Role of protein and lactose interactions in the age gelation of ultra-high temperature processed concentrated skim milk.

Skim milk was pasteurized, diafiltered, and concentrated three times by UF. Lactose or sucrose was then added at 3 or 6%. The five samples containing < .05% lactose, 3 and 6% lactose, and 3 and 6% sucrose were UHT processed at 140 degrees C for 4 s using indirect heating, collected aseptically in presterilized containers, and stored at 4, 20, and 35 degrees C. All samples stored at 4 and 20 degrees C gelled after 21 wk of storage. Samples stored at 35 degrees C did not gel. Browning occurred only in samples containing lactose stored at 35 degrees C. Proteolysis in gelled samples was shown by SDS-PAGE. Bands were due to proteolysis, protein crosslinking, and a streaking pattern in ungelled samples. Electron micrographs of gelled samples showed that various casein particles were connected by hairlike protrusions, but the micelles in ungelled samples were not connected and had few protrusions. The Maillard reaction neither promoted nor deferred age gelation. Protein modifications prevented gelation in samples stored at 35 degrees C. Age gelation was probably a two-step process in which dissociated proteins from the casein micelles reformed on micelles as hairlike protrusions. This process was followed by aggregation of the protein particles.