Heat (85 °C for 20 min) and pressure (600 MPa for 15 min) treatments
were applied to skim milk fortified by addition of whey protein concentrate. Both
treatments caused > 90% denaturation of β-lactoglobulin. During heat treatment
this denaturation took place in the presence of intact casein micelles; during pressure
treatment it occurred while the micelles were in a highly dissociated state. As a result
micelle structure and the distribution of β-lactoglobulin were different in the two
milks. Electron microscopy and immunolabelling techniques were used to examine
the milks after processing and during their transition to yogurt gels. The disruption
of micelles by high pressure caused a significant change in the appearance of the milk
which was quantified by measurement of the colour values L*, a* and b*. Heat
treatment also affected these characteristics. Casein micelles are dynamic structures,
influenced by changes to their environment. This was clearly demonstrated by the
transition from the clusters of small irregularly shaped micelle fragments present in
cold pressure-treated milk to round, separate and compact micelles formed on
warming the milk to 43 °C. The effect of this transition was observed as significant
changes in the colour indicators. During yogurt gel formation, further changes in
micelle structure, occurring in both pressure and heat-treated samples, resulted in a
convergence of colour values. However, the microstructure of the gels and their
rheological properties were very different. Pressure-treated milk yogurt had a much
higher storage modulus but yielded more readily to large deformation than the
heated milk yogurt. These changes in micelle structure during processing and yogurt
preparation are discussed in terms of a recently published micelle model.
1H-Nuclear Magnetic Resonance Studies on the Conformational Changes Related to the Foaming Properties of β-Lactoglobulin
