Laser light-scattering study of ultrasonicated and thermosonicated soy proteins

Abstract

In the food industry, detailed information on the relation between processing conditions and size of aggregates formed is desirable to permit optimization of some manufacturing processes. Ultrasonication (US) and thermosonication (TS, the combination of ultrasound and heat) have been used to modify the structure and function of various food proteins. Detailed investigation on the mechanisms requires accurate determination of the molecular weight and size distribution of proteins. The objective of this study was to investigate the structural characteristic of soy proteins after US and TS treatments using size-exclusion chromatography combined with on-line multi-angle laser light scattering and dynamic quasi-elastic light scattering (SEC-MALLS-QELS). SPI solutions (100 mg/mL) were sonicated for 0, 10, 30, 60, 180 and 300 seconds at power level of 72.02 W/cm2. For thermosonicated samples, protein solutions were first brought up to 25, 40, 50 and 70oC in a temperature-controlled water bath and then sonicated with the same processor for 10 seconds at 72.02 W/cm2. Samples were then analyzed by the SEC-MALLS-QELS system. US treatment caused aggregation of SPI and 7S globulins. Prolonged period of treatment also led to aggregation of 11S globulin to form larger soluble macromolecules. Changes in weight-average molecular weight (Mw) and hydrodynamic radii (rh) suggest that initial formation of large aggregates followed by dissociation. Upon TS treatment, SPI and 7S globulins were associated to form large macromolecules whereas 11S globulin was dissociated. Increases in Mw in TS treated SPI and 7S globulins suggest that heat and ultrasound treatments acted synergistically to promote protein aggregation. TS treatment also led to a slight decrease in Mw of 11S globulin, suggesting protein dissociation. The present results showed that the SEC-MALLS-QELS system is suitable for monitoring changes in molecular weight/size and conformation of soy proteins during ultrasonic treatments.