STUDIES ON THE CHARACTERIZATION OF TRYPTI C IN-VITRO PROTEOLYSABILITYOF STERILIZED MILK

Studies on tryptic in vitro proteolysability of sterilized milk which appears suited to be used as a method for characterizing heat load are described. Further, it was attempted to elucidate whether heating und er the time/temperature conditions as applied in the sterilization reg ion influences tryptic liberation of phosphopeptide-rich fractions. De termination of soluble amino-N has shown that all in all a systematic in- or decrease in OPA-reactive amino groups as a function of heat loa d (sterilization values up to F-0=966 min) is not detectable in steril ized milk which has not undergone proteolysis. The same is true of sod ium hydroxide which was used to maintain a constant pH (pH-stat-method ) during tryptic proteolysis. On the other hand, the contents of solub le amino-N in the supernatant after proteolysis and isoelectric precip itation of protein show a systematic decrease (r = -0.90) in the OPA-r eactive groups with increasing heat load. Separation of the supernatan ts using ion exchangers has shown clearly that phosphopeptide-rich fra ctions are systematically (r = -0.95) eluted to a lesser extent with i ncreasing sterilization values and the associated dephosphorylation de grees of casein. The studies show further that enzymatic dephosphoryla tions may possibly play a greater role in the total deg ree of dephosp horylation of heat-treated milk - as a function of thermal pretreatmen t - than has been known so far.