The stability of gingerol and shogaol in aqueous solutions

Gingerols, pungent principles of ginger (the rhizome of Zingiber officinale), are biologically active components that may make a significant contribution towards medicinal applications of ginger and some products derived from ginger. Gingerols, however, are thermally labile due to the presence of a beta-hydroxy keto group in the structure, and undergo dehydration readily to form the corresponding shogaols. This study investigated the stability of [6]-gingerol [5-hydroxy-1-(4-hydroxy-3-methoxyphenyl)decan-3-one] at temperatures ranging from 37 to 100 degrees C in aqueous solutions, at pH 1, 4, and 7. Quantitative measurements of [6]-gingerol and its major degradation product [6]-shogaol [1-(4-hydroxy-3-methoxyphenyl)decan-4-ene-3-one] were performed by HPLC. Kinetics of [6]-gingerol degradation was characterized by least square fitting of a rate equation. It was found that gingerol exhibited novel reversible kinetics, in which it undergoes dehydration-hydration transformations with shogaol, the major degradation product. Degradation rates were found to be pH dependent with greatest stability observed at pH 4. The reversible degradation of [6]-gingerol at 100 degrees C and pH 1 was relatively fast and reached equilibrium within 2 h. Activation energies for the forward and reverse reactions for [6]-gingerol were calculated from the Arrhenius equation using reaction rates obtained at temperatures ranging from 37 to 100 degrees C.