Preheating intensity affects the properties of age gelation formed in direct ultra-high-temperature skim milk

A preheating treatment before direct ultra-high-temperature (dUHT) treatment may delay age gelation of milk. In this study, to investigate the optimum preheating intensity, the plasmin activities and distribution of proteins in dUHT skim milks with different preheating intensities (80–95 °C for 30 s or 60 s) were investigated during six-month storage at room temperature (25 °C). The results showed the plasmin activities, extents of casein hydrolysis, and protein distribution changes were inhibited with increasing preheating intensity, thus maintaining milk stability and delaying age gelation. The best observations were obtained with the samples preheated at 90 °C or 95 °C for 60 s. The peptides produced by casein hydrolysis had potentially bitter flavor, but some of which could inhibit the activation of plasmin. Our findings defined the optimal preheating intensity for maintaining storage stability of dUHT milk, providing guidance for inhibiting age gelation during shelf life. [Display omitted] •Higher preheating intensity inhibited the extent of plasmin activation.•Higher preheating intensity delayed milk protein destruction and gel formation.•90 °C/60 s was the best preheating condition to prevent age gelation.•New sites for plasmin hydrolysis were identified.•Peptides hydrolyzed from casein may interact with plasminogen activator.