Reducing power assay is used to determine the ability of an antioxidant to donate an electron Yildirim A. et al. (2000). Unlikely most of the other in vitro methods, in this method increase in absorbance is correlated to the reducing capability of the extract. So the best antioxidant will give maximum absorbance value. The basic principle of reducing power assay lies in the capacity of extract having antioxidant potential to react with potassium ferricyanide to form potassium ferrocyanide. The latter then react with ferric trichloride forming a blue coloured complex i.e. ferric ferrocyanide with a maximum absorbtion at 700 nm.
In present study Reducing Power Assay was performed on methanolic, ethanolic and aqueous extracts of both green and black grapes. Extracts were taken in concentration range of 20 µg/ml to 100 µg/ml and in triplets. Ascorbic acid was taken as standard.
In present investigation, absorbance of Methanol Green Grape’s extract ranged from 1.747 to 2.669 nm and absorbance of Methanol Black Grape’s extract ranged from 1.551 to 2.011 nm. Similarly absorbance of Ethanol Green Grape’s extract ranged from 0.542 to 1.146 nm and absorbance of Ethanol Black Grape’s extract ranged from 0.303 to 1.095 nm. Likewise, absorbance of Aqueous Green Grape’s extract ranged from 0.035 to 0.884 nm and absorbance of Aqueous Black Grape’s extract ranged from 0.040 to 0.673 nm. Absorbance of standard i.e. ascorbic acid ranged from 2.628 to 3.180 nm respectively. The results showed that the highest antioxidant properties were present in Methanol Green Grape’s extract as compared to rest of all test extracts.
In a similar research done on grape seed and skin extracts by Nirmala J.G. and Narendhirakannan R.T. (2011) the reducing power increases with increase in concentration.
Similarly Shashidhara T. et al. (2013) documented that methanolic and aqueous extracts of Vitis vinifera L. show strong reducing power activity.
Campos M.S. et al. (2015) have obtained similar reducing power capacity with wild fruit Coccoloba uvifera also known as ‘Sea Grapes’ which exhibited higher absorbance even at low concentrations acting as reducing agents.
The reducing properties are usually related to the presence of reductones. Antioxidant activity of reductones is based upon breaking the free-radical chain by donating a hydrogen atom as documented by Gordon (1990). Reductones react with selected peroxide precursors to prevent peroxide formation.
Present results correlate with the study done by Yesiloglu Y. and Gulen S. (2016) on Vitis vinifera L. leaves extracts.
Since antioxidant activity cannot be fairly determined by just one method, we used three well known methods to find out best antioxidant among methanolic green and black grape extract, ethanolic green and black grape extract and aqueous green and black grape extract. Methanolic green grape extract proved as the best antioxidant followed by methanolic black grape extract while aqueous black grape extract proved as least antioxidant in all three methods.
As discussed earlier methanol is good solvent with the ability to dissolve many bioactive compounds. Though it is polar in nature yet it also allows many non-polar substances to dissolve in it. Besides this green grapes consists flavonols which are considered as a good antioxidant Cantos E. et al. (2002). Reducing power of green grapes is also be due to its hydrogen donating ability. Black grape is also a reservoir of many important bioactive compounds including anthocyanins (a type of flavonoid) and resveratrol which is responsible for its good antioxidant activity Yang J. et al. (2009).