The effects of pH, neutralization after acidification, time, and temperature on the yield of dialkyl thiosulfinates released from garlic powder and garlic cloves were determined. All dipropenyl thiosulfinates (allicin, 1-propenyl allyl, and allyl 1-propenyl) were formed at an optimum pH of 4.5-5.0. The methyl propenyl thiosulfinates (allyl methyl + methyl allyl and 1-propenyl methyl + methyl 1-propenyl) and dimethyl thiosulfinate were optimally formed at pH 6.5-7.0 and pH 5.5, respectively. Below pH 3.6 no thiosulfinates were formed. Neutralization of the pH failed to restore thiosulfinate generation from garlic previously incubated at pH 3 or below. Thus, alliinase is completely and irreversibly inhibited by the acidic conditions found in the stomach. The dipropenyl thiosulfinates were completely formed in 0.3 min at 37 degrees C, while the methyl thiosulfinates were not completely formed until 3.5 min. Allyl 1-propenyl thiosulfinate was the most rapidly formed, and the most unstable, thiosulfinate. The stability of the dipropenyl thiosulfinates was improved at pH 4.5 or lower. Drying garlic at 60 degrees C had no effect on alliin or the rate of formation of the dipropenyl thiosulfinates, but decreased trans-1-propenylcysteine sulfoxide (isoalliin) and the rate of formation of the methyl thiosulfinates. The results demonstrate that there are two alliinase activities in garlic, that a stomach acid-resistant coating on garlic powder tablets is necessary for thiosulfinate release, and that carefully prepared garlic powder can release similar amounts of total thiosulfinates to whole garlic cloves.