Polysaccharides recovery by subcritical water as an additional step for a biorefinery valorisation of grape marc with pressurized fluids

The application of a versatile technology for the sequential extraction of multiple valuable compounds offers a promising and efficient alternative for biorefinery exploitation of food by-products. This study investigated an additional step in the comprehensive valorisation of grape marc, the main winemaking by-product, which has undergone prior defatting and polyphenol extraction phases with pressurized fluids. Specifically, subcritical water (SCW) was employed for the extraction of polysaccharides, considering different temperatures (120 °C, 160 °C, and 200 °C) and the addition of various carboxylic acids (tartaric, citric, and malic acid) at concentrations of 0 %, 5 %, and 10 %. Temperature was the most significant factor on yield and polysaccharide content. Conversely, the type and concentration of carboxylic acid exhibited a comparatively lower impact. The highest yield (13 ± 1 %) and polysaccharide content (1300 ± 23 mg/100 g _d.m.) were achieved with SCW at 120 °C and 10 % (w/v) of tartaric acid, which were 3 times higher than that of conventional method. However, temperature increase or carboxylic acid addition promoted the hydrolysis of polysaccharides, leading to a decrease in their molecular weight, and the formation of monomers and degradation products. SCW allowed a significant increase of the extraction of other valuable components, such as proteins and polyphenols.