High-pressure treatment of milk: effects on casein micelle structure and on enzymic coagulation

Abstract

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.

Plasmin activity and proteolysis in high pressure-treated bovine milk

“…Reduced proteolysis in milk treated at 600 MPa for 30 min (Tabs. II and III), despite considerable disruption of casein micelles [10,14,23], is probably related to the significantly reduced plasmin activity in such samples. On treatment of milk at a pressure ≤250 MPa, plasmin activity was affected only slightly (Tab.…”

“…On treatment of milk at a pressure ≤250 MPa, plasmin activity was affected only slightly (Tab. I) and little disruption of casein micelles occurs [10,14,15,23]; it is therefore not surprising that treatment of milk at such pressures influenced proteolysis during subsequent storage only slightly (Tabs. II and III).…”

“…Treatment at 100-200 MPa has little effect on casein micelle size [6,14,23], but treatment at 250 MPa for ≥15 min increases casein micelle size, possibly through the formation of micellar aggregates [14,15]. Treatment at a pressure ≥300 MPa reduces casein micelle size bỹ 50% [6,8,10,14,15,23]. Dissociation of caseins, particularly β-and κ-caseins, occurs at a pressure ≥100 MPa [2,21].…”

Effects of High-Pressure Technology on the Functional Properties of Milk and Fermented Milk Products

“…With the application of HHP the size and number of casein micelles increases (as casein micelle dissociates into sub-micelles due to weakening of hydrophobic and electrostatic interactions between sub-micelle and further aggregation of sub-micelle to big cluster) as spherical particles change to form chains or clusters of sub-micelles. Size distribution of spherical casein micelles decreases from 200 to 120 nm; maximum changes have been reported to occur between 150-400 MPa at 20 °C [1,22,[27][28][29].…”

“…Furthermore, Needs et al [28] stated that milk subjected to HHP treatment loses its white color and turns to yellowish due to reduction in size of casein micelles.…”