Effect of drying methods on rhelogical properties and particle size characteristics of soy protein isolate

Abstract

Rheological properties and particle size distribution of soy protein isolates (SPI) play a vital role in food formulations along with its physicochemical and functional properties. Ultrafiltration was used to produce SPI solution and obtained solution was dried using different drying methods (oven, freeze and spray drying). Rheological properties, particle size distribution and protein solubility of dried SPI powders were compared with commercially available SPI. With change in angular frequency, commercial SPI shown almost constant storage modulus while others shown a gradual increase whereas loss modulus of all samples shown increment. Commercial SPI shown a decrease in complex viscosity (shear thinning behavior) while other dried samples exhibited a slight increase in complex viscosity values (shear thickening behavior), with increase in angular frequency. Loss tangent values for commercial SPI and oven dried SPI (60°C for 48 hours) were constant and less than unity (solid or gel property), whereas for spray dried SPI (190°C inlet temperature and 0.25-0.3 kg/cm2 high air flow rate) decreased from 1.05 to 0.42 (transition from liquid or sol like characteristics to liquid or gel like characteristics). Commercial SPI had lesser uniformity compared to other dried samples. Highest water solubility index and water absorption capacity was shown by spray dried SPI samples while freeze dried and oven dried samples had comparable lower values. The drying has significant influence on rheological properties, particle size distribution and solubility characteristics of SPI obtained through ultrafiltration which in turn decides the functionality and end use in food applications.