Fruit characteristics and chemical composition of some varieties of velvet beans (mucuna spp) found in Benue State of Nigeria

C D Tuleun, S N Carew* and J A Patrick

Department of Animal Nutrition, University of Agriculture, Makurdi
*Department of Animal Production, University of Agriculture, Makurdi
sncarew2001@yahoo.com

Abstract

Fruits from six varieties of Mucuna (velvet bean) were gathered from various locations in Benue State, north central region of Nigeria. The fruit and seed characteristics were noted, and the seeds were analyzed for amino acids and other nutrients, and also for content of the anti-nutritional factors commonly found in legumes.   

The seeds were found to contain (g/100g dry matter) 27-29 crude protein, 5-9 crude fat, 3-4 crude fibre, 4 ash, 46-50 NFE and 4 kcal/g gross energy.   Mineral content (%) estimates were:  phosphorus 0.18-0.59, calcium 0.45-0.94, sodium 0.08-0.14, magnesium 0.17-0.46. Other estimates (mg/kg) for minerals included  iron 130.7-135, manganese 44.53-85.57, copper 11.90-17.40 and zinc 73.9-114.  The mucuna seeds were found to contain comparatively high amounts of critical amino acids such as lysine (5.51-6.52%), cysteine (0.95-1.32%) and methionine (1.18-1.34%).  Anti-nutrient factor content estimates were: L-3,4 dihydroxyphenylalanine (L-Dopa )  4.75 - 6 .27%; tannin 0.12-0.25%; phytate 0.08-0.22%; oxalate 0.01-0.03%; saponin 0.21-0.74%; alkaloids 1.22-2.30%; HCN 5.3-13.4mg/kg; trypsin inhibitors 8.3-31.4 Tui.

The high content of nitrogen and very good essential amino acid profile of the seeds suggest that they could serve as major sources of essential nutrients to livestock.  Mucuna cochinchinensis (mottled) and Mucuna pruriens seem to be the most promising for feeding monogastric animals, since they contain comparatively lower levels of anti-nutrients.

Keywords: anti-nutritional factors, Mucuna, unconventional feedstuffs, velvet beans

Introduction

The Mucuna bean, commonly called velvet bean, is an annual leguminous climber, with pods that are covered with velvety hairs that irritate the skin when the fruit is mature and dry. The major use for mucuna, at present, is as a green manure/cover crop for small holder farmers in tropical regions of the world.  It produces, on the average, 1-2 tons/ha of seed, resists most pests and diseases, including strains of nematodes which commonly attack legumes (Wanjekechi et al 2003; Skerman et al 1988).  In addition, it has nutritional potential as a rich source of protein (23-35%) (Bressani 2002; Teixeira  et al 2003) and metabolisable energy of about 1kcal/g for raw seeds and 3.2kcal/g for processed Mucuna pruriens seeds (Ukachukwu and Obioha 1997; Ukachukwu et al 1999). 

Mucuna is used as a minor food crop in several countries of Asia and Africa.  In South East Asia, the immature pods and leaves of mucuna beans are used as vegetables.  The seeds of Mucuna sloanei and Mucuna urens are used by ethnic groups of eastern Nigeria as a condiment or as garnishing for the main dish (Ukachukwu et al 2002; Onweluzo and Eilitta 2003).

The limited reports available on its nutrient composition show that mucuna compares favourably with other edible grain legumes (Iyaji and Egharevba 1998; Ahenkora et al 1999).   In common with most other legumes, toxic constituents in the seeds have been reported, which interfere with the digestion and assimilation of nutrients especially in monogastric animals (Udedibie and Carlini 1998; Siddhuraju et al 2000). Twelve Mucuna varieties are recognized at IITA, but there has been only limited systematic evaluation of their nutritional characteristics (Ezeagu et al 2003). 

Adequate information on the chemical composition of a material is a pre-requisite for its effective utilization in animal nutrition.  Given its remarkable nutritional potential, there is a need for detailed physical characterization and nutritional evaluation of all available varieties of Mucuna. In Benue State, North Central Nigeria, a number of mucuna bean varieties exist, and are  utilized in various ways by the local population, including medicinal purposes, and 

this study is designed to evaluate their proximate and mineral composition, amino acid profile and anti-nutritional factor content.

Materials and methods 

Fruit sources and characteristics

Mature mucuna fruits were harvested during January and February 2006 from natural stands at various locations in Benue State, which is located in the north central region of Nigeria. Benue state lies between longitudes 6^o 45'E and 8^o 15' E, and latitudes 7^o 30'N and 9^o 45N. The locations of collection and the fruit and seed characteristics are described in Table 1.

Figure 1 is a photograph of samples of the seeds of the various varieties of Mucuna studied in this report.

Identification of the species and varieties were done at the Herbarium of the Department of Forestry and Wildlife Management, University of Agriculture Makurdi, Makurdi, Nigeria. The fruits were sun-dried for 3-4 days, threshed and winnowed to obtain clean seeds.  The seeds were then stored in plastic containers at room temperature (27-30^oC) for subsequent analysis.

