Determination of the proportion of κ-casein hydrolysed by rennet on coagulation of skim-milk

Extent of κ-casein hydrolysis during renneting of bovine milk: A critical assessment of the analytical and estimation approaches

Renneting is an enzymatic process that turns milk into curd which is then transformed into cheese. Rennet-induced coagulation of caseins (CNs) is the critical step during this process and the key is the primary hydrolysis of κ-CN's Phe105-Met106 bond by chymosin. This article comprehensively reviews the existing data on the extent/degree of κ-CN hydrolysis during renneting of bovine milk and critically evaluates its determination methods. The data show that under normal cheese-making conditions, milk gelation occurs at a degree of κ-CN hydrolysis <80%, which varies due to several factors including analytical and estimation approaches. The common approach involves isolating the macropeptides released, by precipitating whey proteins and residual CN in 1%-12% trichloroacetic acid (TCA), then assuming that the maximum amount obtained is 100% κ-CN hydrolysis. The drawback is that the estimated degree of κ-CN hydrolysis may be higher than the actual value as TCA partially precipitates the macropeptide fractions. Moreover, macropeptide isolation seems unnecessary based on current advances in chromatographic and electrophoretic techniques. The present work proposes a simple mass balance-based approach that will provide accurate estimates in future studies. The accuracy of measuring the degree of κ-CN hydrolysis has implications on the precision of the data in relation to its partitioning (% distribution between the curd and whey) which is essential for improving whey quality.

Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins

Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7-5.3) and pepsin concentration (0.110-2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ∼6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ∼73% to ∼33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.

Effect of Protein Genotypes on Physicochemical Properties and Protein Functionality of Bovine Milk: A Review

Milk protein comprises caseins (CNs) and whey proteins, each of which has different genetic variants. Several studies have reported the frequencies of these genetic variants and the effects of variants on milk physicochemical properties and functionality. For example, the C variant and the BC haplotype of αS1-casein (αS1-CN), β-casein (β-CN) B and A1 variants, and κ-casein (κ-CN) B variant, are favourable for rennet coagulation, as well as the B variant of β-lactoglobulin (β-lg). κ-CN is reported to be the only protein influencing acid gel formation, with the AA variant contributing to a firmer acid curd. For heat stability, κ-CN B variant improves the heat resistance of milk at natural pH, and the order of heat stability between phenotypes is BB > AB > AA. The A2 variant of β-CN is more efficient in emulsion formation, but the emulsion stability is lower than the A1 and B variants. Foaming properties of milk with β-lg variant B are better than A, but the differences between β-CN A1 and A2 variants are controversial. Genetic variants of milk proteins also influence milk yield, composition, quality and processability; thus, study of such relationships offers guidance for the selection of targeted genetic variants.

Composition and effect of blending of noncoagulating, poorly coagulating, and well-coagulating bovine milk from individual Danish Holstein cows

The aim of the present investigation was to study the underlying causes of noncoagulating (NC) milk. Based on an initial screening in a herd of 53 Danish Holstein-Friesians, 20 individual Holstein-Friesian cows were selected for good and poor chymosin-induced coagulation properties; that is, the 10 cows producing milk with the poorest and best coagulating properties, respectively. These 20 selected cows were followed and resampled on several occasions to evaluate possible changes in coagulation properties. In the follow-up study, we found that among the 10 cows with the poorest coagulating properties, 4 cows consistently produced poorly coagulating (PC) or NC milk, corresponding to a frequency of 7%. Noncoagulating milk was defined as milk that failed to form a coagulum, defined as increase in the storage modulus (G') in oscillatory rheometry, within 45min after addition of chymosin. Poorly coagulating milk was characterized by forming a weak coagulum of low G'. Milk proteomic profiling and contents of different casein variants, ionic contents of Ca, P and Mg, κ-casein (CN) genotypes, casein micelle size, and coagulation properties of the 4 NC or PC samples were compared with milk samples of 4 cows producing milk with good coagulation properties. The studies included determination of production of caseinomacropeptide to ascertain whether noncoagulation could be ascribed to the first or second phase of chymosin-induced coagulation. Caseinomacropeptide was formed in all 8 milk samples after addition of chymosin, indicating that the first step (cleavage of κ-CN) was not the cause of inability to coagulate. Furthermore, the effect of mixing noncoagulating and well-coagulating milk was studied. By gradually blending NC with well-coagulating milk, the coagulation properties of the well-coagulating samples were compromised in a manner similar to titration. Milk samples from cows that consistently produced NC milk were further studied at the udder quarter level. The coagulation properties of the quarter milk samples were not significantly different from those of the composite milk sample, showing that poor coagulation traits and noncoagulation traits of the composite milk were not caused by the milk quality of a single quarter. The milk samples exhibiting PC or NC properties were all of the κ-CN variant AA genotype, and contained casein micelles with a larger mean diameter and a lower fraction of κ-CN relative to total CN than milk with good coagulation properties. Interestingly, the relative proportions of different phosphorylation forms of α-CN differed between well-coagulating milk and PC or NC milk samples. The PC and NC milk samples contained a lower proportion of the 2 less-phosphorylated variants of α-CN (α(S1)-CN-8P and α(S2)-CN-11P) compared with samples of milk that coagulated well.

