Rennet-induced Gels Research Articles

Importance of casein micelle size and milk composition for milk gelation

The economic output of the dairy industry is to a great extent dependent on the processing of milk into other milk-based products such as cheese. The yield and quality of cheese are dependent on both the composition and technological properties of milk. The objective of this study was to evaluate the importance and effects of casein (CN) micelle size and milk composition on milk gelation characteristics in order to evaluate the possibilities for enhancing gelation properties through breeding. Milk was collected on 4 sampling occasions at the farm level in winter and summer from dairy cows with high genetic merit, classified as elite dairy cows, of the Swedish Red and Swedish Holstein breeds. Comparisons were made with milk from a Swedish Red herd, a Swedish Holstein herd, and a Swedish dairy processor. Properties of CN micelles, such as their native and rennet-induced CN micelle size and their ζ-potential, were analyzed by photon correlation spectroscopy, and rennet-induced gelation characteristics, including gel strength, gelation time, and frequency sweeps, were determined. Milk parameters of the protein, lipid, and carbohydrate profiles as well as minerals were used to obtain correlations with native CN micelle size and gelation characteristics. Milk pH and protein, CN, and lactose contents were found to affect milk gelation. Smaller native CN micelles were shown to form stronger gels when poorly coagulating milk was excluded from the correlation analysis. In addition, milk pH correlated positively, whereas Mg and K correlated negatively with native CN micellar size. The milk from the elite dairy cows was shown to have good gelation characteristics. Furthermore, genetic progress in relation to CN micelle size was found for these cows as a correlated response to selection for the Swedish breeding objective if optimizing for milk gelation characteristics. The results indicate that selection for smaller native CN micelles and lower milk pH through breeding would enhance gelation properties and may thus improve the initial step in the processing of cheese.

Modification to the renneting functionality of casein micelles caused by nonionic surfactants

Nonionic emulsifiers of small molecular weight such as polysorbates are widely used in dairy products. Nevertheless, the mechanism of interaction between these surfactants and milk proteins is not yet fully understood. This work investigated the effect of Tween 20 on casein micelles by studying the renneting behavior of skim milk in the presence of different amounts of surfactant. The presence of Tween accelerated both the first and second phase of renneting in skim milk. The gel obtained showed a higher elastic modulus than that of a skim milk gel, but also showed similar brittleness. By varying the size of the surfactant (Tween 20 or Tween 80) as well as the colloidal state of the proteins in solution, it was possible to demonstrate that the surfactant did not have a direct effect on the activity of the enzyme, but rather had a direct effect on the casein micelles. The effect of surfactant on the gelation point was reduced by increasing surfactant size. The presence of Tween caused an increase in the size of the micelles without affecting their stability. In addition, Tween did not alter the amount of caseins free in the serum phase. These findings can contribute to improving our ability to custom design final structures in rennet-induced gels, though further studies are needed to fully understand the mechanism at play when casein micelles are enzymatically cleaved in the presence of nonionic surfactants of small molecular weight.

Diffusing wave spectroscopy and rheological studies of rennet-induced gelation of skim milk in the presence of pectin and κ-carrageenan

The effect of the presence of κ-carrageenan on casein micelle stability and their rennet-induced aggregation was studied using diffusing wave spectroscopy and rheology measurements. Different concentrations of κ-carrageenan, below the gelation regime, were added to skim milk at pH 6.7. No changes in casein stability and on the aggregation kinetics of casein micelles were observed at κ-carrageenan concentrations <0.015% (w/v). The effect of mixing different concentrations of high methoxyl pectin (HMP) (0.04, 0.12 and 0.18%, w/v) with 0.015% (w/v) κ-carrageenan in skim milk was then studied. It was observed that κ-carrageenan protects the system from HMP-induced destabilization. The study of the synergistic effect of κ-carrageenan and HMP on the structuring of casein micelles in milk opens new avenues for the development and control of processes where different mechanisms of aggregation and destabilization occur simultaneously.

