Effect of freeze-thaw cycles on the molecular weight and size distribution of gluten

Freezing processing is widely used in food industry, but some key scientific issue is still unclear. In this paper, the effects of freeze-thaw cycles on the molecular weight, radius of gyration, free amino group content, free sulphydryl group content and molecular conformation of gluten were investigated by size-exclusion chromatography in conjunction with multi-angle laser light scattering (SEC-MALLS), spectrophotometry and atomic force microscopy (AFM). The results showed that during the freeze-thaw cycles (frozen at − 18 °C with cycling to 0 °C for 12 h and then back to − 18 °C per 10 days) the molecular weight and radius of gyration of the gluten proteins decreased with the increase in freeze-thaw cycles, implying a depolymerisation in the high-molecular-weight fraction of the gluten. The free amino group content changed only slightly, and the free sulphydryl group content of the gluten increased from 9.8 μmol/g for the control to 13.87 μmol/g for the gluten stored for 120 days and submitted to 12 times freeze-thaw cycling, indicating that the depolymerisation of the gluten during freeze-thaw cycling was due to the breakage of disulphide bonds. AFM images showed that the gluten chain formed a fibril-like branched network which was weakened with increasing freeze-thaw cycles storage time. Some aggregation of the gluten chains was also observed in the AFM images.