Studies were carried out on the precursor of dimethyl sulfide (DMS) that is the characteristic component of off-flavor produced by heating Satsuma mandarin juice. (1) S-methylmethionine sulfonium (MMS) was identified by two-dimensional thin layer chromatography in the juice before heating, and was not found in the juice after heating. (2) It was proved by kinetic studies that MMS is the precursor of DMS. The reaction system, MMS → DMS, was determined to be first order reaction and the pH dependence of the rate constant for the juice was closely similar to that for MMS standard solution. Activation energy (Ea) and frequency factor (A) on the reaction were estimated and coinsided with each other as follows: Ea=35.2 kcal/mol, A=5.37×1016 for the juice ; Ea=35.2 kcal/mol, A=6.17×1016 for MMS standard solution. (3) It has turned out from the GC-MS analysis of MMS degradation products that MMS underwent breakdown predominantly to homoserine and DMS at pH 3.2, while to homoserine lactone and DMS at pH 7.5, and the reasonable reaction mechanisms were then presented. (4) The kinetic studies suggested the possibility to predict theoretically and precisely the amount of DMS formed under any heating condition when the concentration of MMS in the juice is known already.
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