Differences in development of volatiles and bitter peptides between pre- and post-hydrolysed lactose-free UHT milk during storage

Lactose-hydrolysed UHT milk (LHM) has a limited shelf life, especially due to early browning and off-flavour development. This study aimed to directly compare the development in colour, volatile and bitter peptide formation in pre- (lactase addition prior to UHT treatment) and post-hydrolysed (lactase addition after UHT treatment) milk using three different lactases and stored at 25 or 35 °C for up to one year. Pre-hydrolysed milk exhibited more pronounced browning than post-hydrolysed milk. Furthermore, level of Maillard related compounds like acetic acid and furfural were higher in pre-hydrolysed milk compared to post-hydrolysed milk, whereas post-hydrolysed milk was higher in Strecker aldehyde related volatile organic compounds (VOCs). After storage, level of bitter peptides was higher in post-hydrolysed milk using more impure lactases. Generally, lower VOC concentrations and bitter peptides were observed for LHM produced by more pure lactases suggesting higher purity lactases can aid in the development of ambient long-life LHM.