Properties of Acid Casein and Casein Micelles from Milk Destabilized by Frozen Storage

No difference was found in calcium sensitivity, electrophoretic and optical properties between acid caseins prepared from skimmilk before and after frozen storage (up to 180 days at ?7°C).

Destabilization of casein micelles can not be explained either by the reduction of solvation of the micelles or by the liberation of κ-casein from the micelles. However, when storage period was extended (about six months), splitting of a part of κ- and β-casein from the micelles to soluble form was observed, suggesting a drastic change of structure of the destabilized casein micelles.     

Synthesis of α-Adenosine 5′-Monophosphate

The distributions of protein, calcium and inorganic phosphate among casein micelles of skimmilk before and after frozen storage were investigated by gel filtration through Sephadex G-200 with 6.6 m urea solution as eluant.

The results showed that the native casein micelles were fractionated into two fractions. Fast eluting fraction contained large amount of calcium and inorganic phosphorus. Slow eluting fraction contained calcium but was essentially free of inorganic phosphorus. Frozen storage caused the increase of proportion of the fast eluting fraction accompanying the increase of calcium and inorganic phosphorus contents in it. It is suggested that the salt linkage newly formed during frozen storage causes the increase of proportion of the fast eluting fraction.     

Studies on the Changes of the Milk Casein by Various Treatments

A study has been made with the changes occurred in the milk casein during frozen storage. The flocculated protein formed during storage of frozen milk was resolved in 4 ~ 5 peaks in free-boundary electrophoretic pattern, using veronal buffer containing 7 m urea.

The flocculated protein was mainly casein. It may be considered that portions of casein have undergone some modification or denaturation during the destabilization process.

Significant increase has occurred in the relative proportion of β-casein in the casein remaining in supernatant after removal of flocculate, indicating that the partial fractionation of casein component occurred as a result of flocculation.     

Studies on Destabilization of Caseinate Complex during Frozen Storage of Milk

Unpasteurized skim milk was storaged in a frozen state at ?7°C or ?20°C for up to several months. There was no increase of non casein and non protein nitrogens, but a slight increase of free tyrosine and a slight decrease of alkaline phosphatase activity were detected when storage period was prolonged. Destabilization occurred solely in caseinate complex, but non micellar casein appeared to be stable.

The contents of calcium and inorganic phosphate in the caseinate complex separated by ultracentrifugation were increased appreciably after frozen storage. The viscosity characteristics of frozen storaged skim milk was also investigated.

Caseinate complex was ultracentrifugally separated from skim milk before and after frozen storage, and then lyophilized. Skim milk itself was also lyophilized before and after frozen storage. Dispersibility was examined on the reconstituted suspension of the lyophilized samples.

The lyophilized sample from frozen storaged milk was much less dispersible than the lyophilized control sample prepared before frozen storage. However, when lyophilized samples were once resolved with reagents such as urea and potassium oxalate and then dialyzed against fresh milk, stable micelle resulted in both samples prepared before and after frozen storage.

Some reduction of dispersibility occurred during lyophilization and subsequent storage in a dried state in the caseinate complex prepared before frozen storage. This reduction was small when skim milk was lyophilized and stored.     

Caseins are cross-linked through their ester phosphate groups by colloidal calcium phosphate

Artificial casein micelles were prepared by adding 30 mM calcium, 22 mM phosphate and 10 mM citrate to sodium caseinate solutions, and the content of the casein aggregates cross-linked by colloidal calcium phosphate was determined by high-performance gel chromatography on a TSK-GEL G4000SW column in the presence of 6 M urea. The content of the casein aggregates cross-linked by colloidal calcium phosphate in artificial whole casein micelles was 48% of total casein, and their relative casein composition determined by high-performance ion-exchange chromatography was 53.1% for alpha s1-casein, 15.8% for alpha s2-casein, 31.1% for beta-casein and 0% for kappa-casein. The order of cross-linking by colloidal calcium phosphate agreed with that of the ester phosphate content of casein constituents. The content of the casein aggregates cross-linked by colloidal calcium phosphate was higher in alpha s1-kappa-casein micelles than in beta-kappa-casein micelles. kappa- and gamma-caseins and dephosphorylated alpha s1-casein were not cross-linked by colloidal calcium phosphate. Although kappa-casein was not cross-linked, chemically phosphorylated kappa-casein, of which the average phosphate content was 8.5 per molecule, was cross-linked. It is concluded that caseins are cross-linked through their ester phosphate groups by colloidal calcium phosphate.     

