The aim of this work was to investigate the presence of antioxidant and ACE-inhibitory activity in ovine αs2-casein and bovine κ-casein hydrolysates with antibacterial activity. Several pep- tides which had been previously identified in these hydrolysates were selected in order to fulfil cer- tain structural requirements and they were chemically synthesised to evaluate their antioxidant and ACE-inhibitory activity. Hydrolysates of ovine αs2-casein and bovine κ-casein with pepsin strongly inhibited ACE activity, with IC50 values of 41.8 and 9.97 μmol·L −1 , respectively. The κ-casein hy- drolysate also exhibited a significant oxygen radical absorbance capacity, seven times higher than that of Trolox. From the chemically synthesised peptides, two of them, LKKISQ and PYVRYL, both from ovine αs2-casein, exerted potent ACE-inhibitory activity in the range of the most po- tent food-derived antihypertensive peptides described to date (IC50 values of 2.6 and 2.4 μmol·L −1 , respectively). The latter sequence, corresponding to the C-terminal hexapeptide of the ovine αs2- casein molecule, also had antioxidant activity. The activity found is discussed in relation to the peptide sequences. αs2-casein / κ-casein / antibacterial activity / antioxidant activity / angiotensin-converting enzyme-inhibitory activity / bioactive peptide
A reaction kinetic approach for the description of reversible (unfolding) and irreversible (aggregation) denaturation of β-lactoglobulin (β-Lg) was developed in this study. This novel ap- proach allows the calculation of the content of native, partially unfolded and irreversibly denatured β-Lg as a function of the heat treatment time. Moreover, it is able to give a mathematical inter- pretation of the sharp bend on the Arrhenius plot of the formal denaturation rate constant which is characteristic of the denaturation behaviour of both major whey proteins, α-lactalbumin and β- lactoglobulin. The developed theoretical backgrounds were applied for the denaturation kinetics of β-Lg in a protein solution containing 50 g·L −1 of the isolated whey protein fraction and verified by means of differential scanning calorimetry. β-lactoglobulin / thermal denaturation / unfolding / aggregation / reaction kinetic
Cette etude a eu pour but d'examiner le relargage, au cours de l'affinage, de peptides inhibiteurs de l'enzyme de conversion de l'angiotensine (ACE) dans des cheddars fabriques a l'aide de levains lactocoques et Lactobacillus casei 279 ou Lb casei LAFTIĚ® L26, et d'isoler, purifier et identifier ces peptides. L'ajout de probiotiques augmentait l'activite inhibitrice de l'ACE dans les fromages au cours de l'affinage a 4 °C, probablement en raison de la proteolyse accrue. La concentration IC 50 (concentration en ACE necessaire pour inhiber 50 % de l'activite de l'enzyme) etait la plus faible apres 24 semaines d'affinage dans les fromages avec probiotique (0,23-0,25 mg-mL-1), en comparaison aux 36 semaines d'affinage pour les fromages sans ajout de probiotique (0,28 mg.mL -1 ). Les extraits hydrosolubles de chaque fromage ont ete soumis a differentes etapes de fractionnement chromatographique. L'activite inhibitrice retrouvee dans les fractions brutes variait de 0,1 a 2,0 mg.mL -1 . La fraction possedant l'activite la plus elevee a ete purifiee a l'aide d'une seconde etape de chromatographie. Differents peptides inhibiteurs de l'ACE correspondants a la partie N-terminale de la caseine αs1 [(f 1-6), (f 1-7), (f 1-9), (f 24-32) et (f 102-110)] et a la partie N-terminale de la caseine β [(f47-52) et (f 193-209)] ont ete retrouves. Ces resultats suggerent que l'inhibition de l'ACE dans les cheddars dependent dans une certaine mesure de la proteolyse. Les probiotiques utilises dans cette etude ont ete ajoutes de facon efficace dans le cheddar, afin de procurer les effets benefiques pour la sante en meme temps que la production de peptides bioactifs.
