Thermostability of Allicin Determined by Chemical and Biological Assays

Abstract

The garlic-derived antibacterial principle, alk(en)yl sulfinate compounds, has long been considered as very short-lived substance. However, there are some data showing a rather more stable nature of allicin. We determined here the thermostability of allicin by a systematic analyses employing chemical quantification and an antibacterial activity assay. Allicin in an aqueous extract of garlic was degraded stoichiometrically in proportion to the temperature; we estimated the halflife of allicin to be about a year at 4 C (from 1.8 mg/ml to 0.9 mg/ml) and 32 d at 15C, but only 1 d at 37 C (from 2.0 mg/ml to 1.0 mg/ml). The half-life values for antibacterial activity showed a similar trend in results: 63 d or more at 4 C for both antibacterial activities, 14 d for anti-staphylococcal activity, and 26 d for antiescherichia activity at 15 C, but only 1.2 d and 1.9 d for the respective activities at 37 C. Such antibacterial activities were attributable to the major allicin, allyl 2-propenylthiosulfinate. Surprisingly, the decline in the quantity of allicin was not accompanied by its degradation; instead, allicin became a larger molecule, ajoene, which was 3-times larger than allicin.

Antibacterial Potential of Garlic-Derived Allicin and Its Cancellation by Sulfhydryl Compounds

“…The detailed identification of allicin was described previously. 16,17) The standard curve for various amounts of authentic allicin gave a straight line with a correlation coefficient of r 2 ¼ 0:9894 at allicin concentrations of less than 2.0 mg/ml (Fig. 1B).…”

“…Allicin was quantified by a method described previously. [14][15][16][17] Briefly, chromatographic analysis was performed using a C18, MG-II (5 mm, 4:6 mm' Â 250 mm) column (Shiseido, Tokyo), a LC-10AT pump (Shimadzu, Kyoto), a Chromato-PRO data integrator (Run Time Instruments, Kanagawa), and a UV monitor (655A, Hitachi, Tokyo). The eluent was 0.02 M phosphate buffer (pH 6.5) containing acetonitrile and 1, 4-dioxane in a ratio of 7:1:2, and the flow rate was set to 0.5 ml/min.…”

“…The insert shows the structure of allicin as determined by LC/MS/MS, as described in our previous paper. 16,17) B, Standard curve of allicin drawn by analyzing authentic allicin. Each plot was obtained through 3 runs of HPLC, and all the data were averaged.…”

“…However, garlic extract contains sulfinyl compounds other than allicin (allyl 2-propenethiosulfinate), which we quantified by HPLC, 16,17) and thus these additional components should affect the anti-staphylococcal activity of allicin in an additive fashion. On the other hand, it is probable that garlic extract contains a component affording a condition under which allicin can more efficiently attack S. aureus.…”

Factors influencing the stability of garlic thiosulfinates

“…Despite a potent antimicrobial activity, garlic has not been used as a food preservative because of a strong odor (5-7) and instability (2,(8)(9)(10)(11)(12)(13)(14). The stability of garlic thiosulfinates is influenced by temperature (2,(8)(9)(10)(11)(12)(13), pH (11)(12)(13)(14), solvents (2,9), molecular structure (15)(16)(17)(18), and the concentration of garlic (2). In addition to known factors, garlic has been suggested to contain unknown substance(s) that induce faster decomposition of thiosulfinates (2).…”