Storage stability and age gelation of reconstituted ultra-high temperature skim milk

Reconstituted skim milk (12-15% total solids) was processed under different preheat and direct ultra-high temperature (UHT) heating conditions. Samples heated at 75 °C/3 s + 143 °C/3 s showed proteolysis from plasmin and gelled rapidly on storage. Increased preheating (90 °C for 60 or 120 s), UHT holding (143 °C for 15 or 30 s), or combinations of these prevented proteolysis and slowed gelation; however some samples still gelled on storage. None of the properties followed (size, zeta potential, pH, serum mineral levels, colour or serum protein levels) gave any indication whether a sample would gel or not. High levels of κ-casein dissociated from the casein micelles on UHT processing and this did not change further on storage. The gelled material was composed of κ-casein-depleted casein micelles, which is not consistent with the current hypothesis for age gelation. A modified hypothesis for non-enzymatic age gelation is proposed where κ-casein-depleted casein micelles, formed during UHT treatment, slowly sediment due to gravity and cross-link to form a gel once a critical concentration is attained.