The effect of hexametaphosphate addition during milk powder manufacture on the properties of reconstituted skim milk

Skim milk powders with various levels of sodium hexametaphosphate (NaHMP) were prepared. Reconstituted skim milk samples were prepared from these powders. NaHMP slightly reduced the pH, markedly reduced the serum and ionic calcium and markedly increased the serum phase orthophosphate levels of the milks. This shift in the mineral equilibrium resulted in a drastic reduction in casein micelle integrity, with a marked dissociation of casein from the micelles. κ-Casein was the predominant casein dissociated, although significant levels of αS-casein and β-casein were also transferred to the serum phase. This dissociation of the casein micelles caused a marked decrease in size and scattering properties of the casein micelles. In addition, a small decrease in the zeta potential of the casein micelles in the milk was observed. Heat treatment of the milks with added NaHMP induced further dissociation of κ-casein, although much of the αS-casein and β-casein re-associated with the micelles.