Methanethiol formation during the photochemical oxidation of methionine‐riboflavin system

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.

Light-Struck Taste in White Wine: Protective Role of Glutathione, Sulfur Dioxide and Hydrolysable Tannins

“…The latter compound is chemically unstable, photo-sensitive and easily decomposes to MeSH and acrolein through a retro-Michael reaction. Along with the early steps of photo-oxidation, MeSH can be generated by an alternative pathway that involves a direct cleavage of Met side chain [10]. Moreover, two molecules of MeSH Molecules 2021, 26, 5297 2 of 20 can yield DMDS [11].…”

Transient changes of volatile organic compounds (VOCs) during dulce de leche preparation by a direct injection mass spectrometer based on proton transfer reaction (PTR‐MS)

“…The headspace measurements were performed using a high‐sensitivity PTR‐MS (Ionicon, Analytik GmbH, Innsbruck, Austria). The instrumental conditions in the drift tube were as follows: drift voltage 600 V, drift pressure 2.2 mbar and drift temperature +70 °C, resulting an E/N value of 141 TD (1 TD = 10 −17 V cm −2 ) according to the method described by Asaduzzaman et al 2020. For the analysis, dulce de leche samples (1.0 g) were transferred into 40 mL screw‐capped glass vials and incubated at 30 °C for 30 min.…”

Elucidation of the Photochemical Fate of Methionine in the Presence of Surrogate and Standard Dissolved Organic Matter under Sunlight Irradiation

“…14,15 Methional is known to be unstable and able to further degrade into compounds such as DMDS and MeSH. Yet, the formation pathway of MeSH is not fully understood 9 as the direct production of MeSH from MET without methional has also been proposed. 9 Alternative to the pathway via methional, the formation of an [S∴S] + complex has been proposed to produce DMDS via the cleavage of the C−S bond of two MET molecules.…”

“…Yet, the formation pathway of MeSH is not fully understood 9 as the direct production of MeSH from MET without methional has also been proposed. 9 Alternative to the pathway via methional, the formation of an [S∴S] + complex has been proposed to produce DMDS via the cleavage of the C−S bond of two MET molecules. 7 Furthermore, the photoproduction of methanesulfonic acid (MSA) and sulfate from MET in the presence of a natural DOM sensitizer has been reported.…”