  Chemical or enzymatic cross-linking of casein micelles (CMs) increases their
stability against dissociating agents. In this paper, a comparative study of
stability between native CMs and CMs cross-linked with genipin (CMs-GP) as a
function of pH is described. Stability to temperature and ethanol were
investigated in the pH range 2.0-7.0. The size and the charge
($\zeta$-potential) of the particles were determined by dynamic light
scattering. Native CMs precipitated below pH 5.5, CMs-GP precipitated from pH
3.5 to 4.5, whereas no precipitation was observed at pH 2.0-3.0 or pH 4.5-7.0.
The isoelectric point of CMs-GP was determined to be pH 3.7. Highest stability
against heat and ethanol was observed for CMs-GP at pH 2, where visible
coagulation was determined only after 800 s at 140 $^\circ$C or 87.5% (v/v) of
ethanol. These results confirmed the hypothesis that cross-linking by GP
increased the stability of CMs.
