Oxalate Content of Raw and Cooked Oca (Oxalis tuberosa)

The Role of Organic Acids in the Domestication of Oxalis tuberosa: A New Model for Studying Domestication Resulting in Opposing Crop Phenotypes1

“…Despite the wide range of oxalate values, the published literature is consistent in the observation that only soluble oxalate is present in oca tubers in the samples used (Hermann and Erazo 2000;King 1988;Ross et al 1999;Sangketkit et al 2001). King (1988) and Hermann and Erazo (2000) both used light microscopy to search for the presence of calcium oxalate crystals and found none.…”

“…Ross et al (1999) report values of 92-213 mg/100 g FW and attribute the discrepancy with King's data to the unreliability of King's wet titration method as opposed to HPLC quantification and to the fact that King's samples were stored prior to extraction in ethanol, which is known to be an effective oxalate solvent. Sangketkit et al (2001) also found more than two-fold differences in oxalate Hermann and Erazo (2000) reported the highest values, finding 306-539 mg/100 g FW. Hermann and Erazo (2000) attribute their elevated oxalate values to the fact that these ocas were grown in greenhouses and treated with chemical fertilizers, which are known to increase oxalate levels.…”

“…King (1988) and Hermann and Erazo (2000) both used light microscopy to search for the presence of calcium oxalate crystals and found none. Ross et al (1999) and Sangketkit et al (2001) and their colleagues performed both hot 2 M HCl acid extractions for total oxalate content and water extractions for soluble oxalate content and found no significant difference (p<0.05) between the methods, suggesting that all oxalate in oca is of the water-soluble type. However, as discussed above, it is unlikely that any of these studies included the varieties traditionally used for making khaya.…”

“…Of these, oxalic acid is the most studied, due to its potentially deleterious effects on human health (Albihn and Savage 2001a, b;Ross et al 1999;Sangketkit et al 2001). Following usage by these and other authors, we will use "oxalate" as shorthand for both oxalic acid and oxalate ion, unless specified.…”

“…Hermann and Erazo (2000) attribute their elevated oxalate values to the fact that these ocas were grown in greenhouses and treated with chemical fertilizers, which are known to increase oxalate levels. Although some of the samples used by Ross et al (1999) and Sangketkit et al (2001) were New Zealand market samples, presumably grown using conventional Western agricultural methods including chemical fertilizers, several came from traditional Bolivian and Peruvian markets. Other possible explanations for the discrepancies in oxalate values discussed above include a) Differences in oxalate concentrations throughout the tuber; b) Oca usecategory; and c) Potential effects of processing on oxalate solubility.…”