Zinc binding in bovine milk

1989 ◽  
Vol 56 (2) ◽  
pp. 249-263 ◽  
Author(s):  
Harjinder Singh ◽  
Albert Flynn ◽  
Patrick F. Fox
Keyword(s):  
Calcium Phosphate ◽  
Human Milk ◽  
Equilibrium Dialysis ◽  
Bovine Milk ◽  
Skim Milk ◽  
Zinc Binding ◽  
Casein Micelles ◽  
Infant Formulae ◽  
Colloidal Calcium Phosphate ◽  
Acid Whey

SummaryAbout 90% of the Zn in bovine skim milk was sedimented by ultracentrifugation at 100000 g for 1 h. About half of the non-sedimentable Zn was non-dialysable, indicating that it was associated with protein, probably non-sedimented casein micelles. Casein micelles incorporated considerable amounts of Zn added to skim milk as ZnCl2, and at Zn concentrations ≥ 16 mM coagulation of casein micelles occurred. Ca was displaced from casein micelles by increasing ZnCl2 concentration and ˜ 40% of micellar Ca was displaced by 16 mM-ZnCl2. Micellar Zn, Ca and P1 were gradually rendered soluble as the pH of milk was lowered and at pH 4·6 > 95% of the Zn, Ca and P1 were non-sedimentable. These changes were largely reversible by readjustment of the pH to 6·7. About 40% of the total Zn in skim milk was non-sedimentable at 0·2 mM-EDTA and most of the remainder was gradually rendered soluble by EDTA over the concentration range 1–50 mM. This indicates that there are two distinct micellar Zn fractions. No micellar Ca or P1 was solubilized at EDTA concentrations up to 1·0 mM, indicating that both colloidal calcium phosphate (CCP) and casein micelles remained intact under conditions where the more loosely bound micellar Zn fraction dissolved. Depletion of casein micelles of colloidal Ca and P1 by acidification and equilibrium dialysis resulted in removal of Zn, and in colloidal Pi-free milk non-dialysable Zn was reduced to ·-2 mg/1 (˜ 32% of the original Zn). Thus, ˜ 32% of the Zn in skim milk is directly bound to caseins, while ˜ 63% is associated with CCP. Over 80% of the Zn in colloidal Pi-free milk was rendered soluble by 0·2 mM-EDTA, indicating that the casein-bound Zn is the loosely bound Zn fraction in casein micelles. A considerable fraction of the Zn in acid whey (pH 4·6) co-precipitated with Ca and Pi on raising the pH to 6·7 and heating for 2 h at 40 °C, indicating that insoluble Zn phosphate complexes form readily under these conditions. Studies on dialysis of milk against water, or dilution of milk or casein micelles with water, showed that CCP and its associated Zn is very stable and dissolves only very slowly at pH 6·6. The nature of Zn binding in casein micelles may help to explain the lower nutritional bioavailability of Zn in bovine milk and infant formulae compared with human milk.

Influence of aggregation on the susceptibility of casein to proteolysis

1970 ◽  
Vol 37 (2) ◽  
pp. 173-180 ◽  
Author(s):  
P. F. Fox
Keyword(s):  
Calcium Phosphate ◽  
Equilibrium Dialysis ◽  
Limited Proteolysis ◽  
Skim Milk ◽  
Sodium Caseinate ◽  
Casein Micelle ◽  
Casein Micelles ◽  
Micelle Structure ◽  
Colloidal Calcium Phosphate

