 2010; Milhome, Ribeiro, Nascimento, Carvalho, & Queiroz, 2009; Struszczyk, 2002b).

Indeed, the partial deacetylation of chitin imply the generation of free amino groups, randomly distributed along the chitosan backbone (Fig. 1), whose reactivity markedly differentiate the two polymers. In fact, due to the protonation of the -NH2 group, at a pH of <6, KT exhibits a unique polycationic character and increased chelating and antimicrobial activities (Aranaz et al., 2009), making it particularly attractive for the food industry.

The degree of acetylation (DA) and molecular weight (MW) may also affect chitosan behavior since higher bactericidal and antioxidant properties were reported for low MW and low DA chitosan formulations (Sahariah & Másson, 2017; Yang, Shu, Shao, Xu and Gu, 2006).

In addition, solubility itself may depend on the molecular weight of chitosan, with the transition from acid solubility (typical for higher molecular weight) to water solubility (lower molecular weight) reportedly occurring at molecular weights between 4.67 and 3.82 KDa (Tian, Tan, Li, & You, 2015).

In 2011, water insoluble KT has been admitted in oenology as processing aid with clarifying and metal chelating properties, for the adsorption of contaminants or antimicrobial purposes (EU Commission, 2011).

Due to allergenicity concerns, only chitosan from Aspergillus niger had been authorized in wine (European Commission, 2019), excluding shrimps or crustaceans as eligible source for KT extraction. Very recently, fungoid KT from Agaricus bisporus has also been admitted in winemaking (European Commission, 2022) following its introduction in the European list of novel foods even if, to our knowledge, its oenological behavior has not been investigated yet.

Oxidation of stored wines is quite an articulate phenomenon, largely driven by the redox cycle of iron (Danilewicz, 2021; Li, Guo, & Wang, 2008), which involves the initial activation of oxygen by Fe2+ to generate hydroperoxyl radical and then H2O2 (Danilewicz, 2007). If not eliminated by antioxidants in solution (SO2, ascorbic acid or glutathione, for instance), hydrogen peroxide ignites the Fenton pathway where it is reduced by Fe2+ to hydroxyl radical which eventually oxidizes ethanol and tartaric acid (two main constituents of wine) to acetaldehyde and glyoxylic acid respectively (Elias & Waterhouse, 2010). Phenolics with catechol or pyrogallol moieties (e.g. (+)-catechin, (−)-epicatechin, gallic or caffeic acids) fuel this oxidative cascade by reducing Fe3+, generating supplementary hydrogen peroxide and O-quinones. These latter are highly reactive toward nucleophiles such as other phenolics, thiols or amines whose interaction ultimately drive to polymerization, browning and flavor changes of wines (Li et al., 2008). In model wine solutions containing (+)-catechin, the described pathway lead to the formation of yellow/brown pigments identified as xanthylium cations, coming from the oxidative condensation of two (+)-catechin molecules bridged by glyoxylic acid (Barril, Clark, & Scollary, 2008; Es-Safi, Le Guernevé, Fulcrand, Cheynier, & Moutounet, 2000).

Tartaric acid itself plays a further role in oxidation as it may lower the reduction potential of Fe2+/Fe3+ couple by strongly co-ordinating Fe3+, hence promoting Fe2+ oxidation at the expenses of the O2/H2O2 couple, giving rise to the already mentioned O2 activation (Coleman, Boulton, & Stuchebrukhov, 2020; Danilewicz, 2014).

In this overall context, SO2 acts as an effective antioxidant because it i) reduces quinones back to the original o-diphenols, ii) quickly scavenges hydrogen peroxide before it oxidizes other wine constituents and iii) binds with carbonylic compounds (e.g. acetaldehyde or glyoxylic acid) responsible for side-reactions which negatively impact the color and the sensory features of oxidized wine (Elias & Waterhouse, 2010; Li et al., 2008).

In the last years, some investigations shed light on the intriguing capability of KT to acts as an antioxidant in wine-relevant conditions thanks to its radical scavenging properties, its metal chelation activity and the absorption of both native and oxidized phenolic species (Castro Marín et al., 2021; Castro Marín, Colangelo, Lambri, Riponi, & Chinnici, 2021; Chinnici, Natali, & Riponi, 2014). This would enlarge the oenological potential of such a polymer, in particular for the production of sulfite-free wines (Castro Marín et al., 2019), a subject of growing interest among producers, researchers and consumers concerned by the possible adverse effects of sulfites on human health (that include dermatitis, asthma or bronchoconstriction, among others) (Vally, Misso, & Madan, 2009).

The aim of this work was hence to compare for the first time the performance of oenological fungoid KT from Aspergillus niger and Agaricus bisporus in controlling the oxidative decay of model wine solutions in a typical Fenton-like environment where iron, dissolved O2, and tartaric acid are contextually present. Model wines with and without sulfite acted as positive and negative control respectively. An oligomeric hydrosoluble KT derived from Agaricus bisporus was also included to test the effect of this alternative formulation on antibrowning efficacy. Color development, decline of (+)-catechin, generation of phenolic intermediates and iron speciation were used to monitor the progression of oxidative phenomena.

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