Effects of animal selection on milk composition and processability

One goal of animal breeding is to increase the economic output through increased production, improved milk quality, and cow health. The objective of this study was to evaluate genetic progress in relation to milk composition, processability, and yield as a correlated response to selection for the Swedish breeding objective. Dairy cows with high genetic merit, classified as elite dairy cows, of the Swedish Red and Swedish Holstein breeds were used. Milk samples were collected on the farm level in winter and summer from a research herd at Nötcenter Viken, a bovine research farm in Sweden. Comparisons were made with milk from a Swedish Red herd, a Swedish Holstein herd, and a Swedish dairy processor in the same geographical area. Protein, lipid, and carbohydrate profiles as well as minerals were analyzed, and technological properties, including rennet-induced gelation characteristics, lipid oxidation, total antioxidant capacity, and fat globule size, were determined. Higher yields were found for elite cows for components of the protein, lipid, and carbohydrate profiles as well as for minerals, implying genetic progress in relation to milk yield; however, the content of some milk components (e.g., lipid and whey protein contents) had decreased on average. Milk from the elite cows had good gelation characteristics, but was more susceptible to lipid autooxidation and had a lower total antioxidant capacity. These results demonstrate that milk composition and processing characteristics could be used to adjust breeding practices to optimize the quality and stability of milk and dairy products.

Diffusing Wave and Ultrasonic Spectroscopy of Rennet-Induced Gelation of Milk in the Presence of High-Methoxyl Pectin

Rennet-induced aggregation was studied in milk systems containing high-methoxyl pectin (HMP) using ultrasonic and diffusing wave spectroscopy. These two techniques allow for in situ measurements of sol–gel transitions without the need for dilution. At low HMP concentrations, the casein micelles aggregation behavior was similar to that of skim milk, although changes could be noted in the microstructure of the renneted gels. At HMP concentrations between 0.1 and 0.15%, phase-separation kinetics were slower than the rennet-induced aggregation, and different microstructures formed caused by different dynamics of interactions between casein micelles present in HMP-depleted flocs. Higher amounts of HMP failed to create a continuous gel, as phase separation occurred at a faster rate than rennet aggregation. These results highlight the importance of non-invasive techniques in the study of concentration-dependent phase separating and aggregating systems, as only with observations in situ is it possible to determine new ways to control the structuring of protein–polysaccharide mixed systems.

Microencapsulation of probiotic cells by means of rennet-gelation of milk proteins

Probiotic cells were microencapsulated in milk protein matrices by means of an enzymatic induced gelation with rennet. Water insoluble, spherical capsules with a volume-based median of 68 ± 5 μm were obtained from a novel developed emulsifying and subsequent internal gelation process. A high encapsulation yield was found due to the encapsulation procedure for Lactobacillus paracasei ssp. paracasei F19 and Bifidobacterium lactis Bb12. After incubation at low pH-values, microencapsulation yielded higher survival rates compared to non-encapsulated probiotic cells. The viable cell numbers of encapsulated Lactobacillus paracasei and Bifidobacterium lactis were 0.8 and 2.8 log units CFU g −1 higher compared to free cells after 90 min incubation at pH 2.5. The improved survival of encapsulated cells can probably be explained by a higher local pH-value within the protein matrix of the capsules caused by the protein buffering capacity, protecting the cells during incubation under simulated gastric conditions at low pH. The study indicates that rennet-induced gelation of skim-milk concentrates for the microencapsulation of probiotic cells can be a suitable alternative to current available technologies, mainly based on ionotrophic gelation of plant-polymer solutions.

The impact of the concentration of casein micelles and whey protein-stabilized fat globules on the rennet-induced gelation of milk

The rennet-induced aggregation of skim milk recombined with whey protein-stabilized emulsion droplets was studied using diffusing wave spectroscopy (DSW) and small deformation rheology. The effect of different volume fractions of casein micelles and fat globules was investigated by observing changes in turbidity (1/ l*), apparent radius, elastic modulus and mean square displacement (MSD), in addition to confocal imaging of the gels. Skim milk containing different concentration of casein micelles showed comparable light-scattering profiles; a higher volume fraction of caseins led to the development of more elastic gels. By following the development of 1/ l* in recombined milks, it was possible to describe the behaviour of the fat globules during the initial stages of rennet coagulation. Increasing the volume fraction of fat globules showed a significant increase in gel elasticity, caused by flocculation of the oil droplets. The presence of flocculated oil globules within the gel structure was confirmed by confocal microscopy observations. Moreover, a lower degree of κ-casein hydrolysis was needed to initiate casein micelles aggregation in milk containing whey protein-stabilized oil droplets compared to skim milk. This study for the first time clearly describes the impact of a mixture of casein micelles and whey protein-stabilized fat globules on the pre-gelation stages of rennet coagulation, and further highlights the importance of the flocculation state of the emulsion droplets in affecting the structure formation of the gel.

