Abstract:
In the Food Industry, ultrasound processing has become an emerging technology as it can minimize processing time and enhance food quality. On the other hand, starch is one of the most important food ingredients, widely used in food products. However, native starch properties do not always meet desired requirements in all food formulations. Therefore it is necessary to alter starch properties by using different physical (heating) and chemical (crosslinking) treatments. The main objective of this study was to investigate the effects of highpower ultrasound on the physicochemical properties of starch pastes and in particular drawing the relationship between viscosity and the size (hydrodynamic radius) of the sonicated starch pastes. Four commercial starch samples, namely AMIOCA, MAZACA, Melojel and Gelose 50, from the same botanical source (corn starch) but with varying amylose contents (2.72 to 52.71% (w/w)) were considered. The starch pastes were prepared in a controlled manner by using a rheometer, and subsequently, subjected to high-power ultrasound (20 kHz) and sonicated at 13.49 W or 29.91 W powers for different time durations (up to 20 minutes). The physicochemical properties of starch pastes after sonication were characterised by rheological, particle size determination, X-ray diffraction and FT-IR. Sonication induced a marked reduction in apparent viscosity of starch pastes, and in the shearthinning behavior as well as the flow behavior hysteresis. This was due to the reduction in starch molecular weight as measured by dynamic light scattering. Comparing the four types of corn starch, the waxy starches, AMIOCA and MAZACA, were more sensitive to sonication than the other two high-amylose starches. The crystalline structure and chemical structure of the sonicated starches were also affected. Major peaks observed by X-ray disappeared after sonication, except in the case of Gelose 50 due to the high amylose content. In addition, FTIR showed that the molecular scission occurred at the C-O-C bond of α-1,6 glycosidic linkage, and the extend of breakage was inversely correlated with amylose content. Finally, a relationship between the apparent viscosity and hydrodynamic radius was found, allowing us to suggest a master curves representing all the presented data obtained in this thesis.