Heat Stability Of Milk Research Articles

Mammals, Milk, Molecules, and Micelles

After a brief description of my family background and school days, my professional career as a dairy scientist is described under three headings: research, teaching, and writing. My research activities fall into four areas: biochemistry of cheese, fractionation and characterization of milk proteins, heat stability of milk, and dairy enzymology. Finally, I offer some advice to young scientists.

Influence of oestrus on the heat stability and other characteristics of milk from dairy goats

We examined the heat stability, somatic cell count (SCC), pH, fat, protein and lactose content of milk from goats during the oestrous period, in order to investigate evidence of possible oestrus effects on milk physical and chemical properties. Goats free from mammary infections were ranked on average SCC from three tests so that they could be stratified randomly in pairs to synchronized oestrus or left as unsynchronized non-oestrus controls. The synchronisation consisted of insertion of an intravaginal progesterone-releasing device for 17 d, and introduction of the bucks the day of the device removal (D0). The repeated measurements analysis of variance model included the fixed effects of the experimental group (oestrus or control) and day and the corresponding interaction and also the random effect of doe. Reduced milk-heat stability, increased SCC, increased protein content and reduced pH were found in the milk samples of the oestrus group on D1, 2 and 3. The fat and lactose content of the milk was not affected by oestrus. These data indicate that the milk of goats during the mating period has reduced heat stability and, therefore, that dilution into bulk tanks should be recommended to avoid clotting when milk is intended for high thermal treatment.

Influence of composite κ-casein and β-lactoglobulin genotypes on composition, rennetability and heat stability of milk of cows of Slovak Pied breed

The effect of the phenotypic combination of genetic variants κ-CN/β-Lg on the contents of technologically and nutritionally important components of milk as well as technological properties of milk was investigated in this paper. The samples of milk were collected from 72 cows of Slovak Pied breed in the second lactation coming from three farms, once from the different lactation stage. The genotyping of milk samples was determined by the horizontal electrophoretic separation of milk proteins on starch gel with addition of urea and mercaptoethanol. Statistical analysis was carried out in the programmes Statistica 6.0 and Statgraphic Plus 5.1 by the least-squares method using the GLM. A non-significant association of <i>B</i> allele in the κ-CN/β-Lg phenotypic combination with milk fat, crude protein content and true protein content was found out. A positive significant effect of κ-CN/β-Lg <i>BB/BB</i> on casein content and casein number was confirmed. The value of casein number increased in the following order: <i>BB/BB</i> > <i>AB/AB</i> > <i>AA/AA<i> (78.43%:77.53%:75.78%). With regard to whey proteins and heat stability, a negative effect of B allele in the phenotypic combination κ-CN/β-Lg was found out.

Influence of added lyophilized butter serum on the heat stability of unconcentrated and concentrated bovine milk

The influence of added lyophilized butter serum on the heat stability of milk was investigated. The addition of lyophilized serum from salted butter to unconcentrated skimmed milk (SM) reduced the heat coagulation time (HCT) at, and increased the pH of, the maximum in the pH–HCT profile and caused disappearance of the minimum. NaCl had similar effects on the heat stability of SM as lyophilized salted butter serum, whereas lyophilized serum from unsalted butter had little effect. The addition of lyophilized salted butter serum to concentrated skimmed milk (CSM) also shifted the pH of maximum heat stability to a higher value and, at certain concentrations, increased the maximum HCT; similar effects were obtained on addition of NaCl, but lyophilized serum from unsalted butter had little effect. These results suggest that the effects of lyophilized serum from salted butter on the heat stability of SM or CSM are due primarily to the presence of a high level of NaCl in this serum.

Effect of hydrolysis of casein by plasmin on the heat stability of milk

Abstract This study examined the effect of hydrolysis of casein by added plasmin (6 mg L −1 ) on the heat stability of raw, pre-heated, serum protein-free or concentrated skim milk. Plasmin activity markedly affected the heat stability–pH profile of skim milk and serum protein-free milk, apparently by altering the properties of the casein micelles. It is probable that changes in the surface charge of the micelles, as a result of the hydrolysis of caseins, contributed to this effect. Hydrolysis by plasmin reduced the zeta-potential of the casein micelles from ∼−19 to ∼−16 mV. The effect of hydrolysis of casein by plasmin on the heat stability of pre-heated milk was less pronounced, shifting the heat stability–pH profile to more alkaline values; the heat stability of concentrated milk was unaffected by plasmin. A very high (50 mg L −1 ) level of added plasmin resulted in clearing of the skim milk; the L * value decreased from ∼75 to ∼35 after 24 h incubation at 37 °C. Clearing was correlated with a change in casein micelle diameter from an initial value of ∼175 to ∼250 nm. It is suggested that plasmin-induced changes in zeta-potential may promote micellar aggregation or changes in micelle stucture.

