Production and purification of xylooligosaccharides (XOS) with high prebiotic properties from sugarcane bagasse

Abstract

Bacteriocins are proteins/peptides released extracellularly by some strains of probiotic bacteria, especially lactic bacteria, which have a bactericidal or bacteriostatic action. The production of these substances is related to microorganism probiotic potential. Prebiotics are nondigestible oligosaccharides that can positively influence probiotic microorganisms. Thus, substances that stimulate the growth of probiotic flora and the production of bacteriocins should have the highest prebiotic potential.Xylooligosaccharides (XOS) are small oligomers of xylose (2-7 units) with high added value and prebiotic properties that are interesting for pharmaceutical and food industries and should therefore be potential as stimulators of bacteriocin production. XOS can be obtained from a mild hydrolysis of hemicellulose, which is necessary to avoid excessive xylose production, which will hinder the purification step. First, a low severity pretreatment reduces the size of hemicellulose heteropolysaccharides, the main one being xylan, solubilizing it in the form of xylooligomers. A controlled enzymatic hydrolysis can be carried out on this hydrolysate to obtain high value-added XOS.Xylanases (²-1,4-D-xylanase) are enzymes that catalyze the hydrolysis of the glycosidic bonds between the xylose units of the hemicellulosic fraction, and can be divided into endoxylanases and ²-D-xylosidases. Endoxylanases randomly hydrolyze the main chain, while ²-D-xylosidases act at the non-reducing end of the chain, releasing xylose monomers. The use of immobilized enzymes allows its reuse and therefore can help in the economic viability of the process.The composition of XOS resulting from the hydrolysis of xylooligomers with endoxylanases will depend on both the lignocellulosic feedstock, the pretreatment for hemicellulose extraction and the specificity of the enzymatic complex used. Finally, it will further depend on the purification method.The purification of XOS is a complex process, since a large amount of secondary substances are present in the medium, as extractives, solubilized lignin, carbohydrates and proteins. Phenolic compounds, pigments and furfural can be removed by a combination of activated carbon adsorption, solvent extraction and chromatography. New XOS purification techniques include combination of vacuum evaporation and nanofiltration combined with solvent extraction and ion exchange chromatographyIn this context, this postdoctoral proposal focuses on the production and purification of high added value xylooligosaccharides (XOS) from sugarcane bagasse. Different pretreatment techniques will be evaluated for the solubilization of xylan, as well as different free and immobilized endoxylanases, to obtain hydrolysates containing different XOS profiles, which will be characterized in terms of prebiotic capacity, analyzing its influence on the growth of probiotics and in the production of bacteriocins by these strains. The purification will be carried out starting with the simplest and cheapest procedures, increasing the number of steps until reaching the necessary degree of purification (75-95%). The investigation of XOS prebiotic capacity as well as purification methodologies are still necessary for the industrial feasibility of its production. In the context of biorefinery, hemicellulose is currently underutilized and the inclusion of XOS production in the process could add economic value to the hemicellulosic fraction.