Introduction
Exopolysaccharides (EPS) are complex carbohydrate molecules produced by a wide range of microorganisms, including fungi, algae, and bacteria. EPS are high molecular weight carbohydrates secreted by microorganisms into the extracellular environment during growth. EPS have complex and diverse structures that can be classified into homopolysaccharides and heteropolysaccharides, which usually contain one or more monosaccharide units and sometimes also incorporate other components such as acetate or phosphate groups. While homopolysaccharides are usually made up of sucrose, heteropolysaccharides have more than two monosaccharides. These structural differences mainly affect the physicochemical and functional properties of EPS. EPS from lactic acid bacteria (LAB) are generally heteropolysaccharides, usually composed of D-glucose, D-galactose and L-rhamnose. As a biopolymer, EPS not only exhibits numerous properties such as water holding capacity, binding, emulsification and gelation, but also has various beneficial health properties such as antioxidant effects, blood sugar lowering and cholesterol lowering. Before use, it is necessary to conduct detailed research on the production conditions and structure of EPS to evaluate their application potential and identify suitable fields, thereby facilitating the development of LAB exopolysaccharides with industrial applicability. Therefore, the aim of this study was to investigate the functional properties of Lactobacillus rhamnosus EPS and its effect on the rheological behavior of surimi.
Materials and Methods
Lactobacillus rhamnosus isolated from the intestine of the Barbus grypus was isolated and identified using the 16S rRNA gene, and the gene of interest for the identification of the genus and species of the bacteria was confirmed by sequencing. Crude EPS was obtained by a two-step purification process. First, the sample was separated using a DEAE-cellulose anion exchange chromatography column and washed with deionized water, NaCl solutions with concentrations of 0.1 mol/L, 0.3 mol/L, and 0.5 mol/L as the detergent at a flow rate of 1 ml/min. The microstructure and surface morphology of the purified EPS was observed through scanning electron microscopy (SEM). The antioxidant capacities of EPS were investigated by four different methods. The antibacterial activities of the EPS were evaluated against some indicator pathogens. Some microstructural properties of surimi gel were measured.
Results and Discussion
The period of EPS production varied from logarithmic phase to early stable phase, indicating that EPS is a secondary metabolite. EPS yield and properties depend on microorganisms. The absorption peaks in the 1200 to 1000 cm-1 indicated the C=O and C-O-C vibrations of glycosidic bonds or the stretching and bending vibrations of C-O coupled and C-O-H, indicating the presence of carbohydrate. According to the analysis of the monosaccharide composition, the extracted EPS was a heteropolysaccharide. The results showed that with the increase in the concentration of EPS, its antioxidant activity increased. EPS showed a promising and dose-dependent inhibitory effect on α-amylase. The inhibition rate of EPS on α-amylase activity increased slowly with the increase in concentration from 1.5 mg/ mL to 9.5 mg/ mL. Carboxyl, hydroxyl and glycosidic bonds present in EPS may play a role in cholesterol adsorption. The coagulability initially increased with increasing EPS concentration and started to decrease after reaching the highest purification point, which may be because the adsorption of excess coagulants destabilized the particles. At a concentration of 4 mg/mL, EPS showed the highest inhibitory effect on E. coli with 23.4%. L. monocytogenes with 27%, S. aureus with 73%, and S. typhimurium with 61%. EPS were reported to inhibit the spontaneous initial aggregation and cell attachment of bacterial cells either by attenuating cell surface changes or by reducing cell-cell surface interactions. Surimi gel samples containing different concentrations of EPS showed similar gelation patterns characterized by three stages of gel formation, gel weakening, and gel recovery. This study elucidates the conformational relationship of this EPS and provides a theoretical basis for its functionalization in fisheries products.
Conclusion
In this study, we isolated a novel EPS, and characterized its composition, structure, and functions. It has been used and evaluated in fish surimi. The EPS producing novel strains of Lactobacillus rhamnosus were selected based on ropy structure formation. The partially purified EPS was characterized by FTIR, NMR, and SEM techniques and examined for its physicochemical, biological, and rheological properties. EPS exhibited significant biological activities, including antibacterial, antioxidant, antibiofilm, and antidiabetic activities, and was highly thermally stable. Therefore, EPS could be considered a potential biomaterial in the healthcare industry and in thermal processing in the food industry.