A viscometric study of the breakdown of casein in milk by rennin and rennet

Cheese-making with a vegetable rennet from Cardo (Cynara cardunculus)

The milk-clotting enzyme found in the flowers of Cardo (Cynara cardunculus) was investigated as to its suitability as a substitute for traditional animal rennet used in cheese-making. The influence of milk pH, temperature and quantity on the clotting activity of this enzyme was studied. Rheological behaviour of cow's-milk and sheep-milk curds, from renneting to cutting, was determined with the Plint cheese curd torsiometer for various Ca contents and pH values. Edam, Serra and Roquefort cheeses were made and the protein breakdown which occurred in the cheese during the ripening period was determined. Animal rennet was used as a control in all the experiments. The enzyme from Cardo was a satisfactory substitute for animal rennet with cow's milk and was even more suitable for sheep's milk. It was found to be a very good clotting enzyme for soft-bodied cheese like Serra, but because of its high proteolytic activity it presented some problems in Edam cheese-making. In Roquefort cheese it gave satisfactory results, but with some loss in yield.

Coagulation of homogenized milk particles by rennet

The action of rennet on homogenized milk was studied using turbidimetric and light scattering techniques, and compared with results obtained previously for skim milk. The time required for the onset of coagulation was shorter for homogenized milk than for skim milk. The rate of coagulation of fully renneted particles increased with increasing temperature, and with increasing Ca2+ concentration, but was only slightly influenced by changes in ionic strength. The von Smoluchowski rate constant for the coagulation reaction was two orders of magnitude smaller for homogenized milk than for skim milk. Results suggest that coagulation of homogenized milk is controlled in general by the same factors as skim milk, and that the reaction may be inhibited owing to a reduction in the amount of casein available for mutual interaction, rather than to disruption of the micelles on homogenization.

The kinetics of the rennin-casein reaction in abnormal cow's milk and milks of some other species and of the action of rennet substitutes on normal cow's milk

In earlier papers it was shown that the fall in viscosity of milk and caseinate solutions acted on by rennet, follows a first-order reaction equation. The reaction rate constant has been found to be independent of the source of the caseinate substrate used. This seems to be true not only for caseinate from normal cow's milk but also from colostrum, slow-clotting cow's milk and even the milk of buffaloes, sheep and probably goats.

With solutions of caseinate from sow's milk the reaction kinetics and the proportional viscosity loss were similar to those of cow's caseinate but the rate constants were much lower. Rennet showed no measurable reaction with human, whale, mare or rhinoceros caseinates but produced a constant rate of fall in the viscosity of a camel caseinate for more than an hour from rennetting.

Vegetable and microbiological rennet substitutes also produce a fall in the viscosity of sodium caseinate and the reactions follow first-order kinetics. They differ from cheese rennet in that the proteolysis involved is stronger and is not as limited as that of rennin since they attack at least twice as much of the casein ensemble. This might suggest a mixture of enzymes but since the proportionality between enzyme concentration and k1 was very close this seems unlikely.

A simple method for detecting an early stage in coagulation of rennetted milk

An apparatus is described which measures at 10—15 sec intervals the viscosity of rennetted milk taken from a cheese vat. The first signs of an increase in this viscosity mark what is at present the earliest measurable stage in the process of coagulation. Coagulation times so determined agreed fairly closely with the observations of a skilled cheese-maker and, for a given milk, were inversely proportional to rennet concentration over quite a wide range. Clotting times obtained by the method of Berridge (1952) for standardizing rennets came 2 or 3 min after those determined by the new method and were linearly related but not proportional to inverse rennet concentration.

A kinetic model of the clotting of casein by rennet

A kinetic model of casein clotting caused in milk by rennet was defined by measuring changes in viscosity and in the complex modulus of rigidity. Three separate stages of the process were distinguished: the enzymic, the flocculation and the gelification. The kinetic equations and the main activation parameters were defined for each stage.

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.

A method of preparation of x-casein and some observations on its nature

Details are given of the method used in this laboratory for the preparation of k-casein. The recovery is about 25% and the material has a purity, estimated from electrophoretic patterns, of about 90% with β-casein as the main contaminant. Higher temperatures and lower ion concentration caused precipitation of k-casein in the presence of calcium ions, 0·1m-acetate buffer at pH 6·5 being sufficient to stabilize a 0·5% solution in the presence of 0·01–0·2m-CaCl2 at 20 and 30°C but not at all calcium concentrations at 40°C. It was also found that solutions of para-k-casein did not aggregate in the presence of concentrations of electrolytes above about 0·25m.

The rate of release of non-protein nitrogen and decline in viscosity during the enzymic stage of gel formation in solutions of k-casein and rennin had similar apparent first order constants (0·087 ± 0·03 min—1 and 0·086 ± 0·021 min—1, respectively, at 25°C). A gel could be formed by rennin action in a solution containing as little as 6·25mg of the protein per litre. In the non-enzymic stage of gelling of k-casein solutions the calculated activation energy over the range of 20–40°C was much lower than that obtained from the non-enzymic stage of milk coagulation.

