Effects of colloidal calcium phosphate content and free calcium ion concentration in the milk serum on the dissociation of bovine casein micelles

Effects of cations and anions on the rate of the acidic coagulation of casein micelles: the possible roles of different forces

Raw skim milk was diluted 1000-fold using distilled water or various salt solutions as specified. Smooth, hyperbolic profiles of coagulation ratev.pH for casein were calculated from recordings of turbidity (400 nm) with time. The effects of pH, cation type, anion type and cleavage ofk−casein by chymosin (EC 3.4.23.4) were determined. The maximum of pH-coagulation rate profiles decreased by 63, 85 and 94% when the skim milk diluent was changed from water to salt solutions of NaCl (100 mM), CaCl2(50 mM) or MgCl2(50 mM). The maximum of the pH–coagulation rate profile was 15 times greater when the Ca salt was changed from CaCl2to Ca(SCN)2(50 mM). The highest pH at which casein coagulation occurred increased from 4·45 to > 6·0 when Cu2+(1 mM) was included with casein micelles dispersed in CaCl2solution (50 mM). The addition of chymosin to casein micelles suspended in CaCl2solution (70 mM) eliminated the inhibition of casein coagulation by Ca2+at pH 4·5. It is proposed that ions such as Mg2+, Ca2+, and Na+, which generally associate with casein phosphate and carboxylate groups, increased the H+concentration required to initiate the coagulation of casein, because H+must displace bound Ca2+, Mg2+or Na+to reduce repulsive hydration forces between casein micelles, allowing attractive hydration forces (e.g. hydrophobic phenomena) to cause casein coagulation. Furthermore, it is proposed that ions such as Cl−, Br−,and SCN−bind to lysine, arginine and histidine groups and thereby decrease repulsive hydration forces between cationic casein micelles.

Dissociation during dialysis of casein aggregates cross-linked by colloidal calcium phosphate in bovine casein micelles

Casein micelles separated by ultracentrifugation of raw skim milk were dispersed at a casein concentration of 2·5% in simulated milk ultrafiltrate and dialysed against 10 mM-imidazole buffer (pH 7·0) at 5 °C. The amounts of colloidal Ca and inorganic P decreased from 77 to 11 mg and from 31 to 2 mg respectively in 100 ml during 72 h of dialysis. Micellar casein content was reduced to 43 and 11% after 48 and 72 h of dialysis respectively. In high-performance gel chromatography of casein micelles in the presence of 6 M-urea, fraction 1, consisting of the casein aggregates cross-linked by colloidal Ca phosphate (CCP) decreased during dialysis and the retention time of the peak of fraction 1 was prolonged, suggesting that the cross-linkage between CCP and casein molecules was disrupted. The dissociation rates of the individual casein constituents from the casein aggregates cross-linked by CCP during dialysis were in the order β-> αs1 - > αs2-casein. The higher the ester phosphate content, the slower was the dissociation rate of the individual casein constituent. It is suggested that the strength of interaction between CCP and casein molecules depends on the ester phosphate content.

Composition and structure of micellar calcium phosphate

Micellar calcium phosphate has the chemical composition and physicochemical properties that are consistent with it being a complex of the phosphate centres of casein with an acidic amorphous calcium phosphate. Similar acidic amorphous calcium phosphates have been prepared in the laboratory and for these, as well as for micellar calcium phosphate, the most appropriate crystalline model compound from which the short-range structure may be derived is brushite, CaHPO4.2H2O. The predicted secondary structures around sites of phosphorylation in the Ca2+-sensitive caseins often comprise an α-hclix-loop-α-helix motif with the sites of phosphorylation in the loop region. This motif may be important in linking the colloidal calcium phosphate with casein in native casein micelles.

