Thermorheological characteristics of soybean protein isolate

Small amplitude oscillation shear measurement was used to study gel rigidity of commercial soy protein isolate (SPI) dispersions during isothermal and non-isothermal heating. Temperature sweep data (20 degrees C to 90 degrees C at heating rate of 1 degrees C/min) of SPI dispersions demonstrated that elastic modulus (G') predominates over viscous component (G") for all concentrations studied. The gelation kinetics of SPI was evaluated by a non-isothermal technique as a function of elastic modulus (G'). During experiments, it was observed that a critical concentration of 10% was required to form a true SPI gel. Thermorheological data of 10% and 15% SPI dispersions were adequately fitted by 2nd-order reaction kinetics. The reaction order of gelation was initially calculated by multiple regression technique correlating dG'/dt, G' and temperature, which finally was verified by linear regression of kinetic equation at selected order. Isothermal data of 15% SPI was also followed by 2nd-order reaction kinetics. The activation energy during the isothermal technique was significantly higher than non-isothermal gelation of SPI at same concentration level. Gel strength of the non-isothermally heated SPI sample (15% to 20%) was compared with isothermally heated (90 degrees C for 30 min) one. Higher protein concentration (20%) and isothermal heating exhibited significantly higher gel rigidity while the difference between the 2 processes was insignificant at 15% concentration at a similar condition.