Douglas G. Dalgleish

pH-Induced dissociation of bovine casein micelles. I. Analysis of liberated caseins

The dissociation of caseins of different types from casein micelles in milk, acidified to different pH values in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C, has been studied. In contrast to a number of previous findings, it was shown that caseins of all types were dissociated from the micelles, although in all cases β-casein was in highest concentration. The amounts and proportions of all of the caseins were found to be pH- and temperature-dependent, especially the former. Studies of the proportions of the different caseins liberated suggested that, at a defined temperature, the proportions of κ;- and αs2-caseins were independent of pH, while the proportions of β- and αsl-caseins were variable, changes in one being compensated by changes in the other. The manner in which the proportions of the αsl-casein and β-caseins changes with pH was found to be temperature-dependent.

pH-induced dissociation of bovine casein micelles. II: Mineral solubilization and its relation to casein release

Measurements of the release of Ca, Mg and inorganic phosphate(P i ) from the casein micelles of bovine milk have been made, as functions of the pH, in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C. The results are in general agreement with earlier published studies in giving a value of 1·75–1·84 for the micellar Ca:P i ratio. Mg appeared to behave similarly to Ca, although the amounts of micellar material were much smaller. The results on the acid-solvation of calcium phosphate are considered in relation to published quantitative studies of the pH-induced dissociation of the different types of caseins from the micelle, and of the micellar dissociation caused when micellar calcium phosphate is dissolved at neutral pH. It is evident from this that at present it is not possible to derive a universal relation between the dissociation of minerals and of caseins from the micelles at different temperatures and under different conditions.

On the structural models of bovine casein micelles—review and possible improvements

A review is made of the different structural principles involved in the assembly of the casein micellar particles present in mammalian milks. The properties of the constituent casein proteins are described, and how these can lead to different structural models. The evidence for particular models is given, and it is concluded that a model based on calcium phosphate nanoclusters is the most appropriate in view of electron microscopic and scattering experiments. A modification of the nanocluster model is presented, which allows many of the known structural properties of the micelles to be incorporated.

Adsorption of protein and the stability of emulsions

Emulsions (specifically oil-in-water emulsions) are important elements in the formulation of foods, and therefore have to be prepared in such a way as to be stable, often over long periods of time. Reactions such as aggregation or flocculation, which lead to creaming and possibly coalescence, must be avoided. This review article considers the different types of instabilities that may occur in food emulsion systems, what causes them, and how they may be controlled under some circumstances.

The Structure of the Casein Micelle of Milk and Its Changes During Processing

The majority of the protein in cows milk is contained in the particles known as casein micelles. This review describes the main structural features of these particles and the different models that have been used to define the interior structures. The reactions of the micelles during processing operations are described in terms of the structural models.

Adsorption behaviour of whey protein isolate and caseinate in soya oil-in-water emulsions

Abstract A technique to determine directly the amount of adsorbed protein in protein-stabilized oil-in-water emulsions (20 wt% soya oil, pH 7) has been developed employing sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and photometric densitometry. Using this technique the adsorption behaviour of whey protein isolate (WPI) and cascinate at the emulsion droplet surface has been studied. The minimum limiting surface concentrations required to stabilize emulsions containing cascinate or WPI were found to be 1 and 1.5 mg/m 2 respectively. At protein concentrations >2.25 wt% in the aqueous phase, surface concentration of both proteins was at a high limit of ~3.2 mg/m 2 . WPI exhibited behaviour indicative of adsorption in distinct multilayers. Competitive adsorption of proteins in emulsions stabilized by mixtures of WPI and cascinate was studied using the same technique, and it was found that when protein concentration was the limiting factor, WPI and cascinate adsorbed to the same extent. However, with an excess of protein, cascinate adsorbed in preference to WPI.

Competitive adsorption of αs1-casein and β-casein in oil-in-water emulsions

Abstract Competitive adsorption of αs1-casein and β-casein at the emulsion droplet surface and the planar oil-water interface has been investigated. Analysis of the aqueous phase after centrifugation of fresh emulsions made with a mixture of αs1-casein + β-casein shows that β-casein predominates at the interface. Exchange experiments indicate that β-casein will rapidly displace αs1-casein from the emulsion droplet surface; ctsl-casein will also displace β-casein, but to a much lesser extent. Surface viscosity measurements at a planar oil-water interface are consistent with the view that the αs1-casein displaced by β-casein is removed from the interfacial region.

The mechanisms of the heat-induced interaction of whey proteins with casein micelles in milk

Abstract The heat-induced interactions between whey proteins and casein micelles were investigated by defining the final product of the reaction when milk was heated at temperatures up to 90°C. By looking at the changes of the interactions in skim milk and in resuspended casein micelles, to which different amounts of whey protein had been added, information on the mechanisms that determine the heat-induced protein–protein interactions in milk was derived. The ratio of α -lactalbumin and β -lactoglobulin to κ -casein and the ratio of α -lactalbumin to β -lactoglobulin found in the micellar pellet were used as indices of these heat-induced reactions occurring in milk. The results suggested that at these low temperature (70–90°C) with batch heating conditions, whey proteins form soluble complexes which act as intermediates in the heat-induced association of α -lactalbumin and β -lactoglobulin with the micelles. The presence of β -lactoglobulin was necessary for any association of whey protein with casein micelles to occur; furthermore, the amount of β -lactoglobulin found in the micellar pellet after heating seemed to be limited by a discrete number of binding sites available on the micelles.

Effect of temperature and pH on the interactions of whey proteins with casein micelles in skim milk

Abstract Skim milk was heated at temperatures in the range 75–90 °C, at pH values of 6.8, 6.2 and 5.8, The amounts of α-lactalbumin and β-lactoglobulin which interacted with the casein micelles during heat treatment were quantified by SDS-polyacrylamide gel electrophoresis of the micellar fractions isolated by ultra-centrifugation. Both α-lactalbumin and β-lactoglobulin appeared to interact similarly with casein micelles at temperatures up to 85 °C. The amount of whey protein complexed with micelles increased with time, reaching plateau values that, at the highest temperatures, were comparable with the quantity present in the original skim milk. In general, faster reaction of the whey proteins with the micelles was found at lower pH and higher temperatures. The rates and extent of the reaction changed also when additional α-lactalbumin and β-lactoglobulin isolates were added to milk before heating. The reaction between α-lactalbumin and casein micelles depended to a relatively small extent upon environmental variations (pH and temperature), while β-lactoglobulin interactions were more affected, so that a more complex behaviour may be attributed to the latter protein.

Competitive adsorption of β-casein and nonionic surfactants in oil-in-water emulsions

Abstract Competitive adsorption of β-casein and three nonionic surfactants has been investigated in hydrocarbon oil-in-water emulsions (0.4 wt% protein, 20 wt% oil, pH 7). Addition of a water-soluble surfactant, C 12 E 8 (octaethylene glycol n -dodecylether) or Tween 20 (polyoxyethylene sorbitan monolaurate), after emulsion formation leads to a complete displacement of protein from the oil-water interface at surfactant-to-protein molar ratios in excess of 17:1. Addition of the oil-soluble surfactant C 12 E 2 (diethylene glycol n -dodecylether) prior to emulsification leads to reduction in droplet size but no protein displacement. Addition of C 12 E 8 prior to emulsification leads to a slightly greater reduction in droplet size and partial protein displacement at high surfactant-to-protein ratios. The competitive displacement of β-casein from the emulsion droplet surface by C 12 E 8 is broadly consistent with interfacial tension data for C 12 E 8 + sodium caseinate at the hydrocarbon oil-water interface.