RELATION BETWEEN PROTEOLYSIS AND ASTRINGENT OFF-FLAVOR IN MILK

Pasteurized skim milk was treated with 10 proteinases from psychrotrop hic bacteria and with plasmin, the naturally occurring milk proteinase , and evaluated organoleptically for astringency. At comparable levels of digestion (extent of proteolysis equivalent to almost-equal-to .5 mumol/ml of glycine), milk samples treated with plasmin and 5 psychrot rophic proteinases produced varying astringencies. All of the treated samples were extracted by 2:1 (vol/vol) mixtures of chloroform: methan ol, and the isolated compounds were analyzed by fast protein liquid ch romatography on an anion-exchange column and urea-PAGE. Both methods o f analysis showed increased concentrations of gamma-casein only in the extracts from astringent samples. The samples treated with psychrotro ph proteinase that were not astringent apparently were digested nonspe cifically into smaller peptides. Thus, astringent off-flavor has been linked to the production of gamma-caseins, which are specific C-termin al breakdown products of beta-casein (cleavage of peptide bond between residues 28 to 29, 105 to 106, and 107 to 108); treatment of purified beta-casein with plasmin also produced astringent compounds.