Allium species, sulfur compounds

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...

Garlic, Allium sativum L., is a species in the onion family, Alliaceae. Onion, shallot, and leek are close relatives. Garlic has been used throughout history for both culinary and medicinal purposes. It has a pungent hot sensation that mellows and sweetens considerably with cooking. A large number of sulfur compounds contribute to the smell and taste of members of the onion family. Diallyl disulfide and diallyl sulfide are... 

A large number of organic sulfur compounds have been detected in extracts or distillates prepared from garlic, onions, or other Allium species. This chemistry has been reviewed by Block. Polysulfanes have been detected in many of these preparations but most recent results show that fresh extracts of garlic, prepared under mild conditions (20 °C), did not show any polysulfanes R-S -R n > 2) when analyzed by RP-HPLC analysis. ... 

Several sulfiir-containing amino acids, other than cysteine, cystine, and methionine, are found in higher plants. Allium (Liliaceae) species often contain sulfur compounds that are lachrymatory in nature (Fig. 13.17). Among these species are chives, garlic, onion, and leeks. The compounds involved are 5-substituted cysteine sulfoxides and their hydrolysis... 

Fig. 13.15. Sulfur compounds found in Allium species and formation of i yn-propanethial S-oxide (Luckner, 1990).

For years researchers have investigated the sulfur compounds present in various foods. Cooked foods typically contain numerous sulfur compounds, especially heterocyclic compounds like thiazoles, thiophenes, thiazolines, etc. In 1986, Sha-hidi et al. (7) reported that 144 sulfur compounds had been identified in beef. Other heated food systems like bread, potato products, nuts, popcorn, and coffee also contain many sulfur compounds. Aliphatic thiols have been found in fruits, vegetables, dairy products etc., as well as in heated foods. No discussion of the occurrence of sulfur compounds in foods would be complete without mention of their major role in the various allium species. Indeed, more than half of the volatile compounds reported in garlic, onion, leek, and chive contain sulfur (2). Comprehensive reviews of the literature concerning the role of thiazoles, thiophenes, and thiols in food flavor through 1975 can be found in Maga s series of review articles (3-5). 

The identified components and their peak areas by GC are listed in Tables I and II. For both the fruits and leaves, the composition of the volatiles was relatively simple. In the headspace volatiles, ethanal was the main component (96.4%). This compound is probably produced in the fruit during storage from pyruvic acid by decarboxylation under anaerobic respiration (4). Among the other compounds were dimethyl sulfide, dimethyl disulfide, and methane- and propane-thiols, together with two unusual polysulfides which have not been found in the Allium species. The sulfur compounds in the leaves were similar to those found in the fruit, but the concentrations of ( )-2-hexenal and (Z)-3-hexenol, which are well known to be responsible for a green note (5), were much higher than found in the fruit. 

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