Partial renneting of pasteurised bovine milk: Casein micelle size, heat and storage stability (original) (raw)
Related papers
Comparison of casein micelles in raw and reconstituted skim milk
Journal of Dairy Science, 2007
During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spraydrying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.
International Dairy Journal, 1997
This work studied the effect of the absence of a free thiol (-SH) group in fi-lactoglobuhn (p-lg) on the heat stability of a synthetic bovine milk system and the rennet coagulation characteristics of a similar system following heating. The heat coagulation time (HCT) versus pH profiles of serum protein-free bovine casein micelles (SPFCM) dispersed in synthetic milk salts buffer containing added porcine fi-lg did not exhibit a maximum-minimum typical of bovine milk, whereas a similar system containing added bovine fi-lg had a typical maximum-minimum;
Journal of Agricultural and Food Chemistry, 2007
The effects of heat treatment and limited κ-casein hydrolysis on the micelle/serum distribution of the heat-induced whey protein/κ-casein aggregates were investigated as a possible explanation for the gelation properties of combined rennet and acid gels. Reconstituted skim milk was submitted to combinations of 0-67% hydrolysis of the κ-casein at 5°C and heat treatment at 90°C for 10 min. The protein composition of the ultracentrifugal fractions was obtained by reverse-phase highperformance liquid chromatography (RP-HPLC). The aggregates contained in each phase were isolated by size-exclusion chromatography and analyzed by RP-HPLC and sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Upon heating only, 20-30% of the total κ-casein dissociated, while 20-30% of the total whey protein attached to the micelles. When heated milk was renneted, little changes were observed in the distribution and composition of the aggregates. Conversely, the heat treatment of partially renneted milk induced the formation of essentially micelle-bound aggregates. The results were discussed in terms of the preferred interaction between hydrophobic para-κ-casein and denatured whey proteins.
LWT - Food Science and Technology, 2007
Skim milk was adjusted to pH values between 6.5 and 7.1 and heated at 90 1C for times from 0 to 30 min. After heat treatment, the samples were readjusted to the natural pH (pH 6.67) and allowed to re-equilibrate. High levels of denatured whey proteins associated with the casein micelles during heating at pH 6.5 (about 70-80% of the total after 30 min of heating). This level decreased as the pH at heating was increased, so that about 30%, 20% and 10% of the denatured whey protein was associated with the casein micelles after 30 min of heating at pH 6.7, 6.9 and 7.1, respectively. Increasing levels of k-casein were transferred to the serum as the pH at heating was increased. The loss of k-casein and the formation of para-k-casein with time as a consequence of the chymosin treatment of the milk samples were monitored by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The loss of k-casein and the formation of para-k-casein were similar for the unheated and heated samples, regardless of the pH at heating or the heat treatment applied. Monitoring the gelation properties with time for the chymosin-treated milk samples indicated that the heat treatment of the milk markedly increased the gelation time and decreased the firmness (G 0) of the gels formed, regardless of whether the denatured whey proteins were associated with the casein micelles or in the milk serum. There was no effect of pH at heat treatment. These results suggest that the heat treatment of milk has only a small effect on the primary stage of the chymosin reaction (enzymatic phase). However, heat treatment has a marked effect on the secondary stage of this reaction (aggregation phase), and the effect is similar regardless of whether the denatured whey proteins are associated with the casein micelles or in the milk serum as nonsedimentable aggregates.
Interactive Effects of Milk Fat Globule and Casein Micelle Size on the Renneting Properties of Milk
Food and Bioprocess Technology, 2014
The size of the casein micelles (CM) and the milk fat globules (MFG) vary depending on farming factors, such as seasonal variation and stage of lactation, and cow genetics. The MFG and CM size of milk can influence the renneting behavior and texture of manufactured dairy products. In this work, we investigated the combined effects of MFG and CM size on the onset of gelation, the maximum rate of gelation, the value for G′ 60 min (the final storage modulus) and G″ 60 min (the final loss modulus), and tan δ upon renneting. Fractionation of MFG on the basis of size was carried out using laboratorybased centrifugation, whereas milk of predominantly large (184-218 nm) or small (147-159 nm) CM was selected naturally on-farm. Casein micelle size had the dominant effect on curd firmness and gelation rates of milk, where small CM milk formed rennet gels earlier and resulted in a firmer gel than milk with large CM. However, MFG size also influenced the renneting properties. The strongest rennet gels were obtained when large MFG (3.88-5.78 μm) was combined with small CM (153-159 nm). Selecting milk on the basis of the microstructure of key milk components could be achieved by natural selection of dairy cows or via fractionation technologies. Selection may provide a useful tool for efficient manufacturing of different dairy products based on the desirable characteristics specific to each.
