Chitosan from mushroom by-products: sustainable extraction process and winemaking application

Chitosan is a biopolymer industrially obtained from the deacetylation of chitin, the second most abundant polysaccharide on earth, after cellulose. It is extracted from various terrestrial and marine resources, including insects, grasshoppers, shrimps, crabs, lobsters, squids, and fungi. Chitosan has a polycationic character due to the free amine groups along its chemical backbone, and depending on its deacetylation degree (dd) and molecular weight (mw), it shows variable properties that differ from those of other natural polysaccharides. It is an outstanding polymer with low toxicity, high biodegradability, chelating, anticoagulant, antioxidant, and antimicrobial activities. Fungal chitosan from aspergillus niger is the only type accepted in winemaking to avoid any potential concerns about allergenicity because of the crustacean raw material. The addition to wines is currently aimed at controlling unwanted microbial growth, particularly brettanomyces spp., preventing iron and copper casses, and removing heavy metals and ochratoxin a. The addition of fungal chitosan in musts and wines has also been successfully authorized for fining and clarification purposes to reduce turbidity by precipitating suspended particles or proteinaceous matter. The recommended doses of chitosan range from 10 g/hl to 500 g/hl for microbial control, iron and copper haze prevention, and heavy metal and contaminant reduction. This limit is set at 100 g/hl only for fining purposes. The conventional production of chitin and chitosan involves deproteinization, demineralization, decolorization, and deacetylation steps. Acid, alkaline, or enzymatic processes are commonly performed. However, these methods are long-time consuming, low-efficient, and require large volumes of solvents and hazardous chemicals. Several greener strategies and technologies have been proposed. Subcritical water technology is one of the most promising and versatile processes. Subcritical water extraction (swe) is usually conducted at temperatures ranging from 100 to 374°c, under sufficient pressure to keep water in a liquid state. Swe offers several advantages, including fast reaction rates, the replacement of acids/bases with a more environmentally sustainable solvent, and its properties can be finely modulated. Moreover, the identification of new undervalued and unexploited biomasses, such as edible mushroom by-products, should also be promoted for eco-friendlier chitosan production. In the present work, a sustainable extraction process with subcritical water from mushroom (pleourotus ostreatus) by-products was performed. Swe allowed a 4.5-fold increase in extraction yield and a 4-fold reduction in process time. The chitin and chitosan were characterized and compared with conventional ones. Significant effects were pointed out on some chemical properties, such as deacetylation degree, crystallinity index, and chromatic properties, according to the adopted subcritical conditions. The swe chitosan was also studied for winemaking purposes. Several experimental trials were performed at different dosages (0-100 g/hl), on several unstable white wines, aimed to point out significant interactions with proteins, polyphenols, and aroma compounds. Positive effects were highlighted on some analytical indices related to wine protein stability, and a significant decrease in unstable proteins was detected, for both chitinases and thaumatin-like proteins (tlps). The swe chitosan didn’t affect the chromatic characteristics of wines or the total polyphenol content. Moreover, the chromatographic analysis of volatile compounds showed no significant effects on the main chemical classes. Further research is already undergone, aimed at deeply investigating the effect of subcritical water at different conditions on the chitosan polymeric structure and its functionality for winemaking applications.