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Casein microparticles are produced by flocculation of casein micelles due to volume exclusion of pectin and subsequent stabilization by film drying. Transglutaminase post-treatment alters their stability, swelling behavior, and internal structure. Untreated particles sediment due to their size and disintegrate completely after the addition of sodium dodecyl sulfate. The fact that transglutaminase-treated microparticles only sediment at comparable rates under these conditions shows that their structural integrity is not lost due to the detergent. Transglutaminase-treated particles reach an equilibrium final size after swelling instead of decomposing completely. By choosing long treatment times, swelling can also be completely suppressed as experiments at pH 11 show. In addition, deswelling effects also occur within the swelling curves, which enhance with increasing transglutaminase treatment time and are ascribed to the elastic network of cross-linked caseins. We propose a structural model for transglutaminase-treated microparticles consisting of a core of uncross-linked and a shell of cross-linked caseins. A dynamic model describes all swelling curves by considering both casein fractions in parallel. The characteristic correlation length of the internal structure of swollen casein microparticles is pH-independent and decreases with increasing transglutaminase treatment time, as observed also for the equilibrium swelling value of uncross-linked caseins.

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.

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