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

Research was undertaken to estimate the extent of change in milk prote ins brought about by industrial processes that are utilized in the man ufacture of skim milk powder, evaporated milk, sodium caseinate and so dium coprecipitate. Potentiometric titration was used to determine the differences in the accessibility of functional groups. The kinetics o f release of peptides soluble in 2% (w/v) TCA and glycopeptides solubl e in 12% (w/v) TCA from casein substrates by rennin were determined. T he 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 > reconstitut ed skim milk powder. K(m) values for total peptide release were higher (lower affinity) (p less-than-or-equal-to 0.05) in sodium coprecipita te and sodium caseinate than in evaporated milk and skim milk powder. The velocity of glycopeptides release differed (p less-than-or-equal-t o 0.05) for all substrates at pH 6.6. K(m) values for the release of g lycopeptides ranged from 4.8 x 10(-5) to 5.4 x 10(-5) M and were lowes t (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 susceptibili ty of the casein to proteolysis in milk products was influenced by pro cessing.