Probing the location of casein fractions in the casein micelle using enzymes and enzyme–dextran conjugates

The rates of milk clotting and of formation of para-κ-casein in milk, colloidal phosphate-free milk and isolated κ-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete κ-casein hydrolysis at 30 °C and throughout the range of pH 5·6–6·7. κ-Casein hydrolysis by pepsin at pH 6·6 was fastest in milk and slowest in isolated κ-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of κ-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of β-and κ-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. κ-Casein was hydrolysed much faster, and β-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of κ-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the κcasein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.

Effect on the composition and properties of casein micelles of interaction with ionic materials

The effect on the composition and properties of casein micelles of the binding of ionic materials which accelerate the coagulation of milk on rennet treatment, was investigated. When considered in terms of their relative charge concentration, all the materials tested caused similar effects. The casein, inorganic phosphate and Ca contents of the micelles increased slightly. Micelle hydration decreased as additive binding increased. Casein and Ca dissociation on cooling increased at low concentrations of bound material, then progressively decreased at higher concentrations. The mean size of micelles and their electrophoretic mobility was little affected by bound ionic materials. The aggregation of the casein complexes in colloidal calcium phosphate-free milk was markedly increased by adding ionic materials, the efficiencies of these additives paralleling their efficiencies in accelerating the coagulation of milk by rennet. The results suggested that the ionic materials were bound in the interior of the casein micelles and promoted aggregation after rennet treatment by shielding charged groups, thus increasing the micellar hydrophobicity.

Rennet coagulation of milk in the presence of sucrose

The aim of this work was to test the diffusion-controlled hypothesis of milk coagulation kinetics by reducing the diffusion coefficient of casein micelles. This has been achieved by increasing the solvent viscosity of milk through sucrose addition. Milk was reconstituted from skim milk powder and sucrose added at 100–300 g kg–1. Hydrolysis and coagulation were followed by chromatographic determination of caseinomacropeptide content and optical, thermal and viscoelastic measurements. Soluble and ionic calcium were determined by atomic absorption spectrophotometry and ionometry and micelle size was measured by dynamic light scattering. Addition of sucrose resulted in a substantial retardation of both enzymic and aggregation steps, a re-equilibration of calcium because of water reduction, and a micelle size increase. The enzymic rate constant was inversely proportional to the viscosity, according to a diffusion-controlled model, and the lag or characteristic times for the aggregation reaction were inversely proportional to the viscosity. These results are consistent with the involvement of diffusion-controlled steps in the sequence of reactions.

A new calculation of the kinetics of the renneting reaction

A detailed calculation of the growth of molecular weight during the renneting of milk is given, based on a first-order breakdown of κ-casein followed by development of instability caused either by a decrease in the intermicellar repulsive potential or by the formation of holes in the stabilizing surface layer of the micelles. Unlike most of the models which have been described, this model allows a complete analytical solution. The solution is, however, complex and difficult to use simply, although it is shown that the calculations are in accord with experimental observations of the dependence of the coagulation process upon the enzyme concentration and the concentration of the milk. The calculations are also compared with those from other models of the reaction.

Development of texture and flavour in cheese and other fermented products

Cheesemaking is initiated by specific hydrolysis of κ-casein, followed by random aggregation of casein particles to form a network. The structure of this curd determines its subsequent behaviour and the composition and texture of the cheese. It is affected by previous homogenization or concentration of the milk. The composition of the milk and the distribution of components in it, which are affected by season, mastitic infection and cold storage, also influence its curd-forming properties, and the composition and yield of cheese. During ripening of cheese, the final texture and the development of flavour occur. Both are influenced by the type and extent of proteolysis, catalysed by coagulant, bacterial and milk enzymes. These and other enzymic and non-enzymic reactions responsible for flavour development depend on the composition and environment within the product. Some of these reactions can be accelerated by incorporating enzymes and chemical reagents into the curd, resulting in the production of an enhanced flavour. A number of compounds responsible for the flavour of cheese and yoghurt have been identified and mechanisms for their formation and retention in the product have been postulated.

