Gelation of Casein After Modification by Dialdehyde Starch‐chemical and Physical Aspects2

Abstract

The gelation of casein after modification by dialdehyde starch (DAS) was studied by measuring its mechanical and swelling properties, the amount of sol fraction, its melting behaviour, amino acid analysis and polyacrylamide gel electrophoresis. Depending on the concentration of protein and DAS, fusible and infusible gels are formed. The transition from fusible into infusible gels takes place after increasing the amount of DAS, owing to an enhanced blockage of basic amino acid residues and crosslinking of polypeptide chains. The higher the concentration of protein, the lower is the concentration of DAS necessary for gelation. When the DAS concentration is high enough to form a pervading crosslinked network the crosslinking proceeds relatively fast and is nearly completed after one day. The correspondence of the time dependence of chemical andphysical parameters, the irreversible blocking of basic amino acids, especially lysine, the decrease of swelling and nitrogen extractability, the elastic component of creep compliance indicates that during a 7 day storage period the gels are completely crosslinked. Using a corrected Flory relation it was shown that an average of 3.5 crosslinks exist per one polypeptide chain. This number is similar to the number of blocked lysine groups of the same gels (3.6 ‐ 4.5). The melting and creep behaviour of the gels, and especially the effect of sodium do‐decyl sulphate on the creep compliance, point to the formation of a heterogeneous network which includes noncovalently connected mi‐croassociations and covalently connected polypeptide chains as structural elements.