Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 2, 2010
Publication Date: April 21, 2010
Citation: Lester, G.E., Hallman, G.J., Perez, J.A. 2010. Gamma irradiation dose: Effects on spinach baby-leaf ascorbic acid, carotenoids, folate, alpha-tocopherol, and phylloquinone concentrations. Journal of Agricultural and Food Chemistry. 58:4901-4906. Interpretive Summary: Of the eight spinach-leaf phytonutrients (vitamins) assayed, on either a fresh- or dry-weight basis, in this study four (alpha-tocopherol, folate, neoxanthin and phylloquinone) relatively minor compounds exhibited, depending on cultivar, little or no change in concentration with increasing doses (0.0, 0.5, 1.0, 1.5, 2.0 kGy) of irradiation. AsA, on both a fresh- and dry-weight basis, and lutein/ zeaxanthin, violaxanthin and beta-carotene, on a fresh-weight basis, were affected only at 2.0 kGy, and depending on cultivar, at 0.5 and again at 1.5 to 2.0 kGys. Of the three most concentrated phytonutrients (AsA, lutein/zeaxanthin and beta-carotene), AsA was 8-fold and 12-fold more concentrated than lutein/zeaxanthin and beta-carotene respectively, and was the most impacted following increased doses of irradiation. Dihydro AsA, an indicator of stress, coincidently increased as free AsA decreased. The principal role of free AsA in photosynthetic tissue is as an antioxidant, neutralizing reactive oxygen species (ROS) particularly the most hazardous: hydrogen peroxide which is highly generated by ionizing irradiation in hydrated systems e.g. plants. Ascorbic acid, the only compound in our study to have up to a 42% loss in concentration following irradiation, may have been highly affected by ROS likely generated by irradiation. Variation in spinach cultivars’ vitamin sensitivity to irradiation was observed, and variation in ROS scavenging ability is known. It is, therefore, likely the generation of ROS following irradiation, may be more of an impact factor on AsA, beta-carotene, and lutein/zeaxanthin concentrations, than are direct doses of irradiation.
Technical Abstract: Ionizing radiation of fruits and vegetables, in the form of gamma rays or electron beams, is effective in overcoming quarantine barriers in trade, decontamination, disinfestation and prolonging shelf life, but a void of information persists on ionizing radiation effects of vitamin profiles in individual foods. Baby-leaf sized spinach from commercial cultivars flat-leafed ‘Lazio’ and crinkled-leaf ‘Samish’ were grown, harvested, and surface sanitized according to industry practices. Baby-leaf spinach of each cultivar was packaged under air or N(2) atmosphere, representing industry practices, and then exposed to cesium-137 gamma radiation at 0.0, 0.5, 1.0, 1.5, or 2.0 kGy. Following irradiation, leaf tissues were assayed for vitamin [C, E, K, B(9)], and carotenoid (lutein/zeaxanthin, neoxanthin, violoxanthin, and beta-carotene) concentrations. Atmospheres and interaction of atmospheres by irradiation had little effect. Four phytonutrients (vitamins B(9), E, K and neoxanthin) exhibited little or no change in concentration with increasing doses of irradiation. However, total ascorbic acid (vitamin C), free ascorbic acid, lutein/zeaxanthin, violaxanthin and beta-carotene all were significantly reduced at 2.0 kGy, and also, depending on cultivar, at 0.5 and at 1.5 kGy. Dihydro ascorbic acid, an indicator of stress – likely irradiation generated oxidative radicals – in general increased with increasing irradiation doses > 0.5 kGy.