Correlation between the Bioavailability of Magnesium, Other Minerals and Oxalic Acid in Spinach

Abstract

Spinach was evaluated for the bioavailability of important dietary components. The contents of the analyzed substances in spinach were, for minerals (mg/100 g : Mg, 76; Ca, 67; Fe, 2.0; Zn, 0.9), oxalic acid (mg/100 g : total oxalic acid, 843; water-soluble oxalic acid, 459; water-insoluble oxalic acid, 384) and dietary fiber (mg/100 g : NDF, 748; ADF, 636; cellulose, 533; hemicellulose, 112; pectin, 307). The highest elution ratios of the minerals, oxalic acid and pectin in the several fractions during pectin extraction were 74% of magnesium, 39% of iron and 71% of oxalic acid in the aqueous fraction, 39% of zinc in the sodium hexametaphosphate fraction, and 67% of calcium and 75% of pectin in the HCl fraction. It is suggested that the greater part of calcium in spinach did not exist as Ca-oxalate. When the magnesium and/or calcium solution and the oxalic acid solution were mixed, the ratios of insoluble magnesium and calcium were about 20% and about 80% at the physiological pH level, respectively. The binding capacity to oxalic acid increased in accordance with rising pH value, the binding capacity for calcium being 3 to 5 times higher than that for magnesium. When both magnesium and calcium were present in the same solution, the binding capacity for magnesium and calcium to oxalic acid decreased by from 4 to 14% and from 1 to 7%, respectively. When spinach was boiled in 10 volumes of distilled and deionized water or in a 1% NaCl solution for 3 min, the elution ratio of the minerals from spinach was 77% of magnesium, 4.5% of calcium, 40% of iron and 41% of zinc with distilled and deionized water. Although there was no difference in the elution ratio of calcium, iron and zinc between water and the 1% salt solution, the elution ratio of magnesium decreased by about 10% with the 1% NaCl solution. When spinach was boiled in 5 or 10 volumes of distilled and deionized water for 1, 2, 3 or 4 min, the elution ratio of magnesium, calcium, iron, zinc and oxalic acid from spinach was from 40 to 84%, from 2 to 3%, from 26 to 51%, from 23 to 49% and from 40 to 82%, respectively.