Properties of aseptically packed ultra-high-temperature milk: II. Molecular weight changes of the casein components during storage

Abstract

Summary Storage of aseptically packed ultra-high-temperature (UHT) milk has been shown to be accompanied by pronounced changes in the molecular weight distribution of the pH 4·6 insoluble casein. These changes were both time-dependent and temperature-dependent, proceeding much more rapidly at 37 °C than at 30 °C, but even at 4 °C the changes were considerable. The proportions of high molecular weight material present before and after storage have been studied by gel filtration in dissociating solvents using Sephadex G-200. Measurements have also been made on casein material isolated from in-bottle sterilized milks and canned evaporated milk for comparison. The results of gel filtration have been compared with sedimentation coefficients recorded by ultracentrifugation and with changes in the content of amino groups titratable with trinitrobenzene-sulphonic acid. The results are compatible with the suggestion that the Maillard reaction occurred at ambient temperatures, and over a period of several months led to browning and sediment formation due to covalent cross-linking of polypeptide chains. The implications of this are discussed with reference to spoilage of UHT milk on storage and the phenomenon of gelation.