The effect of cooking on the composition and colour of New Zealand grown oca

Oca (Oxalis tuberosa Mol.), known as yam in New Zealand, has been grown commercially for more than a century. More recently, yellow and orange cultivars have been introduced to complement the traditional pink cultivar. This study investigates the composition and colour of three introduced lines, a hybrid between two of these lines and a local yellow cultivar, including the first reported measurements of oca vitamin C content. The CIE L∗a∗b∗ colour of the skin and tissue were measured in the raw tubers and after boiling and baking, the most common way to cook the tubers. The skin and the tissue of all the cultivars were a very similar orange–yellow when cooked. The soluble oxalate, vitamin C, pH and titratable acidity were also measured in the raw and cooked tissue. On a dry matter basis, the mean soluble oxalate of the raw cultivars was 935.5 ± 125.7 mg/100 g DM rising to 1364.8 ± 217.5 mg/100 g DM when baked; in contrast, the mean vitamin C content of the raw tubers was 109.8 ± 28.9 mg/100 g DM falling to a mean of 62.4 ± 12.1 mg/100 g DM when the tubers were baked.