Composition Of Casein Micelles Research Articles

Quantitative multivalent binding model of the structure, size distribution and composition of the casein micelles of cow milk

Compatible theories describing the size distribution of milk casein micelles, the partition of milk salts and the formation of calcium phosphate nanocluster complexes are combined to provide the first quantitative model of the cow casein micelle. Complexes of caseins with calcium phosphate nanoclusters associate with each other and the remaining (free) caseins to form a stable, polydisperse distribution of micelle sizes. The micelles in the distribution have a highly hydrated coat-core structure in which the calcium phosphate nanoclusters, and caseins bound directly to them, occur only in the core whereas the free caseins are found in both the coat and core. The theory is used to describe the structure, substructure, surface potential, size distribution, and the salt and protein composition of casein micelles in a standard cow milk. Multivalent casein–casein interactions through short, linear, amino acid sequence motifs determine many aspects of casein micelle formation and stability.

Effect of dietary crude protein degradability and corn processing on lactation performance and milk protein composition and stability

The present study aimed to evaluate the effect of crude protein degradability and corn processing on lactation performance, milk protein composition, milk ethanol stability (MES), heat coagulation time (HCT) at 140°C, and the efficiency of N utilization for dairy cows. Twenty Holstein cows with an average of 162 ± 70 d in milk, 666 ± 7 kg of body weight, and 36 ± 7.8 kg/d of milk yield (MY) were distributed in a Latin square design with 5 contemporaneous balanced squares, 4 periods of 21 d, and 4 treatments (factorial arrangement 2 × 2). Treatment factor 1 was corn processing [ground (GC) or steam-flaked corn (SFC)] and factor 2 was crude protein (CP) degradability (high = 10.7% rumen-degradable protein and 5.1% rumen-undegradable protein; low = 9.5% rumen-degradable protein and 6.3% rumen-undegradable protein; dry matter basis). A significant interaction was observed between CP degradability and corn processing on dry matter intake (DMI). When cows were fed GC with low CP degradability, DMI increased by 1.24 kg/d compared with cows fed GC with high CP degradability; however, CP degradability did not change DMI when cows were fed SFC. Similar interactions were observed for MY, HCT, and lactose content. When cows were fed GC diets, high CP degradability reduced MY by 2.3 kg/d, as well as HCT and lactose content, compared with low CP degradability. However, no effect of CP degradability was observed on those variables when cows were fed SFC diets. The SFC diets increased dry matter and starch total-tract digestibility and reduced β-casein (CN) content (% total milk protein) compared with GC diets. Cows fed low-CP degradability diets had higher glycosylated κ-CN content (% total κ-CN) and MES, as well as milk protein content, 3.5% fat-corrected milk, and efficiency of N for milk production, than cows fed high-CP degradability diets. Therefore, GC and high-CP degradability diets reduced milk production and protein stability. Overall, low CP degradability increased the efficiency of dietary N utilization and MES, probably due to changes in casein micelle composition, as CP degradability or corn processing did not change the milk concentration of ionic calcium. The GC diets increased β-CN content, which could contribute to reducing HTC when cows were fed GC and high-CP degradability diets.

Effect of temperature on casein micelle composition and gelation of bovine milk

Variation in temperature causes changes in the composition of the casein micelles. Milk samples with 10, 20, 30, 40, and 50% reduction of total β-CN from casein micelles were prepared and analysed. The micelle structure was maintained with minor changes in z-average diameters, zeta potentials, and micellar calcium. During acid gelation decreases in maximum storage modulus, fracture force and firmness were observed with gradual removal of β-CN, but with no significant differences regarding the gelation time among the samples. Decreases in gelation time, maximum G′, fracture force and firmness occurred with gradual removal of β-CN during rennet-induced gelation. Thus, the changes in composition of casein micelle during low temperature treatments had an influence on the gelation properties of bovine milk.

Rennet-induced coagulation properties of yak casein micelles: A comparison with cow casein micelles

