Abstract: In this study, sodium carbonate-soluble pectin fraction (NSF) from two kinds of lotus rhizome with different textures (mealy and crunchy) were evaluated for differences in thermal degradation and its relationship with rheology. After heat treatment at different temperatures, primary structure analysis and nanostructure observation of NSF showed that the thermal degradation degree of NSF from Miancheng lotus rhizome (mealy) was greater than that of NSF from Lulin Lake lotus rhizome (crunchy). The chain of NSF from Lulin Lake lotus rhizome was star-shaped (long branched chain), while the chain of NSF from Miancheng lotus rhizome was mostly short, straight. For both of them, the chain length and height decreased and so did the branching degree after heating. Additionally, the consistency decreased significantly. The pseudoplasticity of NSF from Miancheng lotus rhizome increased slightly, while that of NSF from Lulin Lake lotus rhizome decreased. The chain length, chain height and branching degree of NSF were closely related to the consistency, and the branching degree and the proportion of chains of different lengths (molecular mass distribution) contributed greatly to the difference in pseudoplasticity. Therefore, there was a close relationship between the structure and the rheological properties of NSF, and the difference in thermal degradation of NSF was related to the texture difference between mealy and crunchy lotus rhizome. This study provides a theoretical basis for understanding the relationship between the thermal degradation of NSF and the texture (processing and maturity) of fruits and vegetables.

Key words: lotus rhizome; pectin; atomic force microscope; thermal degradation; rheology