Heat Stability Of Milk Research Articles

Effects of κ-casein glycosylation on heat stability of milk

A study was conducted to determine the relationship between the degree of glycosylation of κ-casein (CN) and heat stability of milk at 140 °C between pH 6.2 and 6.9. Morning milk samples from individual Holstein cows (genotypes κ-CN AA, β-CN A 1A 2, α s1 - CN BB, β-lactoglobulin (LG) AB) in mid-lactation were collected and analyzed for protein and somatic cell count. Samples of low somatic cell count (< 150 000 cells/ml) were skimmed and dialyzed overnight at 4 °C against bulk milk to equilibrate soluble components. In the first series of experiments, the heat clotting time vs pH (HCT-pH) curves were determined for 37 individual milk samples having various degrees of glycosylation of κ-CN, estimated through the N-acetylneuraminic acid (NANA) content in μg/mg of κ-CN (NANA/κ-CN). The mean NANA/κ-CN, HCT max, and HCT min were 50.3 ± 22.3 μg/mg, 17.4 ± 1.2 min and 3.3 ± 0.7 min, respectively. The results of statistical analysis showed that the variations in the degree of glycosylation of κ-CN in normal milk did not significantly affect ( P > 0.1) the heat stability parameters. The effect of NANA depletion on the HCT-pH profile was tested in the second series of experiments. HCT-pH profiles of 14 individual milk samples, untreated and neuraminidase-treated to extensively remove NANA associated with κ-CN, were compared. The average NANA/κ-CN before treatment, the HCT max, and HCT min were 78.3 ± 24.9 μg/mg, 21.4 ± 1.5 min and 3.6 ± 0.8 min, respectively. As no effect of the desialylation was observed for HCT min and pH I and as the effect on HCT max, although significant ( P ≤ 0.05), was very low (0.7 min on average), these results indicate that the charges and the extent of hydrophilicity of the heat-induced κ-CN/β-LG complexes are not the crucial factors for the production of κ-CN depleted micelles upon heating and that the glycosylation of κ-CN does not affect its heat-induced interaction with β-LG.

Effect of starches on the heat stability of milk

SummaryStarches from different botanical sources have been added (0.5–1.5% w/w) to skim milk and pregelatinized by heating at 70°C for 30 min. the heat coagulation time (HCT) of the milk‐starch mixtures was then evaluated at 140°C. the addition of native starches to milk had a destabilizing effect. Increasing the concentration of added starch caused a decrease in HCT's. the reduction in the heat stability of milk proteins was not related to the amylose/amylopectin composition or to the granular structure of the starches. Modification of the molecular properties of the starch components by acid hydrolysis or cross‐linking resulted in an enhancement of the heat stability of the mixtures.

Heat stability of milk: Aggregation and dissociation of protein at ultra-high temperatures

Abstract The aggregation and dissociation of protein in milk heated at 140°C was studied using high performance liquid chromatography on a TSK-GEL G4000SW column in 6 m urea. Initial heating gave rise to the formation of high molecular weight complexes of whey proteins and κ-casein which eluted at or close to the void volume of the column. With continued heating, the quantities of these complexes remained more or less constant but the amounts of intermediate-sized protein material cross-linked through covalent (non-disulphide) bonds increased gradually. The size of the whey protein/κ-casein complexes decreased with increase in pH at heating. Most of the whey protein/κ-casein complexes remained associated with the micelles on extended heating at the natural pH of milk (6·67) although some protein (monomeric in 6 m urea) dissociated from the casein micelles. Increasing the pH at heating resulted in increased quantities of whey protein/κ-casein complexes and monomeric protein in the ultra-centrifugal supernatant. The implications of the above changes are discussed in relation to heat stability of milk.

