Total and soluble oxalate contents in Thai vegetables, cereal grains and legume seeds and their changes after cooking

Abstract

This study was conducted to determine soluble and total oxalate contents in common vegetables, cereal grains and legume seeds and the effect of household cooking on these substances. Each food sample was randomly purchased from three main representative markets in metropolitan area of Bangkok, Thailand. Soluble and total oxalates were determined by high-performance liquid chromatography (HPLC). The limit of quantitation for the oxalates was 3 mg/100 g. All studied vegetables contained relatively small amounts of total oxalate (<100 mg/100 g), except chinese convolvulus ( Lpomoea reptans), acacia pennata ( Acacia pennata), and cultivated bamboo shoot ( Bambusa spp.), contained total oxalate more than 150 mg/100 g which can be significantly reduced after cooking by boiling. Among the legume seeds, soybeans ( Glycine max (L.) Merrill) and peanuts ( Arachis hypogaea L.) contained highest and moderate amounts of total oxalate, 204±14 mg and 142±35 mg/100 g, respectively. Rice contained negligible amount of total oxalate (<3 mg/100 g). There was significant reduction ( P < 0.05 ) in total oxalate due to cooking by boiling, percentage loss ranged form 18% in coconut heart top stems ( Cocos nucifera Linn.) to 76% in A. pennata. Similar findings appeared in soluble oxalate, significant loss ( P < 0.05 ) ranged from 30% in cooked white stems swamp morning glory ( Lpomoea aquatica, Forsk) to 83% in cooked cultivated bamboo shoot ( Bambusa spp.). Loss of oxalates in various foods is likely due to their leaching loss in cooking water.