Biocompatible micro-gel particles from cross-linked casein micelles

The stability of internally cross-linked casein micelles against disruption by urea (which disrupts hydrogen bonds and hydrophobic interactions) and trisodium citrate (which sequesters micellar calcium phosphate) was investigated. Addition of urea (0-6 mol L-1) and/or citrate (0-50 mmol L-1) progressively reduced the turbidity of a suspension of casein micelles cross-linked by transglutaminase and increased particle size (determined by dynamic and static light scattering and small-angle neutron scattering), which was attributed to swelling of the micelles. Furthermore, model calculations, assuming a completely stable casein network, were performed to describe the decreases in turbidity on addition of urea and citrate. Measured and described turbidity values are in agreement, indicating that cross-linking of casein micelles with transglutaminase results in a covalently bound protein network, which is entirely stable to disruption by urea and/or citrate. This may offer potential applications for the use of cross-linked casein micelles as biocompatible protein micro-gel particles.     

Reconstitution of Human Casein Micelle and Its Properties

Human casein micelles were reconstituted from isolated κ- and β-caseins and calcium ions. Micelle formation was recognized in the presence of calcium chloride even at the low concentration of 5mM. At pH levels ranging from 5.5 to 8.0, the re-formed micelles were quite stable so that precipitation of β-casein was not observed. The large micelles were constituted by a higher ratio of β-casein to κ-casein (16:1 by weight) than the small micelles (3: 1). The κ-casein in the small micelles contained carbohydrates to about 43% (w/w) in the molecule, whereas that in the large micelles was only about 25%. When the casein micelles were re-formed from κ-easein and fractionated β-casein components, the extent of phosphorylation of the β-casein component was found to influence the micelle formation; i.e., the β-casein component with no phosphate (the 0-P form) was disadvantageous to form micelles, but the component with 5 phosphates (the 5-P form) formed micelles most easily.     

Rheological properties of highly cross-linked waxy maize starch in aqueous suspensions of skim milk components. Effects of the concentration of starch and skim milk components

The storage modulus of various concentrations of highly cross-linked waxy maize starch was measured in water, a salt solution, a salt solution with lactose, whey, and skim milk. The influence of the skim milk components was deduced from the differences in moduli between these systems. Salts appeared to have no direct influence on the modulus of the starch. Lactose increased the storage modulus, probably due to an increase in the particle rigidity of the swollen starch granules. The whey proteins had only a small effect on the modulus of the starch. However, the concentration of these proteins was very low. Casein micelles increased the modulus of the starch because the swollen starch granules are not accessible to the casein. A model based on the mutual exclusion of swollen starch granules and milk proteins was applied to predict the storage modulus of starch-milk systems as a weighed sum of the moduli of the starch and protein phases. Weight factors were derived from the hydration capacity of starch granules and casein micelles. Although the behaviour of aqueous starch-skim milk systems fell within the boundaries imposed by the model, the predictive power of this model was rather poor.     

Darwinian transformation of a 'scarcely nutritious fluid' into milk

In an early challenge to an aspect of Darwin's theory of natural selection, Jackson Mivart contended that milk could not have evolved 'from a scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland'. The evolutionary change from a gland secretion to milk involves an increase in calcium and protein concentrations by up to 100- and 1000-fold, respectively. Even so, the challenge, we suggest, is not just a problem of scale. An increase in the concentrations of calcium and phosphate brings an increased risk of calcification of the secretory gland because calcium phosphate is highly insoluble. In addition, two of the four constituent milk casein proteins (κ and α(S2)) aggregate to produce toxic amyloid fibrils. It is proposed that both problems were solved through the cosecretion of ancestral β- and κ-caseins to form a stable amorphous aggregate of both proteins with sequestered amorphous calcium phosphate, that is, a primordial casein micelle. Evolutionarily, a gradual increase in the concentration of casein micelles could therefore produce progressively more nutritious fluids for the neonate without endangering the reproductive potential of the mother.     