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Why a bibliographic review on free fat in dairy powders? Pierre Schuck
Cantal is the third French Protected Denomination of Origin cheese. The Cantal cheesemaking process is similar to Cheddar cheese. Both processes involve a milling and a dry salting step which results in a homogeneous salted curd. In order to determine the key parameters of the manufacturing process acting on the specific characteristics of this cheese variety, an extensive and kinetic study repeated three times was carried out at nine stages from the raw milk to 120 d ripened cheese. The microbial ecosystem was explored by numeration and by direct extraction of DNA and PCR-Temporal Temperature Gradient Electrophoresis. Lysis of the lactic acid bacteria was checked by quantification of the intracellular lactate dehydrogenase activity. Carbon sources changes, solubilisation of minerals, proteolysis, lipolysis, and formation of neutral volatile compounds were also assessed. Cheese microstructure modifications and bacterial localisation were determined by confocal scanning laser microscopy. The controlled manufacturing conditions and the use of a complex mixture of starters led to very similar ripened cheeses. The compositional attributes (in particular total solids, fat, nitrogen forms), the lipolysis (0.7 +/- 0.2% fat) and proteolysis final extent (34.4 g.kg(-1) free amino acids in juice) were similar as well as major neutral volatiles compounds (2-butanone, ethanol and 2-butanol). Nevertheless, lactose and citrate metabolisms differed between the three trials and some free amino acids (Arg, Pro, Ser, Cit, Homocys) concentrations varied drastically during the ripening time course. CLSM micrographs showed that fat globules were disrupted during pressing and bacteria were preferentially localised at the fat/protein interface. As the lactic starter species were the same in the three trials and predominant during the cheese manufacture, the observed differences could result of one or several of the followings: (i) the raw milk microflora which showed to be different leading to growth or metabolism activity of particular subdominant populations during the process and the ripening; (ii) the more or less early lysis of the lactic starters; (iii) the possible interactions between raw milk microflora and the added starters. This extensive characterization of Cantal cheese provides a strong basis for building a rational concept of the key steps in which either technological actions or complementary microbial inoculation could be done in order to limit day to day variations.
In this paper, we focused on the characteristics of milk fat (composition, organisation and thermal properties) that may improve the functional, sensory and nutritional properties of food products. Fat is dispersed in raw milk as droplets called milk fat globules, which are enveloped in their biological membrane (MFGM). Dairy processes may greatly affect both the supramolecular organisation and the surface composition of fat. Using confocal laser scanning microscopy, we showed that fat can be dispersed as natural milk fat globules (d(mean) similar to 4 mu m), coalesced fat globules (10 - 20 mu m), aggregates of fat globules (10 - 50 mu m), tiny homogenised fat globules (0.2 - 1 mu m) covered mainly by caseins, and at the extreme opposite, non-globular fat, also called free fat. Fat can also be the continuous phase such as in butter and in its anhydrous form. We focused on the changes in the organisation of fat during the manufacture and ripening of a hard-type cheese and showed that the pressing of curd grains leads to the disruption of the MFGM and the formation of free fat. Furthermore, the pockets of whey surrounding the inclusions of fat after disruption of the MFGM favour the preferential localisation of bacteria at the fat/protein interface and thus contribute to the accessibility of lipolytic enzymes to fat for lipolysis, essential for the quality of cheese. An innovative method was developed using differential scanning calorimetry to determine the solid fat content in food products. We showed that the fat phase is partially crystallised in cheese for T < 41 degrees C, e. g. a mixture of crystals and oil, and calculated that about 56% of fat was solid at 4 degrees C. Finally, we showed that increasing the amount of unsaturated fatty acids in milk fat to improve its nutritional properties changes the solid fat content as a function of temperature, the resistance of fat globules to disruption and thus the functional, textural and sensorial properties of fat-rich products such as butter.