It is essential for yak cheese processing to understand the rennet-induced coagulation properties of gel formation from casein micelles. We have previously discovered that yak milk requires a longer incubation time but forms stronger gels compared with cow milk. In this study, we are aiming to understand the rennet-induced coagulation properties of yak casein micelles comparing with cow casein micelles. Rheological analyses revealed that the gelling times of yak and cow casein micelles were 11.6±0.5 and 8.7±0.4min (P<0.05) respectively, but yak casein gel had a higher elastic modulus G′ (6.5±0.2Pa) than cow casein gel (2.5±0.2Pa; P<0.05). This is consistent with the results obtained by micro-rheology. Confocal laser scanning microscopic images (CLSM) and cryo-scanning electron microscopic images (cryo-SEM) showed that yak casein gel was more homogeneous and had smaller pore size than cow casein gels. Yak casein micelles had higher calcium (26.00mM), phosphate (19.90mM) and β-casein (relative 32%) concentrations. In addition, yak casein micelles were larger (Z-average 218.6nm) than cow casein micelles, and contained lower κ-casein (relative 13%). By comparison with cow casein micelles, yak casein micelle composition corresponding to their micellar calcium phosphate and κ-casein content may greatly contribute to the longer coagulation time and denser gel structure. An initial slower caseinomacropeptide (CMP) release rate and the slower rate of aggregation between para-casein micelles contributed to a more homogeneous yak gel network. Higher colloidal calcium phosphate is crucial for yak casein micelle aggregation and gel firmness because sufficient colloidal calcium phosphates can firmly glue sub-micelles and links casein micelles. This study provides valuable information for yak cheese production.

Alterations to the composition of casein micelles and retentate serum during ultrafiltration of skim milk at 10 and 40 °C

The composition of casein micelles and retentate serum were investigated during four-fold concentration of skim milk by ultrafiltration (UF) at 10 and 40 °C. Analysis of supernatants and pellets obtained by ultracentrifugation showed that while the concentration of soluble casein increased considerably during UF there was a net shift of casein from the serum to micelles at both temperatures. The hydration of casein micelles was higher at 10 °C than at 40 °C and increased slightly during UF. Dynamic light scattering measurements on diluted retentates did not reveal any permanent change in the average micelle diameter. During UF concentration of skim milk the progressive removal of calcium was affected by the partitioning of calcium between the micelles and the serum which was influenced by processing temperature. Performing UF at different temperatures altered the final calcium content of the retentates and composition and hydration of the casein micelles.

Casein Micelles: Size Distribution in Milks from Individual Cows

The size distribution and protein composition of casein micelles in the milk of Holstein-Friesian cows was determined as a function of stage and number of lactations. Protein composition did not vary significantly between the milks of different cows or as a function of lactation stage. Differences in the size and polydispersity of the casein micelles were observed between the milks of different cows, but not as a function of stage of milking or stage of lactation and not even over successive lactations periods. Modal radii varied from 55 to 70 nm, whereas hydrodynamic radii at a scattering angle of 73° (Q² = 350 μm⁻²) varied from 77 to 115 nm and polydispersity varied from 0.27 to 0.41, in a log-normal distribution. Casein micelle size in the milks of individual cows was not correlated with age, milk production, or lactation stage of the cows or fat or protein content of the milk.

Evaporative concentration of skimmed milk: effect on casein micelle hydration, composition, and size.

Understanding the effect of evaporative concentration on casein micelle composition is of high importance for milk processing. Alterations to the hydration, composition and size of casein micelles were investigated in skimmed milk evaporated to concentrations of 12-45% total solids content. The size of casein micelles was determined by dynamic light scattering, and the water content and composition determined by analysis of supernatants and pellets obtained by ultracentrifugation. The mass balance and hydration results showed that during the evaporation process, while micelles were dehydrated, water was removed preferentially from the serum. The amount of soluble casein and calcium in the serum decreased as a function of increasing solids content, indicating a shift of these components to the micelles. The formation of a small proportion of micelle aggregates at high concentrations appeared dependent on the time kept at these concentrations. Upon redilution with water, casein micelles were immediately rehydrated and aggregates were broken up in a matter of minutes. Soluble calcium and pH returned to their original state over a number of hours; however, only a small percentage of original soluble casein returned to the serum over the 5h period investigated. These results showed that casein micelles are significantly affected by evaporative concentration and that the alterations are not completely and rapidly reversible.

Theoretical molecular aspects of colloidal calcium phosphate in bovine milk

A simplified model for the colloidal calcium phosphate (CCP) nanocluster was developed from an active role of phosphate in the precipitation of casein (CN)/Ca mixtures and the composition of casein micelles (CM). The possible shape of the CCP nanocluster was selected as a tetrahedron, and we estimated that 4 CN molecules were involved in crosslinking a single CCP nanocluster. Similar values were obtained for the number of CN molecules involved in stabilizing the nanocluster when the number of CNs attached onto each nanocluster surface was deduced from the composition of CM. If one phosphoserine cluster consisted of 3 phosphoserine residues, the theoretical molecular weight and volume for the nanocluster were estimated to be 4,898 g/mol and 2.88 nm3, respectively. It was also shown that the position of Ca present in our model were reasonably located to accommodate the serine phosphate in CN molecule.

