Properties of strong-heated foods: in vitro proteolysability of sterilisedmilk

Studies on tryptic in vitro proteolysability of sterilised milk which appea rs to be suited as a method for characterising heat load of strong-heated f oods are described. Further, it was attempted to elucidate whether heating under the time/temperature conditions of sterilisation influences tryptic l iberation of phosphopeptide-rich fractions. Determination of soluble amino- N (OPA-method) in non-proteolysed milk revealed no systematic in- or decrea se in OPA-reactive amino groups as a function of heat load (sterilisation v alues up to F-0=966 min). The same is true for the consumption of sodium hy droxide which was used to maintain a constant pH during proteolysis. On the other hand, the contents of soluble amino-N in the supernatant after proteolysis and isoelectric precipitation of protein show a systematic dec rease (r=-0.90) in the OPA-reactive groups with increasing heat load. Separ ation of the supernatants using ion exchangers has shown clearly that phosp hopeptide-rich fractions are systematically (r=-0.95) eluted to a lesser ex tent with increasing sterilisation values and the associated dephosphorylat ion degrees of casein. The studies show further that enzymatic dephosphoryl ation may of more importantance during heat treatment of milk than has been known so far.