The physical and chemical changes in casein during the gelation owing to the chemical modification by dialdehyde starch (DAS) in 8-10% solution were studied using electron-microscopy, measuring the mechanical properties (shear deformation), turbidimetric titration, and determination of amino groups.
The gelation which proceeds during a storage time of 2 weeks is accomplished by the formation of a supermolecular network structure. Depending on the conditions of modification (pH, percentage of DAS, protein concentration) a more or less dense network is formed. An increase of pH (in the range of pH 6,3-9,7) leads to an increase of gelation speed, which favours the formation of irregular network structures. The denser the network formed the smaller is the shear deformation measured by the device of TOLSTOI. During the storage of the gels the plastic component of compliance decreases to a greater amount than the elastic one. Therefore the relative amount of elastic component increases.
During the reaction with DAS the maximum of turbidity of the protein (solubility minimum) shifted to lower pH owing to a blockage of positively charged groups. On the contrary to the changes measured by physical methods, this shift is complete soon after a 1 day storage. Similarly the blockage of amino groups is finished in the most cases this time too.