In this study, the effects of the degree of thermal denaturation of whey protein (WP) added to milk on the dissociation of κ-casein from casein micelles were investigated, since they are related to the strength of acid milk gel and its factors. Acid milk gels were prepared by heating thermally denatured WP isolate (WPI) and undenatured milk mixtures and treating them with glucono-δ-lactone as a coagulant. The strength of these gels was negatively correlated with the WPI denaturation degree and strongly positively correlated with the extent of κ-casein dissociation from casein micelles. This behavior was ascribed to the fact that α-lactalbumin (α-La) and β-lactoglobulin (β-Lg) contained in WPI denatured after heating and engaged in disulfide bond formation with each other. With an increase in the degree of denaturation and disulfide bond formation, the bonding between β-lactoglobulin and κ-casein was suppressed to decrease the amount of κ-casein–WPI complexes. When β-Lg forms SS bonds with α-La, the number of highly reactive, free SH groups decreases, which complicates the formation of SS bridges between β-Lg and κ-casein. Thus, the denaturation degree of WPI largely determined the degree of κ-casein dissociation from casein micelles and, consequently, the strength of acid milk gels. Adding WP to milk increases the strength of acid milk gel, and it can be controlled by changing the degree of thermal denaturation of the WP. Furthermore, it was clarified for the first time that the dissociation of κ-casein from casein micelles influences this effect. Further studies are needed to elucidate the structural features of κ-casein-dissociated micelles.

This research paper addresses the hypothesis that comparative genomics can give a new insight into the functionality of casein genes with respect to the casein micelle. Comparative genomics is a rapidly emerging field in computational biology whereby two or more genomes are compared in order to obtain a global view on genomes as well as assigning previously unknown functions for genes. Casein genes are among the most rapidly evolving mammalian genes, with the gene products mainly grouped into four types (αs1-, αs2-, β- and κ-casein). Functionally, casein genes are central to the casein micelle, the exact structure of which is still a subject of intense debate. Moreover, and adding to this complexity, some mammals lack some of the casein genes, although casein micelles have been observed in their milk. This observation has prompted an investigation into the distribution of casein genes across a host of mammalian species. It was apparent from this study that casein gene sequences are very diverse from each other and we confirmed that many mammalian species lack one or more of the casein genes. The genes encoding β- and κ-caseins are present in most mammals whereas α-casein encoding genes are less represented. This suggests different mechanisms for casein micelle formation in different species as well as the functions that are assigned to each individual casein.

Reconstituted skim milks (10 % w/w total solids, pH 6·7–8·0) were ultrasonicated (20, 400 or 1600 kHz at a specific energy input of 286 kJ/kg) at a bulk milk temperature of <30 °C. Application of ultrasound to milk at different pH altered the assembly of the casein micelle in milk, with greater effects at higher pH and lower frequency. Low frequency ultrasound caused greater disruption of casein micelles causing release of protein from the micellar to the serum phase than high frequency. The released protein re-associated to form aggregates of smaller size but with surface charge similar to the casein micelles in the original milk. Ultrasound may be used as a physical intervention to alter the size of the micelles and the partitioning of caseins between the micellar and serum phases in milk. The altered protein equilibria induced by ultrasound treatment may have potential for the development of milk with novel functionality.

The main requirement for milk processed in most cheese typologies is its rennet coagulation ability. Despite the increasing number of studies, the causes for abnormal coagulation of milk are not fully understood. The aim of this study was to ascertain relationships between milk characteristics and its rennet coagulation ability, focusing on the influence of calcium (Ca) and phosphorus (P). Ca and P are essential constituents of the micelles. Micellar P can be present as part of colloidal calcium phosphate (inorganic-P) or covalently bound to caseins as phosphate groups (casein-P). Eighty one herd milk samples (SCC<400 000 cell/ml) were classified as Optimal (8), Suboptimal (39) Poor (29) and Non-coagulating milk (5), according to their rennet coagulation parameters as assessed by lactodynamographic test. Samples were analysed for their chemical composition (basic composition, protein fractions, minerals and salt equilibria), physicochemical parameters (pH and titratable acidity) and rheological properties. Optimal milk was characterised by the highest contents of major constituents, protein fractions and minerals, lowest content of chloride and highest values of titratable acidity. Non-coagulating milk was characterised by the highest values of pH and the lowest of titratable acidity. At micellar level, Optimal milk showed the highest values of colloidal Ca, casein-P and colloidal Mg (g/100 g casein), while Non-coagulating milk showed the lowest values. Interestingly, there was no statistical difference regarding the content of colloidal inorganic-P (g/100 g casein) between Optimal and Non-coagulating milks. Overall, high mineralisation of the micelle (expressed as g inorganic-P/100 g casein) positively affect its rennetability. However, excessive mineralisation could lead to a reduction of the phosphate groups (g casein-P/100 g casein) available for curd formation.

