The acridity of raphides from the edible aroids

The corms and leaves of most cultivars of the edible aroids, the major staple food of about 200 million people in the tropics, are acrid. This means that if eaten raw they cause swelling of the lips, mouth and throat. This effect is related to the presence of needle-like raphides of calcium oxalate. A new method was developed for the separation of raphides using petroleum ether (PE) and then purification by centrifugation. The acrid raphides were shown by X-ray powder diffraction to consist of calcium oxalate monohydrate (whewellite) and their morphology was studied by scanning electron microscopy. Two distinct types of raphides (thick and thin) were observed and druses (rosette like structures of calcium oxalate) were found only in taro leaves. A comparative survey of raphide morphology was made over four species of aroids. The removal of the acridity of raphides by immersion in hydroxylic solvents, but not in PE, indicates the presence of an irritant on the raphides. Acridity is due to the dual action of the sharp raphides in puncturing soft skin and the irritant (a protease) in causing swelling and soreness.