Comparative micelle structure: II. Structure and composition of casein micelles in ovine and caprine milk as compared with those in bovine milk

A structural comparison of casein micelles in cow, goat and sheep milk using X-ray scattering

The casein micelle is a flexible construct, with its key structural components being casein proteins and colloidal calcium phosphate nanoclusters. According to literature, milk from different species exhibits differences in composition and physicochemical properties. X-ray scattering techniques were used to investigate and compare the nanoscale structure of casein micelles present in cow, goat and sheep milk. Although there were differences in the size and density of larger scale protein structures, at an atomic level the protein structures were similar. There were also strong similarities in the structure of the calcium-containing nanoclusters, namely that they had similar sizes and separations within the casein micelle for all three species.

Salt distribution between the colloidal and soluble phases of ewes' milk

Ewes' milk from two breeds of Greek sheep, Karagouniki and Serron, was analysed throughout mid and late lactation to investigate the salt distribution between colloidal and soluble phases. Separation of the phases was obtained by centrifugation at 48000 g. Effect of stage of lactation was significant only for total Mg in both breeds (P < 0·05) and for colloidal citrate in Serron ewe milk (P < 0·01). Breed differences were not significant (P < 0·05), except for colloidal casein.

In the colloidal phase there was a strong positive correlation of casein with Ca and P, but correlation with Mg was poor. Molar sums of [Ca + Mg] and [P + Cit] were also significantly correlated. The Ca to P molar ratio averaged 2·00 and was higher than that reported for cows' milk. The inclusion of Mg as well as citrate in the colloidal calcium phosphate is indicated. In the aqueous phase there was a strong negative correlation of Ca with pH and a positive one with citrate. Correlation with phosphate was less. It seems that citrate is the main factor influencing Ca concentration in the soluble phase.

Subunit components of casein micelles from bovine, ovine, caprine and equine milks

Subunit components of ovine, caprine and equine casein micelles were separated by gel chromatography using a TSK-G4000SW high-performance column and the subunit components of the fractions analysed and compared with bovine casein. Molecular weights of the casein complexes were determined by the combined use of high-performance gel chromatography and low-angle laser light scattering. The caprine and ovine caseins were separated into three peaks (F2, F3 and F4) which were similar to those of bovine casein with respect to composition and molecular weight (500, 100 and 23 K). These F2, F3 and F4 peaks consisted mainly of αs- and κ-casein, αs- and β-casein and β-casein respectively. The equine casein was separated into two components corresponding to F3 and F4 of bovine casein. These F3 and F4 peaks consisted mainly of αs- and β-casein and β-casein respectively. The molecular weight of equine F3 (850 K) was different from that of the other three species. The contents of F2 and F4 in these caseins were dependent on the contents of κ-casein and β-casein respectively.

Influence of pressure release rate and protein concentration on the formation of pressure-induced casein structures

The formation of pressure-induced casein structures (600 MPa for 30 min at 30°C) was investigated for different pressure release rates (20 to 600 MPa min−1) and casein contents (1 to 15 g/100 ml). Structures from liquid (sol) to solid (gel) were observed. The higher the protein content and the pressure release rate, the higher was the dynamic viscosity. A firm gel was built up at a casein content of 7 g/100 ml for a pressure release rate of 600 MPa min−1, while lower release rates resulted in less firm gels (200 MPa min−1) or liquid structures (20 MPa min−1). In a 5 g/100 ml casein solution and at a pressure release rate of 600 MPa min−1, casein aggregates were generated which were built from smaller casein particles with a larger hydrodynamic diameter and higher voluminosity than in the untreated solution. After a slow release rate casein micelles had a smaller hydrodynamic diameter and a lower voluminosity, but were similar in shape and diameter as compared with the micelles in solution before high pressure treatment.

