Effects of drying methods and solvent types on antioxidant properties and anti-inflammatory potential of two ginger varieties in Malaysia

Ginger (Zingiber officinale) is one the finest medicinal herbs in the world with more than 50% of the world production coming from Asia. In Malaysia, ginger ranks among top ten potential herbs based on its remarkable medicinal properties. However, most usage of ginger from its fresh form which cause huge of postharvest loss. The objective of this study was to systematically evaluate the antioxidant properties and anti-inflammatory activity of Malaysian gingers, Halia Bentong (Zingiber officinale Roscoe) and Halia Bara (Zingiber officinale var. Rubra) as affected by drying methods of shade-, sun-, oven-, vacuum-, and freeze-drying with different extraction solvents of hotwater, 80% aqueous ethanol and ethanol. The changes in dried ginger extracts were analyzed for proximate analysis, phytochemical contents and antioxidant activities using FRAP, TAA by phosphomolybdenum, ability to scavenge ABTS•+, DPPH• and OH• radicals. Chelation on ferrous ion was then evaluated for metal binding capacity. Anti-inflammatory activity was done using cell culture method against inhibition of NO production in LPS-stimulated RAW 264.7 cells. The results show that drying methods have helped to reduce the moisture content of ginger whilst preserving the ginger’s nutritional value, phytochemical contents as well as their antioxidant properties. Phenolic contents of dried ginger, with the highest value of 20.91 mg GAE/g extract for Halia Bentong and 20.07 mg GAE/g extract for Halia Bara were obtained in freeze-dried from ethanol extract and the lowest in fresh ginger with 8.03 mg GAE/g extract and 10.53 mg GAE/g extract, respectively. Flavonoid contents also were highest in dried gingers, which were ranged from 42.98 to 67.82 g RE/100g extract for Halia Bentong and 47.32 to 69.32 g RE/100g extract for Halia Bara when compared to fresh ginger. Meanwhile, ascorbic acid in fresh and dried gingers were less than 1.5 mg AA/g extract, which indicate that both gingers contain less amount of ascorbic acid. Shade-dried of Halia Bentong and sun-dried of Halia Bara from ethanol extracts gave the highest total antioxidant activity of ginger which were 75.23 g AA/100g extract and 81.12 g AA/100g extract, respectively. Sun-dried from ethanol extracts had the highest antioxidant activity as exhibited by FRAP and ABTS•+ scavenging activity. The activity was increased by 15.35-fold and 3.5-fold for dried Halia Bentong and 3.95-fold and 2.07-fold for dried Halia Bara compared to its fresh form. However, different methods of drying did not affect the activity of inhibition of DPPH•, OH• as well as the ascorbic acid contents. Extraction solvent has a significant influence (p < 0.05) on the extraction of antioxidant compounds from ginger. Ethanol extracts of ginger gave the highest level of antioxidant activities as exhibited by phytochemical contents, FRAP, total antioxidant activity (TAA), ABTS•+ scavenging and IC50 values, while hotwater extracts showed the least potent solvent extraction. However, hotwater was found as the most effective solvent in giving the strongest of ferrous ion chelating power with the ranged of 69.45 to 145.47 mg EDTA/g extract for Halia Bentong and 60.73 to 159.17 mg EDTA/g extract for Halia Bara. The extractive yield indicates that aqueous ethanol of 80% which ranged from 2.39 to 17.79% for Halia Bentong and 3.57 to 15.21% for Halia Bara, gave the maximum yield amongst the other extracts. Results suggested that not only the polarity of the solvent affected the effectiveness of extraction, the chemical structure or nature of the solvent also lead to different antioxidant capability. The enhanced phytochemicals and antioxidant activities of dried ginger were also supported by anti-inflammatory studies against inhibition of NO production, which is known as inflammation mediator. Dried ginger from ethanol extracts were recorded as a potent inhibitor of NO at the concentrations 100 μg/mL, while fresh ginger was less potent. At the concentration of 100 μg/mL extracts, NO-inhibitory activity of dried ginger had caused inhibition with ranged from 40.85 to 50.88% for Halia Bentong and 29.16 to 45.13% for Halia Bara. While inhibitory activity of fresh ginger on NO production was 8.65% and 18.23%, respectively. Drying has demonstrated an improved post-harvest treatment of gingers’ bioactivities in regard to enhanced final ginger quality.