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Tramer, Joseph (1956) Correlation between effects of the heat treatment of milk and the enzymes therein. Doctoral thesis, Northern Polytechnic.
The development and aims of the heat treatment of milk are outlined, followed by a description and criticism of various chemical and other control tests for its efficiency.
The writer's published work on the relationship between the official Kay-Graham and the Aschaffenburg and Mullen milkphosphatase test for the control of pasteurisation is reviewed and extended.
Other papers on the assessment of bacterial phosphatases together with a new test based on di-sodium p-nitrophenylphesphate are described and the heat stability and other aspects of bacterial phosphatases discussed.
The recent published findings on reactivation of heat inactivated milk phosphatase were followed up by a study of reactivation of milk peroxidase. A modified peroxidase test was developed, based on the substrate dimethyl p-phenylenediamine sulphate.
Factors governing the reactivation of both enzymes were investigated. Phosphatase reactivation seems primarily a function of the reversion of the denatured protein part of the enzyme, it is greatly enhanced, by reducing conditions and the addition of Mg, Mn and Zn ions in certain concentrations. The addition of small amounts of ethylenediamine tetra acetic acid (EDTA), of copper and of nickel causes inhibition. The interaction between EDTA and zinc, magnesium, copper and cobalt points to zinc as an essential part of the enzyme molecule and to magnesium is important for reactivation.
Peroxidase reactivation is governed by the degree of protein denaturation and its subsequent reversion, it is hardly influenced by the addition of Mg, Zn and Ml ions and by EDTA. Unlike milk phosphatase, variations due to milk supply are small. Homogenisation and laboratory pasteurisation (63°C/30 min.) inhibit reactivation of both enzymes.
The chemical structure of various substrates together with possible reactions on hydrolysis are given.
The possible occurrence and implications of reactivation of phosphatase and peroxidase in various heat treated milks and creams are discussed.
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