The effect of cooking on the location and concentration of oxalate in three cultivars of New Zealand-grown oca (Oxalis tuberosa Mol)

Oca (Oxalis tuberosa Mol) originates from South America but is now also grown commercially in New Zealand. It contains moderate amounts of oxalate, which is only present in its soluble form. The skin of the oca tuber is thought to contain more oxalate than the flesh. In this study the concentration of soluble oxalate was measured in the skin and outer and inner flesh and in the whole tuber of raw, boiled, baked and steamed oca. The analysis was carried out on the traditionally pinkish-red New Zealand cultivar and on the newly introduced cultivars 'Mellow Yellow' and 'Apricot Delight'. In the raw oca tuber the oxalate concentration in the skin is significantly higher than in the flesh (mean value 7.3 g kg(-1) fresh weight (FW) compared to 1.7 and 1.4 g kg(-1) FW in the outer and inner flesh respectively). The highest concentration was found in the skin of the pinkish-red cultivar (10.9 +/- 1.0 g kg(-1) FW). All cooking methods seemed to cause a migration of oxalate from the skin to the underlying flesh. On a fresh weight basis, baking significantly increased the oxalate concentration in the whole tuber, whereas boiling decreased the concentration and steaming had no significant effect (p < 0.001). Boiling might therefore be a better way of cooking oca than baking when a low intake of oxalate is desired. (C) 2001 Society of Chemical Industry.