Effect of the A and B variants of both alpha s1- and kappa-casein on bovine casein micelle solvation and kappa-casein content

Casein micelle solvation, a micelle characteristic that is sensitive to many factors, has been measured by a centrifugation technique at 30 degrees C for a series of uncooled fresh skim milks at pH 6.3, 6.6, 6.9 and 7.1. The relative alpha s-(alpha s1-plus alpha s2-), beta- and kappa-casein contents of all centrifuge pellets and supernatants were determined by a standardized electrophoretic method. The calcium and phosphate contents of a number of the pellets and milk samples were also determined. Solvation of micelles from milks with various genetic variants of beta-lactoglobulin (A and B), alpha s1-casein (A and B) and kappa-casein (A and B) was often found to be lower for milks containing either the B variant of alpha s1-casein or the A variant of kappa-casein. It was also found that these two variant caseins were associated with a lower kappa-casein. It was also found that these two variant caseins were associated with a lower kappa-casein content of the milks and the micelles, which is consistent with the lower solvation as kappa-casein is associated with smaller micelle size and greater solvation. The solvations also seemed to increase during the lactation period. It is possible that some of the other features of milk and its products that have been ascribed to the differences in functional character between the A and B variants of alpha s1-casein may be partly caused by the increased level of kappa-casein. The reason for the association of the A variant of alpha s1-casein with higher concentrations of kappa-casein (and micelle solvation) is not obvious but possibly the haplotype alpha s1-casein A, beta-casein A1, kappa-casein A contains a controlling sequence in the chromosomal DNA that enhances expression of the kappa-casein gene.

Detection of cow milk in goat milk by polyacrylamide gel electrophoresis

Pasteurized goat milk was adulterated with increasing proportions of cow milk and submitted to polyacrylamide gel electrophoresis. A frontal band, missing from the pattern of genuine goat milk and possessing the same electrophoretic mobility as bovine alpha S1-casein, was expressed. The area of this zone was directly proportional to the amount of cow milk added to the goat milk.

Comparative aspects of milk proteins

The current status of knowledge of the major proteins of milks of various species is evaluated. Most of the non-bovine milk proteins are homologous with the recognized families of those of Bos taurus, alpha S1-caseins, alpha S2-caseins, beta-caseins, kappa-caseins, beta-lactoglobulins, and alpha-lactalbumins, each family representing a separate genetic locus specific to the mammary gland. No prominent milk protein not homologous to one of these families has yet been discovered in milk of any species. Genetic polymorphism resulting from substitutions in the polypeptide chains and various degrees of post-translational phosphorylation, glycosylation, and proteolysis have been identified in milk proteins of several species. Total protein production ranges among species from about 0.5 to 10 g/d per kg0.75 maternal weight. Proportions of the several proteins vary greatly among species, but few accurate analytical data are available except for total casein and total whey protein contents.

The voluminosity of bovine casein micelles and some of its implications

A comparison of literature results on the voluminosity of casein micelles yields large differences, largely related to the method of determination. After applying some corrections, methods based on hydrodynamic radius yield a value of roughly 3.9 ml/g dry casein, other methods (microscopy, sediment volume) about 2.2. Roughly half of the discrepancy can be explained by the micelles being not perfectly spherical. To explain the remaining difference, it is assumed that the micelles are hairy, i.e. they have molecular chains protruding into the milk serum. The hairyness would increase with decreasing temperature and be largely removed by the action of rennet. It would cause some entropic repulsion between micelles at close approach.

Casein micelle size from elastic and quasi-elastic light scattering measurements

The average molecular weight, particle radius and size distribution of particles in skim milk from eight cows in mid-lactation have been measured by means of elastic and quasi-elastic light scattering techniques. The properties of sub-micellar casein particles in the milk of each cow were also studied. Particular attention has been given to the effects of particle size heterogeneity in the interpretation of results. The weight average molecular weight of the particles from different cows varied from 2.6-10(8) to 15-10(8) and the corresponding average particle radius varied between 90 and 130 nm. An unusual feature of these particles is their high water content, which was found to vary from 2.4 to 6.4 ml/g with a positive correlation between average particle density and average particle mass. Variations in particle water content can be most readily understood in terms of a gel-like casein micelle.