Grace Oluwakemi Babarinde, Victoria Funmilayo Abioye, Odunayo Omobitan, and Kassim Raji

Department of Food Science and Engineering, Ladoke Akintola University of Technology, P.O.BOX 4000, Ogbomoso, 210001, Nigeria

Introduction

African leafy vegetables are of great nutritional importance as they supply vital nutrients required by human body.They are majorly grown and abundant in the rainy season and become scarce in dry seasons(Kamelaet al., 2016).Different varieties of these vegetables range from wild to semi-wild and they form important parts of our daily diets.Leafy vegetables are the simplest sources of proteins, vitamins, minerals and fibers (Mepbaet al., 2007; Fasuyi, 2006).They are rich sources ofβ-carotene, ascorbic acid, riboflavin and folic acid and the minerals calcium, iron and phosphorous (Guptaet al., 2005).African leafy vegetables contain high levels of vitamin A, vitamin C,iron, calcium and protein and are valuable sources of nutrition in rural areas where they contribute substantially to protein, mineral and vitamin intake(Abukutsa-Onyango, 2003).Vegetables are also good sources of poly unsaturated fatty acids and it is believed that sufficient consumption of vegetable is preventive against cardiovascular diseases (Murciaet al., 1992).

Although, the list of tropical vegetables is enormous, only few had been properly identified,underutilized ones are confined to rural areas where they are not consumed in great quantity (Smith and Eyzaguirre, 2007).They are neglected because they are thought to be high in the anti-nutritional factors(alkaloids, phytate, cyanide, nitrates and oxalates) and due to the flavor and unfamiliar taste impacted on the food.The bioavailability of essential nutrients could be reduced by the presence of the anti-nutritional factors like oxalates and cyanogenic glucoside (Akindahunsi and Salawu, 2005).This phenomenon had limited the importance of vegetables in nutrition.

The nutritional importance of any given food is a function of its nutrient and anti-nutrient composition(Prohpet al., 2006).Some vegetables have been the base of human diets for decades providing consumers with vital micronutrients needed for sustainability and to maintain health and promote immunity against infections (Flyman and Afolayan, 2006).The promoting health attributes of vegetables are associated with their nutritional and phytochemical properties(Oulaiet al., 2014).Example of such component is glucosinolate found inBrassicavegetables which is effective against cancer and heart disease (Adebooye and Opabode, 2004).

Underutilized vegetables have immense potential in contributing to the economic growth of developing nations through sustainable food production and food security since they are tolerant to adverse climatic conditions and can be grown with lower management costs on marginal land (Sheelaet al., 2004).These vegetables are easier to grow and more resistant to pests.In order to improve utilization of these vegetables, it is important to critically evaluate their nutritional values and their implications for consumers.This study was designed to evaluate the nutritional and anti-nutritional contents of some tropical underutilized green leafy vegetables.

Materials and Methods

Sample collection and preparation

The leafy vegetablesAmaranthus spinosusL.,Basella albaLinnaeus,Corchorous tridensL.,Cyrtosperma senegalense(Schott),Erigeron floribundusSch,Ho-slundia oppositeVahl,Lagenaria siceraria(Molina)Standl,Ocimum basilicumL.,Solanum aethiopicumL.andTalinum portulacifolium(Forssk.) Asch.ex Schweinf were obtained from and authenticated at the Institute of Agricultural Research and Training, Moor Plantation, Ibadan (latitude 7°33'N and longitude 3°56'E), Nigeria.Samples were washed with potable water.Samples were sorted into mature leaves and stalk and analyzed separately.

Analyses

Proximate analyses

Protein contents were determined with Kjeldhal method (AOAC, 2005) and values multiplied by 6.25 to obtain the total nitrogen.The total fat contents were determined by using soxhlet extraction method (AOAC,2005) and crude fiber content were determined.Samples were ashed to a constant weight and the weight was determined.Moisture contents were determined gravimetrically at 105℃ for 8 h. Carbohydrate was calculated by subtracting the sum of protein, ash,crude fiber, crude fat and moisture contents obtained from 100.i.e [100- (protein+ash+ crude fiber+crude fat+moisture contents)]%.

