Oxalate contents of Thai rice paddy herbs (L. aromatica and L. geoffrayi) are affected by drying method and changes after cooking

Abstract

Food processing, including drying and cooking, generally influences the physical and chemical qualities of food products. The present study assessed the changes in oxalate contents of two rice paddy herbs (phak-kha-yaeng in Thai), Limnophila aromatica (phak-kha-yaeng khao) and Limnophila geoffrayi (phak-kha-yaeng daeng), as affected by different drying methods, namely hot-air, freeze and vacuum drying. The entire, soluble and insoluble oxalate contents of the cooked samples were also evaluated. Dried samples showed a significant increase in soluble oxalate content compared to fresh samples except for hot-air dried phak-kha-yaeng daeng, while entire and insoluble oxalate contents in dried samples were significantly decreased. Levels of soluble oxalates which were 30% in fresh phak-kha-yaeng khao and 46% in fresh phak-kha-yaeng daeng can be significantly reduced after cooking by boiling. In addition, the cooking of both the hot-air dried phak-kha-yaeng samples also showed greater losses in oxalates than those of other samples studied. Hot-air drying should be considered as a suitable drying method for phak-kha-yaeng with respect to reducing its oxalates. The present study has provided useful information for the drying of vegetables or plant foods to reduce the risk of consuming plant foods which contain high levels of oxalates.