Gelation of casein micelles in β-casein reduced milk prepared using membrane filtration

Abstract

Pasteurized skim milk was subjected to membrane filtration using a molecular weight cut-off of 80 kDa and a plate and frame pilot scale system at temperatures below 10 °C. Via this process, transmission of whey proteins and β-casein through the membrane was achieved. The milk was concentrated to two times (based on volume reduction), and whey protein-free permeate was added to return to the original volume fraction of casein micelles in milk. This diafiltration process was carried out four times, and the retentate obtained was nearly free of whey proteins and with approximately 20% of β-casein removed. The same membrane filtration was also carried out at 25 °C to achieve transmission of whey protein but not of β-casein, and to obtain whey protein-depleted milk without depletion of β-casein. The gelling behaviour of these samples, reconstituted to the original casein volume fraction, was examined using rheology and diffusing wave spectroscopy. When compared to the original skim milk it was found that there were no statistically significant differences in gelation behaviour during acidification, but differences were noted in gelation time and final stiffness modulus for samples undergoing renneting. These differences were attributed mostly to the changes in ionic composition, as when the serum composition of the retentates was re-equilibrated against the original skim milk by dialysis; the gelation behaviour of the samples was comparable to that of skim milk. The results clearly indicate the importance of the milk's overall ionic balance in the early stages of aggregation of rennet-induced gelation of milk.