Heat stability of milk: influence of modifying sulphydryl-disulphide interactions on the heat coagulation time–pH profile

1987 ◽  
Vol 54 (3) ◽  
pp. 347-359 ◽  
Author(s):  
Harjinder Singh ◽  
Patrick F. Fox
Keyword(s):  
Heat Stability ◽  
Casein Micelle ◽  
Coagulation Time ◽  
Casein Micelles ◽  
Ph Profile ◽  
Ph Values ◽  
Stabilizing Effect ◽  
Ph Range ◽  
Β Lactoglobulin ◽  
Minimum Ratio

SummaryAddition of reducing agents such as 2-mercaptoethanol (2-ME), dithio-threitol and Na sulphite to milk markedly reduced its heat stability at pH values below 7·1. 2-ME reversibly destabilized milk or serum protein-free casein micelle dispersions and promoted the release of κ-casein-rich protein from the micelles. Reduction of either casein micelles or β-lactoglobulin (β-lg) with 2-ME and subsequent blocking of the newly formed –SH groups with N-ethylmaleimide irreversibly reduced the maximum to minimum ratio in the heat stability profile. 2-ME disrupted κ-casein/β-lg complexes and stripped κ-casein from the micelles on heating. The milk or caseinate systems were thus destabilized. Addition of KBrO4 or iodosobenzoate to milk at 5 HIM eliminated the minimum but destabilized milk in the region of the maximum. However, KIO3 at 5 mm had a strong stabilizing effect throughout the pH range 6·5–7·3.

Heat stability of milk: pH-dependent dissociation of micellar κ-casein on heating milk at ultra high temperatures

1985 ◽  
Vol 52 (4) ◽  
pp. 529-538 ◽  
Author(s):  
Harjinder Singh ◽  
Partick F. Fox
Keyword(s):  
Whey Protein ◽  
Whey Proteins ◽  
Heat Stability ◽  
Casein Micelle ◽  
Casein Micelles ◽  
Ph Profile ◽  
Ph Values ◽  
Maximum Stability ◽  
Β Lactoglobulin ◽  
Maximum Minimum

SUMMARYPreheating milk at 140 °C for 1 min at pH 6·6, 6·8, 7·0 or 7·2 shifted the heat coagulation time (HCT)/pH profile to acidic values without significantly affecting the maximum stability. Whey proteins (both β-lactoglobulin and α-lactalbumin) co-sedimented with the casein micelles after heating milk at pH < 6·9 and the whey protein-coated micelles, dispersed in milk ultrafiltrate, showed characteristic maxima–minima in their HCT/pH profile. Heating milk at higher pH values (> 6·9) resulted in the dissociation of whey proteins and κ-casein-rich protein from the micelles and the residual micelles were unstable, without a maximum–minimum in the HCT/pH profile. Preformed whey protein–casein micelle complexes formed by preheating (140 °C for 1 min) milk at pH 6·7 dissociated from the micelles on reheating (140 °C for 1 min) at pH > 6·9. The dissociation of micellar-κ-casein, perhaps complexed with whey proteins, may reduce the micellar zeta potential at pH ≃ 6·9 sufficiently to cause a minimum in the HCT/pH profile of milk.

Heat stability of milk: role of β-lactoglobulin in the pH-dependent dissociation of micellar κ-casein

1987 ◽  
Vol 54 (4) ◽  
pp. 509-521 ◽  
Author(s):  
Harjinder Singh ◽  
Patrick F. Fox
Keyword(s):  
Heat Stability ◽  
Casein Micelle ◽  
Casein Micelles ◽  
Degree Of Hydration ◽  
Ph Values ◽  
High Concentrations ◽  
Ph Range ◽  
Β Lactoglobulin ◽  
Micelle Hydration

