Total Phenolic Content
The total phenolic contents were determined by plotting standard calibration curve of different concentration of gallic acid using spectrophotometer at 760 nm. The values of TPC were calculated as gallic acid equivalents (GAE) per 100 gram of dry weight.
The amount of total phenolic content in ginger samples collected from Ayikel and Mandura were influenced significantly by extracting solvent (p < 0.05), and the contents were varied within the range of 690.152 to 1183.813 mg of GAE/100 g of dry weight for acetone and methanol, respectively. Among the solvents, methanol was the most efficient extracting solvent for TPC, followed by ethanol, ethyl acetate and acetone, indicating that the TPC extracted in ginger were higher in polar solvents compared with less polar solvents (Table 1). The variations in the extract yields from ginger using different solvents might be explained by the difference in polarity of different compounds in the samples [20, 21].
Between the two study areas, the higher TPC were found in a ginger sample collected from Ayikel in all extracts. The difference in the quantity of TPC may be attributable to different intrinsic and extrinsic factors, including cultivars, type of soil and growing conditions, maturity state and harvest conditions [22, 23].
Antioxidant Activity
Antioxidant activity of ginger extract was evaluated using ascorbic acid as standard. It is one of the greatest antioxidant compound known by scavenging the stable radical of 1, 1-Diphenyl-2-picryl-hydrazyl (DPPH).
The antioxidant values obtained are presented in Table 2. From the data it is evident the Ayikel ginger samples showed higher value of DPPH (% inhibition) and TPC as compared to Mandura. The value for DPPH (% inhibition) activity of the extracts can be ranked as: methanol extract > ethanol extract > ethyl acetate extract > acetone extract. In addition, the current finding revealed that antioxidant activity was significantly correlated with the phenolic content. Except ethanol and ethyl acetate extracts, there were significant differences (p<0.05) of DPPH radical scavenging abilities between all extracts. As with TPC value, it was observed that methanol extract owned the highest DPPH radical scavenging ability, followed by ethanol, ethyl acetate and acetone. The radical scavenging activities of ginger were close to the positive control, i.e., ascorbic acid with % inhibition of 89.75±0.361).
The result was in agreement with those findings reported in literature, where the phenolic content and antioxidant activity were influenced by extracting solvents. In addition, the current finding revealed that highest DPPH radical scavenging activities of ginger extract was obtained when methanol was used for extracting solvent [1, 24, 25].
The half maximal inhibitory concentration (IC50) is defined as the amount of antioxidant that causes decrease the DPPH concentration by 50%, [26]. IC50 value was calculated from the linear regression plots of percentage inhibition (% DPPH scavenging activity) against concentration of ginger extracts. As depicted in Table 3, the IC50 values of Ayikel were ranged from 0.481 to 0.654 mg/mL for methanol and acetone extracts, respectively. Similarly, it was found that acetone extracts owned the highest IC50 value followed by ethyl acetate, ethanol and methanol extracts in Mandura ginger. This implies that the concentration of acetone extract required decreasing the initial concentration of DPPH solution by 50% is 0.654 mg/mL, whereas for methanol extract is 0.481 mg/mL. The result showed that IC50 value is inversely related to its antioxidant capacity.
It was elucidated that the methanol extracts showed highest antioxidant activities than the other solvents. However, the IC50 values with regards to different solvents used for extraction was as follows: acetone > ethyl acetate > ethanol > methanol. The results are similar to that reported by [17, 27], where by a lowest DPPH radical-scavenging activity of a plant extract had the highest IC50.
In our study ascorbic acid was used as the positive control; with an IC50 value estimated at 0.239 mg/mL while the IC50 values of the ginger extracts ranged from 0.481 to 0.812 mg/mL. This indicates that the extracts are slightly potent inhibitors in comparison with ascorbic acid.
Comparison of current study with results from other countries
There are some reports from different countries on the analysis of the phenolic contents and antioxidants activities of ginger. It is important to compare the results obtained in this study with the values reported in other countries. This comparison helps to identify the differences in composition of samples between countries.
As shown in Table 4, the total phenol contents of ginger extract obtained in this study is higher than that of the results reported by Sharif and Bennett [28] and Adel and Prakash [29].
However, methanol extract reported by Ghorab et al. [30] was found to be higher than the results of this study. The total polyphenol of methanol extract was found to be comparable with the results reported by Mohd and Muhd [31]. Besides, total phenol content of acetone and ethyl acetate extract reported by Mohd and Muhd [31] and Ghasemzadeh et al. [32], respectively were found to be higher than the results obtained in this study at both study sites.
The antioxidant properties of ginger were also compared with the reports from other countries. As shown in Table 5, the antioxidant activities were found to be slightly higher than reported by Ghasemzadeh et al, [1, 32], Mohd and Muhd, [31]. However, the results of present study were in agreement with the reported values by Ghorab et al. [30] and Sharif and Bennett [28]. The differences in the total phenol contents and antioxidant activity of this study with previously reported values were attributed to several factors such as the difference in plant variety, the method and conditions of extraction (temperature and time), environmental conditions, degree of ripeness, plant variety and sun exposure[17, 33, 34]. For instance, the ginger studied by [28] and [29] were extracted 8 h and 3 h respectively, while in study the ginger was extracted for 24 h.