Physicochemical properties of fermented Arabian mares' milk

Due to its numerous benefits, including health-promoting characteristics, there is increasing interest in finding product applications for mares' milk. In the present study, the physicochemical properties, turbidity and conductivity of Arabian mares' milk were studied during acidification by fermentation at 30 °C and compared with those of bovine milk. The data for the acidification rate were modelled using a logistic equation. Mares' milk showed a longer latency phase (284 min) and lower acidification rate ( μ max = 0.0052 dpH min −1) compared with bovine milk (194 min and 0.0098 dpH min −1 respectively). The general shape of the curves of the turbidity versus time and pH were similar in nature for the acidification of both milks. However, the characteristic points were different, due to the differences in casein micelle composition. Measurement of electrical conductivity during acidification showed that maximum demineralization of casein micelles started around pH 6.09 and 5.31, for mares' and bovine milk, respectively. In conclusion, the technological characteristics of mares' milk were very different from those of bovine milk, due to intrinsic physicochemical properties of both milks.

The association of chymosin with artificial casein micelles

The influence of casein micelle composition on the extent of chymosin association is described. The association of chymosin with artificial micelles, made with combinations of αs-or β-casein with κ-casein, in a compositional range of αs or β:κ of 12 : 1–1 : 5, and at various pH values has been studied. Isotherms for chymosin association with micelles were constructed. Those composed of β- and κ-casein retained more chymosin than micelles composed of αs- and κ-casein. The chymosin association increased with higher κ-casein content and with dilution of the system. Furthermore the association did depend on contact time, on casein and chymosin concentration, on temperature and on pH. It is hypothesized that chymosin associates mainly with para-κ-casein but that αs- and β-casein and para-κ-casein itself also associate with para-κ-casein; competition between these caseins and chymosin then determines the extent of association of the latter with the para-casein micelles. A simplified model for this competitive association is presented.The model can explain several observations on rennet retention in curd.

Composition of casein micelles in relation to size in goat milks with A and null αs1-casein variants

Abstract Casein (CN) micelles of goat milks have been fractionated by centrifugation into 7 fractions of decreasing size. Transmission electron microscopy has been used to determine mean diameters of micelles in the fractions, and RP-HPLC to obtain individual casein proportions. Amounts of casein in each fraction have also been determined. Results obtained from goat milks with high levels of αs1-CN (variant A) or no αs1-CN (variant null) are compared. In micelles with increasing diameters (from 29 to 195 nm) we observed decreasing proportions of κ-CN, from 43 to 7%, and increasing αs2-CN proportions, from 5 to 27%. The distribution of the whole casein content in milk according to micelle size was different in A and null milks, with 80% of the casein in a narrow range of micelle size (80–120 nm ∅) in A milk, whereas it was relatively similar over the whole range of sizes in null milk micelles. Individual caseins ( α s 1 , α s 2 , β, κ ) followed the same distribution as whole casein in A milk, but in null milk κ-CN was mainly contained in small micelles and αs2-CN in large-sized micelles. A hypothesis is put forward to explain the variation in the fraction of κ-CN with casein micelle size.

Characterisation of the protein composition of casein micelles after heating

Under a French dairy industry initiative, the changes of protein composition of casein micelles induced by thermal treatment of milk have been studied. To improve the choice of milk powder as raw material for the work, it was established that drying had a low impact on renneting ability (formagraph measurement), non-casein nitrogen values and casein micelle diameter. The effect of storage of powders on the bioavailability of lysyl residues was also studied by complete hydrolysis of the proteins followed by furosine and homo-arginine measurements using an amino-acid auto analyser. After this validation step, thermal treatments of milk using both a laboratory system and an industrial process have been performed. Milk was reconstituted from the powder and heated from 70 to 100°C for 1 or 4 min. Casein micelles were then isolated from whole milk by ultracentrifugation and washed three times in a simulated milk ultrafiltrate, in order to eliminate whey proteins entrapped in micellar casein pellets. Micellar diameter was investigated by photon correlation spectrophotometry (PCS). Further qualitative and semi-quantitative studies of the micellar protein composition were carried out using PAGE systems and gel scanning. PAGE systems were used under complete or partial denaturing conditions, using sodium dodecyl sulfate (SDS) as dissociating agent, with or without dl-dithiothreitol (DTT) as a strong disulfide bond reducer.

