Behaviour of casein micelles at conditions comparable to those in ice cream

The physical properties of ice cream are mainly determined by the processing and the ingredients. Milk (powder) is one of the ingredients and ice cream thus contains casein, the major milk protein. A large proportion of casein in ice cream is present in the plasma phase of ice cream. Since the behaviour of casein in ice cream plasma was not known and could not be predicted and is expected to be important for the properties of ice cream, the behaviour of casein micelles in ice cream plasma and the influence of several processing steps on its behaviour was studied.Properties of casein micelles in ice cream plasma obtained at -10 °C were studied. Structure and behaviour of the casein micelles did not differ greatly from those in milk. A slightly increased amount of salts was associated with the micelles in ice cream plasma and fewer submicelles and small micelles were present. The increase in size was primarily due to fusion of micelles. The properties of the micelles in ice cream could be explained by the two conditions which differ of milk: high concentration of milk components and low temperature. Preheating did hardly affect the properties of casein micelles in ice cream.In a model frozen yoghurt plasma nearly all the calcium and inorganic phosphorus was present in the serum. Frozen yoghurt plasma formed a gel during storage, due to aggregation of casein particles. The rheological properties of the gel changed during storage, presumably due to rearrangement of the casein network.The effects of type of acidulant, addition of various salts, concentration and preheat treatment on aggregation of casein micelles in skim milk in the cold were studied. The pH of aggregation was strongly influenced by the activity of divalent cations. At pH 4.7 at 3 °C casein micelles aggregate if the calcium ion activity is 2.5 mM or lower. At higher calcium ion activities the casein micelles appear stable against aggregation. By varying the calcium ion activity of skim milk, the pH and temperature of aggregation in the cold can be changed.Keywords: casein micelles, ice cream, frozen yoghurt, low temperature, freezing, cold aggregation, divalent kations