At the heart of the milk system are the colloidal casein-calcium-transport complexes termed the casein micelles. The application of physical chemical techniques such as light, neutron, and X-ray scattering, and Electron Microscopy (EM) has yielded a wealth of experimental detail concerning the structure of the casein micelle. From these experimental data bases have arisen two conflicting models for the internal structure of the casein micelle. One model emphasizes protein submicellar structures as the dominant feature, while the other proposes that inorganic calcium phosphate nanoclusters serve this function. In this study, these models are critically examined in light of the two current primary dogmas of structural biology which are: protein structure gives rise to function and that competent and productive protein-protein interactions (associations) will lead to efficient transit through the mammary secretory apparatus. In this light an overwhelming argument can be made for the formation of proteinaceous complexes (submicelles) as the formative agents in the synthesis of casein micelles in mammary tissue. Whether these submicelles persist in milk has been questioned. Recently we have carried out studies on casein micelles and submicelles using Atomic Force Microscopy (AFM) and high resolution Transmission Electron Microscopy (TEM) to gain insights on the nature of protein-protein interactions in submicelles and micelles from a structural biology perspective. The results provide experimental evidence that protein-protein interactions are important in the formation and stabilization of casein micelles. milk / micelle / casein / structure / protein-protein interaction
La fabrication de poudre de lait implique un traitement thermique du lait, qu'on appelle prechauffage, puis sa concentration en matiere seche par evaporation ou sechage par atomisation. Pour les poudres a teneur elevee en proteines, l'ultrafiltration/diafiltration est normalement utilisee avant evaporation ou sechage. Ces procedes provoquent, entre les differents composes du lait, des interactions qui influent sur les proprietes fonctionnelles des poudres. La structure et la stabilite des micelles de caseine sont relativement bien connues dans le lait. Cependant, les effets des procedes de fabrication de poudre sur la structure et les interactions des micelles de caseine sont bien compris. Le but du traitement de prechauffage applique au lait avant evaporation est de produire des poudres ayant des proprietes fonctionnelles et des applications specifiques. Un effet majeur du prechauffage est la denaturation des proteines de lactoserum et leur association avec les micelles de caseine. La nature et l'amplitude de l'association des proteines de lactoserum avec les micelles de caseine, impliquant la caseine K, influent sur le comportement des micelles de caseine au cours des traitements ulterieurs. Pendant l'evaporation, la taille des micelles augmente principalement a cause de l'agregation de certaines micelles et de l'association accrue des proteines de lactoserum avec les micelles. Il y a eu peu de travaux sur les effets specifiques du sechage par atomisation sur les micelles de caseine dans le lait. Dans la fabrication de poudres a teneur elevee en proteines, la concentration du lait par ultrafiltration, en particulier la diafiltration, avant sechage, peut provoquer la dissolution du phosphate de calcium colloidal, resultant dans un relâchement de la structure de la micelle et potentiellement un gonflement des micelles. L'elevation du taux de concentration provoque la rupture progressive de la structure de la micelle qui passe de l'etat intact a un etat diffus gonfle pour finalement s'emietter en plus petits fragments. Ces changements dans les micelles de caseine predisposent le systeme du lait a des interactions proteines/proteines ulterieures au cours du sechage par atomisation et ayant un impact sur les proprietes fonctionnelles du produit.
