Lactose-free dairy products undergo several chemical modifications during shelf life because of the reactivity of glucose and galactose produced by the lactose enzymatic hydrolysis. In this study, proton transfer reaction-mass spectrometry (PTR-MS), coupled with a time-of-flight (TOF) mass analyzer, was applied to get an insight on the phenomena occurring during the shelf life of ultrahigh-temperature (UHT) lactose-free milk (LFM). UHT LFMs produced by three different commercial lactase preparations were evaluated during storage at 20 C over a 150 days period, sampling the milk every 30 days. Production was repeated three times, on three consecutive weeks, in order to take milk variability into consideration. Principal component analysis applied to the whole "volatilome" data demonstrated the capability of PTR-TOF-MS in detecting the milk batch-to-batch variability: Freshly produced milk samples were distinguished based on the week of production at the beginning of shelf life. Additionally, a clear evolution of the volatiles organic compounds (VOCs) profiling during storage was highlighted. Further statistical analysis confirmed VOCs temporal evolution, mostly because of changes in methyl ketones concentration. Differences caused by the commercial lactases did not emerged, except for benzaldehyde. Altogether, data demonstrated PTR-TOF-MS analysis as a valuable and rapid method for the detection of changes in the VOCs profiling of UHT LFM.

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UHT hydrolysed-lactose milk (HLM) is prone to chemical changes, giving rise to off-flavours in the product. To investigate this better, headspace solid-phase micro-extraction gas chromatography (HS-SPME/GCeMS) was applied to monitor the volatiles profile of HLM during production and shelf-life. Optimum extraction conditions for volatiles were explored, focussing on compounds affecting milk aroma. This study is the first investigating industrial scale manufacturing under controlled conditions, allowing direct associations between specific reactions and changes in milk volatiles. The effect of UHT treatment on milk flavour was shown, while the lactase, potentially containing undesirable proteases and odours, did not alter the milk "volatilome" after addition. Commercial samples exhibited different trends in the volatiles along the shelf-life due to differences in production systems, lactase purity and packaging. Merging all results, HS-SPME/GCeMS successfully mapped the phenomena causing modifications in the volatiles profile of UHT HLM at each step of the product life-cycle.