In this study, the properties of casein particles reformed from alkaline disrupted casein micelles were studied. For this purpose, micelles were disrupted completely by increasing milk pH to 10·0, and subsequently reformed by decreasing milk pH to 6·6. Reformed casein particles were smaller than native micelles and had a slightly lower zeta-potential. Levels of ionic and serum calcium, as well as rennet coagulation time did not differ between milk containing native micelles or reformed casein particles. Ethanol stability and heat stability, >pH 7·0, were lower for reformed casein particles than native micelles. Differences in heat stability, ethanol stability and zeta-potential can be explained in terms of the influence of increased concentrations of sodium and chloride ions in milk containing reformed casein particles. Hence, these results indicate that, if performed in a controlled manner, casein particles with properties closely similar to those of native micelles can be reformed from alkaline disrupted casein micelles.

In the study presented in this article, the influence of added α-lactalbumin and β-lactoglobulin on the changes that occur in casein micelles at 250 and 300 MPa were investigated by in-situ measurement of light transmission. Light transmission of a serum protein-free casein micelle suspension initially increased with increasing treatment time, indicating disruption of micelles, but prolonged holding of micelles at high pressure partially reversed HP-induced increases in light transmission, suggesting reformation of micellar particles of colloidal dimensions. The presence of α-la and/or β-lg did not influence the rate and extent of micellar disruption and the rate and extent of reformation of casein particles. These data indicate that reformation of casein particles during prolonged HP treatment occurs as a result of a solvent-mediated association of the micellar fragments. During the final stages of reformation, κ-casein, with or without denatured whey proteins attached, associates on the surface of the reformed particle to provide steric stabilisation.

High pressure (HP) treatment affects many constituents of milk (for reviews see Huppertz et al. 2002; Needs, 2002); particular in the properties of casein micelles in HP-treated milk differ considerably from their counterparts in untreated milk. In milk treated at 100–200 MPa, average casein micelle size differs little from that of untreated milk (Needs et al. 2000a; Huppertz et al. 2004a; Regnault et al. 2004; Anema et al. 2005), but micelle size in milk treated at 250 MPa for [ges ]15 min is considerably higher than in untreated milk, probably due to HP-induced aggregation of casein micelles (Huppertz et al. 2004a,b; Regnault et al. 2004); after treatment at 300–800 MPa, micelle size is ~50% lower than that in untreated milk (Needs et al. 2000a; Huppertz et al. 2004a,b; Anema et al. 2005). HP-induced changes in average casein micelle size are irreversible on subsequent storage, except for the increase in micelle size after treatment at 250 MPa (Huppertz et al. 2004a).

The aim of the present work was to investigate the role of whey protein denaturation on the acid induced gelation of casein. This was studied by determining the effect of whey protein denaturation both in the presence and absence of casein micelles. The study showed that milk gelation kinetics and gel properties are greatly influenced by the heat treatment sequence. When the whey proteins are denatured separately and subsequently added to casein micelles, acid-induced gelation occurs more rapidly and leads to gels with a more particulated microstructure than gels made from co-heated systems. The gels resulting from heat-treatment of a mixture of pre-denatured whey protein with casein micelles are heterogeneous in nature due to particulates formed from casein micelles which are complexed with denatured whey proteins and also from separate whey protein aggregates. Whey proteins thus offer an opportunity not only to control casein gelation but also to control the level of syneresis, which can occur.