Effects of pyruvate group on the gel properties of xanthan gum/konjac glucomannan complex system

Compound gels are mainly formed by mixing two or more natural polymer materials (such as polysaccharides, proteins, and starches). There are better rheological properties, various scopes of use, and cost-saving, compared with a single gel. The irreversible gels can enrich the types and taste of food, currently showing great potential in the application of food production. The current research on compound gels focused mainly on thermo-irreversible gels. However, there is also a type of thermo-reversible gel with a relatively weak gel strength in food. Adjusting the sol-gel transition temperature of the compound system can be used to control the flow state of compound gels and realize the "import" of the food. The design of the "instant" feature can further expand the application of gels in food. The Xanthan Gum (XG) and Konjac Glucomannan (KGM) are commonly-used additives in food. The XG is an extracellular polysaccharide formed by β-1, 4glycosidic bonds connected to D-glucose, where the mannose inside the trisaccharide side chain can be acetylated, and the mannose at the end can be acetonated. The KGM is made up of the D-mannose and D-glucose connected by β-1, 4glycosidic bonds. There are no calories in the high molecular polysaccharide extracted from the tubers of konjac plant, which is hard to be absorbed. It is usually used in meat additives and dietary fiber products to prevent obesity in humans. Neither XG nor KGM can form a gel when they exist alone, but the combined has a good synergistic effect to form a thermoreversible gel. In this study, an acid heat treatment was used to adjust the content of pyruvate groups in XG, aiming to explore the influence of pyruvate content on the rheological properties of XG. The content change of pyruvate group was further utilized to explore the sol-gel transition temperature and the impact of rheology characteristics in the XG/KGM thermoreversible gel. The research results showed that the helical structure of XG gradually tightened with the decrease of the pyruvate group content, and the fluidity increased. The storage modulus of XG showed an overall downward trend, but the XG change temperature sharply increased with the decreasing content of pyruvate group. The orderly temperature and disordered conformational transition increased as well. In the rheological properties of XG/KGM compound gel, the viscosity coefficient and the quasi-solidity decreased, tending to be Newtonian fluids, as the content of pyruvic acid group decreased. At the same angular frequency, the storage modulus and loss modulus in the XG/KGM composite gel showed a decreasing trend with the continuous removal of pyruvate groups, but the storage modulus was always greater than loss modulus, where the tanδ was both less than 1, indicating the solid characteristics. The temperature scan showed that as the temperature decreased, the storage modulus of the compound system gradually increased and reached the maximum at 20℃. However, the increasing initial temperature of storage modulus decreased from 60℃ to 41℃ with the decrease in the pyruvate group content. There was a downward overall trend, indicating that the stability of the complex system that formed with KGM significantly reduced after the removal of the XG pyruvate group, while the sol-gel temperature decreased. The finding can be expected to develop the texture of the melt-in-the-mouth gels in promising food production. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.