Effects of heat treatment and acidification on the dissociation of bovine casein micelles

The effects of heat treatment and subsequent acidification of milk on the distribution of proteins, Ca and Pi, between the serum and micellar phases were examined using ultracentrifugation. After heating milk at 85 °C for 10 min, and storing for 22 h at 4, 20 or 30 °C, there was a marked increase in the concentration of κ-casein in the serum. At 4 and 20 °C there was also slightly more β-casein in the serum from heat-treated milk than in that from the corresponding raw milk. The whey proteins were extensively denatured, and were almost equally distributed between the supernatants and micellar pellets. After storage for 22 h the distribution of Ca and Pibetween soluble and colloidal phases in heat-treated milk was similar to that in raw milk. After acidifying heat-treated milk by the addition of glucono-δ-lactone and storing for 22 h at 4, 20 or 30 °C there was progressive solubilization of colloidal calcium phosphate with decreasing pH, and at pH 5·0 almost all of the Ca and Piwas present in the serum. At 20 °C, and even more so at 4 °C, serum concentrations of the individual caseins increased considerably with decreasing pH, reaching maximum levels of about 25 and 40% of the total casein at pH 5·7 and 5·5 respectively, and then decreasing rapidly at lower pH. Compared with raw milk, maximum dissociation in heat-treated milks stored at 4 and 20 °C occurred at higher pH, and the overall levels of dissociation of individual caseins from the micelles were lower. At 30 °C, the concentrations of individual caseins in the serum of heat-treated milk decreased steadily as the pH was reduced, and did not show the slight increase found previously for raw milk. The role of the denatured whey proteins in interacting with κ-casein and in promoting aggregation of the micelles on acidification is discussed.

Structural changes in sweetened condensed milk during storage: an electron microscopy study

This work aimed to relate changes in rheological behaviour with structural details of different sweetened condensed milk samples as well as to clarify the role of additives in delaying gelation. To study the process of gelation, rheological techniques were applied to fresh and stored condensed milk samples for times and at temperatures known to favour gelation (45 d at 37 °C). Electron microscopy techniques were used to relate the rheological changes with observed structural details. Examination by electron microscopy showed that micellar aggregation and association of casein micelles with fat globules increased during storage. The whey proteins appeared to play an important role in the thickening process.

pH-Induced dissociation of bovine casein micelles. I. Analysis of liberated caseins

The dissociation of caseins of different types from casein micelles in milk, acidified to different pH values in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C, has been studied. In contrast to a number of previous findings, it was shown that caseins of all types were dissociated from the micelles, although in all cases β-casein was in highest concentration. The amounts and proportions of all of the caseins were found to be pH- and temperature-dependent, especially the former. Studies of the proportions of the different caseins liberated suggested that, at a defined temperature, the proportions of κ;- and αs2-caseins were independent of pH, while the proportions of β- and αsl-caseins were variable, changes in one being compensated by changes in the other. The manner in which the proportions of the αsl-casein and β-caseins changes with pH was found to be temperature-dependent.

Effects of the environmental factors on the casein micelle structure studied by cryo transmission electron microscopy and small-angle x-ray scattering/ultrasmall-angle x-ray scattering

Casein micelles are colloidal protein-calcium-transport complexes whose structure has not been unequivocally elucidated. This study used small-angle x-ray scattering (SAXS) and ultrasmall angle x-ray scattering (USAXS) as well as cryo transmission electron microscopy (cryo-TEM) to provide fine structural details on their structure. Cryo-TEM observations of native casein micelles fractionated by differential centrifugation showed that colloidal calcium phosphate appeared as nanoclusters with a diameter of about 2.5 nm. They were uniformly distributed in a homogeneous tangled web of caseins and were primarily responsible for the intensity distribution in the SAXS profiles at the highest q vectors corresponding to the internal structure of the casein micelles. A specific demineralization of casein micelles by decreasing the pH from 6.7 to 5.2 resulted in a reduced granular aspect of the micelles observed by cryo-TEM and the existence of a characteristic point of inflection in SAXS profiles. This supports the hypothesis that the smaller substructures detected by SAXS are colloidal calcium phosphate nanoclusters rather than putative submicelles.

Altering Renneting pH Changes Microstructure, Cell Distribution, and Lysis of Lactococcus lactis AM2 in Cheese Made from Ultrafiltered Milk

