Moving boundary electrophoresis of native and rennet-treated casein micelles

Measurement of electrophoretic mobilities and zeta-potentials of particles from milk using laser Doppler electrophoresis

The use of a commercial laser Doppler electrophoresis apparatus to measure electrophoretic mobilities of milk constituents is demonstrated, and calculations of apparent ζ-potentials from these mobilities are given. Systems studied were: native and renneted casein micelles suspended in milk ultrafiltrate and different buffers containing Ca2+, particles from homogenized milk before and after renneting, fat globules, and homogenized fat/caseinate systems. The results confirmed previous estimates of the ζ-potentials of these materials, but also extended to cover the effect of changing the concentrations of Ca2+ and other salts on the electrokinetic properties of the particles. Increasing concentration of Ca2+ caused considerable decrease in the ζ-potentials of native and renneted casein micelles, and temperature increase caused the apparent ζ-potential to increase in both cases. Particles from homogenized milk showed ζ-potentials very similar to those of native casein micelles, and renneting of the homogenized milk gave behaviour similar to renneted casein micelles. The ζ-potential of the homogenized materials could be distinguished clearly from those of native fat globules. Likewise, milk fat which had been homogenized using whey protein showed distinct differences in its surface properties from particles based on caseinate or casein micelles.

Heat stability of milk: further studies on the pH-dependent dissociation of micellar κ-casein

Whey protein complexed and became co-sedimentable with casein micelles after heating milk at ≥ 90°C for 10 min at pH ≤ 6·9 while at higher pH values (7·3) whey proteins and κ-casein-rich protein dissociated from the micelles on heating. κ-Casein-deficient micelles were more sensitive to heat, Ca2+ or ethanol than whey protein-coated or native micelles and were readily coagulable by rennet. Isolated κ-casein added to skim milk before preheating (90°C for 10 min) did not associate with the micelles at pH ≥ 6·9. Sodium dodecyl sulphate increased the level of both non-sedimentable N (NSN) and N-acetylneuraminic acid (NANA) and shifted the NSN-pH and NANA-pH curves to more acidic values while cetyltrimethylammonium bromide had the opposite effect. It is suggested that the pH-dependent dissociation in micellar κ-casein, which appears to be reversible, depends on the surface charge on the micelles; at a certain negative charge, disruption of hydrophobic and electrostatic forces could result in the dissociation of κ-casein from the casein micelles.

Heat stability of milk: role of β-lactoglobulin in the pH-dependent dissociation of micellar κ-casein

On heating casein micelle systems containing β-lactoglobulin (β-lg) at 90°C for 10 min, β-lg complexed with casein micelles at pH < 6·9, probably as a result of interaction with κ-casein via sulphydryl-disulphide interchange, and co-sedimented with the micelles on ultracentrifugation. Complex formation with β-lg appeared to prevent the dissociation of micellar κ-casein on heating. However, at pH ≥ 6·9, κ-casein/β-lg complexes dissociated from the micelles on heating, thus enhancing the release of micellar κ-casein. High concentrations of β-lg (≥0·8%) induced coagulation at pH 7·3, essentially by promoting the dissociation of micellar κ-casein. It appeared that αs1-, αs2-, β- and κ-caseins dissociated from serum protein-free casein micelles to equal extents, but the presence of β-lg specifically enhanced the dissociation of κ-casein at pH values ≥ 6·9. Micelle hydration increased slightly when casein micelles were heated in the presence of β-lg at pH 6·7, while at pH 7·3 β-lg decreased the degree of hydration of casein micelles. Formation of a complex between β-lg and κ-casein appeared to stabilize the micelles in the pH range 6·5–6·7, possibly via increased micellar charge or degree of hydration or by preventing the dissociation of κ-casein.

Heat stability of milk: influence of denaturable proteins and detergents on pH sensitivity

α-Lactalbumin and SDS in addition to β-lactoglobulin introduced pH sensitivity to the heat stability–pH curve of serum protein free casein micelles particularly by increasing stability in the pH range 6·4–6·7. Bovine serum albumin, ovalbumin and lysozyme caused marked destabilization of milk and casein micelle suspensions throughout the pH range 6·4–7·4. Tetramethyl ammonium bromide caused destabilization of milk at pH values > 7·0, but had no effect in the region of maximum stability while the non-ionic detergents Triton X-100 and Tween 80 had no effect on heat stability.