Plus de dix sortes de Tulum sont produits en Turquie. Ce fromage d'une couleur creme ou blanche possede une teneur elevee en matiere grasse (gras sur sec > 50 %), une pâte semi-dure de texture grumeleuse et une flaveur de beurre piquante. Le plus populaire de ces fromages est le Erzincan Savak Tulum, produit principalement dans la region de l'est de la Turquie et fabrique par plusieurs usines; son poids economique est plus important que celui des autres varietes de Tulum. Les caracteristiques de ce fromage sont passees en revue, i.e., le lait employe, le traitement thermique du lait, les levains et la presure utilises, la technologie de fabrication, la composition chimique et les modifications biochimiques au cours de l'affinage. La seconde variete largement consommee est le fromage de type Izmir Brined Tulum (en saumure), dont les caracteristiques sont egalement presentees en details. Les autres varietes de Tulum sont principalement consommees dans leurs regions de production et ne sont pas rencontrees dans toute la Turquie. Ces derniers ne sont pas aussi bien caracterises et ne sont presentes que brievement. Cependant, l'interet pour la fabrication de ces autres types de Tulum est croissant et leur valeur economique augmente.
Free fatty acids (FFA) liberated from milk fat by lipases and esterases during cheese ripening affect flavour, by themselves or as precursors for the synthesis of other volatile compounds. The effect of the lacticin 481-producing strain Lactococcus lactis subsp. lactis INIA 639 (BP) on the release of intracellular esterases from a Lactobacillus helveticus strain used as adjunct culture and FFA levels during ripening of semi-hard cheese was investigated. Cheeses were manufactured using either L. lactis subsp. lactis INIA 639, the bacteriocin non-producing strain L. lactis subsp. lactis INIA 437, or a combination of both strains, as mesophilic starter cultures. The Lb. helveticus strain, sensitive to lacticin 481, was added to all vats. Cheeses made with the BP strain exhibited on day 50 esterase activity values up to 29% higher than those of cheese made without the BP strain. Levels of most individual FFA in cheese were increased by milk inoculation with the BP strain. On day 50, total (C-4:0-C-18:2) FFA were 628 to 638 mg.kg(-1) in cheeses made with the BP strain, and 576 mg.kg(-1) in cheese made without the BP strain, mostly due to the higher accumulation of FFA from day 25 to day 50 in the former cheeses. Bacteriocin-mediated lysis of the Lb. helveticus strain and subsequent release of its esterases appeared to be responsible for accelerated lipolysis in cheese.
Developing low-fat cheese with flavor to match that of full-fat cheese has been a chal- lenge in the dairy industry. The objective of this investigation was to develop lower fat Cheddar and Parmesan grated cheese using supercritical fluid extraction (SFE) and characterize its flavor profile comparative to a full-fat product. Specifically, enabling flavor compound partition between the matrices of cheese and extracted lipids. Carbon dioxide (CO2) was the supercritical fluid for fat extraction. Extraction took place in a 500 mL SFE vessel using 100 g of grated cheese. Frac- tional factorial design was utilized to investigate two levels of treatment for each pressure (200 and 350 bar), temperature (35 and 40 ◦ C) and CO2 level (500 and 1000 g) for each extraction trial. The most efficient parameters for lipid removal resulted in 51.00% fat reduction (wet basis) for Cheddar extracted at 200 bar, 40 ◦ C, 1000 g CO2, and 55.56% fat reduction for Parmesan extracted at 350 bar, 35 ◦ C, 1000 g CO2. Thin layer chromatography (TLC) was utilized to assess the lipid composition of each cheese and the lipids extracted by SFE. TLC analysis for Cheddar and Parmesan cheeses showed only nonpolar lipids (triaclyglycerides and free fatty acids) in the recovered lipids extracted by SFE; indicating that polar lipids such as phospholipids are being retained in the cheese matrix. Gas chromatography/mass spectroscopy techniques were used to characterize volatile flavor com- pounds for each cheese sample. SFE treatment of the cheeses altered the ability to detect flavor compounds and allowed partitioning of those compounds, which varied with the type of cheese. This study suggests that SFE technology can be used in the dairy industry to develop cheese prod- ucts lower in fat, which retain flavor compounds that may not be typically fully developed with alternative methods of low-fat cheese processing. supercritical fluid extraction / carbon dioxide / Parmesan cheese / Cheddar cheese / fat
