Comparing the antioxidant capacity of various proteins using the oxygen radical absorbance capacity (ORAC) assay

The oxygen radical absorbance capacity (ORAC) assay is a widely used method for assessing antioxidant potential of foods, food ingredients and biological samples. Various protein sources have been shown to possess differences in antioxidant capacity. Soy protein's antioxidant properties may be derived from endogenous peptides as well as its associated isoflavones. To investigate this further, we employed the ORAC assay to assess the antioxidant capacity of the aqueous soluble fraction of various proteins. Soy protein isolates (SPI), milk whey protein (WP), sodium caseinate (CAS), and soy‐dairy protein blend powders were solubilized in phosphate‐buffered saline (PBS) and the soluble fractions (250 μg/mL protein) were tested for their ability to inhibit the oxidation of fluorescein in a standard ORAC assay using fatty acid free BSA (FAF‐BSA) as a reference control. FAF‐BSA had the lowest ORAC value (~ 3000 uM Trolox equiv/mg protein). Milk whey protein (WP) had similar radical scavenging activity to that of FAF‐BSA (~ 2800 uM Trolox equiv/mg protein). Several SPI preparations had significantly higher ORAC values compared to CAS (SPI was 50–80% higher), while other SPI preparations had ORAC values similar to CAS (~ 8400 uM Trolox equivalents/mg protein) and both were significantly higher than the ORAC values for WP (p<0.05). A protein blend of SPI, WP, and CAS (25%:25%:50%, respectively) was similar to CAS and significantly lower than SPI but significantly higher the WP alone (p<0.05). In summary, SPI, on average, has a significantly higher antioxidant capacity compared to dairy CAS and WP as determined by the ORAC assay. CAS has intermediate ORAC values which are significantly higher than those of WP. The addition of SPI in protein blends increases their antioxidant properties and may enhance their potential health benefits. Solae, LLC funded research.