Coagulation of renneted bovine casein micelles: dependence on temperature, calcium ion concentration and ionic strength

The rates of coagulation of completely renneted casein micelles have been measured as functions of ionic strength, temperature, and concentration of Ca2+. At 25 °C and below, the rate constants for the coagulation were found to be low, but increased with temperature so that at 60 °C the particles were coagulating at almost maximum rate permitted by diffusion. This maximal rate at 60 °C was achieved at nearly all of the ionic strengths and concentrations of Ca2+ used. At lower temperatures the rate constant decreased with increasing ionic strength, the dependence being more marked at lower temperatures. Increasing concentration of Ca2+ also increased the rate at low and moderate temperatures. The implications of these results are discussed in terms of specific and non-specific ionic interactions and of hydrophobic bond formation.

Coagulation of homogenized milk particles by rennet

The action of rennet on homogenized milk was studied using turbidimetric and light scattering techniques, and compared with results obtained previously for skim milk. The time required for the onset of coagulation was shorter for homogenized milk than for skim milk. The rate of coagulation of fully renneted particles increased with increasing temperature, and with increasing Ca2+ concentration, but was only slightly influenced by changes in ionic strength. The von Smoluchowski rate constant for the coagulation reaction was two orders of magnitude smaller for homogenized milk than for skim milk. Results suggest that coagulation of homogenized milk is controlled in general by the same factors as skim milk, and that the reaction may be inhibited owing to a reduction in the amount of casein available for mutual interaction, rather than to disruption of the micelles on homogenization.

Interaction of ionic materials with casein micelles

A suspension of casein micelles in a milk-salts solution bound highly cationic polypeptides and charged surfactants extensively, and globular proteins and low molecular weight amines less extensively. All additives were bound equally to native micelles and to a rennet coagulum. The distribution of each additive between the micellar and soluble phases was constant over a wide range of concentration, indicating that the micelle contained a large number of equivalent binding sites. Additives that did not bind Ca2+ caused a decrease in the rennet coagulation time which was related to the charge on the additive and the amount adsorbed. Salmine affected the time course of the viscosity of milk treated with rennet in a similar way to increase in temperature, accelerating the aggregation phase. Extensively-bound additives protected milk against loss of coagulability by rennet on heating, but did not affect the extent of whey protein denaturation. They did not cause aggregation of micelles or produce significant changes in their size distribution, but did cause a decrease in micellar hydration. It was concluded that the additives were bound to the apatite and casein components of the micelle, causing a change in the environment of the κ-casein and para-κ-casein, which affected their properties.

Effect on the composition and properties of casein micelles of interaction with ionic materials

The effect on the composition and properties of casein micelles of the binding of ionic materials which accelerate the coagulation of milk on rennet treatment, was investigated. When considered in terms of their relative charge concentration, all the materials tested caused similar effects. The casein, inorganic phosphate and Ca contents of the micelles increased slightly. Micelle hydration decreased as additive binding increased. Casein and Ca dissociation on cooling increased at low concentrations of bound material, then progressively decreased at higher concentrations. The mean size of micelles and their electrophoretic mobility was little affected by bound ionic materials. The aggregation of the casein complexes in colloidal calcium phosphate-free milk was markedly increased by adding ionic materials, the efficiencies of these additives paralleling their efficiencies in accelerating the coagulation of milk by rennet. The results suggested that the ionic materials were bound in the interior of the casein micelles and promoted aggregation after rennet treatment by shielding charged groups, thus increasing the micellar hydrophobicity.

Rennet coagulation of milk in the presence of sucrose

The aim of this work was to test the diffusion-controlled hypothesis of milk coagulation kinetics by reducing the diffusion coefficient of casein micelles. This has been achieved by increasing the solvent viscosity of milk through sucrose addition. Milk was reconstituted from skim milk powder and sucrose added at 100–300 g kg–1. Hydrolysis and coagulation were followed by chromatographic determination of caseinomacropeptide content and optical, thermal and viscoelastic measurements. Soluble and ionic calcium were determined by atomic absorption spectrophotometry and ionometry and micelle size was measured by dynamic light scattering. Addition of sucrose resulted in a substantial retardation of both enzymic and aggregation steps, a re-equilibration of calcium because of water reduction, and a micelle size increase. The enzymic rate constant was inversely proportional to the viscosity, according to a diffusion-controlled model, and the lag or characteristic times for the aggregation reaction were inversely proportional to the viscosity. These results are consistent with the involvement of diffusion-controlled steps in the sequence of reactions.