Use of Platelet Aggregometer to monitor the chymosin-initiated coagulation of casein micelles

The chymosin-initiated coagulation of casein micelles was followed by monitoring light transmission using a Platelet Aggregometer. The release of macropeptide by chymosin was monitored using fluorescamine. The lag period in the clotting reaction was proportional to clotting time and the reciprocal of enzyme concentration. The average rate of coagulation, which was approximately equal to the reciprocal of clotting time (Tc), increased in proportion to enzyme concentration at low enzyme concentrations and reached a limiting value at high enzyme concentrations. The percentage hydrolysis at the Tc was 47 ± 5% in the presence of 20 mM-CaCl2 and it was calculated that a 5-fold decrease in the speed of the enzyme-catalysed reaction would decrease this value at the Tc to 43 ± 5%. The possible uses and limitations of the Platelet Aggregometer for determining the influence of the chemical environment on the velocity of the chymosin-catalysed reaction and para-casein micelle aggregatability are discussed.

The effect of the chemical structure of additives on the coagulation of casein micelle suspensions by rennet

Casein micelles in milk-salts solution adsorbed charged detergents and highly-charged polypeptides strongly, neutral detergents less strongly and low molecular-weight amines weakly. A tetra-amine was adsorbed more strongly than a tri-amine. The extent of adsorption of proteins tended to rise as the molecular weight increased. Glycerol and lactate were adsorbed to a limited extent but dextran and α-ketoglutarate were not adsorbed at all. Proline was partly adsorbed, indicating that hydrophobic binding sites were available, and caused some disruption of the casein micelles. Additives were bound to approximately the same extent by casein micelles and rennet coagula. The proportions adsorbed were constant over at least 10-fold ranges of concentration. Additives which increased the rennet clotting time (RCT) acted by binding Ca2+. Most additives decreased the RCT, the extent increasing with the amount adsorbed and the positive charge on the additive. The greatest reduction in RCT was observed with those additives which had positively-charged and hydrophobic moieties and bound most strongly to casein micelles. Of the additives tested, only sodium dodecyl sulphate affected the enzymic action of rennet. The reduction in RCT may have resulted from the neutralization of the negative charge of the micelles or enhancement of their hydrophobicity, favouring hydrophobic interactions between the particles.

Increasing cheese yields by high heat treatment of milk

Milk was heated at 97 °C/15 s to denature ∼ 30% of the whey protein and then used to make Cheshire cheese. Measurements of para-κ-casein production indicated that heating milk did not inhibit the enzymic action of rennet, but additional Ca and an initial pH of 6·4 were required for normal coagulation and curd-firming. In the experimental cheeses, about 4·5% more dry matter was recovered compared with controls made with pasteurized milk owing to a 6·7 and 0·7% increase in protein and fat recovery respectively. Experimental cheeses tended to be too moist and the curds did not fuse as well but these problems could be overcome by raising the scald temperature and cheddaring the curds. When the moisture in non-fat solids of control and experimental cheeses was similar (61%), the flavours and textures were not significantly different. The procedure requires little modification to existing commercial plant and should be suitable for varieties of cheese with higher moisture content and more crumbly texture than Cheddar.

Precision conductometry in milk renneting

On renneting, the electrical conductivity of milk decreased as viscosity increased. The sigmoidal time course of the decrease resembled the time course of shear modulus, but was more rapid. The total amount of change was independent of the amount of rennet and proportional to milk conductivity and its casein content. The conductivity change was interpreted as a change in the way casein micelles obstructed the path of the charge-carrying ions.

Application of numerical analysis to a number of models for chymosin-induced coagulation of casein micelles

Four models describing renneting kinetics are evaluated for their ability to describe well documented attributes of the coagulation of casein micelles. The first model is based on a constant flocculation rate parameter. In the second the flocculation rate constant is proportional to the product of the sizes of the aggregating particles. Both models fail to predict proper dependence of rennet coagulation time on enzyme concentration. The third model is based on an energy barrier being reduced in linear proportion to the degree of proteolysis. The enzyme dependency of this model only works when the initial energy barrier is larger than ∼ 50 kBT (where kB is Boltzmann's constant and T the absolute temperature), which does not seem feasible. The fourth model, based on functionality theory, is able to predict proper dependence of rennet coagulation time on enzyme concentration when functionality is ∼ 2.

A method for determination of macropeptide by cation-exchange fast protein liquid chromatography and its use for following the action of chymosin in milk

Cation-exchange chromatography on a Mono S column (Pharmacia) was used to separate macropeptide from whey proteins. Macropeptide was eluted by 0·1 M-NaCl in a 20 mM-KCl–HCl buffer, pH 2. This technique was suitable for quantitative determination of macropeptide in rennet whey and also for following the action of chymosin on κ-casein in skim milk. Precipitation at pH 4·6 was used to remove residual caseins and to keep macropeptide in solution. In comparison with other methods for determining macropeptide, the present one eliminates the need for pretreatment of samples with trichloroacetic acid (TCA) and allows the recovery of all the macropeptide. Quantitative determination of macropeptide in the 8% TCA-soluble fraction by cation-exchange chromatography showed that only 50–75% of the macropeptide was recovered. This chromatographic technique could also be applied for isolating and producing whole macropeptide on a preparative scale.

