A pullulan-type α-D-glucan produced from lactose by Aureobasidium pullulans: structure and potential as a prebiotic agri-food ingredient

Abstract

Abstract Background Microbial exopolysaccharides (EPS) are renewable biopolymers of increasing importance in the agri-food sector, and producing them from low-cost substrates offers a route to valorize agro-industrial resources. Aureobasidium pullulans is an industrially important fungus that secretes the α-glucan pullulan; however, the structure of the EPS it produces under lactose fermentation—a means of upgrading a low-value, dairy-derived carbon source—has not been clearly defined, which limits its rational use as an agri-food bioproduct. This study aimed to produce, structurally characterize, and preliminarily evaluate the physiological activity of this EPS. Results A homogeneous EPS was obtained by ethanol precipitation, Sevag deproteinization, and gel-filtration chromatography. Fourier-transform infrared spectroscopy indicated an α-configured, neutral polysaccharide, and gas chromatography identified glucose as the sole monosaccharide. Size-exclusion chromatography coupled with multi-angle laser light scattering showed a high-molecular-weight polymer. One- and two-dimensional NMR analyses established that the EPS is a linear pullulan-type α-D-glucan with the repeating unit [→6)-α-D-Glcp-(1→4)-α-D-Glcp-(1→4)-α-D-Glcp-(1→] n, in which two α-(1→4)-linked glucose residues alternate with an α-(1→6) linkage. In a high-fat-diet-induced obese rat model, dietary supplementation with the glucan attenuated body-weight gain, improved serum lipid profiles (TG, TC, HDL-C/TC), and lowered serum endotoxin and tumour necrosis factor-α, together with enhanced colonic short-chain fatty acid production, consistent with fermentation of the glucan by the gut microbiota. Conclusions Lactose fermentation by A. pullulans yields a structurally defined, linear pullulan-type α-D-glucan, demonstrating a route to convert a low-value dairy-derived substrate into a value-added biopolymer. Its prebiotic-associated metabolic effects indicate potential as a functional ingredient for the agri-food chain and warrant further mechanistic and safety studies.