High isostatic pressures up to 600 MPa were applied to samples of skim
milk before addition of rennet and preparation of cheese curds. Electron microscopy
revealed the structure of rennet gels produced from pressure-treated milks. These
contained dense networks of fine strands, which were continuous over much bigger
distances than in gels produced from untreated milk, where the strands were coarser
with large interstitial spaces. Alterations in gel network structure gave rise to
differences in rheology with much higher values for the storage moduli in the
pressure-treated milk gels. The rate of gel formation and the water retention within
the gel matrix were also affected by the processing of the milk. Casein micelles were
disrupted by pressure and disruption appeared to be complete at treatments of
400 MPa and above. Whey proteins, particularly β-lactoglobulin, were progressively
denatured as increasing pressure was applied, and the denatured β-lactoglobulin was
incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly
by rennet, but the presence of denatured β-lactoglobulin interfered with the
secondary aggregation phase and reduced the overall rate of coagulation. Syneresis
from the curds was significantly reduced following treatment of the milk at 600 MPa,
probably owing to the effects of a finer gel network and increased inclusion of whey
protein. Levels of syneresis were more similar to control samples when the milk was
treated at 400 MPa or less.
Rethinking Casein Micelle Structure Using Electron Microscopy