Sodium caseinate-stabilized fat globules inhibition of the rennet-induced gelation of casein micelles studied by Diffusing Wave Spectroscopy

Sodium caseinate (NaCas)-stabilized oil-in-water emulsions were added to skim milk and the rennet-induced aggregation was observed in situ using light scattering and dynamic oscillatory rheology. The gelation of the recombined milk was greatly inhibited by the addition of the oil droplets, at volume fractions >0.025. The development of the turbidity parameter, 1/ l*, and the apparent hydrodynamic radius during renneting were determined using diffusing wave spectroscopy. Although the recombined milk samples contained two scattering particles, namely, casein micelles and fat globules, the latter overwhelmingly contributed to the overall light-scattering signal. This made possible to follow the behaviour of NaCas-stabilized fat globules during the gelation process. The enzymatic reaction associated with the hydrolysis of micellar κ-casein was not significantly affected by the presence of the NaCas-stabilized fat globules. However, the emulsion droplets impeded the aggregation of rennet-altered casein micelles preventing the formation of a gel network. The inability of renneted casein micelles to develop a gel network can be attributed in part to an altered equilibrium between soluble and micellar calcium phosphate, caused by the association of soluble Ca 2+ with casein molecules, but mostly can be attributed to the effect of non-adsorbed caseins on the surface of the casein micelles.

Effect of homogenization, microfiltration and pH on curd firmness and syneresis of curd grains

Abstract The impact of the independent variables, homogenization pressure ( p 1 ), concentration factor of microfiltration ( i ) and pH on curd firmness (CF) and syneresis of curd grains was studied. Texture analysis was used to characterize CF of the rennet-induced gels. The analysis of a two-level factorial design revealed that i , p 1 , pH and the interaction of i and pH had the most important influence on CF. Cutting time was therefore individually determined for each milk system using small amplitude oscillatory rheometry for generating comparable conditions for the syneresis experiments. Syneresis of curd grains with a diameter of 11 mm was followed at 35 °C close to semi-hard cheesemaking conditions. The permeate release during microfiltration was taken into consideration, allowing an evaluation of syneresis of grains made from concentrated and unconcentrated milk. It was shown that with increasing milk concentration less curd treatment time was needed to reach a certain syneresis value. Hence, total processing time in cheesemaking is decreased. Analysis of variance revealed that syneresis was affected by the individual variables. Kinetic parameters were satisfactorily estimated through regression ( R 2 >0.98) and it was shown that milk composition and concentration due to microfiltration markedly influenced the endpoint of syneresis, RWR max . The experiments demonstrate that microfiltration and homogenization can be combined to reach CF and syneresis comparable to untreated milk used in conventional cheesemaking. This meets one claim of the cheese industry when implementing both technologies in the manufacture process, since consistency and quality of the ripened cheese are expected to be unchanged.

Effect of Insoluble Calcium Concentration on Rennet Coagulation Properties of Milk

Rennet-induced gels were made from milk acidified to various pH values or milk at pH 6.0 that had added EDTA. The objective was to examine the effect of removing insoluble Ca (INS Ca) from casein micelles (CM) on rennet gelation properties. For the pH trial, diluted lactic acid was added to reconstituted skim milk to decrease the pH to 6.4, 6.0, 5.8, 5.6, and 5.4. For the EDTA trial, EDTA was slowly added (0, 2, 4, and 6mM) to reconstituted skim milk, and the final pH values were subsequently adjusted to pH 6.0. Dynamic low amplitude oscillatory rheology was used to monitor gel development. The Ca content of CM and rennet wheys made from these milks was measured using inductively coupled plasma spectroscopy. The INS Ca content of milk was altered by the acidification pH values or level of EDTA added. In all samples, the storage modulus (G′) exhibited a maximum (GM), with a decrease in G′ during longer aging times. Gels made at pH 6.4 had higher GM compared with gels made at pH 6.7 probably due to the reduction in electrostatic repulsion, whereas the INS Ca content only slightly decreased. The highest GM value of gels was observed at pH 6.4 and the GM value decreased with decreasing pH from 6.4 to 5.4. This was due to an excessive loss of INS Ca from CM. There was a decrease in GM with the increase in the concentration of added EDTA, which was probably due to the loss of colloidal calcium phosphate, which weakens the integrity of CM. Loss tangent (LT) values at GM increased with a reduction in milk pH and the addition of EDTA to milk. Rennet gels at the point of the GM were subjected to constant low shearing to fracture the gels. With a reduction in INS Ca content, the yield stress decreased, whereas LT values increased indicating a weaker, more flexible casein network. Microstructure of rennet-induced gels near the GM point and 2 to 10h after this point was studied using fluorescence microscopy. At GM, gels made from milk acidified to pH 6.4 exhibited more branched, interconnected networks, whereas strands and clusters became larger with a reduction in milk pH to 5.4. Gels made from milk with EDTA added had more finely dispersed protein clusters compared with gels made from milk with no EDTA added. These microscopic observations supported the effect of loss of INS Ca on GM and LT. There was a decrease in apparent interconnectivity between strands in gel microstructure during aging, which agreed with the decrease in G′ after GM. It can be concluded that low levels of solubilization of INS Ca and the decrease in milk pH resulted in an increase in GM. With greater losses of INS Ca there was excessive reduction in cross-linking within CM, which resulted in weaker, more flexible rennet gels. This complex behavior cannot be explained by adhesive hard sphere models for CM or rennet gels made from these CM.

