A new step toward the comprehensive valorisation of grape marc through subcritical water extraction of polysaccharides

Abstract

Winemaking generates a significant amount of waste. Grape marc, the main solid residue, constitutes 20-25% of the pressed grapes and approximately 8-9 million tons are produced globally each year. The huge amounts, combined with the seasonal nature of the winemaking process, pose significant disposal, management, and environmental challenges. In this context, grape marc is already repurposed for various applications, including ethanol distillation, extraction of enocyanins, grape seed oil, and tartaric acid, as well as for energy generation, and animal feed. In recent years, the concept of biorefinery—focused on the sequential utilization of biomass to obtain multiple high-value products—has gained significant interest. This approach involves and combines different chemical, physical and biological techniques to maximize the biomass valorisation and economic profits. However, the integration and maintenance of multiple types of equipment make the economic feasibility challenging. 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 context.

The aim of the present work is to investigate another step towards a biorefinery strategy to complete the grape marc valorisation 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 conversion. 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 was significantly affected by SCW conditions. A comparison with the conventional extraction process revealed the higher efficiency of SCW. Under optimal conditions (120 °C and 10% tartaric acid), SCW achieved an extraction yield of 13.2 ± 0.3%, which was four times higher than the conventional method (3.2 ± 0.2%), with a 30-fold decrease in extraction time. The hydrolysis mechanisms also occured during the SCW process, leading to the depolymerization of polysaccharides, the formation of monosaccharides, and the generation of degradation products such as 5-hydroxymethylfurfural. However, these degradation products are classified as biobased platform chemicals with considerable market demand.