The isolation of milk fat globule membrane (MFGM) material from buttermilk on a commercial scale has provided a new ingredient rich in phospholipids and sphingolipids. In the pharmaceutical and cosmetic industries, highly purified phospholipids extracted from soya oil or egg yolk are used to produce liposomes. Liposomes are spherical structures consisting of one or more phospholipid bilayers enclosing an aqueous core. They may be used for the entrapment and controlled release of drugs or nutraceuticals, as model membranes or cells, and even for specialist techniques such as gene delivery. There are many potential applications for liposomes in the food industry, ranging from the protection of sensitive ingredients to increasing the efficacy of food addi- tives. Our previous work compared the structure and properties of liposomes prepared from a milk fat globule membrane (MFGM) fraction and soya phospholipid material using a high-pressure ho- mogenizer (Microfluidizer). These results identified some potential advantages in the use of MFGM phospholipids for the manufacture of liposomes for use in food systems. This paper compared the general structure and properties of liposomes prepared from the same MFGM phospholipid mate- rial using three different techniques - microfluidization, the traditional thin-film hydration and the heating method. The thin-film hydration technique required the use of organic solvents, while the other two methods do not involve any non food-safe chemicals. The liposomes prepared by both microfluidization and the heating method had high entrapment efficiencies. Liposomes produced via microfluidization tended to be significantly smaller than those produced by the other methods, with a narrower size distribution, and a higher proportion of unilamellar vesicles. There did not seem to be any advantages in the use of the thin-film hydration method, opening the door to the use of food-safe methods for liposome production. liposome / milk fat globule membrane / phospholipid / microfluidization
In this review we summarize the current knowledge on the extensive goat casein polymorphism. Gene structure, and allelic and non-allelic protein variants, characterized by genomic and proteomic analysis, have been described. The effect of genetic casein polymorphism on technological and nutritional properties of goat's milk has been briefly reported. Goat casein genes show a range of different post-translational modifications and complex patterns of splicing, leading to a great heterogeneity of this fraction. In the light of some recent findings, a genomic and proteomic combined approach appears to be the tool of choice in order to gain an exhaustive characterization of these milk proteins.
In an attempt to improve the acid gelation properties of heated milk, or to reduce the heat load necessary to obtain significant acid gelation properties, skim milk adjusted at pH values ranging from 6.7 to 10.5 was heat-treated for 10 min at temperatures ranging from 25 to 95 degrees C then neutralised to pH 6.7. Protein composition of the serum and micellar phases of milk and separation of the protein particles present in the serum phase of milk indicated that formation of micelle-bound heat-induced whey protein/kappa-casein aggregates was prevented from pH 7.5 upward, to the benefit of serum aggregates. Aggregates were formed in milk at pH 9.5 and 10.5 even at mild heating temperature (65-75 degrees C), while temperatures of 85 or 95 degrees C were necessary at lower pH values. Size of the aggregates decreased as pH of heat-treatment increased. Quality of the results was however reduced because of the extensive dissociation of caseins on alkaline treatment and of side mechanisms like Maillard reaction at high pH and temperature. However, some relationships could be found between the occurrence of aggregated whey protein and kappa-casein and higher elastic modulus of the acid gels after heating at low temperature/high pH or high temperature/low alkaline pH values of skim milk.
