Physical stability of caseinate - stabilized emulsions during heating

Abstract

The physical stability of caseinate stabilized emulsions was studied during heating (80- 120°C). Coagulation, coalescence and phase separation of the caseinate emulsions was studied using objective heat stability tests. The physical changes were characterized by light microscopy, particle size measurements and by determination of the solubility of coagula in various reagents. Additional information about physico-chemical changes in caseinate dispersions was obtained by determination of association of salts with caseinates, association of caseins with emulsion droplets, 1H-NMR, 31P-NMR and SDS- Page.The susceptibility towards heat coagulation of caseinate emulsions was mainly determined by volume fraction of oil, decrease of pH during heating and initial Ca 2+- activity. The heat coagulation time could be related to association of calcium and magnesium and caseinate and to association of caseinate and emulsion droplets. Association of calcium and magnesium and caseinate in these conditions would diminish electrostatic repulsion between caseinate molecules, which would lead to aggregation of the caseinate. It was shown that the formation of chemical cross-links is not rate determining in heat coagulation.During heating of caseinate emulsions containing malto-dextrins or a lactose/sucrose mixture, phase droplets were formed. The appearance of these phase droplets was accompanied by multi-layer formation of caseinate on the oil-droplets and a decreased stability to heating, often resulting in a highly viscous mass. Phase separation could be prevented in several ways; by using phosphates as stabilizing salt, by using maltodextrins or glucose syrups with relatively high DE values or by using soya lecithin. The effect of lecithin was only found when lecithin was added prior to homogenization. It appeared that the association of caseinate in solution or with the emulsion droplet was modified and thereby prevented phase separation.Although soya lecithin proved to be a very effective stabilizer, coalescence of the emulsion droplets could be observed when the concentration exceeded a certain value or when the phosphatidylcholine fraction of soya lecithin was used. The lecithins affected the coalescence behaviour, presumably by lowering the interfacial tension and lowering the caseinate surface load, and possibly by changing the structure of the caseinate layer on the emulsion droplets.