Effect of β-Lactoglobulin A and B Whey Protein Variants on the Rennet-Induced Gelation of Skim Milk Gels in a Model Reconstituted Skim Milk System

The effect of fortifying reconstituted skim milk with increasing levels of the β-lactoglobulin (β-LG) genetic variants A, B, and an A-B mixture on rennet-induced gelation was studied by small deformation-sensitive rheology. Free-zone capillary electrophoresis and high-sensitivity oscillatory rheology were used to elucidate the role of potential heterotypic associative interactions between whey proteins and casein in a mixed colloidal system, subjected to moderate heating (65°C for 30min) prior to renneting, on the gelling properties of the system. Increasing levels of added whey protein, in the concentration range of 0.225 to 1.35% of added protein, led to a concomitant progressive increase in the equilibrium shear storage modulus, G′ (recorded after ∼10,800s), in the order β-LG B>β-LG A and β-LG A-B, as the general expected consequence of the setup of denser casein gel networks. The preferential effect of β-LG B over β-LG A on the mechanical strength of the gels may be due to the formation of cross-links and aggregates involving whey proteins and rennet hydrolysis products or an increase in the size of the casein micelle caused by the grafting of β-LG B to its surface, or both. The results of free-zone capillary electrophoresis were consistent with the notion that β-LG B (and not β-LG A) binds to the casein micelle under an optimal stoichiometry of 1:0.045 (mg/mg), even in the absence of heat treatment. The liquid-like character of the gel networks formed, tan δ, was a parameter sensitive to the level of addition of β-LG A in particular. At low concentrations (up to 0.45%) of β-LG A, tan δ increased by almost twice as much, which was interpreted as a result of the increase in the loss modulus, G″, of the sol fraction because of the presence of unbound β-LG A. At greater incremental concentrations of β-LG (>0.45%), the formation of smaller whey protein aggregates confined to the sol fraction may have led to a progressive decrease in tan δ. The critical gel time, tgel, was also affected by the concentration of added whey protein and described 3 zones of behavior, irrespective of the type of whey protein variant. The critical gel time was slightly shorter for β-LG B than for β-LG A at 0.45% of added whey protein, but this difference became larger at 0.67%. Even when only β-LG B was found to associate with casein prior to renneting, both β-LG A and β-LG B, either alone or mixed, had a profound influence on the mechanical strength and coagulation kinetics of the rennet-induced casein gels. This knowledge is expected to be useful to exert better control and optimize processing conditions during the manufacturing of cheese and cheese analogs.

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 ( Φ ∼ 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 κ-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 κ-casein does true aggregation occur, with the appearance of a space-filling gel (defined by rheology as having a value of tan δ < 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.

The suitability of milk from a spring-calved dairy herd during the transition from normal to very late lactation for the manufacture of low-moisture Mozzarella cheese

Bulk milk from a spring-calved dairy herd on a good plane of nutrition was collected and made into low-moisture Mozzarella cheese weekly over 10 weeks from 22 September (218 days in lactation, DIL) to 27 November (284 DIL). The 10 weeks were divided into three lactation periods: normal lactation (NL), 218–240 DIL; late lactation (LL), 241–265 DIL; and very late lactation (VLL), 266–284 DIL. Lactation period did not significantly influence milk composition, percentages of milk fat or protein recovered to cheese, cheese yield and composition, primary proteolysis or cooking properties during storage. The curd firmness of the rennet-induced gels from the LL or VLL milk samples was significantly higher than that of the NL milk. The mean firmness over the 40-d storage period of cheese from the LL and VLL milks was significantly higher than that of the cheeses from the NL milk. Satisfactory Mozzarella cheese making characteristics of milk from spring-calved cows can largely be sustained into late lactation by maintaining good dietary nutrition and herd management practices.

