Abstract
The effects of different drying methods (freeze drying, vacuum oven drying, and shade drying) on the phytochemical constituents associated with the antioxidant activities of Z. officinale var. rubrum Theilade were evaluated to determine the optimal drying process for these rhizomes. Total flavonoid content (TFC), total phenolic content (TPC), and polyphenol oxidase (PPO) activity were measured using the spectrophotometric method. Individual phenolic acids and flavonoids, 6- and 8-gingerol and shogaol were identified by ultra-high performance liquid chromatography method. Ferric reducing antioxidant potential (FRAP) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) assays were used for the evaluation of antioxidant activities. The highest reduction in moisture content was observed after freeze drying (82.97%), followed by vacuum oven drying (80.43%) and shade drying (72.65%). The highest TPC, TFC, and 6- and 8-shogaol contents were observed in samples dried by the vacuum oven drying method compared to other drying methods. The highest content of 6- and 8-gingerol was observed after freeze drying, followed by vacuum oven drying and shade drying methods. Fresh samples had the highest PPO activity and lowest content of flavonoid and phenolic acid compounds compared to dried samples. Rhizomes dried by the vacuum oven drying method represent the highest DPPH (52.9%) and FRAP activities (566.5 μM of Fe (II)/g DM), followed by freeze drying (48.3% and 527.1 μM of Fe (II)/g DM, respectively) and shade drying methods (37.64% and 471.8 μM of Fe (II)/g DM, respectively) with IC50 values of 27.2, 29.1, and 34.8 μg/mL, respectively. Negative and significant correlations were observed between PPO and antioxidant activity of rhizomes. Vacuum oven dried rhizomes can be utilized as an ingredient for the development of value-added food products as they contain high contents of phytochemicals with valuable antioxidant potential.
Highlights
Herbs and spices produce a variety of phytochemicals or secondary metabolites and have been used as food preservatives and flavourings, and as traditional medicines for thousands of years
We shows that all identified compounds had positive and significant (p < 0.05) correlations with DPPH
The findings of the present study indicate that the vacuum oven drying method at 45 ̋ C results in significantly higher content of phytochemicals and may be a suitable method for the drying and preservation of bioactive compounds in Z. officinale var. rubrum
Summary
Herbs and spices produce a variety of phytochemicals or secondary metabolites and have been used as food preservatives and flavourings, and as traditional medicines for thousands of years. A valuable line of study in the medical industry is appraising phytochemicals to ascertain whether they have any biological activities with potential benefits to human health. Ginger contains various phytochemicals and biologically active compounds such as phenolics, flavonoids, 6-gingerol, 6-shogaol, and zingerone [4,5]. From among the identified components, shogaols and gingerols were found to be the most abundant bioactive compounds in ginger, having numerous pharmacological benefits, including antioxidant, analgesic, anticancer, antipyretic, and anti-inflammatory properties [6,7,8]. Herbal materials must be dried under specified conditions in order to retain their nutritional quality and avoid contamination and deterioration, while medicinal herbs should be dried to protect their phytochemical efficiency [10]
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