Free radical scavenging and reducing power tef phenolic extracts

DPPH radical scavenging capacity values of soluble and bound extracts of the seven studied tef varieties are given in Table 2.1.4. The highest and lowest DPPH value of soluble extracts was 6.5 and 3.0 µmol TE/g dm for Zezew and Boset varieties, respectively. There was a significant difference among DPPH values of the soluble extracts among the varieties (P<0.001). An IC50

DPPH value of 0.6, 0.8, 0.9 mg/mL was reported for unknown varieties of red, mixed (brown), and white tef varieties, respectively (Boka et al., 2013) while Forsido et al. (2013) revealed a 29 folds higher IC50 DPPH value (22.4 mg /mL) where tef variety and color was not described. The

soluble extracts of tef varieties showed higher capacity to react and quench DPPH radicals compared to soluble extracts of wheat varieties (1-2 µmol TE/g) (Leoncini et al., 2012), but are within the range of those reported for several rice varieties (1.4-9.0 µmol TE/g dm (Zhang et al., 2015).

Bound phenolic extracts of varieties Dega and Tsedey showed the highest and lowest DPPH radical scavenging capacity (136 and 21 µmol TE/g dm respectively). There was a significant difference of DPPH values among the bound phenolic extracts of tef varieties (P < 0.05), due to the difference in the composition of individual PCs and their extent of reacting to the DPPH free radical assay among the varieties. DPPH radical scavenging capacity values of the bound phenolic extracts are very high as compared to varieties of whole wheat (6-8 µmole TE/g) (Leoncini et al., 2012) and rice (1.7-2.3 µmole TE/g) (Zhang et al., 2015).

Mean value of DPPH free radical scavenging capacity of bound phenolics of the varieties was 17.5 folds higher than that of soluble phenolics and contributed 94.6% to the total values of DPPH free radical quenching potential. In agreement to this result, Liyana-Pathirana and Shahidi (2006) revealed that the values of DPPH free radical scavenging capacity of bound phenolic extracts of white flour, whole flour and bran fractions of hard and soft wheat varieties contributed 63-87% to the total DPPH free radical scavenging capacity.

Similarly, Adom and Liu (2002) also disclosed that bound phenolics contributed 90% in wheat, 87% in corn, 71% in rice, and 58% in oats to the total antioxidant capacity assay. The DPPH free radical scavenging capacity of total (soluble + bound) extracts of tef varieties ranged from 25-142 µmol TE/g dm and decreased significantly in the order of: Dega > Sidam > Boset > Zezew > Zagurey > Quncho > Tsedey (P<0.001).

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The FRAP of soluble and bound extracts of tef varieties are given Table 2.1.4. FRAP of soluble phenolics ranged from 6-16 µmol Fe2+_{/g dm while those of the bound phenolics ranged from 36-}

63 µmol Fe2+_{/g dm and significantly decreased in the order of: Zezew > Zagurey > Dega}

> Boset > Quncho > Simada > Tsedey. The FRAP of the bound phenolics was 5 times higher compared to the one obtained for the soluble phenolics. Soluble FRAP of 0.02 µmole TE/g was reported (Forsido et al., 2013) for unknown tef variety but it is difficult to compare with our results as the standards used are different.

Literature on FRAP of tef is lacking, but the result of the present study were higher compared to soluble and bound FRAP of whole wheat varieties (1.6-3.4) and (9.5-11) µmole Fe2+_/g),

respectively but similar in that bound phenolic extracts of the whole wheat varieties contributed >80% to the total FRAP (Leoncini et al., 2012). Based on DPPH and FRAP results, and due to the fact that the consumption mode of tef is as whole meal, it could be suggested that tef is a better source of antioxidants compared to the widely used conventionally milled hard and soft white wheat flours and rice. The brown tef varieties contained higher TPC, TFC and FRAP compared to the white varieties though this trend was not reflected in the case of DPPH and the individual phenolic content. Zezew variety which is deep brown in color was found to contain the highest TPC, TFC, and FRAP contents followed by Zagurey which is light brown. This result was in agreement with work of Zhang et al. (2015), who revealed deep black rice contained the highest TPC and TFC than their counterpart light purple and white varieties.

Table 2.1.4 DPPH (µmol TE/g dm) and FRAP (µmol Fe2+_{/g dm) capacities of tef varieties}

Variety DPPH FRAP

Soluble Bound Total Soluble Bound Total

Boset 2.88±0.04a* _99.9±0.3de _103±1de _7.03±0.07ab _47.8±1.2b _54.8±1.3b Dega 5.73±0.06d _136±3f _142±3f _11.5±0.5c _50.2±2.6b _61.7±2.8c Quncho 3.65±0.08bc _58.7±2.3b _62.4±2.4b _7.08±0.28ab _39.5±2.5a _46.6±2.7a Simada 3.93±0.05c _106±5e _110±5e _7.37±0.06b _38.7±1.1a _46.1±0.9a Tsedey 3.47±0.03b _21.1±0.4a _25.2±1.5a _6.22±0.15a _36.0±0.4a _42.2±0.5a Zagurey 5.35±0.15d _87.8±5.1c _93.2±5.2c _12.3±0.5c _56.7±1.5c _69.0±1.9d Zezew 6.49±0.17e _93.7±1.4cd _100±1cd _15.7±0.5d _63.4±1.7d _79.1±1.2e p-value <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

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The higher phenolic content and antioxidant capacity of brown colored varieties can be attributed to their seed color which in turn is affected by the higher content of anthocyanins. Anthocyanins are water soluble pigments that contribute to the purple, brown, black and red colors and they are the major component of flavonoid in cereals (Dykes and Rooney, 2006). The reason why the higher phenolic content is not reflected in the individual phenolic acids and flavonoids from HPLC is not clear. The sum of the phenolic acids and flavonoids content from HPLC is very low compared to the results from spectrophotometer. This could explain that there are other abundant individual phenolics but not determined in this study due to time and standard constraints. Therefore, it is difficult to make comparison between these two results without having the full profile of all the individual phenolics.