The effect of the chemical structure of additives on the coagulation of casein micelle suspensions by rennet

Casein micelles in milk-salts solution adsorbed charged detergents and highly-charged polypeptides strongly, neutral detergents less strongly and low molecular-weight amines weakly. A tetra-amine was adsorbed more strongly than a tri-amine. The extent of adsorption of proteins tended to rise as the molecular weight increased. Glycerol and lactate were adsorbed to a limited extent but dextran and α-ketoglutarate were not adsorbed at all. Proline was partly adsorbed, indicating that hydrophobic binding sites were available, and caused some disruption of the casein micelles. Additives were bound to approximately the same extent by casein micelles and rennet coagula. The proportions adsorbed were constant over at least 10-fold ranges of concentration. Additives which increased the rennet clotting time (RCT) acted by binding Ca2+. Most additives decreased the RCT, the extent increasing with the amount adsorbed and the positive charge on the additive. The greatest reduction in RCT was observed with those additives which had positively-charged and hydrophobic moieties and bound most strongly to casein micelles. Of the additives tested, only sodium dodecyl sulphate affected the enzymic action of rennet. The reduction in RCT may have resulted from the neutralization of the negative charge of the micelles or enhancement of their hydrophobicity, favouring hydrophobic interactions between the particles.

Milk coagulation by cationic polyelectrolytes

Positively charged materials, polyethyleneimine, poly-L-lysine and partly estcrified proteins (i.e. methyl bovine serum albumin, methyl casein and ethyl casein), in aqueous solution at the natural pH of milk, caused milk coagulation. This occurred at pH 6–6·7 at ≤ 30 °C using a defined amount of coagulant, which was quantitatively recovered in the curd. The protein and fat recovery yields were higher than with acid and rennet coagulation; the (Ca + Mg) partition between whey and curd was very similar to rennet milk clotting: the curd obtained showed high syneresis rate.

Mineral balance in skim-milk and milk retentate: effect of physicochemical characteristics of the aqueous phase

The effect of physicochemical characteristics (pH, temperature, composition) of the aqueous phase on the mineral balance in milk and milk retentate has been studied. The ratio of colloidal Ca to total protein decreased with pH, but at any given pH the higher the protein concentration, the higher was the ratio of colloidal Ca to total protein. The solubilization of Ca during cooling and the decrease in soluble Ca during heating were approximately the same in retentates and in milk. Among the components of the aqueous phase, soluble Ca and citrate ions were related to the amount of colloidal Ca.

The determination of the zeta potential of casein micelles

The principle of moving boundary electrophoresis has been employed for the measurement of the electrophoretic mobility and subsequent calculation of zeta potential of bovine casein micelles. The zeta potential of casein micelles was observed to increase with increase in temperature (from 10 to 50 °C) and to decrease with decrease in pH. Heat treatment of milk between 90 °C/30 min and 135 °C/50 min had no significant effect on zeta potential. The zeta potential of casein micelles during rennet action decreases until all the κ-casein has been cleaved by the enzyme.

The results of this study indicate that electrostatic interactions alone are not sufficient for an understanding of the absolute stability of casein micelles.

Electrophoretic mobility of casein micelles

A simplified moving boundary electrophoresis technique has been developed for the measurement of the electrophoretic mobility of casein micelles. The zeta potentials of casein micelles from different skim-milk samples were calculated using Henry's equation and shown to decrease with decrease in pH between pH 6.9 and 5.3 and to increase with increase in temperature between 10 and 45 °C. Neither severe heat treatment (up to 135 °C for 51 min) nor centrifugal fractionation of micelles into different micelle size ranges had any significant effect on zeta potential. The ionic composition of the serum phase has been shown to be extremely important in determining the electrophoretic mobility. Casein micelles electrophoresed through milk ultrafiltrate consistently gave a lower mobilities than the same micelles centrifuged through milk centrifugate. The results are discussed in relation to present theories of casein micelle structure; these theories do not accommodate all of the observations.

