Effect of ultrasonic treatments on nanoparticle preparation of acid-hydrolyzed waxy maize starch

Waxy maize starch was dispersed (14.7% solids) in an aqueous sulfuric acid solution (3.16 M), and hydrolyzed by stirring for up to 7 days at 40 °C with ultrasonic treatments at different vibration amplitudes (20 and 40%) and durations (30 and 60 min/day). The amount of starch nanoparticles in the hydrolyzates isolated after 7 days, measured by a dynamic light scattering detector, was raised from 20% to 70% by an ultrasonic treatment (20% amplitude, 30 min). The aggregation of nanoparticles possibly occurring during the hydrolysis was effectively prevented by the ultrasonication. Alternatively, ultrasonic treatments were applied to the re-dispersed suspension of the large microparticles of starch hydrolyzates (2 days) precipitated by a mild centrifugation (500 rpm, 10 min). By an ultrasonic treatment at 60% vibration amplitude for 3 min, the microparticles could be completely transformed to nanoparticles. The inherent crystalline structure of waxy maize starch (A-type in X-ray diffraction) remained after the ultrasonic treatments during acid hydrolysis, but it was disrupted by the ultrasonic treatments for the re-dispersed microparticles.