Role of Water in Starch Mechanical Treatment

The impact of initial moisture content (1.5%-20%) on the ball-milling of pea starch was investigated across multiple structural levels. Scanning electron microscopy revealed moisture-dependent deformation mechanisms: plastic flow and agglomeration in high-moisture granules versus pseudo-brittle fracture in low-moisture samples. Despite these differences, XRD-detectable crystallinity was eliminated within 5 min of treatment, independent of initial moisture content. Fourier transform infrared spectroscopy confirmed the loss of short-range molecular order, including helical and hydrogen-bonded contributions. Gel permeation chromatography demonstrated more severe molecular degradation in low-moisture starch, suggesting a possible two-stage degradation behavior, with a rapid initial decrease of molar mass followed by a slower regime as the material becomes predominantly amorphous. Consequently, ball-milling enhanced solubility in both cold (30°C) and hot (90°C) water, increasing it from less than 1% and ~25% to 60%-70%, respectively. The increased cold-water solubility is explained by the loss of crystallinity. In contrast, the rise in hot-water solubility indicates a more profound structural disintegration. We propose that this is related to the mechanochemical degradation of amylopectin, which may suppress the formation of amylopectin-rich remnants during gelatinization and thereby promote dissolution.