Technological properties of milk as influenced by genetic polymorphism of milk proteins — A review

Abstract Current knowledge of the effects of genetic polymorphisms of milk proteins on technological properties of milk is reviewed. The topics discussed include the rennetability of milk, syneresis of curd, cheese yield and quality, susceptibility of β-lactoglobulin genetic variants to heat denaturation, heat stability of milk and concentrated milk, as well as the few findings concerning the relationships between milk protein polymorphs and micellar stability of casein, water sorption of casein, quality of fermented milk and proteolysis of casein. Although genetic variants of milk proteins clearly have been demonstrated to influence technological properties of milk, in particular those of importance for cheesemaking, further research is necessary to elucidate the underlying mechanisms involved and possible relationships between milk protein loci and traits other than those important for milk processing.

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

SummaryOn heating casein micelle systems containing β-lactoglobulin (β-lg) at 90°C for 10 min, β-lg complexed with casein micelles at pH &lt; 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 colloidal and soluble salts and protein modification on the pH-dependent dissociation of micellar κ-casein

SummaryReducing the colloidal calcium phosphate (CCP) content of milk by 40% or increasing it by 20% did not significantly affect the heat-induced pH-dependent dissociation of micellar κ-casein. However, changes in soluble Ca and phosphate affected the dissociation of κ-casein markedly; decreasing the phosphate concentration or increasing the Ca concentration reduced the formation of non-sedimentable N (NSN) and non-sedimentable 12% TCA-insolubleN-acetyl-neuraminic acid (NANA). Dialysis of milk against water for short periods (∼ 5 h) reduced the formation of both NSN and non-sedimentable 12% TCA-insoluble NANA, as did NaCl at concentrations above 0·05 M. Modification of protein amino groups by succinylation promoted the release of κ-casein while amidation of carboxyl groups had the opposite effect. It appears that the pH-dependent dissociation of κ-casein produced on heating milk above 90°C is controlled by electrostatic interactions. The effects of soluble ions such as Ca2+or Na+appear to be due to shielding of such negatively-charged groups as seryl phosphate and carboxyl on the protein, thus reducing the release of κ-casein.

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

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 KBrO4or iodosobenzoate to milk at 5 HIM eliminated the minimum but destabilized milk in the region of the maximum. However, KIO3at 5 mm had a strong stabilizing effect throughout the pH range 6·5–7·3.

Effects of milk protein genetic variants and composition on heat stability of milk

SummarySkim milk samples from 126 Friesian and 147 Jersey cows in eight commercial herds were preheated at 85 °C for 30 min and concentrated to 200 g l−1 total solids. A heat coagulation time–pH curve was determined at 120 °C for each treated sample. Heat coagulation times ranged from 1 to 50 min at the non-adjusted pH and 1 to 60 min at the pH of maximum stability. The following statistically significant effects were found. Maximum heat stability was affected by genetic variants of κ-casein (B &gt; AB &gt; A; P &lt; 0·001) and β-lactoglobulin (B, AB&gt;A; P &lt; 0·05) whereas natural heat stability was affected only by κ-casein genetic variants (B &gt; AB &gt; A; P &lt; 0·001). Maximum and natural heat stability were corre-lated positively with β-casein and κ-casein concentrations and were negatively correlated with αs1-casein and β-lactoglobulin concentrations. Milk from Jersey cows had greater maximum and natural heat stability than milk from Friesian cows. Differences were found between herds within breed for natural heat stability, but not for maximum heat stability. Maximum heat stability declined with age of the cow. The heat stability of skim milk samples taken from 40 Jersey cows in one of the herds was determined at 140 °C. A considerable variation was found in the coagulation time–pH curves. There was a difference in natural heat stability between κ-casein variants (B &gt; AB; P &lt; 0°05). Natural and maximum heat stability were correlated positively with urea concentration. No relationship was found between the heat stability of preheated concentrated skim milk and the heat stability of the original skim milk. The pH of skim milk samples was associated with αs1-casein genetic variant, age of cow, stage of lactation and concentration of γ-casein.