Intermicellar relationships in rennet-treated separated milk: II. Process of gel assembly

The aggregation of casein micelles in milk by the action of rennet, examined by electron microscopy and viscometry, was correlated with visually-observed coagulation. The spatial relationships between micelles were quantified from measurements made on micrographs. Aggregation did not start until about 60% of the rennet-clotting time, when the enzymic action was almost complete. The micelles tended to form chains which then linked into a loose network. This became more extensive with time as the average size of the aggregates increased. By the clotting time, most micelles were in contact with another and a network was beginning to form. Initially, micelles probably linked by bridges. These then appeared to contract, bringing the particles into contact and eventually causing partial fusion.

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.

A mathematical model for the description of chymosin action on casein micelles

A mathematical model for chymosin action on casein micelles is presented in a two-stage equation which results in a single curve demonstrating the lag time from enzyme addition to the end of coagulum firming. The model uses the Michaelis–Menten enzyme kinetics equation for the first reaction followed by an nth order reaction for the casein micelles agglomeration stage. The computer output using these equations shows that lag time is elongated as enzyme concentration is lowered. Regression analysis of time of gelation against l/E0 shows good correlation. Viscosity of the milk drops at the beginning of the κ-casein hydrolysis and increases thereafter, when the coagulum is being formed.

Viscometric measurements of the activities of commercial rennets using sodium caseinate as substrate

The considerable falls in viscosity of sodium caseinate solutions under the action of rennet are shown to be due to changes in volume and shape (or either alone) of loosely coiled long chain molecules, resulting from the discharge of ionization (‘Third-type electro-viscous effects’). This reduction may be as well effected by small amounts of NaCl or by the separated decomposition products of the rennin-caseinate reaction. Even at the highest practicable caseinate concentrations (9–10% by weight) the solutions are truly fluid over the range of shear-rate of the Ostwald viscometers used

The initial rate of fall of viscosity is found to be proportional to the maximum (initial) specific viscosity, leading to an equation Xk1 = 1/τ where X the proportional viscosity loss, varies with the source of caseinate but is constant for all normal rennets; k1, the first-order reaction constant, varies with the rennet but is independent of the source of casein over a wide range; and τ, which has the dimensions of time, varies with both caseinate and rennet.

The parameter k1 may be found either from the exponential viscosity fall or calculated from the asymptotic initial slope and has been shown to serve as a useful index of rennet activity. This method of rennet testing does not depend on the keeping qualities of rennins, rennets, milks or caseinates. Details of the preparation of caseinates and of the methods for practical testing of commercial rennets are given.

Effect of pH, calcium concentration and temperature on the evolution of the refractometric signal produced during rennet coagulation of milk

An industrial refractometer connected to a microcomputer was used to study the rennet coagulation of milk. The amplitude of the refractometer's signal changes during coagulation was about 104 times more important than the resolution of the computerized refractometer. The signal v. time derivatives were calculated and the inflection point time (ti) was correlated with pH, Ca concentration and temperature. The inflection point time was inversely proportional to the rennet and Ca concentrations and the logarithm of the proportionality factor (k) was linearly correlated with pH. The relation between ti and temperature was curvilinear. It is concluded that the refractometer tested could be used for research on the coagulation process.

Ultrasonic monitoring of yoghurt formation by using AT-cut quartz: lighting of casein micelles interactions process during the acidification

The behavior of weak gels during their formation singularly attracts attention of dairy products factories. In our study we investigate acidified pre-heated milk gels formation that are fairly often used to product yoghurts. The gel formation requires a tight control of the first step of micelles modification process and the kinetics reaction parameters. The most current rheological parameters used to achieve the monitoring are the storage G' and the loss G'' shear moduli and the gelation time. The study of these parameters is commonly performed at very low frequencies (1 Hz). Our technique uses a 6 MHz AT-cut quartz crystal immersed in an acidified milk solution kept at a constant temperature. This method is singularly effective to ensure a complete and a reliable follow-up of the viscoelastic parameters of casein gels. A suitable new model enables a complete follow-up of the micelles evolution from the viscoelastic properties. The experimental results of the G' and G'' moduli versus temperature and versus glucono-delta-lactone (GDL) added to milk are analyzed. In order to understand the micelles modifications, an analysis of the viscoelastic evolution try to explain the validity of the various models of micelles modification. In addition a new accurate kinetics characteristic time is proposed. This time corresponds to the moment for which the elastic effect of material becomes significant. From the kinetics study of casein gels at various temperatures, the Arrhenius relationship and a modified Flory-Stockmayer relationship give us access to the activation energy. By using the proposed technique and the suitable models developed, the structure thus quality of dairy products may be better controlled.

