Effect of Heating on Chain Conformation of Branched β-Glucan in Water

The thermal stability of polysaccharides under heat treatment is an important factor to their functionality in food and pharmaceutical fields. The stiff branched β-glucan coded as AF1-1 isolated from Auricularia auricula-judae was investigated with viscometry, dynamic light scattering (DLS), and size-exclusion chromatography combined with multiangle laser light scattering (SEC-MALLS) in water at 25 to 170 °C. The chain conformation of AF1-1 in the aqueous solution exhibited a sharp decrease in viscosity, hydrodynamic radius (Rh), and weight-average molecular weight (Mw) at elevated temperature in a narrow range of 140 to 160 °C. It was confirmed that the conformation transitions of the AF1-1 chains from rod-like chains to the flexible occurred during heating to 140-160 °C for 30 min, leading to the coexistence of the flexible chains and stiff chains at 155 °C as a result of the breaking of the intra- and intermolecular hydrogen bonds of the AF1-1 macromolecules. The results from scanning electron microscopy and atomic force microscopy further directly proved that the AF1-1 nanofibers in water were destructed into flexible coils consisting of individual chain at the elevated temperature higher than 155 °C, supporting the conformation transition. The conformational transition from stiff to flexible chains at 140-160 °C was irreversible. However, the chain shape and stiffness of AF1-1 was stable below 140 °C and hardly changed with an increase in the temperature. This was important for the application in the fields of food and pharmaceutical.