SummaryOn heating casein micelle systems containing β-lactoglobulin (β-lg) at 90°C for 10 min, β-lg complexed with casein micelles at pH < 6·9, probably as a result of interaction with κ-casein via sulphydryl-disulphide interchange, and co-sedimented with the micelles on ultracentrifugation. Complex formation with β-lg appeared to prevent the dissociation of micellar κ-casein on heating. However, at pH ≥ 6·9, κ-casein/β-lg complexes dissociated from the micelles on heating, thus enhancing the release of micellar κ-casein. High concentrations of β-lg (≥0·8%) induced coagulation at pH 7·3, essentially by promoting the dissociation of micellar κ-casein. It appeared that αs1-, αs2-, β- and κ-caseins dissociated from serum protein-free casein micelles to equal extents, but the presence of β-lg specifically enhanced the dissociation of κ-casein at pH values ≥ 6·9. Micelle hydration increased slightly when casein micelles were heated in the presence of β-lg at pH 6·7, while at pH 7·3 β-lg decreased the degree of hydration of casein micelles. Formation of a complex between β-lg and κ-casein appeared to stabilize the micelles in the pH range 6·5–6·7, possibly via increased micellar charge or degree of hydration or by preventing the dissociation of κ-casein.

Heat stability of milk: influence of κ-casein hydrolysis

1978 ◽  
Vol 45 (2) ◽  
pp. 173-181 ◽  
Author(s):  
Patrick F. Fox ◽  
Catherine M. Hearn
Keyword(s):  
Heat Stability ◽  
Principal Factor ◽  
Coagulation Time ◽  
Casein Micelles ◽  
Ph Profile ◽  
Acetylneuraminic Acid ◽  
Ph Values ◽  
Colloidal Calcium Phosphate ◽  
Maximum Stability ◽  
Ph Range

SummaryThe release of 12% (w/v) TCA-soluble N-acetylneuraminic acid from casein at 5 temperatures between 110 and 150°C was determined and showed a Q10 °C about 3; coagulation occurred when about 20% of the κ-casein was hydrolysed. Renneting of milk or colloidal calcium phosphate-free milk rendered the caseinate very heat-labile at its normal pH when more than about 20% of the κ-casein had been hydrolysed. Heat stability at the pH of maximum stability was not significantly decreased until after very prolonged renneting, but stability at pH values alkaline to the minimum was very sensitive to such hydrolysis suggesting that κ-casein is the principal factor responsible for heat stability in that region. Systems which do not have a maximum or minimum in the heat coagulation time–pH profile, i.e. serum protein-free casein micelles in milk diffusate, or milk which had been dialysed against water, were destabilized by renneting throughout the pH range 6·4–7·4.

Heat stability of milk

1980 ◽  
Vol 47 (2) ◽  
pp. 199-210 ◽  
Author(s):  
Donald F. Darling
Keyword(s):  
Temperature Dependence ◽  
Elevated Temperatures ◽  
Skim Milk ◽  
Heat Stability ◽  
Coagulation Time ◽  
Ph Values ◽  
Protein Polymerization ◽  
Casein Protein ◽  
Apparent Activation Energies ◽  
Β Lactoglobulin

SummaryThe heat stability of a standard reconstituted skim-milk preparation has been investigated as a function of pH, temperature of coagulation, and forewarming treatment. Apparent activation energies have been calculated from the temperature dependence of coagulation time, and a constant value of 144 kJ/mole has been found for milks between pH 6·6 and 6·9. The effect of forewarming resulted in a decrease in stability at the most acid pH values, a slight increase at higher pH but below the pH maximum, and a decrease in the region of the pH minimum. A working hypothesis is proposed for the mechanisms leading to the coagulation of milk at elevated temperatures, based upon Ca induced precipitation of casein, protein polymerization, β-lactoglobulin: κ-casein interaction, and precipitation of insoluble Ca phosphates.

Heat stability of milk: influence of colloidal calcium phosphate and β-lactoglobulin

1975 ◽  
Vol 42 (3) ◽  
pp. 427-435 ◽  
Author(s):  
P. F. Fox ◽  
M. C. T. Hoynes
Keyword(s):  
Calcium Phosphate ◽  
Fold Increase ◽  
Heat Stability ◽  
Protein Charge ◽  
Ph Values ◽  
Colloidal Calcium Phosphate ◽  
Maximum Stability ◽  
Ph Range ◽  
Ph Curve ◽  
Β Lactoglobulin

