Mineral profile and the effect of processing of some leafy vegetables indigenous to Cameroon

Abstract

Leafy vegetables form part of the diet of most people of tropical Africa. The contribution of these leafy vegetables to their mineral needs has not been well elaborated. The aim of this study was to determine the effects of different processing methods on the mineral composition and anti-nutritional components of some tropical leafy vegetables. Twenty-one species of different vegetables locally grown and consumed in the northern region of Cameroon were collected and analyzed for their iron, copper, manganese, zinc, magnesium, calcium, sodium and potassium content. The levels of anti-nutrients like phytates, oxalates, tannins and saponins that are likely to affect the bioavailability of these minerals were also determined. The effects of processing methods on the mineral composition and anti-nutritional components of these leafy vegetables were also determined. Amongst these species, Moringa oleifera, Hibiscus canabinum, Solanum nigrum, Cucurbita maxima and Vernonia calvoana that are used for soup, and found in relative abundance in the region were subjected to two cooking methods (boiling plus squeeze-washing and a combination of boiling in alkaline salt (kanwa) plus squeeze-washing). The levels of iron, zinc, manganese, copper, magnesium, calcium, sodium and potassium varied in the different vegetables. In all vegetables studied, magnesium was the most abundant mineral. The iron levels ranged from 14.99±2.00mg/100g dry weight (DW) in M. oleifera to 167.42 ± 18.63mg/100g DW in H. sabdariffa. Boiling and squeezewashing led to significant losses of minerals with sodium being the most affected. On the contrary, higher values of iron, calcium and sodium were found in samples that were boiled in alkaline salt and squeeze-washed. The levels of antinutrients were also reduced as a consequence of processing. Percentage losses during boiling with or without alkaline salt were between 25.4 and 55.5% for phytates, 11.1 and 80% for oxalates, 16.4 and 68.5% for saponins and 25 and 26.44% for tannins. Boiling, therefore, with or without alkaline salt is an effective means of reducing the levels of these antinutrients in vegetables to tolerable levels. Key words: Leafy-vegetables, Processing methods, Mineral, Antinutrients, Saponins, Tannins, Oxalates, Kanwa