Nutritive value and levels of heavy metals in marketed vegetable amaranth grown with effluent water: a case of ruai and njiru in Nairobi

Abstract

The urban areas release enormous volumes of wastewater into the sewerage system. This water serves for irrigation in urban agriculture. However, the water contain large amounts of nutrients, but also toxic constituents including heavy metals, such as lead and cadmium that could get into the plant tissues meant for food .This study was designed to assess the nutritional value and the levels of the heavy metals lead and cadmium in vegetable amaranth grown with effluent water. The study was in two phases. The first phase was a baseline survey to assess the production, marketing and consumption practices, organized with 91 as farmers and market intermediaries using pretested, structured questionnaires. The second phase consisted of laboratory analyses of proximate composition, vitamin A and C, and the minerals; zinc, calcium and iron, the anti-nutrients; nitrates and oxalates and the levels of lead and cadmium in both raw and cooked vegetables. The study established that 66% of the respondents ranged between 40 to 50 years, and 83% of them were females in full-time vegetable farming and 98 % of them trading. On average 70% of the respondents earned below KES 200, 000 per annum from sale of vegetable amaranth. Up to 77% of the respondents were semi-illiterate. Then 87% of the farmers practiced subsistence farming in less than 3 hectares of land and sold vegetable amaranths to 41% of intermediaries. The study established that 53% of the intermediaries made a profit of between KES.300-400 per 50kg bag of amaranth. Up to 70% of the respondents indicated cooking the vegetable by direct stewing. About 20% of farmers agreed that the waste water come into contact with the leaves, 45% admitted using raw animal manure/municipal/effluent biosolids.The Moisture content of vegetables ranged from 81.03% to 84.69% crude protein from 22.33% to 36.46%, Total ash ranged from 21.23% to 9.75%, crude fibre ranged from 29.03% to 11.57%. The crude fat and soluble carbohydrate ranged between 0.36% to 1.23% and 6.55% to 36.86% respectively. The β-carotene and ascorbic acid contents ranged between10.8 mg/100g to 53.8 mg/100g and from not detectable to 98.5 mg/100g, respectively. There was no significant difference in the levels of the β-carotene content of vegetables. Β-carotene increased with increase in cooking time and highest values were obtained when the cooking water was discarded. Increase in cooking time resulted in increased loss of ascorbic acid. The oxalate and nitrates ranged between 1649.3 mg/100g to 2452.4 mg/100g and 1856.6 mg/100g to 3080 mg/100g, respectively. Boiling the vegetable and discarding the cooking water caused significant reduction in the oxalate and nitrate contents. Cadmium was not detected in the vegetables and in irrigating water, but in soil the values ranged from 62.3 ppb to 97.85 ppb. Lead in vegetables was between 2.8 ppm to 5.1ppm, in irrigation water 2.0 ppm to 4.5 ppm and in soil 3.0 ppm to 34.1 ppm. Cooking did not affect the levels of lead in vegetables from Njiru significantly, but had a significant effect on the levels in vegetables from Ruai. Minerals concentration was as follows: iron 87.5 mg/kg to 443mg/kg and zinc 23.4 ppm to 54.6 ppm. Cooking in large amount of water which was discarded resulted in higher increase in mineral contents than direct stewing. The study concluded that the vegetable amaranth grown in the two places with effluent water contains appreciable levels of nutrients and moderate levels of anti-nutrients. The levels of cadmium were undetectable. However, the levels of lead in the fresh vegetables were above the maximum limits allowed in Kenya, but considering the amounts of vegetables consumed, the amounts of the element ingested would not pose serious public health concerns.