Coagulation Of Casein Micelles Research Articles

Dairy Matrix Effects: Physicochemical Properties Underlying a Multifaceted Paradigm.

When food products are often considered only as a source of individual nutrients or a collection of nutrients, this overlooks the importance of interactions between nutrients, but also interactions between nutrients and other constituents of food, i.e., the product matrix. This product matrix, which can be defined as 'The components of the product, their interactions, their structural organization within the product and the resultant physicochemical properties of the product', plays a critical role in determining important product properties, such as product stability, sensory properties and nutritional and health outcomes. Such matrix effects can be defined as 'the functional outcome of specific component(s) as part of a specific product matrix'. In this article, dairy matrix effects are reviewed, with particular emphasis on the nutrition and health impact of dairy products. Such matrix effects are critical in explaining many effects of milk and dairy products on human nutrition and health that cannot be explained solely based on nutrient composition. Examples hereof include the low glycemic responses of milk and dairy products, the positive impact on dental health, the controlled amino acid absorption and the absence of CVD risk despite the presence of saturated fatty acids. Particularly, the changes occurring in the stomach, including, e.g., coagulation of casein micelles and creaming of aggregated fat globules, play a critical role in determining the kinetics of nutrient release and absorption.

Effect of oil droplet size on the gastric digestion of milk protein emulsions using a semi-dynamic gastric model

A small droplet emulsion (SDE) of D4,3 = 0.877 μm and a large droplet emulsion (LDE) of D4,3 = 2.19 μm, containing 3.5 wt% milk protein concentrate and 4.5 wt% sunflower oil, were made. The digestion of the samples was performed using a semi-dynamic model, which simulates the main dynamics of gastric digestion, including gradual acidification, enzyme secretion and gastric emptying. Both SDE and LDE coagulated in 10 min digestion when the pH was ≥5.2. Clots of SDE gradually creamed, but clots of LDE sedimented as shown in light-scattering curves. Confocal images showed a higher inclusion of oil droplets in SDE clots, which later coalesced inside the protein matrix and formed oil pools. Texture analyzer profiles exhibited that clots of LDE were firmer than SDE. The nutrients emptying profile indicated a delay in delivering protein and lipid from SDE digesta while LDE digesta emptied a significantly (P ≤ 0.05) higher amount of protein than SDE at initial gastric emptying points. The absorption of casein micelles to the vast interfacial area of SDE led to the higher association of lipids and proteins, which reduced the clots density and led to the creaming behaviour and late nutrients emptying. The coagulation of casein micelles in LDE formed firm structures that squeezed out soluble proteins and lipids to the serum phase leading to the sedimentation of clots and earlier nutrients emptying. The results shed some light on variations observed in physiological responses in satiety after consuming milk protein emulsions.

Influence of micellar calcium phosphate on in vitro gastric coagulation and digestion of milk proteins in infant formula model systems

Coagulation of casein micelles in the stomach is an important factor determining gastric transit and digestion kinetics of proteins. The influence of micellar calcium phosphate (MCP) content on the behaviour of milk proteins during in vitro digestion was studied under simulated infant gastric conditions. Samples (1.5% protein, casein:whey protein ratio = 40:60) were prepared using demineralised whey, milk permeate and skimmed milk, from which the MCP content had been adjusted. For milk samples, turbidity decreased and non-sedimentable casein increased with decreasing MCP content. During in vitro gastric digestion, samples prepared with milk with a higher MCP content showed stronger coagulation and these coagulates were found to digest more slowly. SDS-PAGE analysis showed that casein digestion under these conditions was reduced for samples showing stronger coagulation. The outcomes of this study highlight the importance of casein micelle structure and integrity in determining curd formation during in vitro gastric digestion.

Aggregation of Casein Micelles by Interactions with Chitosans: A Study by Monte Carlo Simulations

Recently, it was found that the addition of chitosan, a cationic polymer, to whole or skim milk produces the destabilization and coagulation of casein micelles which takes place without modifications in the milk pH or in the stability of most of the whey proteins. In the present work, Monte Carlo simulations are employed to show that the phase separation of casein micelles induced by chitosan can be explained by a depletion mechanism, where an effective attraction between the casein micelles is induced by the presence of chitosan molecules. This interaction is described on the basis of Vrij's model, where the depletion of polymer from the gap between neighboring casein micelles originates an effective attractive interaction that leads to a phase transition. This model, that considers volume restriction effects, accounts for several qualitative and even quantitative aspects of the experimental data for the coagulation of casein through chitosan addition.