The objective of this study was to investigate the lysis of a highly autolytic strain of Lactococcus lactis ssp. cremoris AM2 in a model cheese made from concentrated ultrafiltered milk. From the same initial ultrafiltered retentate inoculated with L. lactis AM2, 5 cheeses were made by the addition of rennet at different pH values (6.6, 6.2, 5.8, 5.4, and 5.2). Lysis was monitored by measurement of the release of lactate dehydrogenase, an intracellular marker enzyme, and by immunodetection of intracellular proteins with species-specific antibodies. Confocal scanning laser microscopy (CSLM) was used to investigate the cheese microstructure by staining for protein and fat. Dual staining with a bacterial viability kit with CSLM was performed to reveal the integrity and localization of the bacterial cells. Levels of soluble calcium significantly increased when the pH at which the rennet was added decreased. In cheese renneted at pH 6.6, CSLM revealed an open porous structure containing a dense protein network with fat globules of different sizes distributed in the aqueous phase. In cheese renneted at pH 5.2, the protein network was homogeneous, with a less dense protein network, and an even distribution of fat globules. On d 1, bacterial cells were organized into colonies in cheese renneted at pH 6.6, whereas in cheeses renneted at pH 5.2, bacteria were evenly dispersed as single cells throughout the protein network. Lysis was detected on d 1 in cheeses renneted at high pH values and continued to increase throughout ripening, whereas induction of lysis was delayed in cheeses renneted at lower pH values until the end of ripening. This study demonstrates that alterations in the microstructure of the cheese and the distribution of cells play a role in lysis induction of L. lactis AM2.

Dynamic mechanical properties of suspensions of micellar casein particles

Small micellar casein particles, so-called submicelles, were obtained by removing colloidal calcium phosphate from native casein by adding sodium polyphosphate. Aqueous submicelle suspensions were characterized using light scattering and rheology as a function of concentration and temperature. The casein submicelles behave like soft spheres that jam at a critical concentration (C(c)) of about 100 g L(-1). The viscosity does not diverge at C(c), but increases sharply, similarly to that of multiarm star polymers. C(c) increases weakly with increasing temperature, which leads to a strong decrease of the viscosity close to and above C(c). Concentrated submicelle suspensions show strong shear-thinning above a critical shear rate and the shear stress becomes independent of the shear rate. The critical shear rates at different temperatures and concentrations are inversely proportional to the zero-shear viscosity. At much higher shear rates, the shear stress fluctuates strongly in time indicating inhomogeneous flow. The frequency dependence of casein submicelle suspensions is characterized by elastic behavior at high frequencies (concentrations) and viscous behavior at low frequencies (concentrations).

Aggregation and gelation of micellar casein particles

Micellar casein particles (submicelles) are formed by removing calcium phosphate from native casein. The submicelles aggregate and eventually form a gel with a rate that increases strongly with increasing temperature and casein concentration. At low casein concentrations the gel is very weak and collapses under its own weight so that a precipitate is formed. The structure of the aggregates is studied using light scattering and cryo-electron microscopy. It is found that the aggregates have a self-similar structure with fractal dimension 2. The viscoelastic properties of the gel are studied by frequency scans of the loss and storage moduli during the gelation process. The bonds between the submicelles probably involve calcium phosphate complexes.

Application of transmission diffusing wave spectroscopy to the study of gelation of milk by acidification and rennet

Transmission diffusing wave spectroscopy has been used to study and compare three milk gelling systems (acid gelation of heated and unheated milks and rennet coagulation of unheated milk). In all cases, DWS was able to demonstrate the point of gelation as indicated by a rapid increase in particle size, as well as the small decreases in casein micelle radius attributed to the collapse or removal of the hairy kappa-casein layer. More importantly, the photon transport mean free path (l(*)) was measured. This parameter is unique to transmission DWS and can potentially give information about developing microstructures and the mechanical properties between different types of gels. The values of l(*) changed during the gelation processes, and these changes were manifested earlier than any change in particle aggregation or rheology of the systems. All three different gelling systems showed different changes in l(*) with time, showing the development of different interactions as the acidification or renneting reactions proceeded. Although a full analysis of the l(*) parameter and its changes cannot be made, it is concluded that they can provide important information on the pre-gelation states of aggregating systems.

