Heat-induced aggregation and gelation of rapeseed proteins

Heat-induced aggregation and gelation of rapeseed protein isolate (RPI) solutions and their principal protein components napin and cruciferin was investigated as a function of the pH, temperature, protein concentration and ionic strength. Confocal laser scanning microscopy (CSLM) showed that during heating RPI and purified cruciferin form microgels that subsequently aggregate and can form a gel if their concentration is sufficiently large. Purified napin by itself does not gel when heated, but was found to reinforce gels formed by cruciferin. Dynamic mechanical measurements showed that the gelation rate increased strongly with increasing temperature and was controlled by an activation energy of about 300 kJ/mol. The elastic shear modulus of RPI and cruciferin gels increased with increasing protein concentration. The elastic modulus of RPI gels increased with increasing pH between pH 4 and 7, but did not depend much on the pH between pH 7 and 10, nor on the addition of 0.1 M NaCl. However, for purified cruciferin the gels had a larger modulus close to pH 10 and in the presence of NaCl. During cooling the modulus further increased. The yield strain of the gels was found to be approximately 100 % almost independent of the pH and the concentration.