Quantitation of the Intense Aroma Compound 3-Mercapto-2-methylpentan-1-ol in Raw and Processed Onions (Allium cepa) of Different Origins and in Other Allium Varieties Using a Stable Isotope Dilution Assay

A stable isotope dilution assay was developed for the quantitation of the potent onion odorant 3-mercapto-2-methylpentan-1-ol (1) using mass chromatography and synthesized [²H ₂]-3-mercapto-2-methylpentan-1-ol as the internal standard. Application of the newly developed method on onions from different origins revealed amounts between 8 and 32 μg/kg in raw onions, whereas 34-246 μg was found in sliced, stored (50 min), and then cooked onions. In extracts prepared by simultaneous steam distillation-extraction the highest concentrations of 1 were formed, amounting to > 1200 μg/kg. The much higher content of 3-mercapto-2-methylpentan-1-ol in cooked onions suggested its formation from specific, yet unkown, precursors enzymatically formed during cutting of raw onions. 1 was for the first time identified and also quantified in other Allium species such as chives, scallions, and leek, whereas surprisingly garlic and bear's garlic did not contain the aroma compound.