1995 Volume 46 Issue 1 Pages 3-9
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.