The manufacture of miniature Cheddar-type cheeses from milks with different fat globule size distributions

A novel 2-stage gravity separation scheme was developed for fractionation of raw, whole bovine milk into fractions enriched in small (SFG) or large (LFG) fat globules. The volume mean diameter of fat globules in SFG, LFG or control (CTRL) milk was 3.45, 4.68 and 3.58 microm, respectively. The maximum in storage modulus (index of firmness) decreased with increasing fat globule size for rennet-induced gels formed from SFG, LFG or CTRL milks. Miniature (20 g) Cheddar cheeses were manufactured using each of the 3 milks. There were no significant (P > 0.05) differences in the pH, moisture and fat in dry matter levels between cheeses made using any of the 3 milks, however, the fat content of the cheese made using SFG milk was approximately 1% lower than that of cheese made using LFG or CTRL milk in each of the 2 trials. Image analysis of confocal scanning laser micrographs of the cheeses illustrated that the star volume of fat globules in the cheeses decreased significantly (P < or = 0.05) as the size of fat globules in the milks used for cheesemaking was reduced. This indicates that it is possible to manipulate the size distribution of fat globules in Cheddar cheese by adjusting the fat globule size distribution of the milk used for cheese-making. The concentration of free fatty acids (FFA) increased in all cheeses during ripening. At 120 d of ripening, the concentration of FFA varied significantly (P < or = 0.05 and P < or = 0.001 for trials 1 and 2, respectively) with fat globule size, with cheeses made in trial 2 from LFG, SFG or CTRL milks having total FFA levels of 3391, 2820 and 2612 mg/kg cheese, respectively.

Rennet-Induced Gelation of Calcium and Phosphate Supplemented Skim Milk Subjected to CO2 Treatment

A Doehlert design was performed to study the effect of calcium and phosphate supplementation at 0 to 25mmol/kg and 0 to 16mmol/kg, respectively, on the rennet gelation of reconstituted skim milk subjected to pH-reversible CO2 acidification. Supplemented reconstituted skim milk samples were acidified to pH 5.80 by the addition of CO2 under pressure and depressurized under vacuum to restore the initial pH value. The second-order polynomial models satisfactorily predicted the effect of salt addition on the micellar molar Ca:P ratio and the average diameter of the casein micelles, whereas only trends were used in the analysis of the rennet-clotting behavior of salt-supplemented, CO2-treated milk. Whether added Ca was the most determinant factor on the micellar molar Ca:P ratio, added Pi (a mixture of Na2HPO4 and NaH2PO4) was the most determinant factor on the other responses studied, and its effect was most pronounced when Ca was simultaneously added. By comparison with control samples, changes observed in this study were essentially due to salt supplementation and not to the CO2 treatment. Therefore, this CO2 treatment could be considered as an entirely reversible treatment rather than only pH-reversible, and predictions might be applied to untreated milk. In the case of Ca-supplemented milk, the micellar molar Ca:P ratio increased, the average micellar diameter decreased, and the rennet-clotting properties were improved, whereas opposite effects were observed upon Pi supplementation. Since modification of the micellar molar ratio is the result of change in the chemical composition of micellar calcium phosphate, the effect of calcium and phosphate supplementation on the rennet clotting of milk was found to be also dependent on the nature of the interaction between caseins and colloidal calcium phosphate.