Preparation of seed samples 

All analyses were carried out on raw seeds. Just before analysis, 30g of the dry seeds of each variety were ground with a laboratory bench mill equipped with 1mm screen. Samples from these seed meals were subjected to analysis for proximate fractions, minerals, amino acids and anti-nutritional factors.

Chemical  analysis 

All chemical analysis were carried out in triplicate. Proximate analysis was according to the methods of AOAC (2000). Amino acid analysis was performed using a Technicon Sequential Multisample (TSM) amino acid analyser, utilising previously defatted and hydrolysed samples. Ash from each sample was analysed for sodium, potassium and calcium by flame photometer (AOAC 2000).  Magnesium, iron and zinc were determined using an atomic absorption spectrophotometer, while phosphorus was determined by the Vanado-molybdate colorimetric method (AOAC 2000).  Tannin was determined by the Folin-Denis method (AOAC 2000). Phytic acid was determined by the colorimetric method of Stewart (1974). Trypsin inhibitory activity was determined according to the method of Kakade et al (1974) using benzoyl-DL-arginine-P-nitro anilide (BAPNA) as the substrate. The results were expressed as trypsin units inhibited (Tui) per milligram of dry sample.  Hydrocyanic acid was determined by an alkaline titration method (AOAC 2000). Total oxalate content was determined according to the procedure of Abaga et al (1968). Analysis for 3,4 dihydroxyphenyl alanine (L-Dopa) was by the method of Brain (1976). Saponin content of seed samples were determined by the method of Peng and Kobayasli (1995), while the procedure developed by Maxwell et al (1995) was used for alkaloids.

Results and discussion 

Results from proximate analysis of whole seeds are shown in Table 2. 

With the possible exception of NFE and EE, the proximate composition of the mucuna varieties studied showed remarkable uniformity. The range of crude protein content was 26.9% in Mucuna utilis to 28.7% in mottled Mucuna cochinchinensis, with a mean of 27.8%. These mucuna varieties would, therefore, appear to be superior to cowpea (23.7), groundnut (24.7) and pigeon pea (26.3) in crude protein content, but inferior to soybeans (38.7) (FAO 1994).  The range of crude protein content observed in this study is much narrower, and falls entirely within the 20.1-35.5g/100g observed in some earlier studies (Ukachukwu et al 2002; Emiola et al 2003). Mucuna, therefore, shows potential as a protein supplement for low-protein foods and feeds such as cereal grains, a view also held by Ezeagu et al (2003). Ether extract values ranged from 5.04 in cream-coloured Mucuna pruriens, to 8.91% in mottled Mucuna pruriens, the average value being 7.58%.  These low oil content values would appear to preclude Mucuna as a commercial source of oil. Very little differences between varieties in crude fibre content were observed, with a mean value of 3.58%. Mucuna cochinchinensis (mottled) had the highest content of crude fibre (3.82%) while Mucuna utilis had the least (3.24%).  The fairly low crude fibre content of the seeds is an advantage in terms of monogastric animal feeding, since such animals are unable to utilise high fibre diets efficiently. Ash content was also found not to differ much among the mucuna varieties studied, the range being from 4.10%, in Mucuna utilis, to 3.85% in mottled Mucuna pruriens with an average of 3.97%.  Mottled Mucuna cochinchinesis had the highest carbohydrate content (50.3%) while Mucuna utilis  had the lowest  (45.6%).

The amino acid content of the seeds of the Mucuna varieties in this study are shown in Table 3.

The amount of essential amino acids, especially lysine, methionine and cystine, are far superior to those reported for soybeans (FNL 2006). Mucuna content of these critical amino acids is more comparable with those of lesser known legumes such as kidney beans (Phaseolus vulgaris), lima bean (Phaseolus lunatus), pigeon pea (Cajanus cajan) and Bambara nut (Voandzeeia subterranea) (Ologhobo 1992; Ukachukwu and Obioha 1997). Mean gross energy content was found to be 4.25kcal/g. It was highest in mottled Mucuna cochinchinensis and least in Mucuna utilis.

Findings on the macro and micro minerals element content of mucuna seeds are summarized in Table 4.

Calcium, phosphorus, magnesium, sodium and potassium levels were highest in mottled Mucuna cochinchinensis (0.94, 0.59, 0.46, 0.14 and 1.37% respectively), while Mucuna utilis had the lowest (0.45%, 0.12%, 0.17, 0.05% and 0.86% respectively).  Mucuna seeds could, therefore, be a good source of these minerals, assuming they occur in readily available form. The highest values for iron, manganese, copper and zinc were 135, 85.7, 19.9 and 114mg/kg respectively (mottled Mucuna cochinchinesis), while the lowest values (iron, 114; manganese, 5312 and zinc 68.9) were found in Mucuna utilis. Wide variations were found within species, which could be due to such factors as climate, and mineral contents of the soil, as suggested by Aykroyd and Doughty (1982).

Table 5 contains values for anti-nutritional factors found in the seeds of the mucuna varieties investigated in this study.  

