## 3 Results and Discussion
### Antioxidative Activities  
Antioxidative activities by different assays (DPPH, ABTS, and FRAP) of the whey protein modified with phenolic compounds were shown in Figure 6. The DPPH value of the control whey protein was 41.7 umol FAE/mg protein (Figure 6a). These values of the whey protein increased after modification with oxidized phenolic compounds at concentrations of 0.5-5.0% as follows: 4.2-5.0 fold for OGA, 3.3-4.5 fold for OFA, and 6.2-12.0 fold for OTA when compared to the control whey protein. This result showed that the phenolic compounds were incorporated into the whey protein and contributed to the increase in the antioxidative activity of the modified whey protein. The capacity antioxidants to scavenge DPPH radicals was assumed to be related to their hydrogen-donating properties [15]. An increase of DPPH radical scavenging activity was found in all modified whey proteins, especially when higher levels of oxidized phenolic compounds were used (\(p<0.05\)). The highest activity was observed in whey protein modified with 5% oxidized phenolic compound (\(p<0.05\)). This result is in agreement with several studies that have reported that protein-polyphenol complexes have a greater antioxidative activity than the original proteins [29; 30; 31]. [PERSON] [2] revealed that the antioxidative activity of whey protein increased after modification by covalent attachment of chlorogenic acid, rosmarinic acid, and quercetin. At the same concentrations used, the highest activities of all antioxidative parameters were found in the whey protein modified with OTA (\(p<0.05\)), followed by the whey protein modified by OGA and OFA. These results were in agreement with an increase of the total phenolic content for each whey protein modified with different phenolic compounds. This revealed that the antioxidative activity of whey protein modified with phenolic compounds is dependent on the type and concentration of phenolic compounds. The whey protein modified with tannic acid possessed a greater number of phenolic groups and had higher antioxidative activity when compared with the other samples.  
For ABTS assay (Figure 6b), similar results were found when compared with DPPH. ABTS of the control whey protein was 3.26% inhibition, and increased by about 5.5-25.3 fold for OGA, 2.9-6.0 fold for OFA, and 10.0-37.3 fold for OTA after modification with concentrations of 0.5-5.0%. ABTS radical scavenging activity of whey proteins modified with OTA and OFA was higher than whey protein modified with OFA. ABTS assay is used to determine antioxidative activity, in which free radicals are quenched to form ABTS-radical complexes [16]. It indicated that the whey protein modified with OTA and OFA could quench the free radicals and end the radical chain reaction more effectively than the whey protein modified with OFA. Ferric-reducing antioxidant activity (FRAP) of the whey protein after modification with 0.5-5.0% of oxidized phenolic compounds (Figure 6c) was increased by about 8.5-20.4 fold for OGA, 5.7-9.0 fold for OFA, and 9.2-32.0 fold for OTA. FRAP of all modified whey protein was higher than that of unmodified whey protein (123 umol FE/100 g sample). FRAP assay is used to measure the reducing capacity of ferric ion, and is related to radical scavenging capacity [32]. However, differences in the value of each assay were observed, resulting in different mechanical antioxidative activity, but the antioxidative activity result of each assay showed a similar pattern. This reconfirmed that the phenolic compounds introduced to the whey protein contributed to an increase in the antioxidative activities of the whey protein. The maximum activities of all antioxidative parameters were found in the whey protein modified with OTA (\(p<0.05\)), followed by the whey proteins modified by OGA and OFA, respectively.