Change of Salt Distribution in Milk during Frozen Storage and Its Partial Reversion after Thawing

The change of salt distribution of skimmilk during frozen storage at ?7°C and its reversion after thawing were investigated. Losses of ultrafiltrable calcium, inorganic phosphate and citrate were indicated. A part of insolubilized calcium and citrate reversed to a soluble form after thawing, but insolubilized phosphate hardly reversed when the storage period was prolonged. Comparison on the changes in skimmilk and in its dialyzate suggested that the insolubilization of salt constituents in milk due to frozen storage involved the interaction of these constituents with calcium caseinate phosphate complex.     

Alpha casein micelles show not only molecular chaperone-like aggregation inhibition properties but also protein refolding activity from the denatured state

Casein micelles are a major component of milk proteins. It is well known that casein micelles show chaperone-like activity such as inhibition of protein aggregation and stabilization of proteins. In this study, it was revealed that casein micelles also possess a high refolding activity for denatured proteins. A buffer containing caseins exhibited higher refolding activity for denatured bovine carbonic anhydrase than buffers including other proteins. In particular, a buffer containing α-casein showed about a twofold higher refolding activity compared with absence of α-casein. Casein properties of surface hydrophobicity, a flexible structure and assembly formation are thought to contribute to this high refolding activity. Our results indicate that casein micelles stabilize milk proteins by both chaperone-like activity and refolding properties.     

Preparation and characterization of milk calcium salts by using casein phosphopeptide

Milk calcium salt solution was prepared by the following procedures using casein phosphopeptides (CPP). To CPP solution, 1 M citric acid, 1 M CaCl2 and 1 M K2HPO4 were added with stirring, while adjusting the pH to 6.7. The prepared solution was left for 12 hr at 25 degrees C and then used for subsequent experiments, or lyophilized. The concentrations of organic phosphate of CPP, calcium, inorganic phosphate, and citrate in the typical milk salt solution were 7, 30, 22, and 10 mM, respectively, which were close to those in bovine milk. The lyophilized sample was easily dissolved in water. No crystal structure of hydroxyapatite was shown in the lyophilized milk calcium salts by X-ray diffraction analysis, although the pattern of KCl crystal was observed. The X-ray diffraction pattern of commercial whey mineral, which was prepared by precipitation at alkaline pH from rennet whey, was similar to that of hydroxyapatite. It was confirmed by high-performance gel chromatographic analysis that the form of calcium phosphate in the milk calcium salts was similar to that of casein micelles.     

Separation and characterization of 7 and 9 S forms of rat liver aryl hydrocarbon receptor

Two forms of rat liver aryl hydrocarbon receptor were separated by chromatography on DEAE-cellulose in the presence of molybdate. After labeling for 2 h at 0 degrees C, the receptor separated on the DEAE column into a flow-through peak (peak I) and a peak eluting at 80 mM KCl (peak II). It had been reported previously that exposure to high salt in the presence of molybdate caused the appearance of both 9 and 5-6 S receptor forms. After confirming this, I examined the relationship of the peak I and peak II receptors to these receptor forms. In high salt buffer containing molybdate, the peak I receptor sedimented in the 5-6 S region and the peak II receptor at 9 S. High salt buffer lacking molybdate converted both peak I and peak II receptors to forms sedimenting in the 5-6 S region. In low salt buffer containing molybdate, the peak I receptor sedimented at slightly more than 7 S and the peak II receptor at 9-10 S. Thus, the peak II receptor could be stabilized by molybdate as a 9 S form, and the peak I receptor was converted by high salt from a 7 to a 5-6 S form, despite the presence of molybdate. Most of the peak I receptor bound to a DNA-cellulose column and was eluted by high salt. The peak II receptor showed very little DNA binding.     