SummaryReduction of the level of colloidal calcium phosphate (CCP) progressively increased the heat stability of milk at pH values <~7·0 and increased the pH of maximum stability. Removal of 40% CCP also stabilized the system at the pH of minimum stability, but removal of ≥60% CCP rendered milk very unstable at pH values >7·2, an effect not offset by a 4-fold increase in κ-casein concentration. Doubling CCP had a slight destabilizing effect in the pH range 6·5–7·5.Addition of β-lactoglobulin to serum protein-free casein micelles had a marked destabilizing effect at pH values > ~6·8, but increased stability in the pH range 6·4–6·8. β-Lactoglobulin had a similar and more apparent effect on the heat stability of Na caseinate dissolved in milk diffusate.It is suggested that rather than being a stabilizing factor responsible for the maximum in the heat stability-pH curve, the true effect of β-lactoglobulin is to shift the curve to more acid pH values (reason unknown) and to sensitize the caseinate system to heat-induced Ca phosphate precipitation at pH values > ~7·0. Low stability at ~pH 7·0 introduces an apparent maximum in the heat stability-pH curve at ~pH 6·8, but this has no independent existence. At pH values >7·2, increased protein charge more than off-sets the influence of heat-precipitated CCP and stability again increases in micellar but not in soluble casein systems.

Heat stability of milk: the mechanism of stabilization by formaldehyde

1985 ◽  
Vol 52 (1) ◽  
pp. 65-76 ◽  
Author(s):  
Harjinder Singh ◽  
Patrick F. Fox
Keyword(s):  
High Temperatures ◽  
Crosslinking Agent ◽  
Heat Stability ◽  
Cross Linking ◽  
Amino Groups ◽  
Coagulation Time ◽  
Ph Profile ◽  
Ph Values ◽  
Dimethyl Suberimidate ◽  
Ph Curve

SUMMARYThe increase produced by formaldehyde (HCHO) in the heat stability of milk did not occur when milk was treated with HCHO at temperatures up to 60°C followed by dialysis at 5°C. However, the minimum in the heat coagulation time (HCT)–pH curve was irreversibly removed if the milk was preheated at 80–C for 10 min in the presence of 5 mM-HCHO. Although this treatment blocked the ε-amino groups of lysyl residues, the stabilizing mechanism is considered to be due to the cross linking action of HCHO which reduced the level of non-sedimentable, κ-casein-rich protein dissociated from the micelles on heating. The specific crosslinking agent, dimethyl suberimidate, modified the HCT-pH profile of milk in a manner similar to preheating at 80°C for 10 min with 5 mM-HCHO, supporting the crosslinking hypothesis. The results of this study appear to lend some support to the proposal of Kudo (1980) that the minimum in the HCT-pH curve of milk is due to the dissociation of κ-casein from the micelles on heating at high temperatures at pH values greater than 6η7.

Influence of 2-mercaptoethanol on heat stability of concentrated whey-protein-free milk and formation of soluble casein

1984 ◽  
Vol 51 (3) ◽  
pp. 439-445 ◽  
Author(s):  
Takayoshi Aoki ◽  
Yoshitaka Kako
Keyword(s):  
Whey Protein ◽  
Heat Stability ◽  
Coagulation Time ◽  
Casein Micelles ◽  
Ph Profile ◽  
Very High

SummaryThe heat coagulation time (HCT) of concentrated whey-protein-free (WPF) milk measured at 120 and 130 °C was reduced to by addition of 5–20 mM-2-mercaptoethanol (ME). However, although the amount of soluble casein formed on heating was doubled by addition of ME, the shape of the HCT–pH profile was affected only slightly. The proportion of κ-casein in the soluble casein from heated concentrated WPF milk containing ME was very high, though it was somewhat lower than that of the soluble casein from heated concentrated WPF milk containing no ME. No solubilization of colloidal Ca phosphate was observed in either unheated or heated concentrated WPF milk on addition of ME. These facts suggest that ME probably promotes the formation of soluble casein with release of κ-casein from micelles on heating, thus destabilizing the casein micelles.

