Sensory evaluation and oxalate analysis of oca (Oxalis tuberosa)

Abstract

Oca (Oxalis tuberosa Mol.), a starchy South American tuber crop, is grown commercially in New Zealand where it is known as yam. Twelve introduced lines of oca and two local cultivars were obtained from Crop & Food Research Ltd., Lincoln, Canterbury, New Zealand for sensory and oxalate assessments. Five new and one local cultivar of oca were evaluated by a consumer-type panel for the preference for purchasing of raw tubers, and for appearance, flesh colour, flavour, texture and overall acceptability of steamed and baked tubers. The intensity, 'just right' or liking scores were analysed using a proportional-odds model. Skin and flesh colour of tubers were also measured objectively with the CIELAB system using a Minolta chroma meter CR-210. For raw tubers, the local cultivar was most preferred. Panellists preferred bright red tubers (higher a* and C*) but size was also important. Cultivars which had a similar size to the local cultivar (60-70 mm long by 27-30 mm diameter) were more acceptable. Flesh colour, bitterness and mealiness were important variables which described the overall acceptability of steamed oca, while only flesh colour and bitterness were important variables for baked oca. Cultivars which had flesh colour described as 'bright yellow', flavour as 'no bitter taste’ and a 'very slightly mealy' texture were more preferred. Cultivars with higher yellow/blue (b*) and chroma (C*) values for cooked skin and flesh colours were more acceptable. None of the introduced cultivars had a higher acceptability than the locally available market cultivar. The tubers of the 14 raw and cooked oca cultivars were analysed for total oxalates using high performance liquid chromatography. Raw oca was found to contain moderately high levels of oxalates (80-194 mg/100 g fresh weight), but not as high as other commonly consumed foods such as spinach and rhubarb. The levels of oxalates in oca differed significantly among cultivars, but oxalate concentrations were not significantly different among the harvests for three cultivars tested. There was no significant change in oxalate content in oca after boiling or steaming, possibly due to the short duration of cooking. Boiling slightly reduced oxalate content by 1.4-7.4%. Steaming slightly increased in oxalate content by 0.3-6.9 %. Baking greatly increased the concentrations of oxalates by 9.9-29.5 %.

Calcium concentration in raw oca ranged from 7.5 to 15.3 mg/100 g FW. Boiling and steaming did not change the calcium concentration, while baking significantly decreased the concentration. The ratios (mEq) of oxalate to calcium of raw and cooked oca were very high (2.0-13.5). This indicates that oxalate may render the calcium in other dietary sources unavailable for absorption during digestion.

This study first demonstrates the significant relationships between high levels of oxalate and the bitter tastes of steamed and baked oca. Correlation analysis showed raw oca with darker and redder skin colour (larger b*, lower L* and hue angle values) had higher oxalate levels. No correlation was found between oxalate and calcium content.

Overall, oca is a nutritious and tasty vegetable and its oxalate content is unlikely to cause toxicity or health problems to normal people unless consumed in very large amounts. However, oca is not a recommended food for people who are susceptible to calcium deficiency or kidney stone formation.