Determination of oxalates in Japanese taro corms using an in vitro digestion assay

An in vitro assay was used to extract gastric soluble and intestinal soluble oxalates from the corms of four different Japanese taro (Colocasia esculenta (L.) var. Schott) cultivars (Akame, Ishikawa-wase, Yamato-wase and an unnamed cultivar). The oxalate contents were measured in the raw tissue and after boiling and baking the corms.Akame contained the highest levels of gastric soluble oxalates in the raw tissue (168.9 ± 12.0 mg/100 g fresh weight (FW)) and was significantly higher (P < 0.001) than the other three cultivars (mean 86.5 ± 8.1 mg/100 g FW). There was no significant difference in gastric soluble oxalates between Ishikawa-wase, Yamato-wase and the unnamed cultivar. Raw Akame corms contained the highest levels of soluble oxalate after intestinal incubation 134.8 ± 9.8 mg/100 g FW. This was significantly different (P < 0.001) from the other three cultivars, which contained a mean of 66.4 mg intestinal soluble oxalates/100 g FW.Boiling the Japanese taro corms for 40 min reduced the level of intestinal soluble oxalates in the cooked tissue to a low level (mean for all the cultivars, 17.7 mg/100 g FW) as the soluble oxalates were leached into the cooking water. In addition, the mean moisture content of the boiled corms increased to 83.2% compared to the mean moisture content of raw tissue (75.7%).Baking the four cultivars at 180 °C for 40 min led to a significant reduction in the moisture content of the tissue (from a mean of 75.7% moisture content in the fresh tissue to a mean of 50.7% when baked) and an effective concentration of gastric soluble oxalates in the cooked tissue (overall mean 276.1 mg/100 g FW) while the mean intestinal soluble oxalates rose to 187.2 mg/100 g FW. The results confirm that baked taro corms contain moderate amounts of soluble oxalates.The total gastric soluble oxalates of the four different taro corms were similar to the total oxalate contents determined using hot acid method, however the in vitro method used to extract intestinal soluble oxalate appeared to extract more oxalates when compared to the hot water extract method.Baked taro corms should be avoided by people who have an increased risk of renal calcium oxalate stone formation because they contain high levels of intestinal soluble oxalates.