3.4. ABTSþ scavenging activity of rice protein hydrolysates
The cation radical ABTSþ is reactive to most antioxidants and is frequently used to measure antioxidant activity of food and agricultural products. We included this test since our ORAC and DPPH scavenging results are inconsistent, which makes it difficult to select superior antioxidant hydrolysate fractions for later evaluation on meat lipid oxidation. As shown in Fig. 3, all hydrolysate fractions showed significant ABTSþ scavenging activity ranging between 10.8 and 28.3 mmol TE/g dry weight with most samples falling in the range of 15e21 mmol TE/g. The ABTSþ scavenging activity of the hydrolysates are more consistent with their ORAC values though there was no significant correlation between the results of two tests. The 1e3 kDa fractions exerted the highest radical scavenging activities in the same protease group. Similar to the ORAC results, AP-F3 (28.3 mmol TE/g) and Val-F3 (24.8 mmol TE/g) had the strongest ABTSþ scavenging activity among all the fractions, indicating these two fractions are more effective in scavenging free radicals than others. It appeared that ABTSþ scavenging activity of the hydrolysates are more consistent with their ORAC values than DPPH scavenging activity though no significant correctionwas detected. This may be because both ORAC and ABTS assays are hydrophilic than DPPH system. Although APF3 and Val-F3 are not the fractions with the strongest DPPH scavenging results, their activity was only moderately lower than that of the most effective fraction. Therefore, based on all three test tube antioxidant assessments of the hydrolysate fractions, AP-F3 and Val-F3 were selected for further evaluation on meat lipid oxidation.
3.5. Lipid peroxidation in ground beef
Protein hydrolysates and specific food protein-derived peptides have been shown to effectively inhibit lipid peroxidation in meat products (Kong & Xiong, 2006), suggesting that food proteins could be utilized to develop specific hydrolysates/peptides as natural