S-l-Propenyl cysteine

Scheme 7.3 Enzymatic production of sulfur-containing flavour compounds in Allium species from amino acid flavour precursors, a S-Alk(en)yl cysteine sulfoxides and b (+)-S-l-propenyl cysteine sulfoxide (isoalliin) P-5 -Ppyridoxal-5 -phosphate...

Boiling of whole cloves inactivates the enzyme allinase and only 0.5-1% of alliin is converted to allicin which is rapidly converted to diallyl trisulfide. Also 12% of the y-glutamylcysteines are hydrolyzed to S-allyl cysteine (SAC) and S-l-propenyl cysteine. A similar loss of alliin is found [37]. Boiling of crushed or tiny chopped garlic enables tiie formation of allicin... 

Proofs of the presence of 7-L-glutamyl-S-(l-propenyl)cysteine sulphoxide and... 

The lachrymatory factor in onion (thiopropanal-S-oxide) is considered to come from the S-l-propenyl-cysteine sulfoxide precursor [40]. Thiopropanal-S-oxide (CH3-CH2-CH=S=0) is unstable reacting with pyruvate to form propanol, 2-methyl pentanal and 2-methyl pent-2-enal [41]. It is of interest that no lachrymatory properties were found in 25 species of wild onions [42]. This would suggest that the 1-propenyl precursor may be absent from the wild onion. 

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

Onions Allium cepa) were shown to contain similar compounds, S-methyl and S-propyl-L-cysteine sulfoxide (20). The principal flavor precursor in onion is fmn -S( + )-l-propenyl-L-cysteine sulfoxide 21, 22, 23). It is responsible for the lachrymatory properties and bitter taste of freshly cut onion (22). All these compounds were cleaved by an S-alkyl-L-cysteine sulfoxide lyase from onion 24, 25) which yielded pyruvate and ammonia in addition to a sulfur compound. The enzyme has also been demonstrated in Bacillus subtilis 26) and in a number of the Cruciferae where the only substrate known is S-methyl-L-cysteine sulfoxide (27). The product presumably gives rise to dimethyl disulfide which is the odor of cooked cabbage. 

In the case of onion, the principle substrate is trans (+)S-(l-propenyl)L- cysteine sulfoxide which, when acted on by alliinase, produces a distinctive odor profile, a marked pungency, a transient lachrymatory effect, a bitter aftertaste and a pink discoloration. 

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... 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

Alline S-Propenyl-cysteine sulfoxide 7-L-Glutamyl-S-allyl-cysteine... 

PrenCySO+G (+)-S-cis-l-propenyl-L-cysteine sulfoxide + glucose nd not detected... 

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