Natural pH Of Milk Research Articles

Calcium-induced skim milk gels: Effect of milk powder concentration and pH on tribo-rheological characteristics and gel physico-chemical properties

The physico-chemical and tribo-rheological characteristics of calcium-induced skim milk gels obtained by heating of milk dispersions prepared with different concentrations of skim milk powder (10, 20 and 30% w/w) and calcium chloride (30, 60 and 90 mmol kg−1) were studied. The effect of pH on the gelation and on the final gel structure was also analyzed. For this purpose, the pH of selected samples was readjusted to the natural pH of milk of 6.66 after the calcium salt addition. The gelation process was analyzed by rheometry throughout temperature and time sweeps. Gelation at temperatures lower than 70 °C was observed in samples with 10% w/w of skim milk powder (SM) and 30 mmol kg−1 of calcium chloride or similar ratio, when the amount of calcium remaining in the serum phase is enough to induce gel formation. The results obtained from temperature sweeps also suggest that the pH strongly affects the temperature at which gelation initiates. Structuring parameters confirmed these results. From time sweeps, it was observed that the kinetics of gelation depended on both composition and pH. Gels obtained using higher SM concentrations (20 and 30% w/w) showed better physical properties (low syneresis and high water holding capacity). Confocal laser microscopy images also showed a more homogeneous structure in those samples. Higher SM and calcium chloride concentrations improved the lubrication characteristics analyzed by tribology. Friction factors at 10 mm s−1 (typical speed in oral processing) decreased as the SM concentration increased. It is concluded that calcium-induced skim milk gels with different microstructure can be obtained by varying the concentration of skim milk powder and calcium salt added, and pH adjustment.

Effects of milk pH alteration on casein micelle size and gelation properties of milk

ABSTRACTThe effects of changes in pH above and below the natural pH of milk (ca 6.6) on the casein micelle size and the gelation properties of the pH adjusted and restored samples were investigated. The size of casein micelles increased at pH 7.0 and 7.5, then started to decrease above pH 8.5. It is postulated that at pH below 8.5 the casein micelles swell, while elevated pH causes their dissociation. Conversely, the size of casein micelles decreased on acidification to pH 5.5 and increased when the pH dropped below 5.5, due to the shrinkage of casein micelles at lower pH before their aggregation at pH below 5.5. In response to neutralising treated milk back to normal milk pH of 6.6, it was found that the casein micelle size of treated milk with a narrow range of pH change between 6.0 and 7.0 was reversible, while beyond this range the structure of casein micelles became irreversible. The restoration of casein micelle size was followed by the restoration of some parameters such as soluble calcium, ethanol stability, and milk whiteness. Acid-induced gelation did not change the elastic modulus, while rennet-induced gelation was affected by initial milk pH. In reference to the size of reversible range elastic modulus (G’) of acid or rennet gels made from restored milk, the sizes were similar to control milk after 6 h of gelation.

Heat induced casein–whey protein interactions at natural pH of milk: A comparison between caprine and bovine milk

This paper is a study on the distribution of the denatured whey proteins and κ-casein in soluble and micelle-bound complexes in heat treated caprine and bovine milk (90°C, 10min) at natural pH (6.71). Proteins were fractionated using fractionation technique based on renneting and were analysed by three electrophoretic techniques: native PAGE, SDS-PAGE under reducing and non-reducing conditions. Lower than 3% of the total β-LGs remained stable after heat treatment of both milk species, but bovine α-LA was more heat stable than its counterpart in caprine milk (29.6% against 3.82%). Denatured caprine whey proteins (>95%) were part of micelle-bound complexes whereas soluble complexes were not observed. Conversely, about 30% of denatured bovine whey proteins were involved in soluble complexes. About 24.2% of total κ-CN was included into complexes formed in heat-treated bovine milk whereas in heat-treated caprine milk this percentage is about three times higher. Caprine micelle-bound complexes, apart from whey proteins and κ-casein included also β-casein and αs2-casein, which were not found in their bovine counterparts. This knowledge could be very useful in understanding the differences in technological–functional properties of caprine and bovine milk and to enable better control of dairy processes.

Effect of the pH of Heating on the Qualitative and Quantitative Compositions of the Sera of Reconstituted Skim Milks and on the Mechanisms of Formation of Soluble Aggregates

The effect of the pH of heating (6.3-7.3) on the composition of sera in reconstituted skimmed milks was investigated. A combination of SDS-PAGE analysis and size exclusion chromatography (SEC) combined with an original approach to the analysis of the SEC profiles was performed. The composition of the sera varied greatly when the pH of heating was adjusted below and above the natural pH of milk. The formation, composition, and concentration of heat-induced soluble complexes depended on the combination of the effect of adjusting the pH of the milk and the heat treatment. Two types of mechanism for the formation of soluble aggregates appeared to exist, depending on the pH of the milk. The first type results from the formation of WP/kappa-casein aggregates at the surface of the micelle, and these were detached partially into the serum in larger amount as the pH increased up to 6.7, where it reaches a maximum. The second type of complexes, whose amount increased as the pH of heating increased from 6.7 to 7.3, may be formed between caseins (kappa- but also perhaps some alpha(s)-casein) and aggregated WP resulting in complexes that are smaller in size and with a higher kappa-casein/whey protein ratio than the first type.

