Optimization of drying conditions for the conversion of 6-gingerol to 6-shogaol under subcritical water extraction from ginger

6-gingerol can be converted to 6-shogaol, one of the predominant active compounds found in ginger, via processing such as drying and extraction. Subcritical water extraction is the environmentally friendly method of extraction of bioactive compounds using only purified water as a solvent. This study investigated subcritical water extraction (190^oC, 15 min) of 6-gingerol, and 6-shogaol from dried ginger (Zingiber officinale) including drying conditions such as temperature (room temperature, 60, 80^oC, and freeze drying) and time duration for drying (1-4 h). The amount of 6-gingerol was found to be reduced, and that of 6-shogaol was found to be increased depending upon the water content of dried ginger. Upon oven-drying ginger at 60^oC for 2 h, the maximum yields of 6-gingerol (0.18±0.02 mg/g fresh weight), and 6-shogaol (0.47±0.02 mg/g fresh weight) were obtained upon subcritical water extraction.