ADSA Foundation Scholar Award. Formation and physical properties of milk protein gels

Gelation of milk proteins is the crucial first step in both cheese and yogurt manufacture. Several types of milk gels are discussed, with an emphasis on recent developments in our understanding of how these gels are formed and some of their key physical properties. Areas discussed include the latest dual-binding model for casein micelles; some recent developments in rennet-induced gelation; review of the methods that have been used to monitor milk coagulation; and a discussion of some of the possible causes for the wheying-off defect in yogurts. Casein micelles are the primary building blocks of casein-based gels; however, controversy about its structure continues. The latest model proposed for the formation of casein micelles is the dual-binding model proposed by Horne, 1998, which suggests that casein micelles are formed as a result of two binding mechanisms, namely hydrophobic attraction and colloidal calcium phosphate (CCP) bridging. Most previous models for the casein micelle have treated milk gelation from the viewpoint of simple particle destabilization and aggregation, but they have not been able to explain several unusual rheological properties of milk gels. Although there have been many techniques used to monitor the milk gelation process over the past few decades, only a few appear attractive as possible in-vat coagulation sensors. Another important aspect of milk gels is the defect in yogurts called wheying-off, which is the appearance of whey on the gel surface. The factors responsible for its occurrence are still unclear, but they have been investigated in model acid gel systems.

Stable and unstable casein micelles

In milk, casein occurs as colloidal particles with an average size of about 100 nm. These are stabilized against flocculation by an outer layer of several thousands of kappa-casein molecules. Stability of micelles is characterized by the magnitude of the Smoluchowskian flocculation rate constant, which during the renneting of milk nearly approaches the diffusion-controlled limit. The processes of the clotting of milk by rennet and the phenomena of age-thinning and age-thickening of ultra-high temperature-sterilized, concentrated milks bear interesting kinetic resemblances. Both processes are characterized by a lag phase during which viscosity decreases, followed by an explosive increase in viscosity. In the milk-clotting process, the decrease can be explained by the proteolytic action of the renneting enzyme. This strongly suggests that age-thinning and age-thickening are also caused by the action of a protease that survived the sterilization process. A quantitative check of this theory is difficult because of the apparently small amount of enzyme.

The coagulation of differently sized casein micelles by rennet

Fractions of bovine casein micelles of different sizes were prepared by successive centrifugation steps, and dilute suspensions of the different fractions were reacted by rennet. The molecular weight increase with time after addition of rennet was measured by light scattering. After an initial lag stage, where the increase was zero, and a short intermediate stage, the molecular weight became linearly proportional to time, indicating complete conversion of the kappa-casein substrate by the enzyme. The slope of the final portion of the growth profile can be used to define values of the Smoluchowski rate constant governing the aggregation. No significant differences in this constant were found with micelles of different sizes, and the value of the rate constant suggested that the aggregation reaction of completely renneted micelles is only moderately retarded relative to the diffusion-controlled process, even at room temperature.

Effect of chymosin action on the hydrodynamic diameter of casein micelles

Quasi-elastic light scattering shows an initial decrease of about 5 nm in the hydrodynamic radius of casein micelles after adding chymosin, assuming the decrease to be equal for all micelles. This is consistent with the hypothesis that casein micelles have a hairy outer layer that is partly made up of the caseino-macropeptide part of kappa casein.

The voluminosity of bovine casein micelles and some of its implications

A comparison of literature results on the voluminosity of casein micelles yields large differences, largely related to the method of determination. After applying some corrections, methods based on hydrodynamic radius yield a value of roughly 3.9 ml/g dry casein, other methods (microscopy, sediment volume) about 2.2. Roughly half of the discrepancy can be explained by the micelles being not perfectly spherical. To explain the remaining difference, it is assumed that the micelles are hairy, i.e. they have molecular chains protruding into the milk serum. The hairyness would increase with decreasing temperature and be largely removed by the action of rennet. It would cause some entropic repulsion between micelles at close approach.

Casein micelles: the colloid-chemical approach

The colloidal properties of micellar casein are reviewed. It is shown that behaviour of intact micelles is much at variance with the predictions from the Schulze-Hardy rule, and that therefore their stability cannot be explained by the principles of the DLVO theory. Towards electrolyte, micelles behave as a protein rather than a lyophobic colloid. Casein is a strong protective colloid. In the micelle, however, it does not completely cover the inorganic constituent which remains sensitive to changes in the ionic environment. The rate theory of the enzyme-induced clotting of casein micelles is summarized. It is shown that the lag phase in the clotting is due to the second order of the co-agulation reaction. Flocculation rate constants of micelles have been deduced from clotting times. Their relatively low values can be attributed to an orientational constraint. Practical consequences of the theory with respect to clot structure, gelation of sterilized products and cheese manufacture are discussed.