Effect Of High Hydrostatic Pressure On The Structure Of Milk Protein Concentrates And Micellar Casein Concentrates

Milk proteins are used in a wide variety of food and give structure to dairy foods like cheese and yogurt. As consumers increasingly demand high protein foods with minimal processing, high pressure processing (HPP) has received attention as a nonthermal alternative to pasteurization, but this process can also be used as a tool to structurally modify proteins. This research explores structural changes to the main classes of milk, the casein micelles and serum proteins, induced by HPP. Experiments were conducted on suspensions of milk protein concentrate (MPC) and micellar casein concentrate (MCC) at 2.5 and 10% casein (w/v), treated at 150 to 450 MPa for 15 minutes at ambient temperatures. The samples were analyzed for particle size, mineral and protein distribution, and renneting kinetics using rheology and particle sizing. Scanning electron microscopy (SEM) was also used to provide direct imaging of milk protein structure before and after pressure treatment. Increasing pressures increased levels of serum calcium by up to a factor of 4 times as well as increased levels of soluble [alpha]-caseins. In 10% MCC, pressure treatment at 150 or 450 MPa significantly increased the elastic modulus of rennet gels by up to 3 times while treatment at 250 or 350 MPa only resulted in two-fold increases and no reduction in the rennet coagulation time compared to controls. In 10% MPC, treatments led to a monotonic increase in elastic modulus and aggregation rates in rennet gels. Pressurization of 10% samples also led to the formation of weak gel structures. Scanning electron micrographs showed evidence of small spherical substructures, 15-20 nm in diameter, which were tightly networked in 10% samples treated at 450 MPa. These results present some evidence for substructure in the casein micelle and suggest that micelle formation upon pressure release may involve the formation of an intermediate. Interpretations of the data in this experiment and SEM images support the iii existence of casein submicelles as the structural unit of the casein micelle. These results also suggest that casein dissociation and reformation with HPP may be a two-stage process involving the dissociation of micelles into submicelles followed by the disintegration of submicelles into individual casein proteins while reaggregation follows this process in reverse. Pressure induced dissociation of casein was found to be both concentration and pressure level dependent. This research illustrates some effects of HPP on dairy proteins which may enable new applications for these proteins in a variety of new dairy-based and protein-fortified foods. i