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Chemical changes in ultra-heat-treated milk during storage: I. Hydrolysis of casein by incubation with pronase and a peptidase mixture

Published online by Cambridge University Press:  01 June 2009

A. B. Möller
Affiliation:
National Institute for Research in Dairying, Shinfield, Reading, RG2 9AT
A. T. Andrews
Affiliation:
National Institute for Research in Dairying, Shinfield, Reading, RG2 9AT
G. C. Cheeseman
Affiliation:
National Institute for Research in Dairying, Shinfield, Reading, RG2 9AT

Summary

Casein samples from untreated milk and stored ultra-heat-treated (UHT) milk were hydrolysed with pronase (protease ex Streptomyces griseus K-1) and subsequently with a mixture of peptidases prepared from the microsomal fraction of hog kidneys. Incubation of casein from unheated milk with pronase alone hydrolysed 70–80% of peptide bonds involving Ile, Leu, Tyr, Phe and His residues; other amino acids were released less well and proline hardly at all. The pronase/peptidase treatment resulted in 90–100% hydrolysis of peptide bonds involving all amino acids, including proline.

Caseins from stored UHT milks were more resistant to proteolysis than casein from unheated milk. Reduced release of all amino acids was observed from samples taken after storage at 37 °C for 12 months or longer and for Lys, Arg and Asn residues from samples taken after storage at 30 °C for 14 months. Resistance to proteolysis was attributed to the Maillard reaction between milk proteins and lactose during storage of UHT milk.

Type
Original Articles
Copyright
Copyright © Proprietors of Journal of Dairy Research 1977

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