Crosslinking of casein by microbial transglutaminase and its resulting influence on the stability of micelle structure

The influence of enzymatic crosslinking by microbial transglutaminase (mTG) on the stability of casein micelles of ultrahigh temperature (UHT)-treated milk in the presence of EDTA (0-0.45 mM) or ethanol (0-74 vol%) as well as under high hydrostatic pressures up to 400 MPa was investigated. Disintegration of micelles and changes in micelle size were monitored by the measurement of turbidity as well as by dynamic light scattering. The results show that the incubation of UHTtreated milk with mTG resulted in an improved micelle stability toward disintegration on addition of EDTA, ethanol, or pressure treatment. Intramicellar formed isopetides significantly enhanced the stability of casein micelles. It is supposed that net-like crosslinks are formed within the external region of the micelles and they adopt the stabilizing role of colloidal calcium phosphate within the micelles, thus making the micelles less contestable for disrupting influences.     

Water interactions with varying molecular states of bovine casein: 2H NMR relaxation studies

The caseins occur in milk as spherical colloidal complexes of protein and salts with an average diameter of 1200 A, the casein micelles. Removal of Ca2+ is thought to result in their dissociation into smaller protein complexes stabilized by hydrophobic interactions and called submicelles. Whether these submicelles actually occur within the micelles as discrete particles interconnected by calcium phosphate salt bridges has been the subject of much controversy. A variety of physical measurements have shown that casein micelles contain an inordinately high amount of trapped water (2 to 7 g H2O/g protein). With this in mind it was of interest to determine if NMR relaxation measurements could detect the presence of this trapped water within the micelles, and to evaluate whether it is a continuum with picosecond correlation times or is associated in part with discrete submicellar structures with nanosecond motions. For this purpose the variations in 2H NMR longitudinal and transverse relaxation rates of water with protein concentration were determined for bovine casein at various temperatures, under both submicellar and micellar conditions. D2O was used instead of H2O to eliminate cross-relaxation effects. From the protein concentration dependence of the relaxation rates, the second virial coefficient of the protein was obtained by nonlinear regression analysis. Using either an isotropic tumbling or an intermediate asymmetry model, degrees of hydration, v, and correlation times, tau c, were calculated for the caseins; from the latter parameter the Stokes radius, r, was obtained. Next, estimates of molecular weights were obtained from r and the partial specific volume. Values were in the range of those published from other methodologies for the submicelles. Temperature dependences of the hydration and Stokes radius of the casein submicelles were consistent with the hypothesis that hydrophobic interactions represent the predominant forces responsible for the aggregation leading to a submicellar structure. The same temperature dependence of r and v was found for casein under micellar conditions; here, the absolute values of both the Stokes radii and hydrations were significantly greater than those obtained under submicellar conditions, even though tau c values corresponding to the great size of the entire micelle would result in relaxation rates too fast to be observed by these NMR measurements. The existence of a substantial amount of trapped water within the casein micelle is, therefore, corroborated, and the concept that this water is in part associated with submicelles of nanosecond motion is supported by the results of this study.     

Effect of calcium concentration on the structure of casein micelles in thin films

The structure of thin casein films prepared with spin-coating is investigated as a function of the calcium concentration. Grazing incidence small-angle x-ray scattering and atomic force microscopy are used to probe the micelle structure. For comparison, the corresponding casein solutions are investigated with dynamic light-scattering experiments. In the thin films with added calcium three types of casein structures, aggregates, micelles, and mini-micelles, are observed in coexistence with atomic force microscopy and grazing incidence small-angle x-ray scattering. With increasing calcium concentration, the size of the aggregates strongly increases, while the size of micelles slightly decreases and the size of the mini-micelles increases. This effect is explained in the framework of the particle-stabilizing properties of the hairy layer of kappa-casein surrounding the casein micelles.     

