The effect of preparative conditions on the composition of the k -casein complex

The cysteine content of caseion micelles

Following enzymic digestion with pronase, masked SH groups in micellar casein became available for titration in disaggregating media with mercurial reagents. The content of cystine and cysteine was also estimated after reduction with (a) sulphite, and (b) borohydride, and also by reaction in alkaline conditions with Cd(OH)2. The results show that the micelles contain mainly cysteine, and that it is likely that cystine is not present.

Studies on the к-casein complex: II. The isolation of a sialic acid-containing fraction by disrupting the complex at low pH in the presence of sodium chloride

When crude к-casein was precipitated at pH 3 in the presence of 0·4m-NaCl the supernatant contained up to 80% of the total sialic acid but no detectable cystine or cysteine. Two fractions were obtained from this supernatant by chromatography on DEAE cellulose; one containing 4–6% sialic acid and the other only onetenth of this amount.

Most of the sialic acid of the sialic acid-rich fraction was soluble in 12% trichloracetic acid following treatment with rennin. It is suggested that the glycopeptide released by the action of the enzyme on casein originates from this fraction.

Factors affecting the viscosity of caseinates in dispersions of high concentrations

The physical effects of various cations in caseinate dispersions of high concentrations were investigated over a range of temperature and pH.

With calcium and strontium the temperature-viscosity relationships of the caseinates were abnormal in that the viscosity decreased rapidly from 30 to about 40 °C and a gel formed at temperatures in the region of 50–60 °C. On cooling, the gel reliquefied. No gel formed with barium, aluminium or magnesium. On cooling, magnesium preparations separated into 2 phases.

The supernatant phase from the magnesium caseinate and a corresponding phase prepared by centrifuging the calcium caseinate showed depletion ofα-casein and enrichment ofκ-casein andβ-casein. The supernatant phase from the calcium caseinate showed the reversible gel formation on heating. The magnesium supernatant phase did not.κ-Casein and a mixture ofκ- andβ-caseins gave reversible gels at similar levels of calcium and pH.

For reversible gel formation to occur, calcium caseinate was required to be in fairly high concentration, to have a calcium content of about 1·0% of the protein and to be within the pH limits 5·2–6·0. The temperature at which gelation occurred was affected by the concentration of calcium and protein and by pH.

The behaviour of the material was compared with that of methyl cellulose with and without addition of urea.

Some potential commercial applications of the findings on viscosity relationships are outlined.

The action of rennin on casein: the effect of modifying functional groups on the casein

k-Casein and whole casein when photo-oxidized in the presence of methylene blue lose the ability to clot when treated with rennin. Two effects are involved—first the photo-oxidation alters the k-casein fraction so that the rennin is unable to split off the glycopeptide fragment, and secondly, in whole casein the photo-oxidation interferes with the aggregation in the presence of Ca++that normally follows rennin action. As a result of amino acid analysis and specific treatments which affect other photo-oxidizable side chains, it is concluded that both of these effects are caused by the alteration of histidines.

The role of lysine residues in the coagulation of casein

Coagulation of rennin-treated casein was inhibited by treatment of the casein with dimethylaminonaphthalene sulphonyl chloride. The effect was caused by substitution on lysine side chains of theκ-casein fraction, and inhibition was complete when 2–3 lysine residues/molecule were blocked. This level of substitution did not affect other properties of theκ-casein, such as the release from it of non-protein nitrogen (NPN) by rennin and its ability to stabilizeαs-andβ-caseins in the presence of Ca++. The evidence suggests that lysine side chains onκ-casein take part in the coagulation of rennin-treated casein.

Dynamic rheology of renneted milk gels containing fat globules stabilized with different surfactants

Anhydrous milk fat was emulsified with alpha s1-CN (casein), alpha s2-CN, beta-CN, kappa-CN, alpha-lactalbumin, beta-lactoglobulin, Tween 80, or phosphatidylcholine to produce a 30% fat cream in a 0.1 M imidazole pH 7 buffer. The creams were mixed with skim milk to yield a fat content of 3.4% and the viscoelastic properties of the recombined milks clotted with chymosin were measured. Recombined milk containing globules coated with the more amphipathic and phosphorylated alpha s2-CN and beta-CN clotted faster but gel firmness increased more slowly and weaker gels were formed. Gel firmness increased more rapidly for milks containing globules coated with of alpha s1-CN and kappa-CN that possess more uniformly distributed hydrophobic domains.