Onion 3-mercapto-2-methylpentan

The most important flavour compound in raw onions is thiopropanal-S-ox-ide, the lachrymatory factor [145,146]. Other important flavour compounds are 3,4-dimethyl-2,5-dioxo-2,5-dihydrothiophene and alkyl alkane thiosulfonates such as propyl methanethiosulfonate and propyl propanethiosulfonate with a distinct odour of freshly cut onions [35, 36, 147]. Various thiosulfinates that have a sharp and pungent odour may also contribute to the flavour of onions. These compounds, however, are rapidly decomposed to a mixture of alkyl and alkenyl monosulfides, disulfides and trisulfides (Scheme 7.3) of which dipropyl disulfide, methyl ( )-propenyl disulfide, propyl ( )-propenyl disulfide, dipropyl trisulfide and methyl propyl trisulfide are the most important contributors to the aroma of raw and cooked onions (Table 7.5, Fig. 7.6) [148-150]. Recently, 3-mercapto-2-methylpentan-l-ol was identified in raw and cooked onions eliciting intense meat broth, sweaty, onion and leek-like odours [142, 151]. 

A powerful sulphury aroma compound, 3-mercapto-2-methylpentan-l-ol (51), has recently been identified [13] in a complex process flavouring. It has a low odour threshold of 0.15 mg/L of water. The formation could be traced back to the onions present in the process flavouring and its formation is explained from propanal present in onions via aldol condensation, addition of hydrogen sulphide and enzymatic reduction (Fig. 3.31). 

Fig. 3.31 Hypothetical formation of 3-mercapto-2-methylpentan-l-olfrom propanal in onions...

Aliphatic sulfur compounds, in particular disulfides such as methyl propyl disulfide, dipropyl disulfide, and especially cis- and frares-propenyl propenyl disulfide, are mainly responsible for the typical odor of onion oil [233a, 235-235b]. The presence of 2-hexyl-5-methyl-3(2/f)-furanone [33922-66-6] is a characteristic of authenticity [235c], 3-Mercapto-2-methylpentan-l-ol [227456-27-1] is an onion constituent with a very typical onion odor and a remarkable odor intensity [235d, 235e]. 

Mercapto-2-methylpentan- l-ol Meat-like, like onions 0.0016... 

The compound which causes tears (the lachrymatory factor) is (Z)-propanethial-S-oxide (II) which, once the onion bulb is sliced, is derived from trans-(-h)-S-(l-propenyl)-L-cysteine sulfoxide (I) by the action of the enzyme al-liinase. Alliinase has pyridoxalphosphate as its coenzyme (cf. reaction sequence 17.11). Chopping of onions releases 3-mercapto-2-methylpentan-l-ol, which, with its very low threshold of 0.0016 pg/1 (water), smells of meat broth and onions. Raw onions contain 8-32pg/kg, and onions which have been cut, stored for 30 minutes and then cooked contain... 

Not all thiols have unpleasant odours. A component of raw and heat-treated onion with a very intense aroma is 3-mercapto-2-methylpentan-l-ol (2-methyl-l-hydroxypentan-3-thiol, see... 

Granvogl M., Christlbauer M., Schieberle P. Quantitation of the intense aroma compound 3-mercapto-2-methylpentan- 1-olin rawand processed onions (Allium cepa) of different origins and in other Allium varieties using a stable isotope dilution assay. Journal of Agricultural and Food Chemistry, 52 2797-2802 (2004). 

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