Characterization of Casein Micelle Precipitation by Chitosans

We have found that the addition of chitosan, a cationic polymer, on whole or skim milk produces destabilization and coagulation of casein micelles that takes place without changes in the milk pH or the stability of most whey proteins. The amount of lipids recovered in the chitosan-casein aggregates was similar or higher than that obtained with rennet or acid precipitation. Approximately 70% of milk Ca2+ (∼750 mg/L) was found in the chitosan-induced aggregates, which is 10 and 50% higher than the amounts observed with acid or rennet coagulations, respectively. Purified α, β-, and κ-caseins were extensively precipitated by different molecular weight chitosans at pH 6.8. The phosphate groups of caseins seem not to be relevant in this interaction because dephosphorylated α- and β-caseins were equally precipitated with chitosans. Analysis by optical microscopy of the chitosan-casein complex reveals that the size of the aggregates increase as the molecular weight of chitosans increase. Hydrophobic and electrostatic interactions particpate in the association and coagulation of casein micelles with chitosans of different molecular weights. The phenomenon is observed over a broad range of temperature (4 to 70°C) with a reduction in the concentration of chitosan needed to precipitate the caseins that parallels a reduction in the viscosity of the chitosan solutions. Taken together, the results indicate that the electrostatic interactions may contribute energetically to the association between the two biopolymers, but the hydrophobicity of the complex would be the key determinant in the overall energetics of the reaction.

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

SummaryFour 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.

Enzyme-induced coagulation of casein micelles: a number of different kinetic models

SummarySeveral mathematical models are presented in an attempt to describe the kinetics of the enzyme-induced coagulation of casein micelles. In each model the primary phase of the clotting reaction is assumed to follow first order kinetics. The only differences amongst the various models centre on the definition of the flocculation rate constant, which is defined in seven different ways. The rate constants are defined and discussed in terms of activation energy and functionality theory. The first model is such that the number of functional sites is two. The second is such that the number is much larger. The third and fourth are such that there is an exponential energy barrier, one which has a magnitude proportional to the extent of proteolysis caused by the clotting enzyme. These two definitions differ only in the pre-exponent. In one case the pre-exponent is a constant, whereas in the other it is dependent on the size of clotting particles. The fifth and sixth definitions are also energy barrier rate constants, but the energy barrier changes in an arbitrary fashion with respect to time during proteolysis. The seventh definition assumes a large number of functional sites, but such that the number increases with extent of proteolysis. In the Payens nomenclature (Payens, 1989), all models could be considered to be ‘source’ models, and all are derived using the Drake moment equation (Drake, 1972). Only the first model has a truly constant flocculation rate parameter, and only this model has a relatively simple analytical solution. All other models yield analytical solutions only by way of infinite series expansions. Thus, all models are presented in terms of power series expansions, and only through the first five time-dependent coefficients. This confines all models to the early stages of coagulation. In all cases the first three coefficients are virtually the same. The first two coefficients involve only proteolysis, and the third includes initial flocculation information. Time-dependent changes in the flocculation rate constant begin to take effect in the fourth coefficient. When the fourth coefficients of the third and seventh models are compared, a simple relationship is suggested between free energy barrier removal and functional site generation, but only assuming that the number of functionalities is large.

Acidic coagulation of casein micelles: mechanisms inferred from spectrophotometric studies

SummaryThe acidic coagulation of casein micelles was monitored turbidimetrically at 400 nm using a Cary 219 spectrophotometer. Following acidification with HCI, the rate of coagulation during the initial stage of coagulation was proportional to the rate during the immediate subsequent stages, indicating that progressive stages of coagulation were controlled by similar factors and the slope at any portion of the reaction profile could be used to characterize the kinetics of the coagulation reaction. The rate of coagulation following acidification by glucono-δ-lactone was proportional to the reciprocal of the rate of acidification. The effects of environmental factors on the rate of the acidic coagulation of casein micelles over a range of pH values were evaluated from the smooth, hyperbolic profiles of pH v. coagulation rate. Determination of the pH required to initiate coagulation of casein micelles in skim milk ultrafiltrate at 5–25 °C, together with the maximum rates of coagulation at 5–25 °C, suggested that repulsive forces are reduced by acidification and con-sequently facilitate hydrophobic interactions resulting in the coagulation of casein micelles.

Steric effects in the coagulation of casein micelles by ethanol.

Steric effects in the coagulation of casein micelles by ethanol.

The acceleration by cationic materials of the coagulation of casein micelles by rennet

SummaryThree cationic materials markedly reduced the rennet clotting time of casein micelle suspensions, the efficacy of each being primarily dependent on the charge and the amount absorbed by the micelles. The reduction in coagulation time was unaffected by components of the milk serum other than salts. No enzymic action by lysozyme on casein micelles was detected. All materials acted by the same mechanism, increasing the affinity of rennet for the micelles and accelerating the aggregation phase. Coagulation did not occur until a minimum amount of κ-casein had been hydrolysed to para-κ-casein. All additives increased the proportion of added rennet retained by the casein in the coagulum. The results indicated that coagulation occurs by specific interactions between micelles modified by rennet.

Destabilization of casein micelles by lysozyme.

Lysozyme has been demonstrated to bring about coagulation of casein micelles in a manner similar to the action of rennin. Both the rennin and lysozyme catalyzed reactions show similar pH optima and calcium ion dependencies. The differences in specificities of these two enzymes suggest that the carbohydrate moiety of the casein glycoprotein plays a major role in stabilization of casein micelles.