Quantification of heat-induced casein–whey protein interactions in milk and its relation to gelation kinetics

This paper describes a quantitative study on the distribution of denatured whey proteins over whey protein aggregates and whey proteins associated with the casein micelles in heated milk. This was done by combination of enzymatic fractionation and capillary electrophoresis. More severe heat treatment at the natural pH of milk caused more denaturation, but the ratio of denatured whey proteins associated with the casein micelle and present in aggregates remained constant. The distribution is related to changed acid-induced gelation properties and textural properties of milk-derived products. We clearly demonstrated that the observed shift in gelation pH of heated milk is linearly correlated with the distribution of denatured whey proteins. Quantitative description will allow better control and tuning of the final gel properties of milk-derived products.

The effect of intravenous infusion of glucose on ethanol stability of milk in Holstein dairy cows

The alcohol test is used as the initial classification of milk in dairy farms. It’s used as a measure of the natural pH of milk, which is a critical factor for stabilizing casein micelles in milk serum phase during heating (Barros et al., 2000). In practical conditions the test could be also positive immediately after milking, and this type of milk is rejected by dairy processing industry. An experiment was conducted to evaluate the effects of negative energy balance and low level of blood glucose on incidence of alcohol-positive milk in Holstein high milking cows (Sobhani et al., 2002).

Thermal stability of acid proteinases.

Milk-clotting enzymes are used during the production of cheese to coagulate the casein, allowing the formation of a three-dimensional network that entraps the milk fat. Commercially available milk-clotting enzymes differ with respect to source, specificity, optimum pH and thermostability. All are acid proteinases that can cleave κ-casein resulting in the coagulation of milk. Chymosin (EC 3.4.23.4) is specific for the Phe–Met bond in κ-casein at the natural pH of milk (6·7). Recombinant chymosin is available commercially from a variety of sources and has a maximum activity at 40 °C. Recombinant chymosins are purified from the fermentation of recombinant strains of Aspergillus niger, Asp. oryzae or Kluyveromyces marxianus. These enzyme preparations are chemically and functionally identical to calf chymosin. Rennets are purified from the abomasum of bovines and can contain from 60 to 100% chymosin with the remainder being primarily bovine pepsin (Wigley, 1996). Microbial proteinases (EC 3.4.23.6) are generally more proteolytic than chymosin, with varying heat stability. These enzymes liberate more non-protein N from casein and can cleave α- and β-casein as well as κ-casein at the natural pH of milk. Acid proteinases from Cryphonectria parasitica are more heat labile than those from Rhizomucor miehei, which are characterized as thermostable (Ernstrom & Wong, 1974).The objective of this research was to characterize milk-clotting enzymes with respect to thermal inactivation in skim milk. This information has applications in milk and whey processing.

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.

The Ion Interaction Reversed Phase High-Performance Liquid Chromatography and the Voltammetry With Microelectrodes In the Analysis of Milk

Abstract Some anionic species present at the natural pH of milk (6.72), namely hydrogen-orthophosphate, L(+)-ascorbate and hydrogencitrate were detected in samples of crude bovine-milk both by ion interaction reverse phase HPLC chromatography and by amperometry based on microelectrodes. The RSD ranged between 2 and 5 % for the former technique and between 3 and 4 % for the latter. the accuracy was evaluated by comparing the data obtained with the two techniques and discrepancies between the methods never exceeded 9% .

A study of surface hydrophobicity of milk proteins during enzymic coagulation and curd hardening

SummaryThe formation of hydrophobic sites on the surface of casein micelles as a consequence of rennet action has been followed through the binding rate of a fluorescent probe and its distribution between a free and an ‘aggregated’ protein fraction. The variation of this parameter has been related to clotting time and curd hardening kinetics.Results show that a first aggregation of casein through hydrophobic sites interaction began as soon as rennet was added to milk. At the natural pH of milk, the sol-gel transition occurred when all the casein micelles were already involved in large aggregates. This was not the case with slightly acidified milk (pH 6·5 and 6·3) where clotting occurred well before the first aggregation step had been completed. The surface hydrophobicity of casein continued to increase in the curd due to continuing enzymic action and structural rearrangements. When this process has been completed the hardening of curd proceeds at an accelerated pace until it reached its maximum asymptotic value.

Milk coagulation by cationic polyelectrolytes

SummaryPositively charged materials, polyethyleneimine, poly-L-lysine and partly estcrified proteins (i.e. methyl bovine serum albumin, methyl casein and ethyl casein), in aqueous solution at the natural pH of milk, caused milk coagulation. This occurred at pH 6–6·7 at ≤ 30 °C using a defined amount of coagulant, which was quantitatively recovered in the curd. The protein and fat recovery yields were higher than with acid and rennet coagulation; the (Ca + Mg) partition between whey and curd was very similar to rennet milk clotting: the curd obtained showed high syneresis rate.

Natural variations in the average size of bovine casein micelles: II. Milk samples from creamery bulk silos in south west Scotland

SummarySamples of bulk silo milk from 5 creameries in the south west of Scotland were taken over a 16-month period. The average radii of casein micelles were determined by measuring the wavelength dependence of the turbidity of skim-milk diluted with its own ultrafiltrate. In addition, the concentrations of casein, soluble and colloidal Ca and P1 were measured. The average size of casein micelles followed a pronounced seasonal trend with smaller average sizes in the summer compared to the winter period, confirming earlier observations made on milks from individual cows. The average size of casein micelles correlated positively (r = +0·777, P < 0·001) with the amount of colloidal P1 per unit weight of casein and negatively (r = −0·77, P < 0·001) with casein-bound Ca. Average micelle radius was negatively correlated with heat stability at the natural pH of milk (r = −0·61, P < 0·001), but there did not appear to be any causal relation between these 2 variables.