Individual Presentation: Understanding Mechanisms of Light Induced Flavor Development in Milk Using Advanced Analytics

Sulfur-containing amino acids play a crucial role in the generation of initial flavor defects in milk exposed to light. Methionine, in conjunction with riboflavin, is a critical reactant in the formation of off flavors triggered by light exposure. The oxidative degradation pathway of methionine, which converts it to methional and subsequently to methanethiol, is generally well understood. Nonetheless, the detection of methional in milk during the early stages of light exposure has been inconsistent, prompting questions regarding its specific role in the formation of methanethiol. The mechanisms underlying the production of these volatile compounds during initial light exposure remain inadequately explained. To advance our understanding of light-induced oxidation in milk, a series of experiments was conducted in which model methionine solutions were exposed to fluorescent light for up to 6 hours. The volatiles generated during this exposure were monitored in real time using proton transfer reaction mass spectrometry (PTR-MS), gas chromatography (GC‐MS), and proton‐transfer‐reaction time‐of‐flight mass spectrometry (PTR‐ToF‐MS). The findings revealed that methionine undergoes a series of reactions that lead to the early formation of methional, which then oxidizes to generate methanethiol along with other volatiles. Interestingly, the rate of methanethiol (the product) formation was inconsistent with that of methional (the product). Such disagreement was explained here by demonstrating that methanethiol can be formed at very high rates even when methional (the precursor) functionality was blocked using the reductive amination technique. This suggests that methanethiol can arise from at least two concurrent pathways during the early light-exposure phase, both leading to its formation. Moreover, similar compounds were identified in light-exposed skim milk. Overall, these insights will broaden understanding of off-flavor development in light-exposed milk and aid in formulating packaging strategies to prolong product shelf life.