The heat stability of milk

Milk proteins: physicochemical and functional properties

Because of the growing trend toward widespread use of protein ingredients in food formulation and fabrication, an understanding of the relationships between the physical properties of proteins and their behavior in food systems is desirable. A range of milk-derived protein preparations, i.e., dry milk, milk proteins, caseins, whey proteins, and lactalbumin, are used in a range of food products for their specific functional attributes. In this paper some of the apparent relationships between the properties of the protein components and specific functional properties are discussed. Thus, the roles of milk proteins in determining some important physical characteristics (i.e. color, bulk density, sinkability, dispersibility) of milk powders and their involvement in a range of functional properties (water holding, solubility, rheological behavior, gelation, film formation, emulsification, and foaming) are reviewed. Because of the various methods and conditions used in determining functional properties and the variability in composition of preparations it is difficult to compare data and/or reconcile differences in published information. The desirability of developing standard methods is emphasized.

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: interrelationship between assay temperature, pH and agitation

As determined by the standard subjective assay procedure, the minimum in the heat stability--pH curve of milk persisted down to at least 116 degrees C. However when samples were not agitated during heating the minimum became progressively less pronounced as the assay temperature was lowered and it disappeared at approximately 116 degrees C. Activation energies (Ea) for unagitated samples were approximately 30 Kcal/mole at pH 6.87 (maximum) and at pH 7.28, throughout the temperature range 116--145 degrees C and for the pH 6.95 (minimum) sample at 116--125 degrees C; however Ea for the pH 6.95 sample increased to approximately 100 Kcal/mole in the range 127--135 degrees C suggesting that some highly temperature-dependent reaction had occurred and caused premature coagulation at certain pH values, i.e. to a heat stability minimum. The stability of concentrated milk (20% total solids) was very low at pH values above 6.9, regardless of whether the samples were agitated or not during heating and the maximum/minimum in the heat stability--pH curve persisted down to at least 90 degrees C in both agitated and quiescent samples.

A differential scanning calorimetric study of the thermal denaturation of bovine beta-lactoglobulin. Thermal behaviour at temperatures up to 100 degrees C

The thermal behaviour of beta-lactoglobulin in aqueous solutions was followed by differential scanning calorimetry in the temperature range from 40 to 100 degrees C. From the results the following information is obtained. 1. The DSC thermograms show a large transition peak in the temperature range from 60 to 90 degrees C, which reflects the denaturation (unfolding) of beta-lactoglobulin. 2. From the peak surface an apparent denaturation enthalpy delta H = 230 +/- 15 kJ/mol is calculated at pH 6.5. 3. The temperature of maximum deflection of the DSC curve is dependent on the heating rate. Extrapolation to zero heating rate results in a denaturation temperature of 70.4 +/- 0.5 degrees C at pH 6.7. 4. A kinetic analysis of the DSC curves shows that the denaturation of beta-lactoglobulin is of the first order at temperatures between 65 to 72 degrees C. The apparent activation energy amounts to 343 kJ/mol, calculated according to the method of Kissinger. After comparison of the results with data from the literature, it was concluded that 70 degrees C is a critical temperature for the denaturation of beta-lactoglobulin. Above 70 degrees C the denaturation behaviour is changed, probably because of the starting of the aggregation process. This change is indicated by the transition temperature of the DSC curve at the corresponding heating rate.

The role of calcium, phosphate and citrate ions in the stabilization of casein micelles

To obtain greater insight into the interaction of Ca, citrate and phosphate ions with casein, 31PNMR measurements were performed on combinations of these ions with alphas- and kappa-caseins. It was found that addition of alphas-casein to a Ca phosphate solution in D2O at 27 degrees C and pD 6.4 resulted in a downfield shift of the 31P singlet. An almost identical shift was observed with kappa-casein, but no shift was found when only phosphate ions were present or when Ca2+ were added to phosphate ions in the absence of casein. Separate experiments with poly-L-lysine, mol. wt approx. 35,000, resulted in similar downfield 31P chemical shifts of Ca phosphate as with both caseins, whereas no shift was observed when poly-glycine was added. From these results it can be concluded that Ca and phosphate ions associate with casein in a co-operative manner, probably in the way described by ter Horst (1963) as a complex with the NH3+-groups of lysine or arginine in a structure such as: (casein--NH3+)--PO43---Ca2+. The formation of this complex may be enhanced by citrate ions, since preliminary results have shown that addition of Na citrate to a solution of alphas-casein with added Ca phosphate produces a broadening of the 31P signal as well as a chemical shift.

Stability of buffalo casein micelles

Buffalo skim-milk is less heat stable than cow skim-milk. Interchanging ultracentrifugal whey (UCW) and milk diffusate with micellar casein caused significant changes in the heat stability of buffalo casein micelles (BCM) and cow casein micelles (CCM). Buffalo UCW dramatically destabilized CCM, whereas buffalo diffusate with CCM exhibited the highest heat stability. Cow kappa-casein stabilizes alphas-casein against precipitation by Ca better than buffalo kappa-casein. About 90% of alphas-casein could be stabilized by kappa:alphas ratios of 0.20 and 0.231 for cow and buffalo, respectively. Sialic acid release from micellar kappa-casein by rennet was higher than from acid kappa-casein in both buffalo and cow caseins, the release being slower in buffalo. The released macropeptide from buffalo kappa-casein was smaller than that from cow kappa-casein as revealed by Sephadex gel filtration. Sub-units of BCM have less sialic acid (1.57 mg/g) than whole micelles (2.70 mg/g). On rennet action, 47% of bound sialic acid was released from sub-units as against 85% from whole micelles. The sub-micelles are less heat stable than whole micelles. Among ions tested, added Ca reduced heat stability more dramatically in whole micelles, whereas added phosphate improved the stability of micelles and, more strikingly, of sub-micelles. Citrate also improved the heat stability of sub-micelles but not of whole micelles.