SummaryThe susceptibility of the casein in milk to proteolysis was shown to be greatly influenced by its state of aggregation. In normal milk, where the casein is largely in micellar form, the αs1- and β-caseins are almost inaccessible to proteolysis. On removal of the colloidal phosphate, the casein micelles disintegrate, rendering the components, especially the αs1-casein, accessible to proteolysis. The role of colloidal calcium phosphate in the casein micelle is believed to be that of a non-specific aggregating agent which can be effectively replaced by calcium. Dissolved colloidal phosphate can be effectively reformed by elevation of the pH of colloidal phosphate-free (CPF) milk before equilibrium dialysis. Addition of κ-casein to CPF milk also causes aggregation of the component caseins but the micelles formed are smaller than those of normal milk.The behaviour of micellar β-casein differs considerably from that of micellar αs1-casein. The evidence suggests that part of the β-casein freely dissociates either outside or within the micelle when the temperature is reduced. The temperature dependence of the susceptibility of β-casein to proteolysis was similar in skim-milk and in solutions of sodium caseinate, and increased as the temperature was reduced. αs1-Casein was quite resistant to proteolysis in normal milk but became susceptible when the micelle structure was disrupted on removal of colloidal phosphate.It is concluded that limited proteolysis may prove a valuable technique in the study of casein micelle structure.

Some aspects of the ethanol stability of caprine milk

1982 ◽  
Vol 49 (3) ◽  
pp. 459-468 ◽  
Author(s):  
David S. Horne ◽  
Thomas G. Parker
Keyword(s):  
Calcium Phosphate ◽  
Chemical Modification ◽  
Milk Protein ◽  
Bovine Milk ◽  
Skim Milk ◽  
Ph Profile ◽  
Caprine Milk ◽  
Colloidal Calcium Phosphate ◽  
The Individual ◽  
Ethanol Stability

SUMMARYCaprine skim-milk exhibits markedly lower ethanol (EtOH) stability than bovine skim-milk but can still be characterized by a sigmoidal pH profile. As with bovine milk, the position of this profile along the pH-axis was found to be sensitive to available Ca levels. Manipulation of salt levels, either by serum interchange, addition or diminution did not result in any significant increase in the EtOH stability high pH asymptote, Smax, and reduction of the colloidal calcium phosphate of caprine milk also had no significant effect. EtOH stability/pH profiles similar to those of bovine milk were achieved only by chemical modification of the caprine milk protein by reaction with aldehydes and anhydrides. It is concluded that the low EtOH stability of caprine milk as compared with bovine milk is due to the different proportions of the individual caseins present, in particular the lack of an αsl-casein homologue in caprine milk.

Mineral and casein equilibria in milk: effects of added salts and calcium-chelating agents

2000 ◽  
Vol 67 (3) ◽  
pp. 361-370 ◽  
Author(s):  
PUNSANDANI UDABAGE ◽  
IAN R. McKINNON ◽  
MARY-ANN AUGUSTIN
Keyword(s):  
Light Scattering ◽  
Calcium Phosphate ◽  
Chelating Agents ◽  
Skim Milk ◽  
Mineral Salts ◽  
Casein Micelles ◽  
Micellar Size ◽  
Micellar Casein ◽  
Colloidal Calcium Phosphate ◽  
Milk Solids

We have investigated the effects of adding a range of mineral salts and calcium-chelating agents on the distribution of casein and minerals between the non-pelleted and pelleted phases of milk obtained upon centrifugation at 78000 g for 90 min. Adding CaCl2 or mixtures of NaH2PO% and Na2HPO% to reconstituted skim milk (90 g milk solids/kg) at pH 6·65 increased both pelleted casein and pelleted calcium phosphate. Opposite effects were obtained by adding citrate or EDTA. The change in pelleted calcium phosphate was not simply related to casein release from the micelle. Upon adding 5 mmol EDTA/kg milk, 20% of the pelleted Ca, 22% of the pelleted phosphate and 5% of the micellar casein were removed. Increasing the concentration of EDTA to 10 mmol/kg milk decreased the pelleted Ca by 44% and the pelleted phosphate by 46%, and caused 30%of the micellar casein to be released. The effects of adding phosphate, citrate or EDTA at pH 6·65, followed by the addition of CaCl2, demonstrated the reversibility of the dissolution and formation of the micellar calcium phosphate. There were limits to this reversibility that were related to the amount of colloidal calcium phosphate removed from the casein micelles. Adding CaCl2 to milk containing [ges ] 20 mmol EDTA or [ges ] 30 mmol citrate/kg milk did not result in complete reformation of casein micelles. Light-scattering experiments confirmed that the dissolution of moderate amounts of colloidal calcium phosphate had little effect on micellar size and were reversible, while the dissolution of larger amounts of colloidal calcium phosphate resulted in large reductions in micellar size and was irreversible.