Casein–whey protein interactions in heated milk: the influence of pH

Heat-treatment of milk causes denaturation of whey proteins, leading to a complex mixture of whey protein aggregates and whey protein coated casein micelles. In this paper we studied the effect of pH-adjustment of milk (6.9–6.35) prior to heat-treatment on the distribution of denatured whey proteins in aggregates and coating of casein micelles. Proteins were fractionated using an alternative fractionation technique based on renneting. Acid- and rennet-induced gelation of these milks were used to obtain more information on the characteristics of the milk. Acid-induced gelation appeared to be mainly influenced by the presence of whey protein aggregates. Rennet-induced gelation was determined by the whey protein coating of the casein micelles. Both the quantity of whey proteins present on the surface of the casein micelles and the homogeneity of the coating were determining the renneting properties. These results extend the current knowledge on pH dependent casein–whey protein interactions. In order to present a clear picture of the changes occuring during heat treatment of milk at various pH, the results are summarized in a model. In this model we propose that heating at a pH>6.6 lead to a partial coverage of the casein micelles and the formation of separate whey protein aggregates. Heating at a pH<6.6 lead to an attachment of all whey proteins to the casein micelles. At pH 6.55 the coverage is rather homogeneous but lowering the pH further lead to an inhomogeneus coverage of the casein micelles. Surprisingly small changes of the pH at which the milk was heated had considerable effects on the gelation behaviour both in renneting and in acid gelation.

Effects of Added Plasmin on the Formation and Rheological Properties of Rennet-Induced Skim Milk Gels

Elevated plasmin enzyme activity has been suggested as a likely cause of impaired functional properties that occur in milk from cows either in their latelactational period or that are experiencing mastitis. However, there are conflicting reports on the impact of plasmin on rennet coagulation properties of milk. The effects of added plasmin on the rheological properties, at small and large deformation, of rennet-induced gels were investigated. The microstructure of rennet-induced gels was studied, using confocal scanning laser microscopy. Porcine plasmin was added to reconstituted milk, and samples were incubated at 37°C for between 0.5 to 8h. The hydrolysis reaction was terminated using soybean trypsin inhibitor. The extent of degradation of caseins was determined with SDS-PAGE. The extent of breakdown of αs- and β-caseins increased with incubation time with plasmin. Storage modulus of rennet gels decreased linearly with increasing degradation of caseins. There was an increase in the loss tangent parameter of the gels with increasing casein degradation, indicating a more liquid-like gel character. Gelation time decreased until approximately 3h of incubation with plasmin (when the amounts of intact αs- and β-caseins were ∼46 and 50%, respectively); thereafter, gelation time increased considerably. Yield stress of rennet-induced gels decreased with increasing casein breakdown. When the level of casein hydrolysis was high (<40% of intact caseins), the microstructure of rennet-induced gels was drastically altered. Even when there were low levels of casein hydrolysis, the rheological properties of rennet gels were altered, which could have negative impacts on cheese yield and texture.

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.

The relationship between rheological parameters and whey separation in milk gels

The relation between whey separation of rennet-induced gels and rheological properties of those gels is reasonably well understood. A low fracture stress and a high value for the loss tangent at low frequencies have been correlated with a tendency to exhibit syneresis in rennet gels. In contrast, little is known about the relationship between mechanical properties of gels and whey separation in acid-induced milk gels, such as yoghurt, although this continues to be a major defect. In recent work, it has been found that conditions such as high milk heat treatment, fast rates of acidification and high incubation temperatures all gave high levels of whey separation compared with gels made from unheated milk that were incubated at low temperatures and where the rate of acidification was slow (i.e. when bacterial cultures were used instead of the acidogen, glucono-δ-lactone). The tendency to exhibit whey separation in acid gels made from heated milk was related to a low fracture strain and an increase in the loss tangent (observed even at high frequencies) during the gelation process (a high value indicates conditions favouring relaxation of bonds). Excessive rearrangements of particles in the gel network before and during gelation were implicated as being responsible for whey separation and rheological conditions that appeared to indicate this defect are described. It was also concluded that techniques that measure the spontaneous formation of surface whey should be distinguished from those that measure the expression of whey from networks under pressure as the latter tests only measure gel rigidity.

Influence of heat treatment of milk on cheesemaking properties

High heat treatment of milk causes denaturation of whey proteins and complex interactions among denatured whey proteins, casein micelles, minerals and fat globules. It is well established that interactions of whey proteins have both positive and negative implications in cheese manufacture. On the one hand, denatured whey proteins could be incorporated into cheese curd, resulting in a higher yield from a given quantity of milk while, on the other, the interactions of whey protein with casein micelles interfere with the rennet coagulation process, resulting in long coagulation times and weak curd structures. The cheeses made from heated milk differ from traditional cheese in body, texture and flavour profiles. This paper reviews current information on the relationships between whey protein interactions in heated milk and rennet coagulation properties, and discusses possible mechanisms involved. Methods that may be used to improve renneting properties of heated milk are described. In addition some recent results on the interactive effects of heat treatment, ultrafiltration concentration and pH on properties of interest to cheesemaking, in particular rennet-induced gel formation, are presented.