Identification of S-methylmethionine in Petit Manseng grapes as dimethyl sulphide precursor in wine

A procedure for the extraction of free amino acids was applied to isolate S-methylmethionine (SMM) from late harvest Petit Manseng grapes. Grapes were destemmed and crushed, and the obtained clarified must was percolated through cation-exchange resins (Dowex 50 WX4-100). The retained compounds were eluted with ammonia solution and the extract was finally concentrated. Taking into account the potential DMS (PDMS using heat-alkaline treatment assay) of the initial grape juice used (51.5 nmol mL⁻¹) and the concentration factor of the extract (17.9-fold), the PDMS of the final extract (678 nmol mL⁻¹) gave an overall recovery of 73.5% for juice SMM. This compound was identified and quantified (484.5 nmol mL⁻¹ relatively to ²H₃]-SMM used as internal standard) by its selective detection in this extract without derivatization by MALDI-TOF-MS using instrumentation and procedures previously reported to analyze SMM in complex natural extracts. SMM and 22 other amino acids in the initial must and in the final SMM extract were also determined using a Biochrom 30 amino acid analyser with post-column ninhydrin derivatization. SMM peak identification and quantification (401.2 nmol mL⁻¹ relatively to norleucine used as internal standard) were carried out by comparison with commercial SMM.