Obtaining oligo- and monosaccharides from agroindustrial and agricultural residues using hydrothermal treatments.

Agricultural and agroindustrial residues are major sources of cellulose, hemicellulose, and starch that can be converted into bioactive compounds, such as oligosaccharides and monosaccharides, using various chemical and biological methods. These bioactive compounds can be used as raw materials by food, cosmetic and pharmaceutical industries, as well as in the production of intermediate products and the development of biomaterials by chemical industries. In Brazil, the major industrial residues, which are corn residues, soybean residues, sugarcane bagasse, palm and coconut fibers, and grape and tomato seeds, among others, are produced at a rate of approximately of 600 million tons per year. Thus, the utilization of these residues using sustainable technology is of great interest. Hydrothermal treatment is a green technology that includes autohydrolysis as well as subcritical and supercritical hydrolysis, in which water is used at high pressures and temperatures to recover polysaccharides from complex vegetal matrices. The hydrolytic mechanisms can be improved by changing the ionic product or the polarity and electrical conductivity of water in subcritical and supercritical states. These properties promote the selective dissolution of the starch, hemicellulose and cellulose in the residues. The conversion of starch and hemicellulose into oligosaccharides and monosaccharides is preferentially performed at temperatures of less than 200°C. In contrast, the conversion of cellulose into oligosaccharides is promoted at temperatures greater than 200°C, with the highest amount oligosaccharide formation occurring at close to the critical point. In this article, the main biomass components, the properties of water under subcritical and supercritical conditions, and the latest studies of polysaccharide conversion in biomasses using hydrothermal treatments are reviewed.