The influence of main milk components on the hydrophobic interactions of milk protein system in the course of heat treatment

The effect of the concentration of protein, calcium and lactose on heat-induced changes in the surface hydrophobicity and interactions of milk proteins were examined. It was observed that the protein–protein interactions increased with increasing concentration of protein during the heat treatment of milk, and the micelles reorganised into a more compact structure. However, the heat stability of milk increases with decreasing concentration of protein and the changes that occur in protein structure are less, apparently in heat treatment. It was observed that the critical temperature, when the drastic changes in protein surface hydrophobicity start, was higher in lactose concentrated-milk than others. The heat stability of milk increased with increasing concentration of lactose. An increase in the calcium ion concentration of milk caused an increase in colloidal stability during the heat treatment. It is found that when calcium concentration of milk decreased, the critical temperature needs for hydrophobic interactions between the proteins increased. Overall changes occurred above 80 °C.

Impact of source and level of calcium fortification on the heat stability of reconstituted skim milk powder.

Calcium enrichment of food and dairy products has gained interest with the increased awareness about the importance of higher calcium intake. Calcium plays many important roles in the human body. Dairy products are an excellent source of dietary calcium, which can be further fortified with calcium salts to achieve higher calcium intake per serving. However, the addition of calcium salts can destabilize food systems unless conditions are carefully controlled. The effect of calcium fortification on the heat stability of reconstituted skim milk was evaluated, using reconstituted skim milks with 2 protein levels: 1.75 and 3.5% (wt/wt) prepared using low and high heat powders. Calcium carbonate, phosphate, lactate, and citrate were used for fortification at 0.15, 0.18, and 0.24% (wt/wt). Each sample was analyzed for solubility, heat stability, and pH. The addition of phosphate and lactate salts lowered the pH of milk, citrate did not have any major effect, and carbonate for the 1.75% protein samples increased the pH. In general, changes in solubility and heat stability were associated with changes in pH. Calcium addition decreased the solubility and heat stability. However, interestingly, the presence of carbonate salt greatly increased the heat stability for 1.75% protein samples. This is due to the neutralizing effect of calcium carbonate when it goes into solution. The results suggested that the heat stability of milk can be affected by the type of calcium salt used. This may be applied to the development of milk-based calcium enriched beverages.

Heat stability of milk

The heat stability of milk has been the subject of a considerable amount of research for about a century. This research has been aimed mainly at understanding the effects of compositional and processing factors on heat stability and elucidating the mechanisms of protein coagulation. This paper provides an overview of the factors that influence the pH dependence of the heat stability of normal and concentrated milks. The principal heat‐induced changes in the milk system that contribute to coagulation are discussed. Current knowledge of the mechanisms of heat coagulation in normal and concentrated milks is also reviewed.

Heat-induced coagulation of goat milk: modification of the environment of the casein micelles by membrane processes

Unlike bovine milk, goat milk is characterised by a very low heat stability at its natural pH (i.e., 6.6-6.7). In the present work, combined membrane processes were used to evaluate how modi- fying the environment of the casein micelles affects the heat stability of milk. It was shown that the in- crease of 1.4 times of the caseins: whey proteins ratio, either through a reduction of whey proteins content or selective concentration of caseins, increased the heat coagulation temperature from 135 o C to 141 o C. Further improvement of the stability was obtained by concomitant dilution of both whey proteins and soluble low molecular weight components. From these results, we propose that the de- stabilising effect of whey proteins at the natural pH is linked to their ability to sensitise casein mi- celles to heat-induced precipitation of calcium phosphate. Goat milk / heat stability / membrane processing / casein:whey proteins ratio

Effect of Transglutaminase on the Heat Stability of Milk: A Possible Mechanism

Treatment of milk with transglutaminase (TGase) affects its heat stability, but the manner in which it does so depends on whether or not the milk had been preheated before incubation and on the temperature of preheating. In raw milk, it appears that cross-link formation between the individual caseins is responsible for preventing the dissociation of κ-casein from the micelles at pH values in the region of minimum stability. In milks preheated before incubation with TGase, denaturation of whey protein may have allowed the formation of cross-links by TGase between denatured whey proteins and the individual caseins which, in combination with cross-linking of the caseins, contributed to greatly improved heat stability at pH>6.5. It appears from the results of this study that TGase has potential commercial applications as a food-grade additive capable of improving the heat stability of milk.

