ARS | Publication request: Alkylperoxyl Radical Scavenging Activity of Red Leaf Lettuce Phenolics

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 14, 2003
Publication Date: N/A

Interpretive Summary: Certain fruits and vegetables may provide specific health benefits by providing dietary antioxidants. The antioxidative activities of vegetables have been measured and used to provide dietary guidelines. However, certain vegetables, such as lettuce, had low levels of antioxidative compounds. In this report, the antioxidative activities of red leaf lettuce extracts and their components were reconsidered. The leaf lettuce contained compounds with high antioxidative activity. Since the concentrations of these compounds are particularly sensitive to environmental factors, growth and storage conditions may be developed to increase the nutritional quality of lettuce and other vegetables. This information will benefit food scientists, human nutritionists and plant physiologists by providing insight on the antioxidant properties of plant phytonutrients.

Technical Abstract: Although lettuce may contain relatively low levels of antioxidative phytochemicals which have been shown to contribute to human health, lettuce leaf extracts contained compounds with high specific peroxyl radical scavenging activities. After determining the extraction conditions that minimized phenolic oxidation and produced the highest oxygen radical absorbance capacity (ORAC) values, the phenolic compounds from red leaf lettuce were separated by reverse-phase high performance liquid chromatography (HPLC). The primary phenolic compounds in the leaf tissue extracts were mono- and dicaffeoyltartaric acid (CTA and DCTA), mono- and dicaffeoylquinic acid (CQA and DCQA), quercetin 3-malonylglucoside (QMG), quercetin 3-glucoside (QG), cyanidin 3-malonylglucoside (CMG) and an unknown phenolic ester (UPE). Significant levels of DCQA were only found after wounding. Using the new fluorescein-based ORAC assay procedures, fractions from the HPLC analyses were assayed for peroxyl radical absorbance capacity. The decreasing order of contribution to the total ORAC value of an extract from wounded tissue was QMG > DCQA > CMG > DCTA > UPE > CTA > QG > CTA. The decreasing order the specific peroxyl radical scavenging activities was CMG > QG > DCTA > DCQA > QMG > UPE > CQA > CTA. Since the concentrations of plant flavonoid and phenolic acid esters are sensitive to environmental factors, this information may be used to develop pre- and postharvest conditions which increase the dietary benefits of leaf lettuce.