Rheological properties of the structurally characterized biopolymers produced by leuconostoc strains isolated from wild strains in northern Argentina

Abstract

Certain lactic acid bacteria (LAB) can produce an extracellular α–D–glucan homopolysaccharide (HoPS) formed by glucose units when growing in sucrose-enriched media. HoPS from LAB are natural bio-thickeners that improve rheological properties by providing viscosity and reducing syneresis. The structural diversity of biopolymers is linked to their vast commercial value and applications in the food, cosmetic, medical, and biotechnology fields, reaching much attention in recent years. Previously, we reported the bioprospection of thirty-one HoPS producer LAB strains, isolated from wild fruits of northern Argentina and belonging to the Weisella and Leuconostoc (Ln.) genera in a sucrose-enriched MRS medium (MRS-S, 4%). In addition, some techno-functional properties of selected HoPS identified as glucans dextran-type were studied. Currently, we aimed at the chemical structure and rheological properties of HoPS from selected Ln. strains (Ln. pseudomesenteroides F-G2-22 and Ln. citreum F-Cq1-496) since their high polymer production in MRS-S. The two studied HoPS were isolated and purified from MRS-S agar (gr.Kg-1 medium) after 48 h at 30 °C. The molecular structure was studied by HPLC (Knauer Wellchrom with a Waters Ultrahydrogel column), NMR (400 MHz, Agilent Technologies), DRX (Bruker D8 Discover A 25), ξ (Zetasizer Nano ZS, Malvern Instruments Ltd.), and rheological properties at different concentrations: 1, 2.5, and 5 % (TA Instruments, mod. ARES-G2). F-G2-22 and F-Cq1-496 strains grew up in MRS-S agar produced 6.27 and 6.78 gr.Kg-1 of HoPS, respectively. In addition, we not observed contaminants in the samples (protein, nucleic acid, or pigment by UV spectra). These HoPS were biopolymers of high molecular weight: 7442.7 kDa and 6857.3 kDa for F-G2-22 and F-Cq1-496 strains, respectively. NMR studies revealed typical signals of a linear dextran without branches structure. Only a single α-1,6 glycosidic binding signal ( ̴ 5 ppm) in both biopolymers were observed. On the other hand, the XRD spectra reflected a semi-crystalline arrangement with crystallinity indices lower than 30%. Also, both HoPS exhibited neutral charges in their structures (ξ ̴ 0 mV) commonly observed in glucan polysaccharides. The oscillatory rheology, the storage modulus G' and the loss modulus G" showed G' < G'' at low oscillation frequencies. A decrease in the modules' slope at higher frequencies was possible to exhibit typical viscoelastic fluids characteristics in all studied solutions. Dextran is produced by many food-grade organisms such as LAB. It improves the texture of products containing plant protein to make vegan replacements for dairy products. Our results suggest that the biopolymers produced by the Ln strains possess a dextran polymeric structure similar to each other. The rheological properties of both studied biopolymers confer great potential to be used as bio-additives in the food industry.