The effect of Indian cooking style on the nutritional and anti-nutritional properties of spinach

Spinach (Spinacea oleracea) is a commonly consumed plant with a high nutritional value that is rich in antioxidants but also contains high level of oxalates, which are anti-nutritive factors. In this present research four different Indian dishes were prepared from spinach: palak, plain palak saak, palak saak with ginger and palak paneer. Spices - caraway, cumin, cardamom, cinnamon, ginger and Malabathrum leaf - were added following traditional Indian recipes. The samples were collected at different stages, raw, boiled saak and saak bhaji, which were uncooked, and cooked for 15 and 25 minutes, respectively to determine the levels of total, soluble and insoluble oxalates and the antioxidant activities of spinach. High performance liquid chromatography was used to measure the total and soluble oxalate contents and an oxygen radical absorption capacity (ORAC) assay was used to measure the antioxidant activity of all spinach dishes. Initial analysis of the total, soluble and insoluble oxalate contents of the spices showed that the total oxalate contents ranged from 194 (nutmeg) to 4014 (cardamom green) mg /100 g DM, while the soluble oxalate contents ranged from 41 (nutmeg) to 3977 (cardamom) mg/100 g DM. Overall, the percentage soluble oxalate content of the spices ranged from 4.7 to 99.1% of the total oxalate content. Raw spinach contained mean total oxalate, soluble oxalate and insoluble oxalate of 7.56, 6.42 and 1.14 g/100 g DM, respectively, with a mean ratio of soluble oxalate to total oxalate of 85%. Cooking in a wok resulted in a marginal to 91% reduction in soluble oxalates due to their combination with free minerals (e.g. Ca, Fe, Mg) present in the spinach or spices. The greatest reduction (91%) of soluble oxalates occurred when paneer (a milk product) was added to the palak paneer dish; the proportion of soluble oxalate to total oxalate content was then reduced from 74-85% to 20-31%, depending on the cooking time. Spices showed ORAC values in the range of 469 (turmeric) to 794 (caraway) μmol TE/g DM. In the raw spinach dishes antioxidant activity ranged from 49 to 163 μmol TE/g DM. The antioxidant activities of cooked spinach dishes decreased from 9 to 39% depending on the ingredients added. Addition of spices showed low to moderate effects on the antioxidant activities of the cooked spinach dishes. Antioxidant activity of the palak paneer decreased in the range of 67% to 78% with the addition of paneer due to the dilution factor. This study suggested that Indian cooking styles can reduce soluble oxalate contents, where the antioxidant activity depends on the type of spices added to the dish.