Assessment of nutritional quality of cooked Swazi leafy vegetables

Abstract

Swazi leafy vegetables are cheap, seasonal, locally grown and easily available, easy to propagate and store, highly nutritious food substances that are often used to supplement diets and whose nutrient content is affected by processing treatments. The effects of cooking on the nutrient composition of some Swazi vegetables were investigated. Common Swazi vegetables and some exotic cultivars were collected from the Manzini region, then cooked strictly according to Swazi traditional practice, digested and analysed for calcium, copper, iron, magnesium, manganese, potassium and zinc content using flame atomic absorption spectrophotometer and inductively coupled plasma spectrometer. The vegetables investigated were: Hibiscus  escolentus [upright and exotic cultivars], Corchorius olitorus, Amaranthus  spinosus, Amaranthus caudatus, Bidens pilosa, Solanum nigrum and Momordica involucrata. Brassica oleracea and Spinacia oleracea were also analysed and compared with other vegetables. Sampled vegetables were divided into two  categories. All vegetables in category one were cooked without any additives. In category two Hibiscus escolentus and Corchorius olitorus were cooked with  bicarbonate of soda and common salt; Amaranthus spinosus, Amaranthus caudatus, Bidens pilosa, Solanum nigrum, Brassica oleracea and Spinacia oleracea were prepared with sunflower cooking oil and salt. Only salt was added to Momordica involucrata. Results of the study show that cooking significantly (p £ 0.05) reduces zinc, calcium, iron, magnesium, manganese,  potassium and copper content of all the vegetables analysed. The percent  reductions were: 4.2 – 100% for zinc, 2.0 – 75.6% for calcium, 2.4 – 79.2% of iron, 1.2 – 73.2% for magnesium, 6.6 – 100% for manganese, 1.9 - 67.5% for potassium and 46.4 – 100% for copper. It was also found that addition of salt and bicarbonate of soda enhances loss of minerals. The cooked Swazi vegetables had significantly higher minerals content when compared to exotic varieties such as Brassica oleracea which lost most of the minerals when cooked. The loss of minerals in the vegetables may be due to leaching effect during the cooking process. It appears that this leaching effect is exacerbated by addition of sodium bicarbonate, sodium chloride and sunflower cooking oil. Cooking the common vegetables without additives or with small quantities of additives is, therefore, recommended.

Key words: Additives, Minerals, Nutritional quality, Cooked, Swazi, Malvaceae, Amaranthaceae, Asteraceae, Solanaceae, Cucurbitaceae