Skim milk powders (SMPs) of different heat classifications are used in recombined milks and milk products. These SMPs are broadly classified as low-, medium- and high-heat powders, based on their whey protein nitrogen index (WPNI). The WPNI is a measure of undenatured whey protein nitrogen (WPN) content (expressed as milligrams of WPN per gram of powder). This heat classification, based on the WPNI, gives an indirect indication of the denaturation and aggregation of whey proteins and thus the severity of the heat treatments that were used during the manufacture of milk powders. The severity of heat treatment has an impact on the functional properties of the resultant powders or their suitability for different applications. In the present study, we measured the WPNIs of a range of SMPs using a dye binding method (reference method) and attempted to corre- late these results with the WPNI predicted using Fourier transform near infra-red (FT-NIR) spectra, and with denaturation and aggregation of proteins as analysed using various polyacrylamide gel electrophoresis (PAGE) and capillary electrophoresis (CE) methods. In most cases, FT-NIR spec- troscopy provided a rapid method for predicting the WPNI of milk powders, with good correlation (R 2 = 0.985). The correlation was used to successfully predict the WPNI of a new set of powders, and the method could potentially be used to determine the WPNI routinely using appropriate con- trols. The denaturation and aggregation of native monomeric whey proteins as analysed by PAGE and CE correlated well with the WPNI of the respective SMP samples. Modified one-dimensional sodium dodecyl sulfate (SDS)-PAGE and two-dimensional SDS- and then reduced SDS-PAGE gave an indication of the type and composition of disulfide-linked protein aggregates and showed some interesting differences between possible protein-protein interactions involved in the manufacture of low-, medium- and high-heat SMPs. The low-heat powder (WPNI 6.76 mg WPN·g −1 powder) retained most of the whey proteins in the native state. In contrast, the high-heat powder (WPNI 0.33 mg WPN·g −1 powder) contained a comparatively small proportion of native whey proteins, although some α-lactalbumin was present. The degree of denaturation of β-lactoglobulin appeared to be crucial and could be related to the WPNI. skim milk powders / WPNI / protein denaturation and aggregation / 1D and 2D gel elec- trophoresis / FT-NIR
An overview of the present knowledge on the influences of dairy processing and storage on the content of conjugated linoleic acids ( CLA) and to possible differences between organic and standard products is given. In organic dairy products CLA was reported to be from not significantly up to 135% higher. Newer studies on the effect of heating steps show no changes in CLA content or isomer profiles, with the exception of microwaving, where CLA was decreased by up to 53%. In commercial dairy products no effects of fermentation on CLA content were observed. Recent studies on cheese showed no changes in the CLA content during manufacturing or ripening. CLA content was stable during butter-making out of CLA-enriched milk. In several more recent investigations with probiotic bacteria ( lactic acid bacteria such as Lactobacillus rhamnosus or Lactobacillus acidophilus, and propionibacteria and bifidobacteria such as B. breve and B. dentium) or other strains of these bacteria groups on a laboratory scale, an increase in CLA could be observed under the condition that free linoleic acid ( LA) was available in the culture medium. Conversion rates reached up to 87% with Propionibacteria freudenreichii ssp. shermanii. In cultivated form, B. breve reached a comparably high concentration of 398 mg CLA . L-1 broth. Especially high concentrations of up to 40 g CLA . L-1 broth could be produced with resting cells of Lactobacillus plantarum and Lb. acidophilus or with immobilised cells of Lb. delbrueckii ssp. bulgaricus. CLA formation in yoghurt could be observed under the condition that free LA was added. After 14 days of storage the increase was 77%. Specific procedures allow one to increase the content of CLA in a fraction. These procedures are dry fractionation (63% increase), fractionation using supercritical carbon dioxide (89% increase) and crystallisation ( concentration 2.5 times). Numerous studies on the shelf-life stability of CLA-enriched dairy products showed no significant differences in flavour quality parameters.
Nowadays, fat-filled spray-dried dairy powders have acquired an economic importance. Increasing the fat content leads to further quality implications such as off-flavors, and poor rehydration and flowing properties. However, the control of the drying operation and of the powder properties is still rather empirical in this case and few studies have focused on the whole issue up to now. This review aims at giving an overview of research work on fat-filled powders. The so-called "free fat" is often related to fat-filled powders, but the review also highlights the relevance of "surface fat" to powder physical properties. Quantitative and qualitative analytical techniques, mainly referring to free fat, are described here. This review focuses on the specific and combined influence of composition constituents ( fat, protein, lactose and mineral elements) and process steps ( such as heat treatments, homogenization, drying and storage) on physical properties of fat-filled powders ( oxidation, wettability, dispersibility, solubility and flowing properties).
