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

Abstract

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.

Ultrasonic Monitoring of Biocatalysis in Solutions and Complex Dispersions

“…Adapted with permission from Reference [80]. Copyright © Proprietors of Journal of Dairy Research 2005.…”

“…Figure 17 represents real-time ultrasonic monitoring of enzymatic removal of the 'hairy' layer of hydrated protein nanoparticles (casein micelles, 120 nm diameter approx. ), followed by particle aggregation and gelation in milk at 30 • C [80]. The κ-casein 'hairy' layer provides steric stability of the particle dispersion.…”

“…This is followed by the increase of the size, which demonstrated the beginning of the aggregation of 'bald' particles. At the reaction time 30 min, further aggregation of particles results in a formation of gel network, as indicated by rheological (rise of shear storage modulus) data [80]. parameters of the casein micelles and of the continuous medium utilized in these calculations were taken from data published by Griffin et al [82].…”

“…parameters of the casein micelles and of the continuous medium utilized in these calculations were taken from data published by Griffin et al [82]. Minor amendments of some parameters were introduced to account for small differences in temperature to which the parameters were originally attributed and the temperature at which the measurements were performed [80]. Casein micelles, volume fraction 0.1 in milk at 30 °C.…”

Ultrasonic Characterisation of W/O Microemulsions - Structure, Phase Diagrams, State of Water in Nano-Droplets, Encapsulated Proteins, Enzymes

“…The capability of this and other ultrasonic techniques were demonstrated for a variety of reactions catalysed by enzymes, namely, urease, -amylase and catalase (Kudryashov et. al, 2003), chymosin (Dwyer et al, 2005) proteinase K (Buckin & Craig, 2005), invertase (Resa et al, 2009), and cellobiase (Resa & Buckin, 2011) as well as other types of bioprocesses (McClements, 1995;Povey, 1997). It provided the detailed reaction extent and rate profiles in a broad range of concentrations, allowed analysis of the reverse reaction, determination of equilibrium constant and of the molar Gibbs free energy of reactions as well as analysis of reactions kinetic models (Resa & Buckin, 2011).…”