Studies on the heat stability of milk: II. Association and dissociation of particles and the effect of added urea

SummaryThe sizes of the casein particles in milk heated at 130°C were measured as functions of heating time and of initial pH. The effect of adding 10 mM-urea to the milks was also studied. Turbidities of the milks before and after dispersing the caseinate particles in 8 m-urea/EDTA/β-mercaptoethanol were measured, the latter measurement giving an indication of the extent of covalent cross-linking. Urea was found to modify the behaviour of the milk: diameters of the caseinate particles were considerably altered, and the extent of covalent cross-linking was diminished. These results agree with the observation that more protein is solubilized from the casein micelles when urea is present, indicating that a competitive reaction scheme involving cross-linking in opposition to micellar dissociation may be responsible for the ultimate precipitation of milk by heat.

Studies on the heat stability of milk: I. Behaviour of divalent cations and phosphate in milks heated in a stainless steel system

SummarySkim milk was heated rapidly to 130 °C in stainless steel tubing, and was then held at this temperature for periods of up to 1 h in a stainless steel holding vessel. Samples taken at intervals during the holding period were analysed for cations, inorganic and organic phosphate and protein in the total sample and in the supernatant after centrifugation at 60 500g. The cation and total phosphate (organic+inorganic) contents of the sedimentable material remained constant throughout the heating, although the caseins became extensively dephosphorylated. Dephosphorylated protein dissociated from the casein micelles during the first 20 min of heating, after which its concentration in the serum began to decrease, perhaps indicating the onset of the heat coagulation reaction.

Heat stability of milk: further studies on the pH-dependent dissociation of micellar κ-casein

SUMMARYWhey protein complexed and became co-sedimentable with casein micelles after heating milk at ≥ 90°C for 10 min at pH ≤ 6·9 while at higher pH values (7·3) whey proteins and κ-casein-rich protein dissociated from the micelles on heating. κ-Casein-deficient micelles were more sensitive to heat, Ca2+or ethanol than whey protein-coated or native micelles and were readily coagulable by rennet. Isolated κ-casein added to skim milk before preheating (90°C for 10 min) did not associate with the micelles at pH ≥ 6·9. Sodium dodecyl sulphate increased the level of both non-sedimentable N (NSN) andN-acetylneuraminic acid (NANA) and shifted the NSN-pH and NANA-pH curves to more acidic values while cetyltrimethylammonium bromide had the opposite effect. It is suggested that the pH-dependent dissociation in micellar κ-casein, which appears to be reversible, depends on the surface charge on the micelles; at a certain negative charge, disruption of hydrophobic and electrostatic forces could result in the dissociation of κ-casein from the casein micelles.

Nutritional quality of diacetyl/urea treated caseinates

Diacetyl markedly improves the heat stability of milk, especially in the presence of urea. The present study was undertaken to investigate the effects of severe heat treatments in the presence of diacetyl and urea on the nutritional value of casein. Acid casein solutions (7% w/v protein) at pH 6·6 were heated at 120°C for 20 min in the presence of the following: (1) no additive; (2) 10 m m urea; (3) 1·5 m m diacetyl; (4) 5 m m diacetyl; (5) 1·5 m m diacetyl + 10 m m urea; (6) 5 m m diacetyl + 10 m m urea. All caseinates treated with diacetyl or diacetyl plus urea were visibly browned. When compared with caseinate heated alone, available lysine was reduced by 8%, 9·5%, 8% and 7% in the caseinates treated with 10 m m urea, 1·5 m m diacetyl + 10 m m urea, 5 m m diacetyl and 5 m m diacetyl + 10 m m urea, respectively, while treatment with 1·5 m m diacetyl, 5 m m diacetyl and 5 m m diacetyl + 10 m m urea reduced arginine content by 10%, 15% and 11%, respectively, and tryptophan content by 5%, 8% and 8%, respectively. PER, NPR and available methionine were unaffected by diacetyl and/or urea except for a 9% reduction in PER in caseinate treated with 5 m m diacetyl + 10 m m urea.

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

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 &lt; 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 (&gt; 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 &gt; 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: the mechanism of stabilization by formaldehyde

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.

