Determination Of Antioxidant Capacity Of Diverse Fruits By Electron Spin Resonance (ESR) And UV-vis Spectrometries

Twenty-one kinds of fruits were measured for antioxidant capacity based on their ability to scavenge 1,1-diphenyl-2-picryl-hydrazyl(DPPH)free radical.Vitamin C equivalent antioxidant capacity(VCEAC)was used to quantify the antioxidant capacity of the studied fruits.Each fruit was divided into two parts: fresh part(i.e.fresh fruit analyzed immediately)and frozen part(i.e.fruit frozen and pulverized in liquid nitrogen).To measure the antioxidant capacity of the fruits,two methods including UV-vis spectrophotometry(UV-vis method)and electron spin resonance spectroscopy(ESR method)were applied.When UV-vis was applied,the concentration of DPPH working solution was 0.10 mmol/L.For the fresh fruit samples,the VCEAC values range from 11.19 to 500.85 mg/100 g.Mulberry has the highest antioxidant capacity and white dragon fruit has the lowest antioxidant capacity.For the frozen fruit samples,the VCEAC values range from 11.94 to 421.82 mg/100 g.Mulberry has the highest antioxidant capacity and sapodilla has the lowest antioxidant capacity.The VCEAC values obtained for the frozen fruit samples are highly correlated to those for the fresh fruit samples(r = 0.97).The experiment results showed that when the fruits were fast frozen and then pulverized into powder in liquid nitrogen,the total antioxidant capacity of the fruits was not affected.We have tried fast freezing four fruits(banana,green kiwi fruit,strawberry,and orange)in small slices in liquid nitrogen combined with thawing them at room temperature,in refrigerator at 4°C,or in microwave oven.However,compared with the fresh fruits,none of these three methods could satisfactorily maintain the antioxidant capacity.The VCEAC values,obtained by thawing the small slices at room temperature,in refrigerator at 4°C,and in microwave oven,declined 6 to 51%.The VCEAC values obtained by fast thawing the powder pulverized in liquid nitrogen only decreased 3-5%.Therefore only freezing in powder could lead to fast thawing thus keep original antioxidant capacity of fresh fruits.Thus fast freezing fruits in powder form could extend the storage life of the fruits.When the ESR method was applied,the UV-vis method was also applied for comparison.In the experiments,the frozen fruit samples and 0.50 mmol/L DPPH working solution were used.The antioxidant capacity were measured by both ESR and UV-vis methods.The VCEAC values obtained by the UV-vis method range from 11.48 to 345.75 mg/100 g,and those obtained by the ESR method range from 7.01 to 366.26 mg/100 g.The experimental results indicated that the VCEAC values obtained by the two methods were highly correlated.ESR spectroscopy is a method unique to free radical measurement and other substances have no effect on the signal.The absorption spectra of some antioxidant compounds,such as carotenoids,overlap with that of DPPH when measured by UV-vis method.Therefore,the measurement might be interfered.As we known,UV-vis method should determine uniform solution and is easily affected by the clarity and color of the sample solution.If the sample solution is turbid,the sample might not be measured by UV-vis method.As the sample solution's color of blueberry,black plum,and mulberry is similar to the color of DPPH solution,the measurement might be influenced.We found some advantages for the ESR method compared with the UV-vis method.First,when UV-vis spectrophototry was applied,the shape of the absorption curve is distorted when the concentration of the analyte is too high.Whereas in the ESR measurement,the spectrum is not affected when the analyte concentration is high.Second,in ESR spectroscopy,only the signal from DPPH was measured,and any turbidity,clarity,or color from sample itself will not affect the quantification.