Short Communication: Influence of Transglutaminase on the Heat Stability of Milk

Skim milk powders were prepared from control and transglutaminase-treated skimmed milk. The heat stability of reconstituted transglutaminase-treated skimmed milk (9.0% total solids) was markedly increased in the pH region of minimum stability (pH 6.8 to 7.1) compared with control milk, while the heat stability of reconstituted concentrated transglutaminase-treated skimmed milk (22.5% total solids) increased progressively as a function of pH relative to control milk. The effect of transglutaminase treatment on the heat stability of skimmed milk may have commercial applications, but extensive research is necessary to gain a better understanding of the mechanism by which transglutaminase improves heat stability.

Effect of beta-lactoglobulin and precipitation of calcium phosphate on the thermal coagulation of milk.

The effect of beta-lactoglobulin and heat-induced precipitation of calcium phosphate on the pH dependence and mechanism of thermal coagulation of milk throughout the pH range 6.3-7.3 was studied using serum protein-free milk and sodium caseinate as models for micellar and non-micellar milk protein systems respectively. It appears that the specific effect of beta-lactoglobulin at the pH of maximum stability may be related to its ability to chelate calcium. The effect of beta-lactoglobulin at the pH of minimum stability does not appear to be directly related to heat-induced dissociation of K-casein or micellar integrity but may be due to its ability to sensitize casein micelles to heat-induced precipitation of calcium phosphate, by increasing micellar hydrophobicity. The extent of heat-induced precipitation of calcium phosphate, as a function of pH, is an inverse reflection of the pH dependence of heat stability. Micellar integrity appears to play a critical role in the heat stability of milk but for reasons not previously appreciated.

Correlación entre la termoestabilidad y prueba de alcohol de la leche a nivel de un centro de acopio lechero

Resumen en: The alcohol test is used by the milk industry as reception test for milk in the plant, to measure the heat stability of milk. The initial recommended eth...

Effect of high pressure on heat and ethanol stabilities of milk

The effect of high pressure (in range from 200 to 1000 MPa) and periods of exposure time (from 15 to 35min) on the changes of selected physico-chemical characteristics of skim milk, particularly considering its heat and ethanol stability, was studied. Due to High pressure the conformation of milk proteins and the milk salt system were changing. Pressurization caused slightly change in the ethanol stability of milk. The influence of pressurization on heat stability was more evident. The heat stability of milk increased with an increase in pressure and exposure time.

The Two-Stage Coagulation of Milk Proteins in the Minimum of the Heat Coagulation Time-pH Profile of Milk: Effect of Casein Micelle Size

Milks with casein micelles larger or smaller than control milk were prepared by differential centrifugation. The heat stability of these modified milks increased markedly throughout the pH range 6.4 to 7.1 with decreasing casein micelle size. Within the region of the minimum in the heat coagulation time-pH profile, the control milk coagulated by a two-stage process, but the modified milks, because of their narrower casein micelle size distribution, coagulated by a single-stage process at the pH of minimum stability. The content of κ-CN and protein hydration increased as the size of the casein micelles decreased, and the level of glycosylation of κ-CN and protein surface hydrophobicity increased as a function of micelle size. The effect of casein micelle size on the heat stability of milk is likely to be related to changes in the above physico-chemical properties.

Effects of phenolic compounds on the heat stability of milk and concentrated milk

A methanol extract of green tea was fractionated on Sephadex LH-20. The compounds eluted were identified by thin layer chromatography as catechin–epicatechin, gallocatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate. When added to milk at 2·0 g/l, these polyphenols, apart from the catechin–epicatechin mixture, increased the heat stability of skim milk, particularly in the region of the minimum (pH 6·8–7·1). When added at 0·4 g/l, green tea polyphenols also increased the heat stability of concentrated milk. The effects of other phenolic compounds on the heat stability of milk were also examined. Chlorogenic acid, guaiacol, thymol, vanillin, butylene hydroxyanisole, propyl gallate and butylene hydroxytoluene did not affect the heat stability of milk or concentrated milk. Quinic acid markedly reduced the heat stability of skim milk. Pyrogallol, catechol, tannic acid, ellagic acid, phloroglucinol and gallate converted a type A heat coagulation time–pH profile to a type B profile. Ferulic acid and vanillic acid increased heat stability in the region of the maximum, with little effect on the minimum, and stability did not recover at pH values on the alkaline side of the minimum. Caffeic acid increased the heat stability of milk while the related non-phenolic compounds 2,5-dimethoxycinnamic acid and 3,4-dimethoxycinnamic acid had no effect.

