Relation Between Proteolysis and Astringent Off-Flavor in Milk

Abstract

Pasteurized skim milk was treated with 10 proteinases from psychrotrophic bacteria and with plasmin, the naturally occurring milk proteinase, and evaluated organoleptically for astringency. At comparable levels of digestion (extent of proteolysis equivalent to ≈.5 μmol/ml of glycine), milk samples treated with plasmin and 5 psychrotrophic proteinases produced varying astringencies. All of the treated samples were extracted by 2:1 (vol/vol) mixtures of chloroform: methanol, and the isolated compounds were analyzed by fast protein liquid chromatography on an anion-exchange column and urea-PAGE. Both methods of analysis showed increased concentrations of γ-casein only in the extracts from astringent samples. The samples treated with psychrotroph proteinase that were not astringent apparently were digested nonspecifically into smaller peptides. Thus, astringent off-flavor has been linked to the production of γ-caseins, which are specific C-terminal breakdown products of β-casein (cleavage of peptide bond between residues 28 to 29, 105 to 106, and 107 to 108); treatment of purified β-casein with plasmin also produced astringent compounds. © 1993, American Dairy Science Association. All rights reserved.