Effects of high pressure on some constituents and properties of buffalo milk

2005 ◽  
Vol 72 (2) ◽  
pp. 226-233 ◽  
Author(s):  
Thom Huppertz ◽  
Mathias R Zobrist ◽  
Therese Uniacke ◽  
Vivekk Upadhyat ◽  
Patrick F Fox ◽  
...  
Keyword(s):  
High Pressure ◽  
Buffalo Milk ◽  
Casein Micelle ◽  
Coagulation Time ◽  
Casein Micelles ◽  
Rennet Coagulation ◽  
Micelle Size ◽  
Significant Difference ◽  
Β Lactoglobulin

In this study, effects of high pressure (HP) on some constituents and properties of buffalo milk were examined. HP treatment at 100–600 MPa for 30 min affected casein micelle size only slightly, whereas treatment at 800 MPa increased it by ~35%. Levels of non-micellar αs1- and β-caseins were increased by treatment [ges ]250 MPa, and were highest after treatment at 400–800 MPa. The level of non-micellar calcium increased with increasing pressure up to 600 MPa. The L*-value of the milk decreased gradually with increasing pressure, from ~82 for untreated milk to ~65 for milk treated at 800 MPa. Milk pH was increased by ~0·07 units after treatment at 100–800 MPa, with no significant difference between treatment pressures. Denaturation of α-lactalbumin occurred at pressures [ges ]400 MPa, and reached >90% after treatment at 800 MPa, whereas β-lactoglobulin (β-lg) was denatured >100 MPa, reaching ~100% after treatment at 400 MPa; after treatment [ges ]400 MPa, all β-lg was associated with the casein micelles. The rennet coagulation time of buffalo milk increased with increasing pressure, whereas the strength of the coagulum formed decreased after treatment at 250–800 MPa. Overall, HP treatment affected many constituents and properties of buffalo milk; some of these effects have also been observed in the milk from other species, but the extent of the effects, and the pressure at which they occurred, differed considerably.

Physico-chemical characteristics of casein micelles in dilute aqueous media

1979 ◽  
Vol 46 (2) ◽  
pp. 357-363 ◽  
Author(s):  
Patrick F. Fox ◽  
Bridget M. Nash
Keyword(s):  
Aqueous Media ◽  
Heat Stability ◽  
Coagulation Time ◽  
Casein Micelles ◽  
Second Stage ◽  
Rennet Coagulation ◽  
Physico Chemical ◽  
Micelle Size ◽  
Low Levels ◽  
Ph Range

SUMMARYThe heat stability and rennet coagulation time (second stage) of milk were reduced by brief dialysis against water. Destabilization appears to arise from a developed imbalance between Ca and phosphate plus citrate due to the very slow diffusion of Ca on dialysis. Average micelle size as indicated by permeation chromatography in porous glass CPG 10 was slightly reduced by dialysis for 24 h. Direct addition of low levels (10–100 mM) of NaCl to milk markedly reduced heat stability at pH > 7·0 (normal minimum) possibly due to dissociation of κ-casein, but increased rennet coagulation times; higher levels of NaCl decreased heat stability throughout the pH range 6·4–7·4.

Heat stability of milk: influence of denaturable proteins and detergents on pH sensitivity

1978 ◽  
Vol 45 (2) ◽  
pp. 159-172 ◽  
Author(s):  
Patrick F. Fox ◽  
Catherine M. Hearn
Keyword(s):  
Tween 80 ◽  
Heat Stability ◽  
Ph Sensitivity ◽  
Ammonium Bromide ◽  
Casein Micelle ◽  
Maximum Stability ◽  
Ph Range ◽  
Triton X ◽  
Ph Curve ◽  
Β Lactoglobulin

Summaryα-Lactalbumin and SDS in addition to β-lactoglobulin introduced pH sensitivity to the heat stability–pH curve of serum protein free casein micelles particularly by increasing stability in the pH range 6·4–6·7. Bovine serum albumin, ovalbumin and lysozyme caused marked destabilization of milk and casein micelle suspensions throughout the pH range 6·4–7·4. Tetramethyl ammonium bromide caused destabilization of milk at pH values > 7·0, but had no effect in the region of maximum stability while the non-ionic detergents Triton X-100 and Tween 80 had no effect on heat stability.

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