Spinach is one of the most desirable dark green, leafy vegetables as a result of its high content of beta carotene (provitamin A), lutein, folate, vitamin C, calcium, iron, phosphorous, and potassium (Morelock and Correll, 2008). Spinach is currently grown on ≈25,000 ha in the United States annually for both fresh and processed markets with a crop value of $190 million (National Agricultural Statistics Service, USDA, 2006).
In addition to its high nutrient content, spinach is also known to have a greater amount of oxalic acid than most crops. Oxalic acid is an organic acid present in fungi, algae, lichens, higher plants, and animals, including humans (Oke, 1969) formed as a secondary metabolite of vitamin C (Hodgkinson, 1977). Oxalic acid exists as free acid and mineral crystals that include soluble salts of potassium, sodium, and zinc and insoluble salts of calcium, magnesium, and iron. Insoluble oxalate crystals formed in the gut are not absorbed and are carried out with the feces, thus reducing the bioavailability and absorption of calcium and iron in diets (Bataille and Fournier, 2001). Human urine always contains small levels of calcium oxalate (Oke, 1969) that may be deposited in the kidneys of certain people as a common form of kidney stones (Massey et al., 1993). In healthy adults, with U.S.- or European-type diets, 90% of oxalic acid excreted in the urine comes from endogenous metabolic synthesis (Massey et al., 1993). However, reduction of foods with high levels of oxalic acid is recommended as a dietary change for kidney stone-formers.
Besides spinach, other vegetables with a high level of oxalic acid include amaranth, cassava, chives, parsley, and purslane (USDA, 1984). Calcium oxalate is the form principally found in spinach (Kitchen et al., 1964). Oxalic acid oxidase catalyzes the breakdown of oxalic acid into carbon dioxide and hydrogen peroxide. Although the enzyme is present in spinach leaves, it is usually inactive (Oke, 1969). Studies on beet root extracts showed that the sole inhibiting factor was nitrate, and a low concentration of nitrate was sufficient to inhibit the enzyme (Oke, 1969). Palaniswamy et al. (2004) found that the oxalic acid concentrations in purslane leaves were reduced with increasing ammonium to nitrate ratios in hydroponic solutions. In soil, however, nitrifying microorganisms are very abundant and convert the ammonium first into nitrite and then into nitrate.
Kaminishi and Kita (2006) found that spinach oxalate concentration on a fresh weight basis was the lowest in the fall, followed by the summer and the spring, and was the highest in the winter. Some investigators found oxalate increased with plant age (Hodgkinson, 1977), whereas others reported spinach oxalate concentration decreased with age (Okutani and Sugiyama, 1994). Okutani and Sugiyama (1994) also found that the higher the leaf position from the plant base, the lower the oxalate concentration in three spinach cultivars. Oxalic acid contents were lower in stems and petioles than in leaf blades (Elia et al., 1998; Maynard et al., 1976; Pandey and Kalloo, 1993).
Genetic variation in spinach oxalate content has been reported. Kitchen et al. (1964) found significant differences among 39 breeding lines, hybrids, and F2 populations in the amount of anhydrous oxalic acid present. In a study of 182 open-pollinated and F1 hybrid cultivars and breeding lines available in Japan, Kaminishi and Kita (2006) found that fast-growing cultivars contained higher nitrate and lower oxalate, whereas slow-growing cultivars had lower nitrate and higher oxalate concentrations. Conversely, Kohman (1939) observed no significant differences in the oxalate content in 53 commercial and experimental varieties, averaging 9.0% anhydrous oxalic acid on a dry weight basis. Anhydrous oxalic acid levels as high as 15% of the dry weight have been reported in spinach (Moir, 1953).
Reducing the oxalic acid concentration would increase the nutritional value and consumer acceptance of spinach. A breeding program for improvement must start from germplasm evaluation to identify sources of low oxalate content. This experiment was conducted to screen the USDA spinach germplasm collection for oxalate concentration and to study the possible association of oxalate content with other traits.
Anonymous 2008a Sun rise and sun set in Salinas 19 May 2008 <http://www.timeanddate.com/worldclock/astronomy.html?n=883>.
Kaminishi, A. & Kita, N. 2006 Seasonal change of nitrate and oxalate concentration in relation to the growth rate of spinach cultivars HortScience 41 1589 1595
Kitchen, J.W., Burns, E.E. & Perry, B.A. 1964 Calcium oxalate content of spinach (Spinacia oleracea L.) Proc. Amer. Soc. Hort. Sci. 84 441 445
Massey, L.K., Roman-Smith, H. & Sutton, R.A.L. 1993 Effect of dietary oxalate and calcium on urinary oxalate and risk of formation of calcium oxalate kidney stones J. Amer. Diet. Assn. 93 901 906
Morelock, T.E. & Correll, J.C. 2008 Spinach 189 218 Prohens J. & Nuez F. Handbook of plant breeding, Volume 1, Vegetables I, Asteraceae, Brassicaceae, Chenopodicaceae, and Cucurbitaceae Springer New York, NY
National Agricultural Statistics Service, USDA 2006 Spinach national statistics 26 June 2007 <http://www.nass.usda.gov/QuickStats/index2.jsp>.
Okutani, I. & Sugiyama, N. 1994 Relationship between oxalate concentration and leaf position in various spinach cultivars HortScience 29 1019 1021
Palaniswamy, U.R., Bible, B.B. & McAvoy, R.J. 2004 Oxalic acid concentrations in purslane (Portulaca oleraceae L.) is altered by the stage of harvest and the nitrate to ammonium ratios in hydroponics Sci. Hort. 102 267 275
Pandey, S.C. & Kalloo, G. 1993 23: Spinach Spinacia oleracea L 325 336 Kalloo G. & Bergh B.O. Genetic improvement of vegetable crops Pergamon Press Oxford, UK
USDA. 1984. Oxalic acid content of selected vegetables. In: Agriculture handbook, No. 8-11, Vegetables and vegetable products. 15 Jan. 2008. <http://www.nal.usda.gov/fnic/foodcomp/Data/Other/oxalic.html>.