Changes in antioxidant and antibacterial activities as well as phytochemical constituents associated with ginger storage and polyphenol oxidase activity

BACKGROUND Herbal materials should be stored at optimal conditions in order to retain their nutritional quality. Proper storage has a significant impact on the quality of the herbs and spices. METHODS The effects of storage temperature (5 and 15 °C) and time (4 and 8 months) on the phytochemical constituents associated with the antioxidant and antibacterial activities of ginger varieties (Halia bentong and Halia bara) were evaluated to determine the optimal storage conditions for ginger rhizomes. Total flavonoid content (TFC) and Total phenolic content (TPC) were measured using the spectrophotometric method. Individual phenolic acids and flavonoids, 6-gingerol and 6-shogaol were identified by ultra-high performance liquid chromatography. Ferric reducing antioxidant potential (FRAP) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays were used for evaluation of antioxidant activities. An antibacterial property of ginger varieties was evaluated using well diffusion method. RESULTS Dry matter, TPC, TFC and individual phenolics and flavonoids content, 6-gingerol and 6-shogaol content noticeably decreased at 5 and 15 °C during the storage times from 4 to 8 months. Highest content of flavonoids, phenolic acids, 6-gingerol, and 6-shogaol was observed in fresh samples followed by rhizomes stored at 5 °C for 4 months. Storage at 15 °C for 4 months reduced the phytochemical content significantly. Cinnamic acid and tannic acid were not detected in those variety stored at 15 °C for 4 and 8 months. Polyphenol oxidase (PPO) activity was associated significantly with storage time and temperature. Highest and lowest PPO activity was observed in stored and fresh rhizomes respectively. Antioxidant and antimicrobial activities gradually declined with the increase of storage temperature (from 5 to 15 °C) and duration (from 4 to 8 months) in both the varieties. Freshly harvested Halia bara variety had higher antioxidant and antibacterial activity compared to the Halia bentong variety. CONCLUSIONS Halia bara exhibited valuable phytochemical content and antioxidant and antibacterial activities at higher levels compared to that exhibited by Halia bentong rhizomes. In general, storage of Malaysian ginger varieties at temperature of 5 °C is recommended and the storage time should be not more than 4 months. This storage condition will provide greater stability to the concentration of the phytochemical constituents more similar to the fresh material.