Effects of physical, enzymatic and acid treatments on the structure and physicochemical properties of chestnut starch

Starch is one of the most important constituents of chestnuts, accounting for 38% to 71% of the dry mass. Its structure and physicochemical properties can also dominate the edible quality of chestnuts, and their suitability for processing in food and non-food industries. Natural chestnut starch has been limited to the application, due to the low thermal stability and susceptibility to aging during processing. The physical, chemical, and enzymatic treatments can be expected to fully meet industrial needs and suitability. However, only one to three treatments have been reported to compare the structure and physicochemical properties of chestnut starch under various pre-treatments. In this study, the chestnut starch was subjected to a series of treatments, including the pregelatinization, pressure heat, microwave cooking, ultrasound-pressure heat, acid hydrolysis-pressure heat, and pullulanase-pressure heat, with the untreated chestnut starch as the control group. The present study aims to reveal the significant differences in the structural and physicochemical properties of chestnut starch from different varieties and origins. The structural and physicochemical properties of chestnut starch were characterized using scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier infrared spectroscopy (FTIR), laser particle size analysis, and differential scanning calorimetry (DSC). A comparison was then performed on the structural and physicochemical properties of chestnut starch under different pre-treatment, including physical, enzymatic, and acid treatments, as well as their effects on the structure and digestive properties of chestnut starch. The results showed that there was no new functional group produced in the chestnut starch under physical, enzyme, and acid treatments, indicating the only variation in the internal order of starch molecules. The particles of chestnut starch were decomposed into small molecules, and then recrystallized to form the denser starch crystals; The diffraction peaks of chestnut starch were replaced by two broad peaks at 23.1°, 22.80° and 24.13° under physical, enzyme, and acid treatments, respectively, with one weak diffraction peak near 19.89°. Their crystal structures were transformed into B+V-type crystal structures, as the particle sizes increased. The distributions of D10, D50, and D90 were in the ranges of 7.31-65.89 μm, 101.96-119.96 μm and 147.54-199.19 μm, respectively. There was a decrease in the short-range ordered degree, relative crystallinity, enthalpy of pasting, and range of pasting temperature. While the degree of double helix all increased. The shape of chestnut starch granules was changed from oval, sunflower seed-shaped, and triangular to amorphous agglomerates or agglomerated external morphology, with rough and uneven surfaces. In terms of physicochemical properties, the swelling power and solubility of chestnut starch increased under physical, enzyme, and acid treatments. Specifically, the contents of total starch, fat, and branched-chain starch significantly decreased (P<0.05), with the content of straight-chain starch ranging from 23.70% to 76.17%. The highest content of straight-chain starch was found in the Pullulanase-pressure heat treatment (76.17%). In addition, Pullulanase-press heat treatment significantly improved the RS content (Resistant starch, RS) in the chestnut starch from 48.69% to 61.78% (P<0.05). Therefore, the purulanase-pressure heat treatment can be expected to serve as the denser crystal structure and higher content of resistant starch. The finding can also provide a strong reference for the deep processing of chestnut starch in the food field.