Mechanism of aggregation of casein micelles in rennet-treated milk

Rheology of Swiss Cheese Fondue

Cheese fondue is a popular Swiss dish prepared by melting cheese under the addition of wine, starch, and seasoning. The flow behavior or rheology of fondue is crucial for mouthfeel, flavor release, and the tendency of fondue to cling to the bread. Fondue is a complex multiphase system whose rheology is determined by the interactions of its colloidal ingredients. We establish cheese fondue as a water-continuous system with dispersed fat droplets, charged caseins, and starch granules. Irreversible phase separation, a common issue in fondue preparation, may be prevented by addition of a critical minimum starch concentration. Fondue was found to be a shear-thinning yield stress fluid, which is desirable for mouthfeel and facilitates fondue to cling to the bread for consumption. Fondue showed a viscoelastic stress response around the gel point (G' ≈ G″), which is proposed as crucial for the balance of orally perceived gumminess (G') and liquidity (G″). Ethanol addition and lowering pH toward the isoelectric point of casein, as associated with wine addition, decrease fondue viscosity due to a decrease in casein micelle size. Below the isoelectric point of casein, fondue is unstable and phase separates, potentially impeding fondue digestion. Thus, fondue rheology is governed by the complex colloidal interactions within its ingredients, and ultimately determines fondue eating experience.

The occurrence of noncoagulating milk and the association of bovine milk coagulation properties with genetic variants of the caseins in 3 Scandinavian dairy breeds

Substantial variation in milk coagulation properties has been observed among dairy cows. Consequently, raw milk from individual cows and breeds exhibits distinct coagulation capacities that potentially affect the technological properties and milk processing into cheese. This variation is largely influenced by protein composition, which is in turn affected by underlying genetic polymorphisms in the major milk proteins. In this study, we conducted a large screening on 3 major Scandinavian breeds to resolve the variation in milk coagulation traits and the frequency of milk with impaired coagulation properties (noncoagulation). In total, individual coagulation properties were measured on morning milk collected from 1,299 Danish Holstein (DH), Danish Jersey (DJ), and Swedish Red (SR) cows. The 3 breeds demonstrated notable interbreed differences in coagulation properties, with DJ cows exhibiting superior coagulation compared with the other 2 breeds. In addition, milk samples from 2% of DH and 16% of SR cows were classified as noncoagulating. Furthermore, the cows were genotyped for major genetic variants in the αS1- (CSN1S1), β- (CSN2), and κ-casein (CSN3) genes, revealing distinct differences in variant frequencies among breeds. Allele I of CSN2, which had not formerly been screened in such a high number of cows in these Scandinavian breeds, showed a frequency around 7% in DH and DJ, but was not detected in SR. Genetic polymorphisms were significantly associated with curd firming rate and rennet coagulation time. Thus, CSN1S1 C, CSN2 B, and CSN3 B positively affected milk coagulation, whereas CSN2 A(2), in particular, had a negative effect. In addition to the influence of individual casein genes, the effects of CSN1S1-CSN2-CSN3 composite genotypes were also examined, and revealed strong associations in all breeds, which more or less reflected the single gene results. Overall, milk coagulation is under the influence of additive genetic variation. Optimal milk for future cheese production can be ensured by monitoring the frequency of unfavorable variants and thus preventing an increase in the number of cows producing milk with impaired coagulation. Selective breeding for variants associated with superior milk coagulation can potentially increase raw milk quality and cheese yield in all 3 Scandinavian breeds.

Milk protein genetic variants and isoforms identified in bovine milk representing extremes in coagulation properties

A gel-based proteomic approach consisting of 2-dimensional gel electrophoresis coupled with mass spectrometry was applied for detailed protein characterization of a subset of individual milk samples with extreme rennet coagulation properties. A milk subset with either good or poor coagulation abilities was selected from 892 Danish Holstein-Friesian and Jersey cows. Screening of genetic variants of the major milk proteins resulted in the identification of common genetic variants of β-casein (CN; A(1), A(2), B), κ-CN (A, B), and β-lactoglobulin (LG; A, B), as well as a low frequency variant, κ-CN variant E, and variants not previously reported in Danish breeds (i.e., β-CN variant I and β-LG variant C). Clear differences in the frequencies of the identified genetic variants were evident between breeds and, to some extent, between coagulation groups within breeds, indicating that an underlying genetic variation of the major milk proteins affects the overall milk coagulation ability. In milk with good coagulation ability, a high prevalence of the B variants of all 3 analyzed proteins were identified, whereas poorly coagulating milk was associated with the β-CN variant A(2), κ-CN variant A or E, and β-LG variant A or C. The β-CN variant I was identified in milk with both good and poor coagulation ability, a variant that has not usually been discriminated from β-CN variant A(2) in other studied cow populations. Additionally, a detailed characterization of κ-CN isoforms was conducted. Six κ-CN isoforms varying in phosphorylation and glycosylation levels from each of the genetic variants of κ-CN were separated and identified, along with an unmodified κ-CN form at low abundance. Relative quantification showed that around 95% of total κ-CN was phosphorylated with 1 or 2 phosphates attached, whereas approximately 35% of the identified κ-CN was glycosylated with 1 to 3 tetrasaccharides. Comparing isoforms from individual samples, we found a very consistent κ-CN isoform pattern, with only minor differences in relation to breed, κ-CN genetic variant, and milk coagulation ability.

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.

Application of polymer cross-linking theory to rennet-induced milk gels

A general theory of gel formation by cross-linking of polymer molecules, orginally developed for the study of rubber vulcanization, is proposed as a model for milk gel formation. The model divides gel formation and development into 4 stages, an enzymic and a non-enzymic stage preceding gelation, then a stage of material transition from the sol to the gelled state and a stage of increase in gel strength due to internal cross-link formations. The 2 post-gelation stages overlap to a variable degree depending upon particular circumstances. Gel formation and properties are interpreted in terms of the number of initial units which combine to form the gel and the number of cross-links eventually formed by each unit. When combined with a kinetic expression for the rate of cross-link formation the time course of gel shear modulus development or of the incorporation of material into the gel may be predicted. The classical asymmetric sigmoid shape of gel shear modulus development curves results from the third stage of material transition into the gel, which in turn depends upon the number of cross-links eventually formed by each initial unit. The modulus of mature gels is found to depend upon the square of material concentration, and gel breaking strength is found to be proportional to the modulus, in agreement with the predictions of the theory.

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

The 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.

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.

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.

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.

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.

Rennet-Induced Milk Coagulation by Continuous Steady Shear Stress

The effect of continuous steady shear stress (CSSS) on rennet-induced coagulation of milk was studied by measuring the change in viscosity of the system with time. Continuous shear stress (< or =0.5 Pa) applied during coagulation did not counteract the network formation at standard cheese-making conditions. In fact, CSSS of 0.2 Pa promoted coagulation by possibly increasing diffusion, collision, and hydrolization rates due to near-field, attractive hydrodynamic reactions. This was evidenced by the high viscosity of the resultant coagulum. However, the viscosity profile of the coagulum formed in the presence of CSSS followed the same trend as that formed in the absence of CSSS. Viscosity versus time profiles in both cases displayed an initial lag phase followed by a steady increase until a plateau value. The viscosity plateau reached under CSSS was marked with several sudden peaks, indicating the dynamic structure of the coagulum. These peaks in viscosity profiles began to appear after about 1380 s since rennet addition at standard cheese-making conditions. The time at which viscosity first exceeds 40 kPa s was verified to coincide with manual determination of the time at which the coagulum is cut (i.e., cutting time) during cheese making.

Structures and functionalities of milk proteins

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.