ON THE FRACTAL NATURE OF THE STRUCTURE OF ACID CASEIN GELS

Acid casein gels are formed by aggregation of casein particles in an acid environment. Two different types of gels were formed and studied by rheological measurements at small (< 2%) and large deformations (like creep measurements). Gel type 1 was made by quiescent warming of a sodium caseinate dispersion of pH 4.6 from 4 to 30°C and gel type 2 was made by acidification of a sodium caseinate dispersion with glucono-δ-lactone (GDL) at 30°C. For small deformations, the dynamic moduli of type 1 gels were much higher than those of type 2 gels. The relative difference between the moduli of the two gel types was greater at lower casein concentrations, probably caused by a difference in the way the big aggregates of casein particles are linked together. The difference supposedly stems from the difference in voluminosity of casein particles in the cold (3.5-4.5 ml g-1) and at 30°C (about 2.5 ml g-1). For gel type 1 the gelation occurred at a high voluminosity; after the gel was formed the voluminosity of the casein particles decreased due to temperature increase, and this led to a stretching of the strands in the gel. The structure of gel type 2 did not change significantly after the gel was formed. The proposed explanation agreed with the results of creep measurements, showing the deformation at which fracture occurred to range between 0.5 and 0.6 for type 1 and between 1 and 1.2 for type 2 gels. Little difference was observed between the shear stresses at which fracture occurred for either type of gel and the relative difference was independent of the casein concentration. The rheological behaviour of both gels could be explained quantitatively using the concept of fractal geometry. © 1990.