STRUCTURE OF ACID CASEIN GELS .1. FORMATION AND MODEL OF GEL NETWORK

The formation and the resulting structure of acid casein gels was studied by rheology, permeametry and electron microscopy. Most gels were formed by quiescent heating of casein solutions acidified in the cold at 0-2°C. After acidification of a skim milk powder or a sodium caseinate dispersion to pH 4.6 at low temperature a physically stable suspension of casein particles was obtained. Gelation results from the aggregation of these casein particles at temperatures above about 10°C. These particles have a complex structure due to the association of numerous different casein molecules. Lowering of the temperature after gel formation to 4°C shows that the gel for mation is irreversible in this respect. It is shown that the formation of acid casein gels is subject to an activation Gibbs energy, which markedly decreases with increasing temperature and depends on at least two factors. At ageing temperatures above 10°C, the dynamic moduli G′ and G linearly increase with the logarithm of time over at least a week. Their absolute values depend, primarily, on the heteroge neity of the network. From the strong dependence of the dynamic moduli on the casein concentration, and from permeability measurements and electron microscopy it follows that the gel network is very heterogeneous. It consists of rather big conglomerates and holes each with a diameter of 1-10 μm. The big conglomerates are thought to be built of smaller ones, which in turn consist of casein particles aggregated in strands and nodes. This picture implies that acid casein gels, although they are basically built of macromolecules, have a particulate structure. The strands and nodes themselves can be regarded as concentrated (∼25%) protein gels with a modulus of about 105N m-2. © 1990.