Milk was concentrated to 3X (volume reduction) by ultrafiltration. Disodium phosphate and sodium citrate were added, and the milk concentrates were homogenized. The concentrates were then heated at 135°C for 50 s in a laboratory ultra-high temperature (UHT) heating system. Rennet gels were made from heated and unheated milk concentrates and their curd firmness measured using a Formagraph. Gel microstructures were examined by electron microscopy. When rennet was added to unhomogenized milk concentrate before UHT heating, the resultant gel consisted of a strong protein network that encapsulated the fat globules. Pockets of milk serum were associated with the fat. Homogenization caused the fat droplets to be coated with casein micelles and become tied into the protein network as an integral part of the gel structure. The microstructure of UHT milk concentrate gels was different from gels made from unheated milk. Gelation of UHT milk proceeded more slowly and the gels were weaker. Much of the casein in such samples had lost their micellar identity and was present as a homogeneous mass around the fat droplets. Large areas in the gel lacked protein network, which weakened the UHT milk gels. Samples with disodium phosphate added did not gel after UHT treatment, even if high concentrations of rennet were added. Samples with sodium citrate added formed only a weak rennet gel after UHT treatment. 10 micrographs, 2 diagrams, 32 references, discussion with reviewers
