Mercapto onion

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]. 

Mercapto-2- Allium cepa (onion) (Liliaceae) [bulb] OD-R... 

Thiiranes occur naturally in cruciferous vegetables. Thiirane and 2-methylthiirane have been detected by GC-MS in the aroma of canned beef. Beer with an onion-like off-flavor was found to contain (l-methylethyl)thiirane and 2-mercapto-3-methyl-l-butanol <88Mi 105-01). Thiirane is in the aroma of cooking mutton and is formed in the brewing of foods. 1 -Cyano-4,5-epithiopentane was the major component of rape seeds at 3 days of germination <85ABC217>. 

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]. 

Numerous odour-active VSCs have been detected in beers. Examples of beer VSCs encompass methanethiol, ethanethiol, H2S, dimethyl sulphide, dimethyl disulphide, methional, methionol, 3-(methylthio)propyl aceate and 2-mercapto-3-methyl-l-buta-nol (Angelino, 1991 HiU Smith, 2000). Most VSCs cause off-odours such as rotten egg-like, cabbage-like, onion-like and garlic-like. However, some VSCs have a positive impact on beer flavour by accentuating fruitiness (e.g. 3-mercaptohexanol and 3-mercaptohexyl acetate). 

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... 

Fischer, K.-H., Grosch, W. Volatile compounds of importance in the tiroma of mushrooms (PsalUota bis-pond). Lebensm Wiss. Technol. 20.233 (1987) GianvogJ, M., Christlbauer, M., Schieberle, P. Quantitation of the intense tiroma compound 3-mercapto-2-methylpenttm-l-ol in raw and processed onions Allium cepa) of different origins and in other Allium varieties using a stable isotope dilution assay. J. Agric. Food Chem. 52, 2797 (2004)... 

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... 

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