Casein micelle size in relation with casein composition and αs1, αs2, β and Κ casein contents in goat milk

The relationship between micelle size and casein micelle composition was studied on 21 individual goat milks from animais homozygous for as1casein variants A, B2, C, E, F and O. A large variation in milk composition was obtained, as asl secretion levels varied from 7.2 g-kg! in A milks to none in 0 milks. Mean micelle size (MMS), determined by photon correlation spectroscopy, var- ied between samples from 192 nm to 287 nm. This was explained by the different aspects of the histograms of casein distribution according to the size, determined from transmission electron microscopy data, which showed a maximum either in the low diameter range (20-130 nm) or in the large diameter region (130-260 nm), or even intermediary figures with a bimodal distribution. The caseins, as1CN, as2CN, ~CN and KCN were determined in milks from nitrogen malter determinations (N x 6.38) and RP-HPLC analysis of casein. Polynomial relations were calculated between micelle size and milk composition al parameters. MMS was correlated on one side to the as1CN and KCN lev- els in milks (g-kg') and, on the other side, to the proportions of as1CN %, as2CN % and ~CN % in total casein. These polynomial relations allowed the prediction of the mean micelle size in milks from the casein levels, with aiS % accuracy. Monofactorial correlations also showed a significant effect of as1CN (r = -0.77), but not any of KCN. Mineral composition of milks was determined, calcium by atomic absorption spectrophotometry and phosphorus, by a colorimetrie method. Goat milks were characterized by a constant colloidal inorganic P level (12.4 (SO = 1.7) mmol-kg ). In contrast, colloidal Ca (Cac), SerineP and total colloidal P (Pc) were correlated to the total casein content. The ratio CaclPc was the most constant parameter in goat milks, amounting 1.22 (SO = 0.05), presumably characterizing an unique mode of association of caseins in milk. No significant correlation was obtained between the colloidal Ca and P levels in milks and the size of micelles. © InralElsevier, Paris. / 1 1

Association of chymosin with artificial casein micelles as influenced by micelle composition and pH

Abstract The flavour of cheese develops partly due to retained milk-clotting enzymes in curd during cheese ripening. In this paper, the influence of the casein micelle composition, and the pH of the micelle solution on the amount of chymosin adsorbed onto casein micelles, are studied. After incubation of chymosin with artificial casein micelles of four compositions and at four pH values, the amounts of chymosin remaining in the supernatant were measured with an Enzyme Linked Immunosorbent Assay (ELISA) and a Milk Clotting Test (MCT). Results confirmed earlier findings i.e. the lower the pH, the higher the adsorption of chymosin onto casein micelles. The β κ- casein micelles adsorbed more chymosin than α s κ- casein micelles of the same ratio of individual caseins and pH value. Micelles of β κ - or α s κ- casein of a ratio 12:1 were more influenced by pH than those of 3:2. Micelles of ratio 3:2, except at pH 5.2 for α s κ- casein , adsorbed more chymosin than those of ratio 12:1 at all pH values studied.

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

SummaryThe casein micelles of sheep (Ovis aries), goat (Capra hircus) and cow (Bos taurus) milks have been examined by electron microscopy. The ovine micelles were smaller than bovine micelles, most being below 80 nm diam. The caprine micelles were either large (c. 200 nm diam.), easily sedimented and electron-dense, or small (below 80 nm) and similar to the ovine micelles. Examination of the micelles using the freeze-etch technique showed, however, that they were all composed of subunits of similar dimensions (10–15 nm diam.).Electrophoresis of the caseins showed that there were differences between them, with the caprine casein containing a smaller proportion of its protein in the more mobile components. The mineral contents of the milks were also different, but these were apparently related to the different pH and casein contents of the milks. It is likely that the diiferences in micelle size distribution are determined by the physical and chemical properties of the casein components.

On the composition of casein micelles isolated under different conditions of ultracentrifugation.

AbstractThe authors describe the yield and composition of casein micelles isolated at gravitational field intensities of 50000 × g to 165000 × g at 5 °C and 20 °C respectively. The content of residual casein in milk serum obtained on centrifugation at 165000 × g was about 10% of the total casein content. This residual casein, presumably, represented non‐micellar dissolved casein. At lower gravitational field intensities, a certain amount of casein micelles remained in the supernatant. The yield of casein micelles was higher at 20 °C than at 5 °C due to dissociation of casein fractions, mainly β‐casein, from the micelles at 5 °C. Casein micelles of a smaller size contained a higher proportion of β‐casein. Casein micelles isolated at 5 °C had a lower calcium content.