Physicochemical Properties of Repetitive Heat-treated Ginger and Its Quantitative Conversion of Gingerol to Shogaol

Ginger was steamed at $121^{\circ}C$ and $1.5lb/in^2$ for 30 min, dried at $60^{\circ}C$ for 12 h, and each step was repeated nine times. During processing, the lightness ($L^*$ value) and yellowness ($b^*$ value) decreased from $85.65{\pm}0.33$ and $26.99{\pm}0.20$ in the non-treated ginger to $56.91{\pm}0.25$ and $16.69{\pm}0.06$ in ginger treated for the ninth treatment. On the other hand, redness ($a^*$ value) increased from $-1.51{\pm}0.03$ to $7.34{\pm}0.08$ on the eight treatment and then decreased to $7.21{\pm}0.04$ on the ninth theatment. The contents of 6-gingerol decreased from $3.257{\pm}0.067mg/g$ in the non-treated ginger to $0.567{\pm}0.036mg/g$ on the theatment, whereas the contents of 6-shogaol increased from $1.299{\pm}0.050mg/g$ to $2.999{\pm}0.089mg/g$ on the sixth treatment and decreased to $2.099{\pm}0.039$ on the ninth treatment. The contents of 10-gingerol decreased slightly from $1.106{\pm}0.125mg/g$ to $0.806{\pm}0.026mg/g$. Unlike the 6- and 10-gingerol, the contents of 8-gingerol did not change greatly, with values between $0.916{\pm}0.005mg/g$ and $1.106{\pm}0.005mg/g$ being observed during processing. The tyrosinase inhibitory activities were increased from $43.42{\pm}11.45%$ in the non-treated ginger to 100% on the sixth treatment and then decreased to $51.98{\pm}7.36%$ on the theatment. The antioxidative activity was retained during processing.