The rennet coagulation mechanism of skim milk as observed by transmission diffusing wave spectroscopy

The technique of forward-scattering diffusing wave spectroscopy has been used to study the rennet-induced gelation of skim milk. The results allow the comparison of a colloidal suspension at a realistic concentration (Phi approximately 10%) compared with well-established measurements made on highly-diluted milk samples. It is shown that the partially renneted casein micelles do not begin to approach one another until the extent of breakdown of kappa-casein has reached about 70%; above this point, they interact increasingly strongly with the extent of proteolysis. This interaction initially restricts the diffusive motion of the particles rather than causing true aggregation. Only after more extensive removal of the protective kappa-casein does true aggregation occur, with the appearance of a space-filling gel (defined by rheology as having a value of tandelta<1). The results show in greater detail than hitherto the progress of interactions between the particles in a system where the steric stabilization is progressively destroyed, and suggest that the renneting of milk at its normal concentration cannot be described simply by reactions between freely-diffusing particles.

Impaired rennetability of heated milk; study of enzymatic hydrolysis and gelation kinetics

Casein micelles in milk are stable colloidal particles with a stabilizing hairy brush of kappa-casein. During cheese production rennet cleaves kappa-casein into casein macropeptide and para-kappa-casein, thereby destabilizing the casein micelle and resulting in aggregation and gel formation of the micelles. Heat treatment of milk causes impaired clotting properties, which makes heated milk unsuitable for cheese production. In this paper we compared five different techniques, often described in the literature, for their suitability to quantify the enzymatic hydrolysis of kappa-casein. It was found that the technique is crucial for the yield of casein macropeptide and this yield then affects the calculated enzymatic inhibition caused by heat treatment, ranging from 5 to 30%. The technique, which we found to be the most reliable, demonstrates that heat-induced calcium phosphate precipitation does not affect the enzymatic cleavage, while whey protein denaturation causes a very slight reduction of enzyme activity. By using diffusing wave spectroscopy, a very sensitive technique to monitor gelation processes, we demonstrated that heat-induced calcium phosphate precipitation does not affect the clotting. Whey protein denaturation does not affect the start of flocculation but has a clear effect on the clotting process. This work adds to a better understanding of the processes causing the impaired clotting properties of heated milk.

Alterations of the physical characteristics of milk from transgenic mice producing bovine kappa-casein

kappa-Casein is the protein fraction of milk that allows formation of micelles and determines micelle size and function, thus affecting many of the physical characteristics of milk. Several lines of transgenic mice were generated bearing the B allele of the bovine kappa-CN gene under the control of the regulatory sequences of the caprine beta-CN gene that specifically directed expression of bovine kappa-CN to the lactating mammary tissue of these mice. High expression of bovine kappa-CN protein was observed in the lines studied; the total level of protein in milk was not significantly affected. A high degree of conservation in the amino acids involved in the predicted three-dimensional structure exists between murine and bovine kappa-CN. Milk from transgenic lines expressing high bovine kappa-CN had a significantly smaller micelle size than did control milk. Therefore, bovine kappa-CN appears to have effectively participated in assembly of murine casein micelles. There was no effect on the time of rennet coagulation, but the association was significant between the milk of transgenic lines and the production of a stronger curd in rennet-induced gels. We conclude that bovine kappa-CN is an appropriate candidate for transgenic technology that would increase the ratio of kappa-CN to the calcium-sensitive caseins, therefore affecting the physical properties of the colloidal casein suspension.

Specificity of chymosin on immobilized bovine B-chain insulin

1. The specificity of chymosin on immobilized bovine B-chain insulin is studied. 2. Eight sites of hydrolysis are determined.

Analysis by fast protein liquid chromatography of variants of kappa-casein and their relevance to micellar structure and renneting

Fast protein liquid chromatography was used to study the kappa-casein fraction of casein micelles from bulk milk and from milk from individual animals homozygous for kappa-caseins A and B. The extent of glycosylation of the kappa-casein appeared to have no effect on its distribution in casein micelles of different sizes, nor did it affect the rate at which kappa-casein was destroyed during renneting. The rate of breakdown of kappa-casein during renneting was also almost independent of micellar size. The results may indicate a difference between methods which analyse for intact kappa-casein or for the product macropeptide during renneting.