Addition of sodium caseinate to skim milk inhibits rennet-induced aggregation of casein micelles

The objective of this paper was to observe the rennet-induced aggregation behaviour of casein micelles in milk in the presence of additional sodium caseinate. Analysis of the centrifugal supernatants by size exclusion chromatography confirmed an increase in the soluble protein in the milk serum phase after addition of sodium caseinate. Although the total amount of κ-casein hydrolyzed over time was not affected, there was a significant effect of soluble casein on milk gelation, with a dose-dependent decrease of the gelation time as measured by rheology. Light scattering experiments also confirmed that the addition of soluble caseins inhibited the aggregation of casein micelles. Addition of 1 mM CaCl₂ prior to renneting increased the extent of rennet aggregation in samples containing additional sodium caseinate, but the inhibiting effect was still evident. The amount of soluble casein (as measured by chroma tography) significantly decreased after renneting, suggesting its association with the micellar fraction. Supporting experiments carried out with purified fractions of soluble caseins demonstrated that both α_s-casein and β-casein played a role as protective colloids (increasing steric repulsion) during renneting. It was concluded that the inhibiting effect observed during gelation was caused by the adsorption of soluble casein molecules on the surface of rennet-altered casein micelles.