Effect of pullulan on molecular chain conformations in the process of starch retrogradation condensed matter

Starch retrogradation is an inevitable process in the natural state caused by motion of starch molecular chains. However, traditional starch retrogradation research methods cannot explain this process from the aspect of chain motion. Therefore, the objective of this study was to investigate the effect of pullulan (PUL) on potato starch (PS) retrogradation and analyze this using condensed matter theory. The entanglement concentration (c_e) was determined by Ubbelohde viscometer. The c_e values of PS and PS-PUL were 0.22 and 0.32 g/dL, respectively. Small-angle X-ray Scattering (SAXS) was used to obtain the radius of gyration (R_g) values and investigate conformation evolution of PS and PS-PUL in different solution concentrations ranging from extremely dilute to concentrated. The R_g values of PS increased by 38.7% upon transition from single chain (0.001 g/dL) to an entanglement state (0.22 g/dL) and the percent reduction of R_g was only 3.3% from entanglement state (0.22 g/dL) to stabilization (1 g/dL). The percent increase of R_g values of PS-PUL reached to 34.7% from extremely dilute (0.001 g/dL) to concentrated (1 g/dL) solutions. The trend of R_g indicated that c_e played a major role in starch retrogradation. Dynamic light scattering (DLS) was used to acquire the hydrodynamic radius (R_h) which was a significant parameter to calculate form factor (ρ) by the ratio: ρ = R_g/R_h. The ρ values of PS-PUL were generally lower than those for PS. This result illustrated that PUL delayed the process of starch retrogradation by increasing flexibility of PS molecular chain. Furthermore, the ρ value of PS-PUL was higher than PS at 0.22 g/dL (c_e of PS). This result illustrated that entanglement could decreased flexibility of molecular chain. The present research provides a novel research route for exploring the influence of hydrocolloids on starch retrogradation from the viewpoint of chain motion.