Effect of ground salt and crop residue ash filtrate on retention and bioavailability of selected micronutrients in vegetables

Abstract

Leafy vegetables form part of the diet of most people in tropical Africa. Different processing and preparation methods are used to prepare a vegetable meal including use of ground salt and crop residue ash filtrates. However, the effect of the use of ground salt and crop residue ash filtrates on the nutrient profile of the leafy vegetables is not well understood. This study examined the effect of use of ground salt, crop residue ash filtrate from simsim and bean husks on the retention of micronutrients, antinutrient factors and mineral bioavailability in the black-eyed pea leaf vegetable, pumpkin leaf vegetable and ‘malakwang’. The micronutrients studied were iron, zinc, calcium, phosphorus and magnesium based on the foundation of their public health importance. Polyphenols, tannins and phytates were the antinutrition factors analyzed because of their ubiquitous nature in vegetables. The vegetables were analyzed for iron, zinc, magnesium, calcium and potassium content using AAS and spectrophotometry for potassium. The antinutrition factors were analyzed using spectrophotometry. The bioavailability of iron and zinc was assessed using in-vitro digestion method. pH of the ground salt and crop residue ash filtrate was measured using a pH meter. The retention of micronutrients was dependent on the quantities of that particular micronutrient in the salt treatment. Retention of iron of 108.8% in ‘boo’ boiled with simsim ash filtrate indicates possibility of supplementation from the salt treatments. Simsim ash filtrate had the highest iron concentration of 5.58mg/g. Highest pH of 10.37 was in simsim ash while ground salt had the lowest at 10.18. The alkaline cooking environment introduced by use of salt treatments was probably responsible for leaching into cooking water, autooxidation, flavonoid acylation, formation of tannin complexes affecting the levels of antinutrition factors. Additionally, polyphenol oxidase being inactivated at alkaline pH had an impact on the levels of polyphenols in these vegetables. Denaturation of fiber in the alkaline cooking water probably explains higher percentage mineral bioavailability in ‘malakwang’ and pumpkin leaf vegetables boiled with the salt treatments. This study provides evidence-based insights into the effect of ground salt and crop residue ash filtrates in vegetables that guide nutrition education efforts towards optimization of the traditional cooking methods in the context of improving dietary quality and combating micronutrient deficiencies.