S. We have investigated the effects of adding a range of mineral salts and calcium-chelating agents on the distribution of casein and minerals between the nonpelleted and pelleted phases of milk obtained upon centrifugation at 78 000 g for 90 min. Adding CaCl # or mixtures of NaH # PO % and Na # HPO % to reconstituted skim milk (90 g milk solids\kg) at pH 6n65 increased both pelleted casein and pelleted calcium phosphate. Opposite effects were obtained by adding citrate or EDTA. The change in pelleted calcium phosphate was not simply related to casein release from the micelle. Upon adding 5 mmol EDTA\kg milk, 20 % of the pelleted Ca, 22 % of the pelleted phosphate and 5 % of the micellar casein were removed. Increasing the concentration of EDTA to 10 mmol\kg milk decreased the pelleted Ca by 44 % and the pelleted phosphate by 46 %, and caused 30 % of the micellar casein to be released. The effects of adding phosphate, citrate or EDTA at pH 6n65, followed by the addition of CaCl # , demonstrated the reversibility of the dissolution and formation of the micellar calcium phosphate. There were limits to this reversibility that were related to the amount of colloidal calcium phosphate removed from the casein micelles. Adding CaCl # to milk containing 20 mmol EDTA or 30 mmol citrate\kg milk did not result in complete reformation of casein micelles. Lightscattering experiments confirmed that the dissolution of moderate amounts of colloidal calcium phosphate had little effect on micellar size and were reversible, while the dissolution of larger amounts of colloidal calcium phosphate resulted in large reductions in micellar size and was irreversible.

The effects of adding CaCl2, orthophosphate, citrate, EDTA, or a mixture of these, to reconstituted skim milk (90 g of solids/kg solution) on the gelation of renneted milk were mediated by changes in Ca2+ activity and the casein micelle. At pH 6.65, the addition of citrate or EDTA, which removed more than 33% of the original colloidal calcium phosphate with the accompanying release of 20% casein from the micelle, completely inhibited gelation. Reformation of the depleted colloidal calcium phosphate and casein in the micelle, by the addition of CaCl2, removed this inhibition. When the minimum requirements for colloidal calcium phosphate and casein in the micelle were met, the coagulation time decreased with increasing Ca2+ activity, leveling off at high Ca2+ activity. The storage modulus of renneted gels, measured at 3 h, increased with increasing colloidal calcium phosphate content of micelles up to a level at which it was approximately 130% of the original colloidal calcium phosphate in the micelles. Further increases in colloidal calcium phosphate by the addition of CaCl2, orthophosphate, or mixtures of these, which did not change the proportion of casein in the micelle, decreased the storage modulus. The gelation of the renneted milk was influenced by Ca2+ activity, the amounts of colloidal calcium phosphate, and casein within the micelle, with the effects of colloidal calcium phosphate and casein within the micelle clearly dominating the storage modulus. These results are consistent with the model of Horne (Int. Dairy J. 8:171-177, 1998) which postulates that, following cleavage of the stabilizing K-casein hairs by rennet, the properties of the rennet gel are determined by the balance between the electrostatic and hydrophobic forces between casein micelles.

No abstract

The effects of addition of EDTA on the dissociation of caseins and foaming properties of milks (100 g solids/l) reconstituted from skim milk powders given a low-heat (72°C for 30 s) or high-heat (85°C for 30 min) treatment during powder manufacture were determined. The EDTA-induced dissociation of caseins was independent of heat treatment but in high-heat milk was accompanied by release of denatured whey proteins. EDTA changed the proportions of individual caseins in the supernatant. EDTA addition improved both foam overrun and foam stability of low- and high-heat milks. The increase in serum protein on addition of EDTA contributed to the improvement in foaming properties of milks by increasing the availability of the proteins for formation of the air–water interface.

No abstract

No abstract

Graphene oxide/polystyrene (GO/PS) nanocomposite capsules containing a two-compartment cargo have been successfully fabricated using a Pickering emulsion strategy. Highly purified GO sheets with typically micrometer-scale lateral dimensions and amphiphilic characteristics were prepared from the oxidation reaction of graphite with concomitant exfoliation of the graphite structure. These GO sheets were employed as a stabilizer for oil-in-water emulsions where the oil phase comprised toluene or olive oil. The stability and morphology of the emulsions were extensively studied as a function of different parameters including GO concentration, aqueous phase pH, ultrasonication time, effects of added electrolytes and stability to dilution. In selected conditions, the olive oil emulsions showed spontaneous formation of multiple w/o/w emulsions with high stability, whereas toluene formed simple o/w emulsions of lower overall stability. Olive oil emulsions were therefore used to prepare capsules templated from emulsion droplets by surrounding the oil phase with a GO/PS shell. The GO sheets, emulsions and composite capsules were characterized using a variety of physical and spectroscopic techniques in order to unravel the interactions responsible for capsule formation. The ability of the capsules to control the release of a model active agent in the form of a hydrophilic dye was explored, and release kinetics were monitored using UV-visible spectroscopy to obtain rate parameters. The composite capsules showed promising sustained release properties, with release rates 11× lower than the precursor GO-stabilized multiple emulsion droplets.

Sedimentation Field Flow Fractionation (SdFFF) was combined with Photon Correlation Spectroscopy (PCS), to characterize changes in the structure of the colloidal particles of reconstituted skim milk of diameter >50 nm (aggregates of casein and calcium phosphate known as casein micelles) with the changes in partitioning (with the addition of salt) of calcium (Ca), inorganic phosphate (Pi) and casein between the serum and colloidal phases of the milk. The number weighted particle size distributions are determined. These are well represented by a log-normal distribution. Methods are presented for estimating the relative contributions of scattering and absorbance to the SdFFF detector signal and for taking both into account when analysing SdFFF data. The values found for the effective density of the casein micelles were in good agreement with the literature and ranged from (1·06–1·08 g cm−3) according to the composition of micelles. The changes in the scattering intensity as determined by PCS correlated with the changes in the particle composition. Although the concentrations of colloidal calcium phosphate (CCP) (1·1–3·5 g/kg milk) and micellar casein (18·1–27·2 g/kg milk) varied considerably only small changes in the size distribution of particles >50 nm diameter were observed except for milk to which 30 mmol Pi+10 mmol Ca/kg milk had been added where the particle size distribution shows a swelling of the particles consistent with a lower than expected value for the particle density. These observations suggest that the micelles have the ability to both lose (depleted micelles) and accommodate (enriched micelles) more casein, calcium and inorganic phosphate in their interior, thus confirming the model of the micelles which postulates an open structure allowing freedom of movement of casein and small ions.