Effect of pH and ionic strength on competitive protein adsorption to air/water interfaces in aqueous foams made with mixed milk proteins

Quantitative analysis of competitive milk protein adsorption to air/water interfaces in aqueous foam was performed by capillary electrophoresis (CE). Foams were made by whipping protein solutions, in which skim milk powder (SMP) and whey protein isolate (WPI) were mixed at 0.5% protein in different proportions at different pH values and NaCl concentrations. Preferential adsorption of beta-casein into foam phases occurred under most solution conditions, if partial dissociation of the casein micelles had occurred. Preferential adsorption of beta-casein was not observed with added Ca2+, due to the re-association of casein micelles. Enrichment of caseins into the foam phase was more apparent than that of whey proteins. The foamability of SMP demonstrated a continuous improvement due to the gradually increasing dissociation of casein micelles when the concentration of NaCl increased from 0 to 0.8 M. The foamability of WPI increased when NaCl concentration rose from 0 to 0.1 M, and decreased with further increase in NaCl concentration. NaCl at low concentration (I < or = 0.4) did not show a significant effect on the competitive adsorption among milk proteins, indicating that electrostatic interactions do not play a key role in competitive adsorption. NaCl at higher concentration, e.g., 0.6 M, caused less whey protein to be adsorbed to the air/water interfaces. The whippability of WPI was highest at pH 4.5 and lowest at pH 3, and that of SMP was the opposite. The proportions of beta-lactoglobulin and alpha-lactalbumin in the foam phase were lower at acidic pH and higher at basic pH, compared with that at natural pH of WPI.

Production of a Novel Ingredient from Buttermilk

The presence of material derived from the milk fat globule membrane (MFGM) makes buttermilk (the byproduct of butter making) distinct from any other dairy product. Membrane filtration of commercial buttermilk was carried out to obtain isolates rich in MFGM material. The separation of MFGM from the skim milk proteins present in commercial buttermilk was carried out by the addition of sodium citrate followed by microfiltration through a membrane of 0.1-microm nominal pore size. The sodium citrate caused the dissociation of casein micelles and allowed permeation of a large proportion of the skim-milk derived proteins through the membrane. This process successfully concentrated MFGM material in the retentate, and demonstrated that membrane filtration can be employed to produce MFGM fractions from commercial buttermilk. The utilization of MFGM isolates from buttermilk is of increasing importance in light of recent studies suggesting the role of phospholipids in many health-related functions: buttermilk is an untapped resource of these functional components.

The use of lithium chloride to study human milk micelles

Various methods have been used to study the dissociation of milk micelles in attempts to determine their structure and the interactions that stabilize them. These include the addition of urea, cooling to alter hydrophobic bonding, the addition of EDTA to sequester calcium, and changes in pH to alter molecular charge. For this study, the mild chaotropic agent LiCl was added to human milk micelles, and measurements were made on the relative percentages of the six different phosphorylation levels of beta-casein (CN) at various LiCl concentrations for different lengths of time and at different temperatures. Added LiCl had little effect at 37 degrees C but caused maximal dissociation, mainly of the beta-CN species with higher phosphorylation levels, at 23 degrees C and 4 degrees C between 1 and 2 M concentration. Comparison was made with 2-M additions of NaCl, MgCl2, and KCl at 4 degrees C, with LiCl showing the only appreciable change. The results suggest that Li+ may displace Ca2+ in protein-Ca2+-protein or protein-colloidal calcium phos+ phate-protein salt bridges and that the nonphosphorylated form of human beta-CN may change its conformation and mode of interaction upon phosphorylation. Lithium chloride may be useful to study the dissociation of the different CN in bovine milk micelles.

Effect of salt addition on the micellar composition of milk subjected to pH reversible CO2 acidification

Response surface methodology was used to investigate the effect of salt supplementation on the micellar composition of reconstituted skim milk subjected to acidification by CO2 pressure to pH 5.8, followed by depressurization under vacuum. Using a Doehlert design, calcium and phosphate were added to skim milk in the range of 0 to 25 mmol/kg and 0 to 16 mmol/kg of milk, respectively, and the pH was adjusted to 6.65 +/- 0.02. After carbonation, the milk sample was depressurized, and the pH returned to its initial value without modification of the ionic strength. Micellar composition was assessed by the concentration of micellar Ca, P, Mg, and protein, and the buffering properties of milk. The second order polynomial models satisfactorily predicted the effect of salt supplementation on the micellar composition (R2adj > 0.75). Added calcium was the most determinant factor, and favored the removal of Ca, P, Mg, and proteins from the soluble phase to the micellar phase when this addition was less than 17.5 mmol/kg of milk. Above this concentration, only the concentration of micellar Ca increased. The buffering response surface showed that the amount of micellar calcium phosphate increased to a maximum upon addition of 17.5 mmol of Ca/kg. By comparison with a control sample (supplemented but untreated skim milk), changes were essentially due to salt supplementation and not to the CO2 treatment. We suggest that Ca formed micellar calcium phosphate when added at a concentration less than 17.5 mmol/kg; whereas above this concentration, Ca bound directly to micellar proteins.