Incorporation of individual casein constituents into casein aggregates cross-linked by colloidal calcium phosphate in artificial casein micelles

1989 ◽  
Vol 56 (4) ◽  
pp. 613-618 ◽  
Author(s):  
Takayoshi Aoki
Keyword(s):  
Calcium Phosphate ◽  
Inorganic Phosphate ◽  
Bovine Milk ◽  
Cross Linking ◽  
Casein Micelles ◽  
Colloidal Calcium Phosphate ◽  
Soluble Phase

SummaryArtificial casein micelles were prepared at a casein concentration of 2·5% with 10–40 mM-Ca, 12–27 mM-phosphate and 10 mM-citrate and cross-linking of casein by colloidal Ca phosphate (CCP) was examined. No casein aggregates cross-linked by CCP were formed at 10 mM-Ca, 12 mM-phosphate and 10 mM-citrate, which are the approximate concentrations found in the soluble phase of bovine milk. Although the amounts of casein aggregates cross-linked by CCP and of colloidal inorganic phosphate increased with increasing Ca and phosphate concentrations, the effect was not uniform. The incorporation rates of individual casein constituents into casein aggregates cross-linked by CCP were in the order > > β-casein. This was the reverse of the order of dissociation rates during dialysis of casein aggregates cross-linked by CCP reported previously.

High pressure-induced denaturation of α-lactalbumin and β-lactoglobulin in bovine milk and whey: a possible mechanism

2004 ◽  
Vol 71 (4) ◽  
pp. 489-495 ◽  
Author(s):  
Thom Huppertz ◽  
Patrick F Fox ◽  
Alan L Kelly
Keyword(s):  
High Pressure ◽  
Calcium Phosphate ◽  
Crucial Role ◽  
Bovine Milk ◽  
Blocking Agent ◽  
Casein Micelles ◽  
Colloidal Calcium Phosphate ◽  
Β Lactoglobulin ◽  
Interchange Reactions

In this study, high pressure (HP)-induced denaturation of α-lactalbumin (α-la) and β-lactoglobulin (β-lg) in dairy systems was examined. In both milk and whey, β-lg was less baroresistant than α-la; both proteins were considerably more resistant to HP-induced denaturation in whey than in milk. HP-induced denaturation of α-la and β-lg increased with increasing proportion of milk in mixtures of milk and whey. Addition of a sulphydryl-oxidising agent, KIO3, to milk or whey increased HP-induced denaturation of β-lg, but reduced the denaturation of α-la. Denaturation of both α-la and β-lg was prevented by adding a sulphydryl-blocking agent, N-ethylmaleimide, to milk or whey prior to HP treatment, highlighting the crucial role of sulphydryl-disulphide interchange reactions in HP-induced denaturation of α-la and β-lg. Removal of colloidal calcium phosphate from milk also reduced HP-induced denaturation of α-la and β-lg significantly. The higher level of HP-induced denaturation of α-la and β-lg in milk than in whey may be the result of the abscence of the casein micelles and colloidal calcium phosphate from whey, which facilitate HP-induced denaturation of α-la and β-lg in milk.

Relation between micelle size and formation of soluble casein on heating concentrated milk

1983 ◽  
Vol 50 (2) ◽  
pp. 207-213 ◽  
Author(s):  
Takayoshi Aoki ◽  
Yoshitaka Kako
Keyword(s):  
Calcium Phosphate ◽  
Skim Milk ◽  
Differential Centrifugation ◽  
Slight Effect ◽  
Casein Micelles ◽  
Milk Samples ◽  
Micelle Size ◽  
Colloidal Calcium Phosphate ◽  
Concentrated Milk ◽  
Milk Casein