Ascorbic acid contents of samples were determined using 2, 6-dichlorophenol indophenols titration method.Carotene was extracted from randomly chosen 100 g of each sample.Twenty mL of acetone was added while grinding.After the third extraction,water was added.The filtrate was transferred to a separate funnel.The extraction of the acetone phase with a small volume of petroleum ether was continued until no color was visible.The petroleum ether extract was filtered through anhydrous Na2SO4.Extraction with petroleum ether was repeated, until the alcoholwater layer was colorless.The intensity of the color at 452 nm was measured using 3% acetone in petroleum ether as blank.The concentration ofβ-carotene was determined.

To determine magnesium content, the ash content was digested by adding 5 mL of hydrochloric acid to the ash in crucibles and heating to dryness on a heating mantle.Five mL of hydrochloric acid was added and the mix heated to boil and filtered through a Whatman No.1 filter paper into a 100 mL volumetric flask.The filtrate was made up to mark with distilled water and concentration of magnesium determined with a Jenway Digital flame photometer.

Iron and iodine contents were determined from digests with 5 mL of hydrochloric acid solution and heated to dryness on a heating mantle.Diluents were aspirated into a Buck 200 Atomic Absorption Spectrophotometer (Buck Scientific, Norwalk) through the suction tube.Each trace element was read at the respective wavelength.

For tannin, 10 g of sample was measured into a 50 mL beaker and 2 mL of 50% methanol was added and the beaker was covered with paraffin, placed in a water bath at 77-80℃ for 1 h and shaken to ensure a uniform mixing.The extract was filtered using double layered Whatman No.4 filter paper in to a 100 mL volumetric flask.Absorbance of tannic acid standard solution and the samples were read after color development on a 21D spectrophotometer (Michigan,USA) at a wave length of 760 nm.

Saponin was determined using Brunner (1984)method.Ten g of finely ground sample was weighed into a 250 mL beaker and 100 mL of isobutyl alcohol added.The absorbance of samples and standards were determined with a Jenway V6300 spectrophotometer(Cole-Parmer Ltd., Staffordshire, United Kingdom)at a wavelength of 380 nm.Phytate was determined using Maga (1982) method.Two g of sample was weighed into a 250 mL conical flask.A hundred mL of 2% hydrochloric acid was added and the sample was left to stand for 3 h.The mix was filtered through a double layer of hardened filter paper.Fifty mL of each filtrate was placed in 250 mL conical flask and 100 mL of distilled water was added into each solution.Titration was with a standard iron (III) chloride solution containing 0.00195 g · mL-1of iron.

Oxalate was determined using Chai and Liebman(2005) method.Two g of sample was boiled in 40 mL of water for 30 min in a re flux condenser and 10 mL of 20% Na2CO3was added and the mix boiled for another 30 min.The liquid extract was filtered and washed with hot water until the wash water did not show any alkaline reaction.The combined wash water and filtrate were concentrated to a small volume and cooled.An aliquot of this filtrate was transferred into a 400 mL beaker, and diluted with water to 200 mL.The filtrate was titrated against 0.05N KMNO4solution and was reported as oxalic acid.

Cyanide content was determined using Bradburyet al.(1999) method.One g of sample was weighed into a flat bottom plastic bottle with a screw-capped lid.A 0.5 mL of 0.1 mol · L-1phosphoric acid at pH 6 was added to the sample and a yellow picrate paper attached to a plastic strip and placed in the bottle.The cap of the bottle was closed.A blank was prepared as the above.The absorbance of all the solutions, including linamarin standards, was measured against a blank on a Spectronic 2ID spectrophotometer at 510 nm.

pH and titratable acidity were determined using Illandet al.(2000) method by using a pH meter with pH buffer solutions of pH 4, 7 and 9.