Structures and functionalities of milk proteins

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.

Behavior of calcium and phosphate in artificial casein micelles

This study investigated the behavior of Ca and phosphate in artificial casein micelles using 45Ca and 32P by means of UF, equilibrium dialysis, and gel permeation HPLC in the presence of 6 M urea. Artificial casein micelles were prepared at a casein concentration of 2.5% with 30 to 40 mM Ca, 22 to 27 mM phosphate, and 10 mM citrate using 45Ca or 32P. About 75 and 65% of colloidal 45Ca and 32P, respectively, in artificial casein micelles that were formed in the presence of 30 mM Ca and 22 mM phosphate were exchanged after dialysis of artificial casein micelles against a simulated milk ultrafiltrate at 4 degrees C for 72 h. The percentages of 45Ca and 32P in the fraction of casein aggregates that were crosslinked by micellar Ca phosphate were 26.9 and 27.6%, which decreased to about 10% after equilibrium dialysis against a simulated milk ultrafiltrate at 4 degrees C for 72 h. The proportion of exchanged colloidal 45Ca and 32P in artificial casein micelles during dialysis for 72 h were higher at 25 degrees C than at 4 degrees C, suggesting that the exchange of Ca and phosphate between the diffusible and colloidal phases depended on temperature. The results suggest that the exchange of Ca and phosphate between diffusible and colloidal phases proceeds slowly, and a portion of the Ca and phosphate in the artificial casein micelles is difficult to exchange.

Phosphopeptides interacting with colloidal calcium phosphate isolated by tryptic hydrolysis of bovine casein micelles

After extended tryptic hydrolysis of large bovine casein micelles, a mineral-rich peptide fraction was recovered by ultracentrifugation. Its mineral part contained 72% of the colloidal Ca and 49% of the colloidal P1 originally present in the native micelle. Colloidal nitrogenous components were also present, amounting to 27% of the original N content. They contained most of the phosphopeptides and 82% of the micellar phosphoseryl residues. These tryptic peptides were characterized by reversed-phase HPLC on-line electrospray ion source-mass spectrometry analysis. Among the peptides produced 14 phosphopeptides were identified: alpha s2-CN(1-24), alpha s2-CN(1-21), alpha s1-CN(43-79), alpha s1-CN(35-79)7P, alpha s1-CN(35-79)8P, alpha s1-CN(37-79), alpha s1-CN(104-119), alpha s1-CN(104-124), beta-CN(1-25), beta-CN(1-28), beta-CN(1-29), beta-CN(30-97), beta-CN(33-97) and beta-CN(29-97). The proportion of the phosphopeptides interacting with colloidal calcium phosphate was correlated with their relative content of phosphoserine residues, since phosphopeptides containing more than four phosphoserine residues were consistently present within this fraction. It also appeared that other types of peptides, some of them hydrophobic in nature, were also partly or completely present within the colloidal fraction, including alpha s1-CN(91-100), alpha s1-CN(152-193), alpha s1-CN(23-34), alpha s1-CN(125-193), alpha s1-CN(125-199), beta-CN(177-209). beta-CN(184-209), beta-CN(114-169) and beta-CN(108-169). Their possible involvement in the micellar backbone is discussed.

Interrelationships of the concentrations of some ionic constituents of human milk and comparison with cow and goat milks

1. Milk and milk ultrafiltrate samples from four women (Homo sapiens) during the first 16 weeks of lactation were analysed for the main salt constituents, protein and lactose and the interrelationships of these constituents were determined. 2. A good positive correlation was observed between ultrafiltrate calcium and citrate concentrations, as previously observed for cow (Bos taurus) and goat (Capra hircus) milk but the expected negative correlation of lactose with ultrafiltrate salts was poor. 3. An invariant ion activity product for milk calcium phosphate was found for a dicalcium phosphate stoichiometry but the milk serum was less supersaturated with respect to calcium phosphate and citrate salts than ruminant milks. 4. The observed interrelationships are discussed in relation to the constraints on human milk composition arising from a fixed osmotic pressure, overall charge neutrality and saturation with respect to calcium phosphate.