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 b y UF. Lactose or sucrose was then added at 3 or 6%. The five samples c ontaining <.05% lactose, 3 and 6% lactose, and 3 and 6% sucrose were U HT processed at 140-degrees-C for 4 s using indirect heating, collecte d 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 w k of storage. Samples stored at 35-degrees-C did not gel. Browning occ urred only in samples containing lactose stored at 35-degrees-C. Prote olysis in gelled samples was shown by SDS-PAGE. Bands were due to prot eolysis, protein crosslinking, and a streaking pattern in ungelled sam ples. Electron micrographs of gelled samples showed that various casei n particles were connected by hairlike protrusions, but the micelles i n ungelled samples were not connected and had few protrusions. The Mai llard reaction neither promoted nor deferred age gelation. Protein mod ifications prevented gelation in samples stored at 35-degrees-C. Age g elation 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.