Ultrasonic analysis of rennet-induced pre-gelation and gelation processes in milk

Dynamics of micro-structural changes in milk during the renneting process were analysed using high-resolution ultrasonic spectroscopy in combination with dynamic rheology and NIR transmission measurements. Two independent ultrasonic parameters, velocity and attenuation were measured in the frequency range 2 to 15 MHz, as a function of time after addition of rennet to milk. The results show an initial decrease of 20 nm for the average diameter of micelles caused by hydrolysis of the kappa-casein 'hairy' layer followed by an aggregation of the micelles into small clusters (effective aggregation number of 3) and then formation of the gel structure. It was found that evolution of ultrasonic attenuation in the renneting process could well be described by the scattering of the ultrasonic waves on aggregates. The evolution of ultrasonic velocity is well described by the scattering theory but deviates from the predicted curve at the gelation stage of the process, which shows the difference in propagation of ultrasonic waves in a gel structure compared with dispersions. Overall, we found high-resolution ultrasonic spectroscopy to be a powerful tool for analysis of microscopic processes in the formation of milk gel. It allows the characterisation of the pre-gelation processes, such as hydrolysis and aggregation, and the initial stages in the formation of the gel network as well as monitoring of the microscopic evolution in the gel at the post-gelation stage.