Onion (Allium cepa) and garlic (Allium sativum) have been shown to inhibit growth and toxin production of many microorganisms including B. cereus, C. botulinum type A, E. coli, Lactobacillus plantarum, Salmonella, Shigella, and S. aureus, and the fungi A. flavus, A. parasiticus, Candida albicans, and species of Cryptococcus, Rhodotorula, Saccharomyces, Torulopsis and Trichosporon

(Saleem and Al-Delaimy, 1982; Conner and Beuchat, 1984; Beuchat, 1994; Gonzalez-Fandos et al., 1994). Cavallito and Bailey (1944) isolated the major antimicrobial compounds from garlic by using steam distillation of ethanolic extracts. They identified the antimicrobial component as allicin (diallyl thiosulfinate; thio-2-propene-1-sulfinic acid-5-allyl ester). Allicin is formed by the action of the enzyme, allinase, on the substrate alliin [S-(2-propenyl)-L-cysteine sulfoxide]. The reaction occurs only when cells of the garlic are disrupted, releasing the enzyme to act on the substrate. A similar reaction occurs in onion except that the substrate is [S-(1-propenyl)-L-cysteine sulfoxide] and one of the major products is thiopropanal-S-oxide. The products responsible for antimicrobial activity are also apparently responsible for the flavor of onions and garlic. In addition to antimicrobial sulfur compounds, onions contain the phenolic compounds protocatechuic acid and catechol, which could contribute to their antimicrobial activity (Walker and Stahmann, 1955). The mechanism of action of allicin is most likely inhibition of sulfhydryl-containing enzymes (Beuchat, 1994).

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