Chemistry and biological properties of onions and garlic

Abstract

The fresh flavors of onions, garlic, and other members of the Allium genus are produced by enzymic decomposition of S‐alkyl‐ and S‐alkenyl‐L‐cysteine S‐oxides from which the primary products are thiolsulfinates containing alkyl and alkenyl substituents (alkyl‐ and alkenyl‐disulfide S‐oxides). The amino acid flavor precursurs of garlic are (+)S‐methyl‐, (+)S‐propyl‐, and (+)S‐(2‐propenyl)‐L‐cysteine S‐oxides, with the last amino acid predominant. The initial flavor of freshly chopped garlic is due mainly to the enzymic product 2‐propene‐2‐propenylthiolsulfinate. Small quantities of the corresponding methyl and propyl derivatives are also present. On standing, these compounds are converted to disulfides, trisulfides, and more complex sulfur‐containing compounds. The flavor precursors of the onion are (+)S‐methyl‐, (+)S‐propyl‐, and (+)S‐(l‐propenyl)‐L‐cysteine S‐oxide, with the last amino acid predominant. Enzymic decomposition of the propenyl derivative yields the characteristic transitory lachrymatory substance. The lachrymator has the unusual structure propanethial S‐oxide. On standing, or more rapidly on heating, the thiolsulfinates decompose to yield a mixture of disulfides and trisulfides containing methyl, propyl, and 1‐propenyl groups, and smaller quantities of thiophene derivatives and other cyclic sulfur‐containing compounds. The minor members of the Allium genus, chives, leeks, and seal‐lion, generally contain the same amino acid flavor precursors as the onion and similar fresh flavors, but in much weaker intensities. Onions and garlic possess weak antibiotic properties. Onions have been shown in a number of cases to reduce blood sugar levels in humans. A number of investigators have reported that onions and garlic reduce blood cholesterol in experimental animals and in some cases in humans, and inhibit platelet aggregation. This may be of interest in the treatment of atherosclerosis and related cardiovascular problems.