Statistical analyses

All the analyses were done in three replicates.Data were subjected to analyses of variance and means separated with Duncan's Multiple Range Test (SAS,2001).

Results and Discussion

Proximate analysis

Proximate compositions in all the samples analyzed differed (Table 1).Protein was the highest inC.tridensand the lowest inE.floribondus.The range of the values (2.10%-5.93%) fell within those for selected commonly consumed tropical green vegetables (Bailey, 1992; Akindahunsi and Salawu,2005).According to Aberoumand (2012), plants that derived more than 12% of its calorific values from proteins were a good sources of proteins which made vegetable good sources of proteins.Guptaet al.(2005)reported a high protein content of 7.1 and 4.3 g · 100 g-1inDelonix elataL andDigera arvensisForsk, respectively.Similar values had been reported for the protein contents of some tropical leafy vegetables of Nigeria (Aletor and Adeogun, 1995).

The highest fat content was forH.oppositaand the least forE.floribondus.Fat contents of these vegetables compared favorably to that for edible green leafy vegetables of southern India and Nigeria(Midmoreet al., 1991).A diet providing 1%-2% of its calorie as fat was sufficient for humans, as excess fat consumption was implicated in certain cardiovascular disorders such as arteriosclerosis and cancer.

Crude fiber content was the highest inB.albaand the lowest inC.tridens; these values were lower than that inIpomea batatasLam,Piper guineenseandVernonia amagydalina(Akindahunsi and Salawu,2005; Antiaet al., 2006).Non-starchy vegetables were the richest sources of dietary fiber (Agostoniet al.,1995) and were employed in treatment of obesity,diabetes, cancer and gastro-intestinal disorders(Swaminathan, 1995).The required dietary allowance of fiber for children, adults, pregnant and lactating mothers were 19-25, 21-38, 28 and 29 g, respectively (Akubugwoet al., 2007).The vegetables studied were capable of contributing to daily requirements when l00 g dried leaves were consumed and could be a valuable source of dietary fiber in human nutrition.

Ash content was the highest inA.spinosusand the lowest inB.alba(Table 1).Ash was an index of the total mineral content.The highest carbohydrate content was forB.albaand the lowest forC.tridens.Though generally low, when compared to some carbohydrates, the studied vegetables could meet required dietary allowance of children (130 g), adults(175 g), and pregnant and lactating mothers (210 g)(Akubugwoet al., 2007).Moisture content was the highest inC.tridensand the lowest inH.opposita;the values were within the range for some Nigerian leafy vegetables and pacific green leafy vegetables(Akindahunsi and Salawu, 2005; Guptaet al., 2005;Akubugwoet al., 2007).

Table 1　Proximate composition of some tropical under-utilized vegetables

Anti-oxidant components and mineral contents differed for vegetable samples (Table 2).Ascorbic acid content was the highest inA.spinosusand the lowest inC.senegalense.All the samples had significant amounts of ascorbic acid.Guptaet al.(2005) reported large variations in the ascorbic acid contents of some green leafy vegetables.Ascorbic acid was an antioxidant which was used as additive in many processed foods to prevent nutrient loss and spoilage.Ascorbic acid found in these green leaves had the capacity in varying degrees to counteract free radicals.The highestβ-carotene content was inA.spinosusand the lowest inO.basilicum.The values obtained were close to the values reported by Shokunbiet al.(2011).β-carotene was a precursor of vitamin A and needed for maintenance of skin mucous membrane, bones and teeth.The highest iodine level was inA.spinosusand the lowest inH.opposita.Iodine levels of the vegetables were within the range reported by Bailey (1992).Iodine was needed by the body to prevent goiter disease.