L-Dopa content ranged from 4.75 to 6.63% with an average of 5.81%. Maroon Mucuna cochinchinensis had the highest level  (6.63%) while that of mottled M  pruriens were the lowest (4.75%).  Varietal variations in L-Dopa concentration has been reported in Mucuna (Siddhuraju and Becker 2001), and a pattern similar to that found in the present study was observed by Ezeagu et al (2003). Vadivel and Janardhanan (2000) and Janardhanan et al (2003) have reported values in the range of 5.60 - 6.63% in a study involving five Indian varieties of Mucuna. There were significant differences in seed HCN concentration among the Mucuna varieties examined in this study (Table 5). Mucuna pruriens (cream-coloured) contained the lowest concentration of HCN (5.25mg/kg) while the highest value of 13.36mg/kg was found in Mucuna poggei and the average was 10.14mg/kg.

These levels are much lower than those found in cowpea (40mg/kg), and is within acceptable limits (Oke et al 1996), since the FAO stipulates an upper limit of 10mg/kg HCN as safe for human consumption (Makkar and Becker 1997). This study, therefore, confirms the report of Ukachukwu et al (2002) that, generally, the levels of cyanide been found in Mucuna beans are low. It should be noted, however, that prolonged intake of even low levels of HCN, as happens when the carrier is a staple food, still results in chronic toxicity, which leads to damage to the central nervous system and the thyroid gland, which is manifested as ataxia and goiter respectively (Ekpechi 1967; Delange and Ermans 1971; Makkar and Becker 1997).

There was wide variability in trypsin inhibitor activity in the seeds of  mucuna varieties under study. Mucuna poggei had the highest value of 31.38 Tui/mg protein while the lowest value of 8.25Tui/mg protein was obtained in maroon Mucuna cochinchinensis, the mean being 17.86.  These values are similar to those in an earlier report by Vadivel and Janardhanan (2002), but higher than those in another study on two varieties of Mucuna beans (Siddhuraju and Becker 2001). Mottled M.cochinchinensis contained the lowest concentration of tannins (0.12%), while the highest value (0.25%) was observed in Mucuna poggei, with an average of 0.20%. These values are similar to those reported by Vadivel and Janardhanan (2000) and Gurumoorthi et al (2003) (0.14-0.24%), but higher than those reported by Siddhuraji et al

(2000).  Siddhuraju et al (1996) had reported some lower values for Indian Mucuna varieties.  High intakes of plant products rich in tannin and oxalate have been reported to cause neurotoxicity, behavioural changes and severe diarrhoea in human beings and animals (Akanji and Osho 2007).  Phytic acid content ranged from 0.08% to 0.22% with mottled Mucuna cochinchinensis having the lowest value and Mucuna poggei the highest.  These values were lower than those reported in a previous investigation (Siddhuraju and Becker 2001). Phytates are known to increase requirements for minerals, especially phosphorus, which forms insoluble complexes with phytic acid. As common with legumes, mucuna seeds also contain oxalates, though at low levels. The mean total oxalate content is 0.018%.  The highest level of 0.027% occurred in Mucuna poggei seeds, while mottled Mucuna cochinchinensis variety contained the minimum (0.007%).  Similar levels were also observed by Ezeagu et al (2003).  Oxalates make calcium and other bivalent cations unavailable by forming insoluble chellates with them.

The saponin content of the mucuna seeds range from 0.21 to 0.74%.  Again, the lowest values were found in Mucuna cochinchinensis mottled. The amounts of saponin observed was substantially below the 3% which was reported by Kumar (1992) to be responsible for cattle losses when they grazed on Drymaria arenroides.  Saponins are known to cause gastroenteritis, manifested as diarrhoea and dysentery in man. It also suppresses rumen protozoa by reacting with protozoan cell membrane cholesterol, causing the cells to lyse. On the other hand, Oakenfull and Sidhu (1990) reported that it reduces body cholesterol by reducing cholesterol absorption and increasing its excretion, thereby reducing blood pressure. Alkaloid content ranged between 1.22% in mottled Mucuna cochinchinensis and 2.44% in  maroon Mucuna cochinchinensis. Alkaloids are a wide range of mildly alkaline nitrogenous compounds of mostly plant origin, of high activity, and could be extremely toxic. According to Redmond (2008) 0.2g of coniine, an alkaloid found in the seeds of hemlock, is fatal to an adult human, others, such as nicotine and cocaine are dangerous addictive drugs. Many of these same alkaloids are, however, of great medicinal value, when properly used.

Conclusion 

• The overall nutrient and anti-nutrient factor profiles found in these Mucuna varieties from Benue State of Nigeria are comparable to those found in Mucuna of other regions of the World and, in many respects, to those of traditional food legumes such as cowpea, soybean and groundnut. 

  
  

• Since anti-nutrients can be eliminated or reduced below toxic levels by proper processing, these mucuna bean varieties are potential sources of both food and feed.

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Received 23 May 2008; Accepted 15 June 2008; Published 3 October 2008

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