Abstract
Methionine and riboflavin have been identified as key reactants in the development of off-flavours induced by light exposure. However, the mechanism behind the production of volatiles organic compounds (VOCs) that are generated during the early stages of light exposure is still unclear. To provide new insights into the development of light-induced off-flavours, fluorescent light was applied for up to 6 hours to model solutions of methionine and riboflavin and the released VOCs were continuously monitored by proton-transfer-reaction mass spectrometry (PTR-MS). Upon light exposure, methanethiol was rapidly generated, with methional not being released until about 3 minutes later. 2-Propenal and formic acid, which are methanethiol coproducts from oxidation of methional, were not released until after approximately 22 and 29 min exposure, respectively. These observations coupled with the results from the methional reductive amination blocking experiment suggest that methanethiol could be formed directly from methionine rather than only via the methional intermediate. This means that the light-induced oxidation of methionine can undergo, at least, two parallel pathways, both leading to the formation of methanethiol. The finding was confirmed in semi-skimmed milk subjected to similar fluorescence light exposures where consistent changes in VOC signal intensities were observed. The results from these trials demonstrate the advantages of direct injection mass spectrometry techniques like PTR-MS which enable reactions to be followed in real time and offer the potential to uncover new insights into reaction pathways.