for the production of garlic powders that can be used for culinary spices or production of medicines such
as garlic tablets or capsules [35]. Other postharvest preparation methods used to generate medicinal forms
of garlic include oil extraction of the bulb to generate garlic oils and steam distillation to generate an es-
sential oil preparation from volatile compounds present in the bulb [35,37]. As oil extraction selects for
compounds soluble in oil and steam distillation selects for more volatile compounds, the array of active
chemicals may differ in these preparations [37].
The pharmacological activity of garlic appears to be associated with the sulfur-containing chemicals
that are accumulated in the bulb. Of the sulfur-containing compounds present in the bulb, about 95% can
be found in two classes of compounds: the -glutamyl-S-alkyloysteines and the S-alkylcysteine sulfox-
ides (Figure 4A). Of these compounds alliin is most prominent, amounting to about 6–14 mg/g fresh
weight [36,37]. This is followed by -glutamyl-S-t-1-propenylcysteine (3–9 mg/g fresh weight), -glu-
tamyl-S-allylcysteine (2–6 mg/g fresh weight), methiin (0.5–2 mg/g fresh weight), and cycloalliin
(0.5–1.5 mg/g fresh weight) [36]. Although alliin and methiin may be prominent compounds in the intact
bulb, these chemicals are not present in any garlic preparations for which the garlic cloves have been
crushed or powdered. This is due to the activity of the enzyme alliinase, which is released upon crushing
of the tissue. A major result of this alliinase activity is the production of allicin (Figure 4B) and this is sig-
PHYTOMEDICINAL CHEMICAL PRODUCTION BY PLANTS 491
Figure 4 The -glutamyl-S-alkyulcysteines and S-alkylcysteines present in garlic (Allium sativa) (A) and the
reaction conducted by alliinase with the crushing of garlic bulb tissue (B). (Adapted from Refs. 15 and 36.)