Demonstration de la viabilite cellulaire et de l'innocuite de Enterococcus faecium CRL 183 dans des experiences sur le long terme. Enterococcus faecium CRL 183, souche isolee a partir de bacteries lactiques non levain en transformation fromagere, a ete l'objet de nombreuses recherches sur sa capacite a etre un probiotique potentiel, bien que sa survie dans le tractus digestif ne soit pas encore bien demontree a ce jour. Afin de determiner la capacite de E. faecium CRL 183 a survivre dans de telles conditions, cette souche a ete administree quotidiennement a des rats pendant 30 semaines. Les animaux ont ete divises en 3 groupes experimentaux: le groupe 1 ne recevant pas E.faecium, le groupe II recevant une culture pure de E. faecium CRL 183, et le groupe III recevant E. faecium CRL 183 sous la forme d'un produit fermente a base de soja. Les echantillons de feces ont ete collectes au debut et a 50 %, 75 % et 100 % de la periode d'experimentation. L'isolement et la numeration des Enterococcus ont ete realises sur milieu selectif KF. Des tests biochimiques (API Strep 20) et moleculaires (PCR) ont ete realises pour distinguer les especes isolees des feces. Initialement, E. faecium etait absent de la flore intestinale des rats; cependant, apres 15 semaines d'administration, E. faecium a pu etre retrouve dans les feces des groupes II et III, demontrant que E. faecium CRL 183 etait capable de survivre dans les conditions du transit gas-trointestinal. L'innocuite de la souche a egalement ete etudiee au regard du poids corporel et des analyses biochimiques plasmatiques.
Casein micelles are the basis of many new and traditional dairy products, and in all of these products the aggregation properties of the casein micelles are of primary importance. Either we seek to avoid the aggregation of the milks (as in long-storage liquid products) or we wish to promote aggregation and structure formation (as in cheese and yogurt manufacture). In either case, therefore, we need to be able to understand and control the surface of the casein micelle, because the stability of the particle is defined by the structure and properties of what is on its surface. To some extent, the interior of the particle is not relevant to its gelation properties (that is, the coming together of particles to form a network), although it may be very important in defining the properties of the gel once it is formed. Over the last 30–40 years, many models of the casein micelle have been proposed, and it must be admitted that a consensus has not been achieved. It is only necessary to compare two recently-published reviews [3, 5] to see how far apart the points of view can be, especially as regards the interior structure of the casein micelle. The aspect that seems to be generally accepted is that the κ-casein of the micelle is located mainly, if not completely, on the micellar surface. This surface location permits the macropeptide of the κ-casein to protrude into the surrounding serum, to give a sterically-stabilizing “hairy layer” to the particles. This layer is also negatively charged, although it has been calculated that the charge alone would not be sufficient to prevent close approach of the casein micelles. Breakdown or removal of the hairy layer by acidification or renneting removes its stabilizing influence and the casein micelles coagulate. Much evidence has been collected on the change of hydrodynamic size of casein micelles during acidification or renneting, consistent with the existence of this hairy layer. It should be noted, however, that this hairy layer need not be intact, in the sense that it is not necessary for the casein micelles to have a complete “skin” of κ-casein [2]. Other caseins can be present on the micellar surface. All that is necessary is that there should be sufficient steric stabilization to prevent the casein micelles from coming too close. In a sense, native bovine casein micelles are “over-stabilized” because we know that it is possible to remove a sizeable proportion of the micellar κ-casein (by, for example, treatment with rennet) without causing the milk to coagulate. How then can we modify the properties of the surfaces of the casein micelles so as to control their properties? It is surprising that relatively little has been done in this direction, except for a rather “all-or-nothing” approach. That is, there are many traditional cheeses that depend on the complete removal of the hairy layer, and there are acid products that