Photolysis of Aryl Glycosides with Ultraviolet Irradiation

Bovine casein components (α_sl-, β-, and κ-caseins) were chemically phosphorylated and the properties of the modified components were compared with those of the native to clarify the function of the intrinsic phosphate groups of casein components in casein micelle formation. The calcium binding ability of casein components increased after chemical phosphorylation. The concentrations of calcium chloride required to precipitate modified α_sl- and β-caseins were higher than those for native components. However, phosphorylation of α_sl- and β-caseins did not affect their properties of forming micelles through interaction with κ-casein. The stabilizing ability of κ-casein for α_sl- and β caseins was impaired by its phosphorylation, but the stability was recovered by treating phosphorylated κ-casein with phosphoprotein phosphatase. The results show that the phosphate content of κ-casein must be low to form a stable casein micelle. The results also explain why the specific phosphorylation of casein components in the mammary gland is required.     

Casein aggregates built step-by-step on charged polyelectrolyte film surfaces are calcium phosphate-cemented

The possible mechanism of casein aggregation and micelle buildup was studied in a new approach by letting α-casein adsorb from low concentration (0.1 mg·ml(-1)) solutions onto the charged surfaces of polyelectrolyte films. It was found that α-casein could adsorb onto both positively and negatively charged surfaces. However, only when its negative phosphoseryl clusters remained free, i.e. when it adsorbed onto a negative surface, could calcium phosphate (CaP) nanoclusters bind to the casein molecules. Once the CaP clusters were in place, step-by-step building of multilayered casein architectures became possible. The presence of CaP was essential; neither Ca(2+) nor phosphate could alone facilitate casein aggregation. Thus, it seems that CaP is the organizing motive in the casein micelle formation. Atomic force microscopy revealed that even a single adsorbed casein layer was composed of very small (in the range of tens of nanometers) spherical forms. The stiffness of the adsorbed casein layer largely increased in the presence of CaP. On this basis, we can imagine that casein micelles emerge according to the following scheme. The amphipathic casein monomers aggregate into oligomers via hydrophobic interactions even in the absence of CaP. Full scale, CaP-carrying micelles could materialize by interlocking these casein oligomers with CaP nanoclusters. Such a mechanism would not contradict former experimental results and could offer a synthesis between the submicelle and the block copolymer models of casein micelles.     

Casein Microparticles from Blend Films Forming Casein/α-Tocopherol Emulsion Droplets in Solution

Microparticles continue to receive widespread attention from food professionals because of their potential use as carriers of bioactive substances. This study demonstrates a novel method to prepare casein microparticles, which co-assemble with α-tocopherol (αT) into emulsion droplets. When the particles were extracted from the pectin matrix via enzymatic degradation, they remained stable in a buffer solution for at least 3 weeks. Optical microscopy showed that the size distribution of the microparticles is between 5 μm and 50 μm, which is in accordance with previous observations in blend films. High-performance liquid chromatography revealed that the amounts of α_S1- and κ-casein in the microparticles are significantly higher than those in native casein micelles. Confocal Raman microscopy showed that in the presence of α-tocopherol, the microparticles assemble into emulsion droplets, with phenol in the core and casein in the shell. Herein, we demonstrate a new method to form casein-based emulsion droplets for potential use as carriers of bioactive substances.     

Structure of bovine casein oligomers

We have characterized the size, molecular weight, and composition of the oligomeric particles produced by dialysis of bovine casein micelles against solutions lacking calcium ion. The particles were stabilized against further dissociation after dialysis by glutaraldehyde fixation. The progress of the dissociation was monitored by Biogel A-15 gel permeation chromatography, sucrose density gradient ultracentrifugation, inelastic laser light scattering, sedimentation velocity and equilibrium, and urea starch gel electrophoresis. The casein oligomers isolated after dialysis against either 0.5 m NaCl/SMUF A/azide or calcium-free SMUF/ azide have a hydrodynamic radius of about 5.5 nm and a molecular weight of about 95,000 corresponding to roughly four casein monomers (SMUF, simulated milk ultrafiltrate). The oligomers are highly hydrated and contain one-third of the calcium ion found in native micelles. During the course of dialysis, the micelles gradually break down into a broad distribution of intermediatesized particles and then into the oligomers described above.