Physicochemical characterization of exopolysaccharides conjugated to phenolic compounds: a novel acidic exopolysaccharide containing tartaric acid derived from Rhodotorula taiwanensis

Abstract

Background Polysaccharides that are derived from different sources, in particular those from microorganisms, constitute a hot topic in contemporary research thanks to their high-value applications in different biotechnological sectors. Objective Considering limited existing studies concerning yeasts, the current study was designed to search for promising exopolysaccharide (EPS)-producing yeasts from samples obtained from different biological sources, adopting the strategies of isolation and screening. Materials and methods The present study focused on isolation and screening of EPS-producing yeasts from samples obtained from different biological sources, namely, soil rhizosphere, rotten fruits, local beverages, dairy products, and mixture pickles; identification of the selected promising yeast isolates phenotypically and genetically; extraction and chemical composition of crude exopolysaccharides (C-EPSs) in terms of their contents of carbohydrate, protein, and phenolics; and physicochemical characterization of the partially purified exopolysaccharides (PP-EPSs) by high-performance liquid chromatography (HPLC), Fourier transformation infrared, proton nuclear magnetic resonance, thermogravimetric analysis, X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray analysis. Results and conclusion The most potent isolates that provided the highest yields (2.5 and 2.25 g/l) were identified phenotypically and genetically as Rhodotorula mucilaginosa A1 and Rhodotorula taiwanensis G1. The chemical compositions of C-EPSs of both strains differed in terms of their contents of carbohydrate, protein, and phenolic components. HPLC analysis of the phenolic compounds of C-EPSA1 revealed the presence of eight different constituents, of which quercetin followed by kaempferol, hesperetin, and gallic acid represented 99.81%. However, C-EPSG1 contained only seven, in a much smaller quantity. HPLC analysis demonstrated that both PP-EPSs were acidic heteropolysaccharides; PP-EPSA1 consisted mainly of 69.52% fructose and 30.48% uronic acids. PP-EPSG1 is probably unique; it showed remarkable differences as it contained tartaric acid (1.22%) besides glucose (50.04%), fructose (39.65%), and uronic acid (9.09%). Spectral analyses of both PP-EPSs confirmed their polysaccharide nature through the presence of characteristic functional groups and glycosidic linkage regions. PP-EPSs were semicrystalline in nature, similar in porosity and surface smoothness, and showed resistance to high temperatures. Elemental analysis indicated the participation of both PP-EPSs in five elements (O, C, N, S, and P) in close proportions; PP-EPSA1 contained Ca as an additional element.