SummaryIn order to examine the influence of micelle size on the formation of soluble casein brought about by heating concentrated milk, casein micelles were separated into 3 fractions by differential centrifugation. The micelles were dispersed in the ultrafiltrate of skim-milk and the preparations were concentrated to 1/2·5 of the original volume. When the concentrated milk samples were heated at 135–140 °C, a larger amount of soluble casein was formed from smaller micelles than from larger micelles. Only a slight effect on the formation of soluble casein on heating was observed when colloidal calcium phosphate (CCP) was removed from the large micelles to a level lower than that associated with the small micelles or when the CCP in the small micelles was increased to a level higher than that of the large micelles. The soluble caseins formed on heating from different sizes of micelles were similar in composition. The fact that the formation of soluble casein on heating was affected by micelle size may be ascribed to variation in the κ-casein content of micelles rather than to variation in CCP content.

pH-Induced dissociation of bovine casein micelles. II. Mineral solubilization and its relation to casein release

1989 ◽  
Vol 56 (5) ◽  
pp. 727-735 ◽  
Author(s):  
Douglas G. Dalgleish ◽  
Andrew J. R. Law
Keyword(s):  
Calcium Phosphate ◽  
Inorganic Phosphate ◽  
Bovine Milk ◽  
Universal Relation ◽  
Casein Micelles ◽  
Neutral Ph ◽  
Quantitative Studies ◽  
A Value ◽  
Different Types ◽  
Different Temperatures

SummaryMeasurements of the release of Ca, Mg and inorganic phosphate(Pi) from the casein micelles of bovine milk have been made, as functions of the pH, in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C. The results are in general agreement with earlier published studies in giving a value of 1·75–1·84 for the micellar Ca:Pi ratio. Mg appeared to behave similarly to Ca, although the amounts of micellar material were much smaller. The results on the acid-solvation of calcium phosphate are considered in relation to published quantitative studies of the pH-induced dissociation of the different types of caseins from the micelle, and of the micellar dissociation caused when micellar calcium phosphate is dissolved at neutral pH. It is evident from this that at present it is not possible to derive a universal relation between the dissociation of minerals and of caseins from the micelles at different temperatures and under different conditions.

Intestinal absorption of zinc: a role for a zinc-binding ligand in milk.

1978 ◽  
Vol 235 (5) ◽  
pp. E556
Author(s):  
J R Duncan ◽  
L S Hurley
Keyword(s):  
Human Milk ◽  
Intestinal Mucosa ◽  
Neonatal Period ◽  
Bovine Milk ◽  
Zinc Binding ◽  
Acrodermatitis Enteropathica ◽  
Zinc Absorption ◽  
Human Infants ◽  
Normal Human ◽  
Old Rats

This study examined the proposal that a low molecular weight, zinc-binding ligand (ZBL) in certain milks is important for zinc absorption in the neonatal period. Ten-day-old rats, in which intestinal ZBL is absent, fed (by stomach intubation) 65Zn-labeled ZBL from rat milk, absorbed significantly more 65Zn than those fed free 65ZnCl2 or bovine milk fractions. ZBL from human milk appeared to have an intermediate effect, possibly due to species specificity. 65Zn was found in the ZBL fraction in intestinal mucosa of 10-day-old rats fed rat or human milk fractions, but not in those fed bovine milk or free 65ZnCl2. In contrast, in 18-day-old rats, which have an endogenous intestinal ZBL, there were no differences in zinc absorption, and any of the labeled milk fractions or free 65Zn caused localization of 65Zn in the ZBL fraction of intestinal mucosa. These results support the hypothesis that the intestinal ZBL plays a role in zinc absorption and that in the neonatal period before its development the milk ZBL is valuable for this function. This mechanism may be important in normal human infants as well as in acrodermatitis enteropathica patients.

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

1982 ◽  
Vol 49 (1) ◽  
pp. 87-98 ◽  
Author(s):  
Margaret L. Green
Keyword(s):  
Calcium Phosphate ◽  
Inorganic Phosphate ◽  
Casein Micelles ◽  
Low Concentrations ◽  
Colloidal Calcium Phosphate ◽  
Ionic Materials ◽  
Mean Size ◽  
Charge Concentration ◽  
The Mean ◽  
Micelle Hydration

SummaryThe 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.

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