Proposed mechanism for the effect of polyphenols on the heat stability of milk

Caffeic acid at 5.5 mmol l−1 markedly enhanced the heat stability of milk at 140°C, the minimum was eliminated and stability increased as a function of pH; the apparent activation energy for coagulation was increased. The stabilizing effect of caffeic acid was reduced or eliminated when milk was heated at 150°C or when the air in the headspace was replaced by N2. The stabilising effect of caffeic acid was reduced when stability was assessed as the temperature required to cause instantaneous coagulation, i.e., a maximum and minimum were evident. The thermal degradation/transformation of caffeic acid in milk was a zero-order reaction with an activation energy of ∼40 kJ mol−1 and Q10 value of ∼1.3 from 130–140°C; when the air in the headspace was replaced by N2, thermal degradation/transformation of caffeic acid was reduced. The addition of caffeic acid resulted in a reduction of the reactive lysine and sulphydryl content and inhibited the dissociation of κ-casein-rich protein from the casein micelles in milk on heating. It is postulated that on heating in milk, caffeic acid is thermally oxidised to quinones which then interact with nucleophilic amino acid residues to inhibit κ-casein dissociation from the casein micelle.

Influence of κ-casein and β-lactoglobulin phenotype on the heat stability of milk

Abstract The combined effect of genetic variations of β -lactoglobulin and κ -casein on the heat stability of milk was studied. The phenotype combination β -lactoglobulin AA with κ -casein BB was associated with the highest maximum coagulation time.

Heat stability of milk emulsions: phospholipid–protein interactions

Abstract Certain types of lecithin increase the heat stability of recombined and conventional full-cream evaporated milk but the mechanism is unknown. Previous studies have indicated the importance of fat in the system. New studies have indicated the nature of protein–lecithin interactions involved in improving heat stability. Two likely mechanisms were considered: (a) that lecithin increases heat stability by displacement of or by interaction with surface-adsorbed protein and (b) that lecithin interacts with free protein in solution thereby influencing heat-induced free protein/bound protein interactions. These possibilities were investigated by examining the effect of lecithin on the heat stability of fresh whole milk, unhomogenized and homogenized, and the corresponding skim milk and recombined milk in the absence and presence of indigenous serum protein. Lecithin does not require fat per se to affect heat stability. In addition, it does not exercise its effect on heat stability by interaction with free protein in solution but changes heat stability by targeting the membrane proteins. These findings were obtained for lecithins containing non-acidic and acidic phospholipids.

Influence of κ-casein and β-lactoglobulin genetic variants on the heat stability of milk

SUMMARYHeat coagulation time-pH curves at 140°C were obtained for 43 blended skim milk samples from Holstein cows to determine the effects of genetic variants of κ-casein and β-lactoglobulin on milk heat stability. The blended milk samples were similar in terms of protein content and milk salts, but were genotypically different for κ-casein (AA, AB) and β-lactoglobulin (AA, AB, BB). Type A curves were obtained for all milks. Maximum heat stability was affected by the κ-casein genotype (AB > AA, P < 0·01) but the influence of the β-lactoglobulin genotype was only significant when the κ-casein AA genotype was present (β-lactoglobulin AA > BB, P < 0·0001). Minimum heat stability was significantly higher (P < 0·0001) for milk genotyped κ-casein AB:β-lactoglobulin BB. The effects of milk genotyped κ-casein BB on maximum and minimum heat stability were determined by analysing individual milks: κ-casein BB:β-lactoglobulin AB (n=8) and reconstituted milks: κ-casein BB:β-lactoglobulin AA, AB and BB (n = 17). Type B curves were obtained on three occasions for individual κ-casein BB:β-lactoglobulin AB milk and on five occasions in the case of reconstituted milks with κ-casein BB:β-lactoglobulin AA, AB and BB. This suggests a relationship between the type B curve and the κ-casein B genetic variant. Comparison of the mean values of heat stability at the pH of maximum heat stability of each individual and reconstituted milk genotype suggested that the best genotype for κ-casein in terms of heat stability was BB.