Novel biorefinery step for grape marc valorisation: polysaccharides extraction by subcritical water

Abstract

The exploitation of food by-products has garnered significant attention over the past few decades, particularly within the framework of the European Green Deal. This strategic initiative emphasizes sustainability and resource efficiency, promoting the use of innovative technologies to transform food by-products into valuable resources. This aligns with the principles of a circular economy, aiming to reduce waste and improve the overall environmental footprint of industrial processes. The winemaking process generates a large amount of by-products, mainly represented by grape marc. It is estimated that 8-9 million tons of grape marc are generated worldwide each year. It is evident that these quantities pose enormous storage and management challenges. In this context, the biorefinery approach, aimed at valorising biomass through multiple sequential steps to obtain several products, has attracted significant interest. Generally, the biorefinery approach involves various technologies¹. An innovative alternative could be the application of a biorefinery strategy based solely on the sequential extraction of different classes of high-value bioactive compounds by a unique, versatile and green technology. The extraction technology using pressurized fluids, particularly supercritical and subcritical fluids, may represent the best solution in this context².

The aim of the present work is to investigate another step towards the complete valorisation of grape marc by extracting polysaccharides using subcritical water (SCW). Subcritical water refers to liquid water at a temperature above its boiling point and below its critical point (Tc= 374°C, Pc= 221 bar). The increased temperature and pressure conditions enhance extraction mechanisms in solid matrices. SCW can be exploited not only as a solvent, but also as an outstanding and sustainable reaction medium, making it an environmentally friendly method for biomass conversion³. Several experimental trials at different temperatures (120, 160, and 200°C), adding different carboxylic acids (tartaric, malic and citric acid) at different percentages (0, 5 and 10%) were performed.

The extraction yield of polysaccharides, as well as their chemical characteristics, are significantly affected by SCW conditions. It is important to note that not only extraction phenomena but also chemical processes occur during the process, primarily hydrolysis, which lead to depolymerization of polysaccharides, formation of monosaccharides, and also their degradation products⁴. These degradation products can still add value as they are defined as biobased platform chemicals, which have a significant market demand for several purposes. The comparison with the conventional process highlighted an outstanding efficiency of SCW. At optimal conditions, SCW allowed an extraction yield (13.2±0.3%) that was 4 times higher than the conventional process (3.2±0.2%), with a 30-fold decrease in extraction time.