The cysteine content of caseion micelles

The effects of variations in pH, of the removal of calcium and of the addition of sulphur-bond inhibitors on the rate of setting of renneted milk

The rate of increase of the complex rigidity modulus of renneted skimmilk was measured, both in a modified form of the torsiometer previously described and in the thrombelastograph, an instrument designed for the study of blood coagulation. Milk diluted with water was also tested.

The effect of holding the milk is to slow down the process of coagulation without affecting the ‘coagulation time’ (i.e. its starting time). In undiluted milk a fall in pH from 6.66 to 6.18 does not affect maximum firmness but a later softening of the curd is accentuated in diluted and undiluted milk. Again, the coagulation time is not affected but the rate of firming up is reduced. Potassium oxalate, used to reduce the calcium content, shows similar effects.

N-Ethyl maleimide, a reagent specific for sulphydryl groups, had no effect on undiluted milk; but with dilute milk, at a pH of about 6·0, there was a decrease in maximum firmness and there were also differences in the later softening of the curd.

The significance of these effects on bonds formed during postenzymic coagulation is discussed.

Sulphydryl and disulphide groups in casein

The method of Gaitonde (1967) was adapted for the measurement of cysteine and cystine in acid hydrolysates. Analysis of 3 different casein preparations indicated the presence of both cysteine and cystine in whole casein in the proportions of 1:1 by weight and in the molecular ratio of 2:1. If all the cysteine and cystine is associated with κ-casein then it appears that one mole of the latter contains 2 moles of cysteine and 1 mole of cystine. The sulphydryl group in whole casein appears not to be highly labile.

Colorimetric Analysis of Protein Sulfhydyl Groups in Milk: Applications and Processing Effects

Methods for protein sulfhydryl (SH) group analysis in food systems have been largely overlooked. Nevertheless, changes in SH group concentration affect both physical and nutritional characteristics of high protein foods and ingredients. Food scientists and technologists require improved understanding of protein SH chemistry in order to design processes that minimize loss of thiol groups. This article surveys colorimetric methods for food protein SH group analysis with applications to fluid milk and dried milk powder. Most colorimetric assays (chloromeribenzoate, pyridine disulfide, Nitrobenzo-2-oxa-1,3-diazole, papain reactivation assay, etc.) were found to be inferior to the Ellman method based on the use of 5,5'dithio (bis-2 nitro benzoic acid). Techniques for SH group analysis in fluid milk and dried milk powder are described, along with typical results, their interpretations, and current research related to processing effects and the role of milk SH content on a wider range of technological issues, such as development of cooked flavors, fouling and cleaning of plate heat exchanges, protein-protein interactions, and the storage stability. Finally, a number of areas requiring further research are presented.

The isolation and properties of whole casein: some implications for comparative studies

1. Problems in the isolation of whole casein from bovine milk are considered. A summary is given of our experiences in its isolation. 2. The physical characteristics, sedimentation velocity, heterogeneity, absorptivity, nitrogen, phosphorus and carbohydrate contents of whole casein prepared from normal and sub-clinical mastitic milk samples by a variety of methods are compared. The methods are acid precipitation, high-speed centrifugation, with and without added calcium (II), and ammonium sulphate precipitation. 3. A description is given of the low temperature ammonium sulphate procedure preferred for the isolation of whole casein, especially when it is to be used for subsequent fractionation for conformation and micelle studies. 4. The question of the use of bovine casein as a paradigm for the caseins of other mammalian species and the need for further studies of the physical, chemical and biological properties of the caseins are discussed.