The effect of dietary‐induced changes in milk urea levels on the heat stability of milk

AbstractTwo experiments were carried out to investigate the relationship between milk urea content and the heat stability of the protein in the skim milk. In experiment 1, four cows were offered a diet of grass silage with different amounts of hay and a protein concentrate. Although there were individual differences between the cows in the relationship between coagulation time and milk pH, there was a significant correlation between milk urea content and the maximum coagulation time. In experiment 2, two groups of five cows were given a basal diet of hay with a supplement of either barley or soya bean meal. These diets were exchanged weekly over a 3 week period. A significant correlation between milk urea content and maximum coagulation time was observed, and a close relationship between milk urea content and the mean urea content of the blood found. It is concluded that, although milk urea content is not the sole determinant of coagulation time, there is considerable potential for manipulating the urea content of milk in order to increase the heat stability.

Influence of some polyvalent organic acids and salts on the colloidal stability of milk

Fifteen polyvalent organic salts and acids increased, some very markedly, the heat stability of milk progressively with increasing concentration; acetate and mesaconic acid had no effect, and maleate caused destabilization. Citrate, oxalate and EDTA markedly reduced the [Ca+] in the milk and increased its ethanol stability by shifting the ethanol/pH profile to lower pH values. These additives ≥ 20 mmol/l prevented the coagulation of milk by rennet, but the other additives caused only slight increases in RCT.Citrate, EDTA and oxalate disrupted casein micelles and would therefore not be suitable additives: however, the other acids and salts did not alter the appearance of the milk and might be recommended as suitable additives to improve the heat stability. In terms of the response to the level of Ca2+ and colloidal calcium phosphate in milk, there appeared to be a correlation between ethanol stability and rennet coagulation time.

Effect of homogenization on the heat stability of milk

SummaryThe effect of homogenization on the heat stability characteristics of milk was examined. The heat stability of homogenized milk, as determined by the time taken for protein clots to form when heated at 140 °C, was reduced with increasing pressure in the range 3·5–34·5 MPa. The heat stability of homogenized milks was greater for samples obtained in the summer months than for those obtained in the winter. The general destabilizing effect of homogenization could be partly offset by 2-stage homogenization (20·7 MPa followed by 3·5 MPa), addition of phosphate stabilizers (0·08% w/v) or homogenization at a high temperature (65 °C). Whilst homogenized and unhomogenized milks reacted similarly to the addition of Ca, phosphate stabilizers, sulphydryl-blocking and oxidizing agents, the effects of season, addition of urea or formaldehyde were different for homogenized milk.

Heat stability of milk: influence of cationic detergents on pH sensitivity.

The heat stability--pH profile of milk is shifted to more acidic values by anionic compounds such as SDS, lysolecithin and beta-lactoglobulin and to more alkaline values by cationic compounds such as quaternary ammonium compounds. Proline, which reduces the hydrophobicity of proteins, destabilized milk slightly. The results suggest that the heat stability of milk and the shape of the HCT-pH curve may be dependent on micellar charge effects.

Heat stability of milk: influence of modification of lysine and arginine on the heat stability-pH profile.

Several dicarbonyl compounds (glyoxal, substituted glyoxals, diacetyl and 1, 2-cyclohexanedione) had a marked stabilizing effect on the heat stability of milk, especially in the presence of urea. These reagents are believed to modify arginine more or less specifically suggesting an important role for arginine residues in heat stability. In contrast, modification of lysine residues with dansyl chloride, acetic anhydride or cyanoborohydride had little effect on maximum heat stability although it did alter the HCT-pH profile. Since diacetyl is a natural constituent of fermented milks and cheese, it may be acceptable as an additive to increase the heat stability of milk.

Heat stability of milk: synergic action of urea and carbonyl compounds

SUMMARYUrea stabilizes milk to heat only in the presence of a carbonyl compound, such as a reducing sugar. Low molecular weight carbonyl compounds can increase stability in the absence of urea whereas larger molecules, e.g. hexoses and reducing disaccharides, cannot. However, the efficacy of all carbonyl compounds is increased considerably in the presence of urea. Aldehydes and ketones of similar molecular weights appear to be equally effective stabilizers. The optimum concentration of lactose in milk for stability was approximately 1% irrespective of urea concentration within the range 5–20 mM. However, modifying the lactose content of concentrated milks (approximately 8.5% protein) had little effect on heat stability.