Addition of sodium caseinate to skim milk inhibits rennet-induced aggregation of casein micelles
Food Hydrocolloids, 2012
The objective of this paper was to observe the rennet-induced aggregation behaviour of casein micelles in milk in the presence of additional sodium caseinate. Analysis of the centrifugal supernatants by size exclusion chromatography confirmed an increase in the soluble protein in the milk serum phase after addition of sodium caseinate. Although the total amount of k-casein hydrolyzed over time was not affected, there was a significant effect of soluble casein on milk gelation, with a dose-dependent decrease of the gelation time as measured by rheology. Light scattering experiments also confirmed that the addition of soluble caseins inhibited the aggregation of casein micelles. Addition of 1 mM CaCl 2 prior to renneting increased the extent of rennet aggregation in samples containing additional sodium caseinate, but the inhibiting effect was still evident. The amount of soluble casein (as measured by chroma tography) significantly decreased after renneting, suggesting its association with the micellar fraction. Supporting experiments carried out with purified fractions of soluble caseins demonstrated that both a scasein and b-casein played a role as protective colloids (increasing steric repulsion) during renneting. It was concluded that the inhibiting effect observed during gelation was caused by the adsorption of soluble casein molecules on the surface of rennet-altered casein micelles.
Physicochemical Properties of Acylated Casein Micelles in Milk
Journal of Food Science, 2002
The objective of this study was to determine the effects of acetylation or succinylation on the physicochemical properties of casein micelles in milk. Analysis of untreated and modified milk samples revealed solubilization of caseins and calcium phosphate from the micelle under mild acylation treatments. Solvation of micelle was affected, whereas no influence in size dia and potential zeta arose from the acylation treatments. Physicochemical characteristics of acetylated milk sample were intermediate between untreated and succinylated ones. Changes were also observed after calcium addition to reconstituted milk. Calcium supplementation counteracted the effect of chemical modification of caseins by reducing the susceptibility of casein micelle to disassembly.
2010
Mastitis deteriorates the quality of raw milk and higher proteolytic activity in mastitic milk has been reported. There are no published studies on the effects of health status of the animal on the casein micelle microstructure, therefore the specific objective of this study was to determine the effects of somatic cell count (SCC) on the dimension and microstructure of casein micelles in raw milk. Raw milk samples were collected from the individual quarter of the dairy cattle in three different levels of SCCs (i.e., low: SCC<200,000, medium: 200,000<SCC<800,000 and high: SCC >800,000 cells/ml) and prepared for study by Scanning Electron Microscopy (SEM). The results suggested that the size of casein micelle changes in mastitic milk, and there were significant differences in casein micelle microstructure between high and low SCC milk samples. The mean diameter of micelles in medium and high SCC specimen decreases dramatically and SEM micrographs revealed that aggregation of casein micelles increases considerably in raw milk samples, especially in high SCC milk, except for that of low SCC. Increase the activity of proteolytic enzymes such as plasmin and lysosomal enzymes (Elastase, Cathepsin B, D and G, etc) was realized as the main reason of hydrolyzing casein in milk, especially in milk with high SCC, which can be the cause of changes in size and tendency to aggregation of casein micelles in mastitic milk. In addition, decreased milk synthesis ability and reduced steric and electrostatic repulsion among casein micelles might be other causes for this phenomenon.
Effect of temperature and pH on the interactions of whey proteins with casein micelles in skim milk
Food Research International, 1996
Skim milk was heated at temperatures in the range 7559O"C, at pH values of 6.8, 6.2 and 5.8. The amounts of a-lactalbumin and p-lactoglobulin which interacted with the casein micelles during heat treatment were quantified by SDS-polyacrylamide gel electrophoresis of the micellar fractions isolated by ultracentrifugation. Both o-lactalbumin and ,&lactoglobulin appeared to interact similarly with casein micelles at temperatures up to 85°C. The amount of whey protein complexed with micelles increased with time, reaching plateau values that, at the highest temperatures, were comparable with the quantity present in the original skim milk. In general, faster reaction of the whey proteins with the micelles was found at lower pH and higher temperatures. The rates and extent of the reaction changed also when additional cr-lactalbumin and P-lactoglobulin isolates were added to milk before heating. The reaction between cl-lactatbumin and casein micelles depended to a relatively small extent upon environmental variations (pH and temperature), while /I-lactoglobulin interactions were more affected, so that a more complex behaviour may be attributed to the latter protein. Copyright 0 1996 Canadian Institute of Food Science and Technology