Comparison of the estimates of the oxalate content of taro leaves and corms and a selection of Indian vegetables following hot water, hot acid and in vitro extraction methods

Total, soluble, gastric soluble and intestinal soluble oxalates were determined using three groups of foods. The total and soluble oxalate contents were extracted using hot acid (80 °C, 0.2 mol/l HCl) and hot water (80 °C). Gastric soluble and intestinal soluble oxalates were extracted using an in vitro method; the food samples were incubated for 2 h at 37 °C in gastric and intestinal juice. The extracted oxalates obtained from both methods were then determined by HPLC chromatography. The oxalate contents of taro leaves were determined in the raw leaves and after being baked at 150 °C for 1.5 h either alone or with additions of cows’ milk or coconut milk prior to baking. The oxalate contents of four different cultivars of taro corms were determined in the raw corms and after the corms had been either boiled or baked. The oxalate contents of a selection of green leafy Indian vegetables were also determined in the dried raw material. Overall, the total oxalate contents of the foods analysed in this study ranged from 209 mg oxalate/100 g dry matter (DM) for raw Indian radish leaves to 12,576 mg oxalate/100 g DM for raw Indian spinach leaves, while the soluble oxalate contents of the samples ranged from 143 mg soluble oxalate for raw taro corms to 11,900 mg soluble oxalate/100 g DM for raw Indian spinach leaves. Regression analysis of all the gastric soluble oxalate values against the total oxalate values showed that there was a good fit (R2 = 0.955). However, the gastric soluble oxalate values were, on average, 14.2 ± 3.7% lower than those for total oxalate obtained by hot acid extraction. Regression analysis of all the intestinal soluble oxalate contents against the soluble oxalate values showed a good fit between the values (R2 = 0.967). Overall, the intestinal soluble oxalate values were 10.0 ± 6.6% lower than those for soluble oxalate obtained by hot water extraction.