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

Starch is the main constituent of chestnuts, accounting for 38% to 71% of their dry mass, and its structure and physicochemical properties determine the edible quality of chestnuts as well as their suitability for processing in food and non-food industries. Natural chestnut starch has very limited processing applications due to defects such as poor thermal stability and susceptibility to aging during processing. These natural defects can be addressed by physical, chemical, and enzymatic treatments to meet industrial needs and enhance suitability. The present study revealed significant differences in the structural characteristics and physicochemical properties of chestnut starch from different varieties and origins. Existing reports on the study have used one to three treatments, which made it difficult to obtain and compare data on the changes in the structure and physicochemical properties of chestnut starch under various pre-treatment methods. In this study, chestnut starch was subjected to pregelatinization, pressure heat, microwave cooking, ultrasound-pressure heat, acid hydrolysis-pressure heat, and pullulanase-pressure heat treatments, with untreated chestnut starch serving as the control group. The structural and physicochemical properties of chestnut starch were determined using scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier infrared spectroscopy (FTIR), laser particle size analysis, and differential scanning calorimetry (DSC). These analyses enabled in-depth comparisons of the structural and physicochemical properties of chestnut starch under different pre-treatment methods, including physical, enzymatic, and acid treatments, as well as their effects on the structure and digestive properties of chestnut starch. The results showed that no new functional groups were generated in chestnut starch under physical, enzyme and acid treatments, which only changed the internal order of starch molecules. The results showed that the chestnut starch under physical, enzyme and acid treatments did not produce new functional groups, and only changed the internal ordering of starch molecules, so that the chestnut starch particles decomposed into small molecules and then recrystallized to form denser starch crystals; the diffraction peaks of the chestnut starch under physical, enzyme and acid treatments were replaced by two broad peaks at 23.1°, 22.80° and 24.13°, and one weak diffraction peak near 19.89°, and their crystal structures were transformed into B+V-type crystal structures, and the particle sizes increased, and 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. The short-range ordered degree, relative crystallinity, enthalpy of pasting, and range of pasting temperature all decreased, and the degree of double helix all increased. The shape of chestnut starch granules changed from oval, sunflower seed-shaped, triangular and other shapes 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, and the contents of total starch, fat and branched-chain starch were significantly decreased (P < 0.05), with the content of straight-chain starch ranging from 23.70% to 76.17%, and the highest content of straight-chain starch was found in its Pullulanase-pressure heat treatment (76.17%). In addition, Pullulanase-press heat treatment significantly increased the RS content (Resistant starch, RS) in chestnut starch from 48.69% to 61.78% (P < 0.05). The purulanase-pressure heat treatment can make chestnut starch have dense crystal structure and higher content of resistant starch, and this study can provide a certain reference for further deep processing of chestnut starch in the food field.