The acceleration by cationic materials of the coagulation of casein micelles by rennet

Three cationic materials markedly reduced the rennet clotting time of casein micelle suspensions, the efficacy of each being primarily dependent on the charge and the amount absorbed by the micelles. The reduction in coagulation time was unaffected by components of the milk serum other than salts. No enzymic action by lysozyme on casein micelles was detected. All materials acted by the same mechanism, increasing the affinity of rennet for the micelles and accelerating the aggregation phase. Coagulation did not occur until a minimum amount of κ-casein had been hydrolysed to para-κ-casein. All additives increased the proportion of added rennet retained by the casein in the coagulum. The results indicated that coagulation occurs by specific interactions between micelles modified by rennet.

Estimation of casein micelles' surface energy by means of contact angle measurements

The surface energies of highly hydrated casein micelle layers isolated from variously pretreated skim milks have been determined by means of contact angle measurements. The long range Lifshitz-Van der Waals (LW) and the short range hydrogen bonding (SR) components of surface energy were determined using α-bromonaphthalene and water for contact angle measurements. Casein micelles isolated from untreated and heat treated milks showed similar surface energy values of about 63·5 mJ.m-2 with an LW component of 19·2 mJ.m-2 and an SR component of 44·3 mJ.m-2. The calculated attraction potential energy was − 0·7 mJ.m-2. Casein micelles isolated from renneted milk showed a surface energy of 33·0 mJ.m-2 with an LW component of 30·7 mJ.m-2 and an SR component of 2·3 mJ.m-2. The attraction potential energy of renneted micelles was nearly two orders of magnitude higher than those of micelles from other milks ( − 63·3 mJ.m-2). The SR component of interfacial energy accounted for 98% of this attraction potential. The importance of attractive forces in relation to casein micelle stability is discussed.

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was −18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0·2 εm, in good agreement with previously reported values. The particle size increased as the pH was decreased.

Interaction between casein and sodium dodecyl sulfate

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) with 2.0 mg/ml casein was first investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and fluorescence spectra. ITC results show that individual SDS molecules first bind to casein micelles by the hydrophobic interaction. The micelle-like SDS aggregate is formed on the casein chains when SDS concentration reaches the critical aggregation concentration (c1), which is far below the critical micellar concentration (cmc) of SDS in the absence of casein. With the further increase of SDS concentration to the saturate binding concentration c2, SDS molecules no longer bind to the casein chains, and free SDS micelles coexist with casein micelles bound with SDS aggregates in the system. DLS results show that the addition of SDS leads to an increase in the hydrodynamic radius of casein micelles with bound surfactant at SDS concentration higher than 4 mM, and also an increase in the casein monomer molecule (or submicelles) at SDS concentration higher than 10 mM. Fluorometric results suggest the addition of SDS leads to some changes in the binding process of hydrophobic probes to casein micelles.

Mammary gland expression of transgenes and the potential for altering the properties of milk

Transgenic animals are a useful in vivo experimental model for assessing the ability and impact of foreign gene expression in a biological system. Transgenic mice are most commonly used, while transgenic sheep, goats, pigs and cows have also been developed for specific, "applied" purposes. Most of the work directed at targeting expression of transgenes to the mammary gland of an animal, by using a milk gene promoter, has been with the intent of either studying promoter function or recovering the desired protein from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk resulting in novel manufacturing properties. The properties of milk have been altered by adding a new protein with the aim of improving the milk, not of recovering the protein for other uses.

The influence of temperature on the flocculation rate of renneted casein micelles

The flocculation rate constant of completely renneted casein micelles in milk ultrafiltrate was measured by Rayleigh light scattering between 20 and 35 degrees C. In this temperature range an apparent energy of activation of 103 kJ mol (+/-11 kJ mol : n = 50) was measured. At 15 degrees C clotting was not longer perceptible. The activation of the flocculation between 20 and 35 degrees C is explained not so much by the height of the energy barrier separating the clotting micelles, as by the very negative temperature coefficient of that barrier. In line with this conclusion it is suggested that renneted micelles adhere through hydrophobic bonding. The flocculation rate constant of renneted casein micelles is independent of micelle size at the four temperature levels studied.

Heat stability of milk: influence of cationic detergents on pH sensitivity

The heat stability--pH profile of milk is shifted to more acidic values by anionic compounds such as SDS, lysolecithin and beta-lactoglobulin and to more alkaline values by cationic compounds such as quaternary ammonium compounds. Proline, which reduces the hydrophobicity of proteins, destabilized milk slightly. The results suggest that the heat stability of milk and the shape of the HCT-pH curve may be dependent on micellar charge effects.