Small and large deformation rheology of acid gels from transglutaminase treated milks

The purpose of this study was to evaluate the effects of microbial transglutaminase on the rheological and mechanical proper-ties of gels from a commercial UHT milk made by acidification with D-gluconic-acid-delta-lactone. Although further heating of UHT milk, primarily carried out to inactivate the enzyme, showed a significant impact on gel firmness, we observed no enzyme-related effects at small deformation. The impact of the additional thermal treatment was also evident in large deformation measurements. In these experiments, gels from enzyme-treated milk revealed much higher elasticity and rupture force. Prolonged incubation time or increased enzyme concentration resulted in lower gel firmness (small deformation), indicating a reduced number of total bonds as a result of the restriction of proper rearrangements during acidification. Large deformation measurements revealed a heat-induced amplification of this effect, i.e., lower rupture forces at almost constant elasticity levels. In gets from milk, which was not subjected to thermal enzyme inactivation, we observed a further increase in both parameters. This is presumably caused by a combination of residual enzyme activity and of additional substrate, liberated during acidification.