Table 2　Anti-oxidant and mineral contents of under-utilized tropical leafy vegetables

The highest iron content was inA.spinosusand the lowest inH.opposita, but compared favorably with values reported for some green leafy vegetables(Antiaet al., 2006; Akubugwoet al., 2007).Iron was an essential trace element for hemoglobin formation,normal functioning of the central nervous system and in the oxidation of carbohydrates, protein and fats (Adeyeye and Otokiti, 1999).The difference in nutrient concentration in these leafy vegetables may be due to environmental and genetic factors.Magnesium content was the highest inT.portulacifoliumand the lowest inH.opposita.The magnesium contents of the studied vegetables were within the range reported by Antiaet al(2006).This mineral is important, due to its connection with is chemic heart disease and calcium metabolism in bones.

The plants differed in contents of anti-nutritional compounds (Table 3).Values obtained for tannins inT.portulacifoliumandC.senegalensewere among the highest and there was few difference among other vegetables.Other types of plants had higher tannin levels (Vander-Poel, 1990; Apenaet al., 2004).Tannin binds to exogenous and endogenous proteins including enzymes of the digestive tract and affects their utilization (Soteluet al., 1995).Tannin is known to inhibit the bioavailability of proteins and mineral.

The saponin content inC.tridenswas significantly higher than some of the other plants but similar to most.Saponin values were lower than those in bitter leaf (Vernonia amygdalina) as reported by Apenaet al(2004).Saponins were practically non-toxic to man and could lower plasma cholesterol concentrations.High levels of saponin have been associated with gastro enteritis (Awe and Sodipo, 2001).

Phytate content was the highest inC.senagalenseand the lowest inL.sicerariaandA.spinosus.The values were comparable to those reported in other crops (Adeyeyeet al., 2000; Apenaet al., 2004).Vander-Poel (1990) obtained higher levels of phytic acid in other crops.The effect of high phytic acid consumption was induction of mineral deficiency through formation of insoluble salts with divalent metals, so that they were unavailable to the body(Iyawe, 2009).Oxalate values were generally similar for all the plants.The oxalate contents were lower than the lethal dose, 4-5 g, dose reported for adults (Roseet al., 1999).The major problem associated with excessive oxalate consumption was its ability to form water soluble salts with some mono- and divalent ions and reducing its bioavailability (Iyawe, 2009).The highest hydrogen cyanide (HCN) value was inA.spinosusand the lowest inL.siceraria.High dose of HCN could inhibit respiration (Onigbinde, 2005);however, the values obtained were very low and within the recommended value that must be present in a diet.

Table 3　pH, titratable acidity and anti nutrients composition of some under-utilized tropical leafy vegetables

The highest pH was inC.tridensand the lowest inC.senegalense.Plants with no HCN, and low values of other anti-nutrients had high pH.Acidic and alkaline foods did not usually translate in to acid and alkaline yielding foods.Jack (2009) reported that citrus fruit and tomatoes were acidic, but had a net alkaline yield once their constituents got to the kidneys.Vegetables were rich in potassium salts, a natural buffer.Acidosis would decrease the body's ability to repair damaged cells, decrease ability to detoxify heavy metals, make tumor cells thrive, and make persons more susceptible to fatigue and illness (Jack, 2009).The total titratable acidity was the highest inS.aethiopicumand the lowest inA.spinosus.The total titratable acidity indicated acid content of vegetables, but not the strength of the acid.

Conclusions

The under-utilized leafy vegetables contain appreciable amount of proteins, fat, fiber, carbohydrate, vitamins A and C and mineral elements.All the analyzed vegetables were high in one nutritional attribute or two nutritional attributes.These vegetables could contribute to the nutrient requirements of human and should be used as a source of nutrients to supplement other sources.The anti-nutritional constituents in the vegetables included oxalate, phytate, tannin, saponin and cyanide at values lower than the established toxic levels andL.siceraria,C.tridens,S.aethiopicum,O.basilicumandB.albahad the least amounts of anti-oxidants.It is recommended that vegetables with higher levels of anti-nutrients be well processed before consumption.

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