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On heating, bovine 8-lactoglobulin A (8-lg A) dimers dissociate and then aggregate. The extent of aggregation of 8-19 A after a heat-quench treatment was studied quantitatively using photon correlation spectroscopy (PCS) ; circular dichroism (CD) spectroscopy was also carried out on heated solutions of 8-1s A to examine changes in the secondary and tertiary structure of t h e protein. In pH 7.0 solution the protein underwent a change in tertiary structure at 67 "C; by contrast, the secondary structure, containing ca. 50% 8-sheet, showed very little change up to 90°C. PCS showed that t h e extent of aggregation of 8-lg A depended on pH and buffer concentration; under some conditions it was observed that there was a temperature above which t h e rate of aggregation decreased. The pH-dependence of t h e temperature of onset of aggregation correlated with the conformational stability of 8-19 A. Detailed examination of the light scattering data suggested initial aggregation of the protein molecules to form linear, rod-like particles. It is proposed that, at later stages in the aggregation process, the rods themselves aggregate and start to form a network.

The effect of depletion of Ca2+ on the composition and size distribution of casein micelles in milk has been examined using chemical analysis, size exclusion chromatography, fast protein liquid chromatography, turbidimetry and photon correlation spectroscopy. Partial removal of Ca2 + by EDTA and subsequent dialysis resulted in disaggregation of some of the casein inicelles; as the EDTA concentration increased, the proportions of CaZ + and phosphate relative to protein in the micelles remaining intact decreased. However, the composition of the intact micelles, with respect to the different caseins, and the number-frequency size distribution were essentially unchanged.The caseins (asl, m2, / l and k--casein polypeptides) account for more than 80% of the protein in bovine milk [l]. They exist predominantly as large stable aggregates (20-600 nm diameter), which are known as casein micelles, in dynamic equilibrium with smaller aggregates and monomers [2, 31. These casein micelles contain appreciable deposits of calcium phosphate and calcium citrate (approximately 7% by mass) [2]. The calf absorbs milk calcium and phosphorus very efficiently [4] and it is likely that one important biological function of casein micelles is the transfer of calcium and phosphate to the calf in a compact but easily assimilable form. An efficient, natural, controlled release mechanism for calcium and phosphate seems to be operating in milk and it is of interest to understand the mechanism of action, which may be relevant to the devising of artificial controlled-release delivery systems for drugs and dietary supplements.While Ca2' is essential to the maintenance of casein micelle structure (the removal of Ca2+ ions from the aqueous environment results in disaggregation of the micelles [2]), it has been reported that some micellar depletion of Ca2 + is possible while retaining some structural integrity as indicated, for example, by measurement of average hydrodynamic diameter [5]. In an earlier paper [6] we described the estimation of the total refractive index of the casein micelles, which is compounded from contributions arising from the protein and inorganic constituents; this present work was begun with the intention of identifying these different contributions by selective removal of micellar calcium phosphate. However, we found that it was not possible to remove significant amounts of Ca2 + without some micellar disaggregation. The experiments described here were undertaken to examine whether, during the disaggregation, there was progressive loss of material from the micelles resulting in a decrease in micelle size.Correspondence to M. C . A. Griffin, AFRC Institute of Food Research, Reading Laboratory, Shinfield, Reading, RG2 9AT, England Abbreviations. PCS, photon correlation spectroscopy; CPG, controlled-pore glass. MATERIALS AND METHODSRaw milk was obtained from a bulk storage tank; 16 1 was centrifuged in batches at 1600 x g,, for 20 min at 5 "C, and the cream was removed. NaN3 was added to the skim milk to A method similar to tha...

SummaryWhole milks concentrated 1·5–4-fold and acidified and citrated milks concentrated 2·8-fold by ultrafiltration at 50 °C were analysed for chemical changes relevant to further processing, storage or nutrition. Fat and protein were entirely retained in the concentrate. The retention of water-soluble vitamins, Ca, Mg, phosphate and trace minerals depended on the proportion bound to the protein. Ascorbic acid was rapidly destroyed during concentration. Because of the differential retention of nitrogenous components, protein comprised a progressively higher proportion of the total N as the milk became more concentrated. No denaturation of whey protein or disruption of casein micelles was detected during concentration of whole milk, but some solubilization of the casein occurred after citration. Reduction of fat globule size occurred early in the concentration process, damage to the fat globule membrane was indicated and the milk became more susceptible to lipolysis. Apart from a tendency for preacidified or precitrated concentrates to gel, no change in the susceptibility of the milks to heat damage was detected.