Garlic Compounds

Raw, intact garlic contains various chemical compounds, all of which are converted to other sulfur-containing compounds when processed. All of these compounds are derived from the compound allicin. Allicin is formed from alliin, by the action of allinase, which is released when garlic is chopped or chewed (8). Allicin is extremely unstable and further breaks down to pro duce hundreds of organosulfur compounds such as diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide, ajoenes, methyl allyl di- and trisulfides, vinyl dithiins, and other sulfur compounds, depending on how the garlic is prepared. By the formation of these compounds, allicin is responsible for most of the biological activity of garlic; however, it is also a major contributor in garlic's characteristic odor (5).

Different methods of processing garlic, resulting in products containing different sulfur-containing thiosulfinate derivatives, have been discussed (8). A bulb of raw garlic, on average will contain up to 1.8% alliin, a small amount of SAC, which is a less-odorous biologically active compound, but no allicin. When garlic is chopped or crushed, 1 mg of alliin is converted to 0.48 mg of allicin (3). Cooking whole or coarsely chopped garlic destroys allinase, the enzyme necessary for production of allicin, ajoene, diallyl sulfide, diallyl disulfide, and vinyl dithiins; only cysteine sulfoxides such as alliin remain.

Crushing or finely chopping garlic followed by boiling in an open container leads to volatilization and loss of many chemically unstable, but potentially medicinal thiosulfinates. Steam distillation produces an oily mass of active compounds including diallyl, methyl allyl, dimethyl, and allyl 1-pro-penyl oligosulfides that originate from the thiosulfinates. Maceration of garlic in vegetable oil or soybean oil produces vinyl dithiins, ajoenes, and diallyl and methyl allyl trisulfides. These latter two methods are used to prepare some commercially available garlic capsules. When garlic is allowed to ferment (cold aging, AGE products), water-soluble SAC, S-allyl-mercaptocysteine, and other biologically active compounds are produced.

Garlic powder is produced by drying and pulverizing sliced or crushed garlic. The drying process is thought to cause powders to lose approximately one-half the amount of alliin found in whole garlic cloves. If dried at low temperatures, the garlic powder will remain odor-free until the product reaches the gastrointestinal (GI) tract after ingestion (7). In contrast to common belief, odorless garlic products still produce the same adverse drug reactions as those nonodorless products. Kwai brand coated odorless garlic powder tablets contain dried garlic powder prepared by freeze-drying fresh garlic (9). After the tablets are ingested, the alliin is converted to allicin in the GI tract by the enzyme allinase, which can come into contact with alliin once the coated tablets disintegrate and mix with intestinal water. Kwai is one of the most common garlic preparations used in studies.

Regardless of the processing procedure used, no garlic preparation available contains allicin, because of its high volatility. Many products report an "allicin yield" potential when consumed. However, studies have shown that allicin is not produced in significant amounts after ingestion of garlic prod ucts, which may be owing to inactivation of alliinase in the acidic stomach. Therefore, allicin is likely not an appropriate marker of the potential activity of the product (5). Enteric-coated products may preserve the activity of allinase, by delaying dissolution of the product. AGE products are standardized to SAC, which is found in detectable levels in the body and may therefore be a better standardization marker for garlic products than allicin yield (5).

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