Ethanol stability of casein micelles – a hypothesis concerning the role of calcium phosphate

Ultraviolet spectroscopic studies and the prediction of the feathering of cream in filter coffees

Cream feathering was studied in sixteen samples made from six brands of filter coffee. We have used a predictive rule, deduced from our studies of instant coffees, to determine the likelihood of feathering occurring in filter coffee samples upon the addition of coffee cream. The predictive rule makes use of 1,3,4,5-tetra-hydroxycyclohexane carboxylic acid-3-[(3,4) dihydroxycinnamate] (chlorogenic acid) concentration in the coffee, and the ratio of ultraviolet absorbances at 261 nm and 340 nm. We found good agreement between the predicted and observed behaviour of samples of filter coffee and conclude that the predictive rule is reliable for both instant and filter coffees. The effect of increased fat content in the cream was also studied.

Crosslinking of casein by microbial transglutaminase and its resulting influence on the stability of micelle structure

The influence of enzymatic crosslinking by microbial transglutaminase (mTG) on the stability of casein micelles of ultrahigh temperature (UHT)-treated milk in the presence of EDTA (0-0.45 mM) or ethanol (0-74 vol%) as well as under high hydrostatic pressures up to 400 MPa was investigated. Disintegration of micelles and changes in micelle size were monitored by the measurement of turbidity as well as by dynamic light scattering. The results show that the incubation of UHTtreated milk with mTG resulted in an improved micelle stability toward disintegration on addition of EDTA, ethanol, or pressure treatment. Intramicellar formed isopetides significantly enhanced the stability of casein micelles. It is supposed that net-like crosslinks are formed within the external region of the micelles and they adopt the stabilizing role of colloidal calcium phosphate within the micelles, thus making the micelles less contestable for disrupting influences.

Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and alcohol

We have investigated the combined effect of ionic calcium and ethanol on the visual creaming behavior and rheology of sodium caseinate-stabilized emulsions (4 wt% protein, 30 vol% oil, pH 6.8, mean droplet diameter 0.4 microm). A range of ionic calcium concentrations, expressed as a calcium/caseinate molar ratio R, was adjusted prior to homogenization and varying concentrations of ethanol were added shortly after homogenization. A stability map was produced on the basis of visual creaming behavior over a minimum period of 8 h for different calcium/caseinate/ethanol emulsion compositions. A single narrow stable (noncreaming) region was identified, indicating limited cooperation between calcium ions and ethanol. The shear-thinning behavior of the caseinate-stabilized emulsions is typical of systems undergoing depletion flocculation. Addition of calcium ions and/or ethanol was found to lead to a pronounced reduction in viscosity and the onset of Newtonian flow. The state of aggregation was correlated with emulsion microstructure from confocal laser scanning microscopy. Time-dependent rheology (18 h) with a density-matched oil phase (1-bromohexadecane) revealed that the visually stable emulsions were time-independent low-viscosity fluids. Surface coverage data showed that increasing amounts of caseinate were associated with the oil-water interface with increasing R and ethanol content. A decrease in free calcium ions in the aqueous phase with moderate increases in R and ethanol content was observed, which is consistent with greater calcium-caseinate binding (aggregation). Ostwald ripening occurred at the high-ethanol emulsion compositions that were stable to depletion flocculation. While the coarsening rate was low, this can account for the cream plug formation observed during gravity creaming experiments. The caseinate emulsion with no ionic calcium or ethanol exhibits depletion flocculation from excess nonadsorbed caseinate submicelles. Addition of calcium ions reduces the submicelle number density via specific calcium-binding in the aqueous phase (fewer, larger calcium-caseinate aggregates) and at the droplet surface (increased surface coverage). Nonspecific ethanol-induced (calcium-dependent) caseinate submicelle aggregation in the bulk phase and on the droplet surface (increased surface coverage) culminates in a reduction in the number density of caseinate submicelles. A narrow window of inhibition of depletion flocculation occurs in systems containing both calcium ions and ethanol, both species combining to aggregate the protein and so reduce the density of free submicelles.