Effect of cooking on the oxalate content of selected Thai vegetables

Total and soluble oxalate content levels were measured in thirteen selected vegetables purchased from a local market in Surin Province in the northeast of Thailand. Total oxalate contents of the leaves, shoots and roots of the fresh vegetables ranged from 249.5 ± 12.1 to 7597.9 ± 77.6 mg oxalate/100g dry matter (DM) while soluble oxalate content ranged from 205.0 ± 2.3 to 2677.6 ± 19.0 mg oxalate/100g DM. Very high levels of total oxalates were found in three of the selected Thai vegetables, Polygonum odoratum (7597.9 ± 77.6 mg/100g DM), Piper aurantaucum (7026.6 ± 76.9 mg/100g DM) and Limnophila aromatica (6179.0 ± 23.6 mg/100g DM). However, the soluble oxalate content of L. aromatica was low and the highest soluble oxalate contents of fresh vegetables were found in P. odoratum, P. aurantuacum and Neptunia oleracea at 2677.6 ± 19.0, 2152.2 ± 65.3 and 1640.8 ± 3.4 mg/100g DM, respectively. Boiling the vegetables reduced the soluble oxalate content between 30.4 and 65.0%. The insoluble oxalate levels increased in eleven of the cooked vegetables while small decreases were observed in L. aromatica and N. oleracea