Onion thiosulfinates

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]. 

The generation of unsaturated bis(sulfines) by a dithio-Claisen rearrangement is discussed in Sect. 2.6. The intermediacy of these products is proposed to take place in nature from unsaturated thiosulfinates formed in onion and garlic, and to be the source of zwiebelanes [91-93]. Studies by Block and his group produced a great deal of information about this fascinating chemistry. Activation parameters as low as 15.5 kcal/mol for AH+ have been measured. 

In onions, things start much the same way but the initial amino acid is not quite the same. The skeleton is the same as that of the garlic compound but the double bond is conjugated with the sulfoxide. Elimination and dimerization of the sulfenic acid produce an isomeric thiosulfinate. 

Oxidation of the thiosulfinate ester up to the sulfonate level gives the compound responsible for the lell of raw onions, while a hydrogen shift on the conjugated sulfenic acid (not possible with the garlic impound) gives a sulfine, the sulfur analogue of a ketene. The compound has the Z configuration cted from the mechanism and is the lachrymator that makes you cry when you cut into a raw onion. 

Members of the allium family (onions, garlic, and leeks) contain cysteine sulfoxide derivatives (allyl sulfur compounds), such as allicin and alliin (see Figure 14.9). When the plant cells are damaged, the enzyme alliinase is released from vacuoles and catalyzes the formation of thiosulfinates and thiols, including the lachrymator thiopropanal S-oxide. Their function in the plant is presumably to provide protection against attack by pests. 

Calvey, E.M., Matusik, J.E., White, K.D., DeOrazio, R., Sha, D. and Block, E. (1997) Allium chemistry supercritical fluid extraction and LC-APCI-MS of thiosulfinates and related compounds from homogenates of garlic, onion, and ramp. Identification in garlic and ramp and synthesis of 1-propanesulfinothioic acid S-allyl ester. J. Agric. Food Chem. 45(11) 4405-4413. 

As for the inhibiting action of garhc oil, onion oil, and diallyl sullide on skin cancers, thrae is evidently a beneficial action on epidermis cells. As for stomach and colon cancer, it is advanced that the antimicrobial effect of the thiosulfinates in garlic inhibits some of the less desirable gut flora. Otho- findings are repeated as follows ... 

But the goal of onion research is more than just culinary. For example, researchers in West Germany have discovered that compoimds in onions called thiosulfinates can relieve asthma. And Bela Karoly, the renowned former Olympic gymnastics coach, swears by the old Transylvanian remedy of applying a cooked onion to an inflamed joint. 

Actual (and Predicted) Shelf Life (Days) of Dried Onion Flakes Based on Browning and Thiosulfinate Loss at Different Temperatures... 

We have used chiral y-cyclodextrin GC columns under isothermal GC-MS conditions (column temperature 9 120 ) and found that enantiomers of /ran -zwiebelane (9a, Figure 2) and the thiosulfinates MeS(0)SMe, MeS(0)SPr-/i, and MeSS(0)Pr-/i (Figure 3) can be resolved and that individual enantiomers are stable under the analytical conditions (25). However, analysis of an onion extract on the chiral column showed that all of these con unds were present as racemic mixtures, su esting that asymmetric induction is not involved in their formation from achiral sulfenic acid ... 

The flavor precursors, 5 -alk(en)yl-L-cysteine sulfoxides, themselves do not have any specific flavor. However, when garlic or onion cells are damaged by cutting or crushing and the 5 -alk(en)yl-L-cysteine sulfoxides meet the C-S lyase (alliinase), they are converted into various volatile sulfuric compounds. Stoll and Seebeck (1951) first proposed the production of diallyl thiosulfinate (allicin) from the 5 -allyl-L-cysteine sulfoxide (alliin) by the C-S lyase. Through the reaction catalyzed by the C-S lyase, 5 -alk(en)yl-L-cysteine sulfoxides yield alk(en)yl sulfenic acid and aminoacrylic acid, the latter being spontaneously degraded... 

When onion is cut or cmshed, ( )-5 -l-propenyl-L-cysteine sulfoxide (isoalliin) is converted into ( )-l-propene-l-sulfenic acid. Different from garlic, (Z)-thiopropanal S-oxide, a lachrymatory factor, is formed from the sulfenic acid by lachrymatory factor synthase (Figure 18.6). The remaining ( )-l-propene-l-sulfenic acid and methyl sulfenic acid produced from 5 -methyl-L-eysteine sulfoxide ean form methyl 1-pro-pene-thiosulfinate and 1-propenyl methane thiosulfinate that are further converted to sulfides such as 1-propenyl methyl disulfide and 1-prope-nyl methyl trisulfide. 

Even considering the difference in the amount of 5 -alk(en)yl-L-cysteine sulfoxide, the total amount of thiosulfinates produced from onion is significantly lower than that from garlic being approximately 0.01-0.035 mg/g-fresh weight (Table 18.2). The reason for this is because most of the ( )-l-propene-l-sulfenic acid produced from E)-S-1-propenyl-L-cysteine sulfoxide is converted into a lachrymatory factor, and not so much can be used for the production of thiosulfinates. 

This scarcity in thiosulfinates results in the low production of sulfides. Sulfides majorly detected in onion are dipropyl disulfide, dipropyl trisulfide, methyl propyl disulfide, methyl 1-propenyl disulfide, 1-prope-nyl propyl disulfide, and methyl propyl trisulfide (Boelens et al. 1971 Kallio and Salorinne 1990). Interestingly, more amount of sulfides with propyl group is produced in spite of the lower amount of S-propyl-L-cysteine sulfoxide than that of S-methyl-L-cysteine sulfoxide (Randle etal. 1995). 

The kinetics of quality deterioration in dried onion flakes (NEB and thiosulfinate loss) and dried green beans (chlorophyll-u loss) were studied as a function of water activity and temperature and empirical equations and mathematical models developed that successfully predicted the shelf life of the dried products as a function of temperature and (Table 25.11 and Table 25.12) [88]. Above the mono-layer (flw, 0.32-0.43) for onion, increasing moisture contents resulted in greater reaction rates for browning and thiosulfinate loss. Very little browning was observed over a storage period of 631 d at 20 C and... 

Fig. 1.3 Major thiosulfinate compounds found in AUiaceae family (a) organosulfur compounds in intact plants, (b) compounds Produced from allyal cystein sulfoxide (in garlic) and (c) 1-propenyl cystein sulfoxide (in onion) by aUinase...

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