MANUFACTURING PROCESSES INFLUENCE THE PROTEOLYTIC ACTION OF RENNIN ON CASEIN IN SEVERAL DAIRY-PRODUCTS

Research was undertaken to estimate the extent of change in milk proteins brought about by industrial processes that are utilized in the manufacture of skim milk powder, evaporated milk, sodium caseinate and sodium coprecipitate. Potentiometric titration was used to determine the differences in the accessibility of functional groups. The kinetics of release of peptides soluble in 2% (w/v) TCA and glycopeptides soluble in 12% (w/v) TCA from casein substrates by rennin were determined. The casein substrates showed a diversified susceptibility to the action of rennin. The velocity of total peptides release at pH 5.6 decreased (p less-than-or-equal-to 0.05) in the following order: reconstituted condensed milk > sodium coprecipitate > sodium caseinate > reconstituted skim milk powder. K(m) values for total peptide release were higher (lower affinity) (p less-than-or-equal-to 0.05) in sodium coprecipitate and sodium caseinate than in evaporated milk and skim milk powder. The velocity of glycopeptides release differed (p less-than-or-equal-to 0.05) for all substrates at pH 6.6. K(m) values for the release of glycopeptides ranged from 4.8 x 10(-5) to 5.4 x 10(-5) M and were lowest (p less-than-or-equal-to 0.05) in skim milk powder at pH 6.6 and 5.6 and evaporated milk at pH 5.6. It was concluded that the susceptibility of the casein to proteolysis in milk products was influenced by processing.