Effect of Boiling and Wet Frying on Nutrient Bio Accessibility, Antinutrients Content and Bioactive Compounds in Traditional Leafy Vegetables Consumed in Malawi

Abstract

Apart from provision of nutrients, traditional vegetables also provide phytochemicals with health benefits. Micronutrient deficiencies of iron, zinc and vitamin A are particularly common in most developing countries yet traditional vegetables that can provide these nutrients grow wildly in those areas. However, diets containing vegetables may lead to reduced availability of minerals from the diets due to presence of antinutrients. Such diets are also associated with lower bioaccessibility of minerals such as iron and zinc. Previous studies have shown that different cooking methods may have different effects on the nutritional, antinutritional and phytochemical properties of fresh vegetables. Other heat treatment methods have also been reported to influence the bioaccessibility of many nutrients in food.The main objective of this study was to investigate the effect of short time boiling and wet frying on nutrient retention, nutrient bioaccessibility, antinutrients content and bioactive compounds content in five traditional leafy vegetables commonly consumed in Malawi. Atleast 1Kg each of the vegetable leaf samples of Amaranth hybridus (Amaranth ssp.), Moringa oleifera (drum stick), Ipomea batatas (sweet potato), Corchorus olitorius (jute mallow) and Bidens pilosa (black jack) collected and investigated. The edible portions of the leafy vegetables were either boiled or wet fried for 10 minutes, after which they were air dried alongside raw vegetables under the shade.To determine nutrients retention of the vegetables, beta-carotene, vitamin C, and minerals (iron, zinc, calcium, magnesium and copper) were analysed in both the cooked and raw dried vegetables. The different vegetable species had differences in retention of various minerals and vitamins. Wet frying increased the oil content of the vegetables by a range of 15.49% to 28.40 % and was hence associated with lower % ash and mineral contents. Wet frying significantly reduced (P<0.05) beta-carotene content in all the vegetables except in jute mallow. Boiling had no significant effect on beta-carotene content of most of the vegetables. Boiling significantly reduced (P<0.05) vitamin C content in all the vegetables while wet frying retained this vitamin in all the vegetables. Generally, boiling was associated with better retention of minerals and beta-carotene while wet frying was more advantageous in retaining vitamin C.To determine the effect of boiling and wet frying on antinutrients, oxalates, phytates and tannins were analysed in both the cooked and raw dried vegetables. Both boiling and wet frying significantly reduced (P<0.05) oxalates in all the vegetables except in black jack. Both boiling and wet frying significantly (P<0.05) reduced the concentration of phytates in most of the vegetables. However, boiling was more effective in reducing the amount of phytates than wet frying. Boiling reduced concentrations of tannins in all the vegetables as compared to wet frying. Generally, boiling was associated with greater reduction of antinutrients in most of the vegetables than wet frying.To determine the effect of boiling and wet frying on bioactive phytochemicals, total phenolic content (TPC), total flavonoid content (TFC), total carotenoid content (TCC) and 2, 2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity were determined in both the cooked and raw dried vegetable samples. Boiling decreased TPC significantly (P<0.05) in all the vegetables except jute mallow while wet frying decreased TPC in all the vegetables except amaranth. Wet frying increased TFC in drum stick and jute mallow but reduced TFC in black jack and sweet potato leaves. Boiling either increased or retained TCC in all the vegetables while wet frying had negative impact on TCC of most of the vegetables except for jute mallow and drum stick leaves. Both boiling and wet frying decreased the DPPH radical scavenging activity of all the vegetables except jute mallow where boiling increased its radical scavenging activity. Wet frying reduced the DPPH radical scavenging activity of most of the vegetables more than boiling. The study demonstrated that boiling and wet frying have different effects on the phytochemicals and antioxidant activity of different vegetables and the effects depend on the vegetable species.To investigate the effects of boiling and wet frying on nutrient bioaccessibility, bioaccessibility of iron, zinc and pro-vitamin A (beta-carotene) in the five vegetables were determined using an in-vitro gastro-intestinal digestion coupled with dialyzability. Bioaccessibility of iron, zinc and beta-carotene varied in the raw dried vegetables species. The highest % bioaccessibility of iron, zinc and beta-carotene from the raw dried vegetables were 73.0%, 72.5% and 46.5%, respectively. Both boiling and wet frying showed varying effects on the bioaccessibility of beta-carotene. Wet fried vegetables had higher bioaccessible beta-carotene than most boiled counterparts. Bioaccessibility of iron was higher in most of the cooked vegetables compared to the raw dried counterparts. Iron bioaccessibility was higher in most of the wet fried vegetables compared to their boiled counterparts. Bioaccessibility of zinc was higher in most of the wet fried vegetables compared to their raw dried and boiled counterparts. Bioaccessibility of iron, zinc and beta-carotene was higher in most wet fried vegetables than their boiled counterparts.In conclusion, nutritional content, antinutrients content, phytochemicals constituents and bioaccessibility of nutrients in the traditional vegetables varied from one plant species to another. Generally, the effect of boiling and wet frying on phytochemicals and DPPH radical scavenging activity varied in different vegetables and depended on the type of vegetables. Bioaccessibility of iron, zinc and beta-carotene was higher in most wet fried vegetables than their boiled counterparts. Wet frying can therefore make substantial contribution to the recommended daily intakes of iron, zinc and vitamin A and hence may help to reduce the risk of iron, zinc and vitamin A deficiency.