Effect of genipin cross-linking on the structural features of skim milk in the presence of ethylenediaminetetraacetic acid (EDTA)

Abstract

Casein nanogels formed using genipin as a cross-linking agent were studied by dynamic light scattering (DLS), transmission electron microscopy (TEM), small angle X-ray and neutron scattering (SAXS, SANS). The size, substructure, and stability of casein nanogel particles at various genipin concentrations were investigated upon the addition of ethylenediaminetetraacetic acid (EDTA) up to a concentration of 30 mM. Particle size was retained up to 30 mM EDTA for particles with a genipin concentration of 2.5 mM or greater. In contrast, small casein aggregates were formed with size of ∼20−30 nm as a result of casein micelle disintegration on adding EDTA. SAXS revealed that, for both control and cross-linked milks, the homogeneity within the casein protein matrix is increased upon EDTA addition. SANS confirmed that the colloidal calcium phosphate (CCP) particle is an amorphous calcium phosphate core stabilized by a protein shell. Adding EDTA into milk induced a complete solubilisation of the CCP. In the genipin cross-linked milk only the amorphous calcium phosphate core is dissolved, while the protein shell surrounding the CCP is kept intact. These results demonstrate that genipin cross-linking can maintain the structural integrity of the casein protein matrix and the protein shell layer surrounding the CCP particles.