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Chemical composition and nutritive value of some wild-gathered tropical plant seeds

Ikechukwu E. Ezeagu, Cornelia C. Metges, Jürgen Proll, Klaus J. Petzke, and Akintunde O. Akinsoyinu

Editorial comment

The following article gives the chemical composition and nutritive value of some wild-gathered tropical plant seeds. The authors recognize that without information on yield, acceptability, and possible toxic and antinutrient factors, this information is of limited immediate practical value. This article is published in the hope that it will promote further research and that some of the seeds described will prove to be a useful addition to indigenous food supplies.

Abstract

As part of the search for alternative sources of food to alleviate hunger, this study reports the approximate total contents of soluble sugars, starch, and gross energy of nine lesser-known wild-gathered plant seeds. High crude protein levels occurred in Lonchocarpus sericeus (28.03%), Albizia zygia (32.90%), and Gliricidia septum (34.15%). High levels of fat occurred in Entandrophragma angolense (59.30%), L. sericeus (34.15%), and Millettia thonningii (30.66%). Low levels of crude protein but high levels of total carbohydrate were found in Diospyros mespiliformis (77.21% carbohydrate), Daneillia ogea (74.32% carbohydrate), and Afzelia belle (53.96% carbohydrate). Starch contents were high in A. zygia (40.46%), D. ogea (69.62%), and A. belle (51.43%). The seeds with high fat content invariably showed high gross energy levels: E. angolense (30.9 kJ/g), L. sericeus (24.37 kJ/g), and M. thonningii (25.12 kJ/g). The results are compared to those for soya bean and other common staples. It is concluded that these less familiar wild seed plants should not be ignored, but further investigation into possible toxic and antinutrient factors, amino acid patterns, digestibility, and fatty acid composition is still required before recommendations are made.

Introduction

In most developing tropical countries the food situation is worsening owing to increasing population, shortage of fertile land, high prices of available staples, and restrictions on the importation of food [1, 2]. This has resulted in a high incidence of hunger and malnutrition, a situation in which children and women, especially pregnant and lactating women, are most vulnerable [3, 4].

Predictions of future food needs based on the current rates of population increase and food production emphasize the seriousness of this problem [5]. There can be no immediate single solution to the problem of food sufficiency. An interdisciplinary approach is necessary [6]. All information on new sources of food will be of value in dealing with the food problem [7].

While every measure is being taken to boost food production by conventional agriculture, a lot of interest is currently being focused on the possibilities of exploiting the vast numbers of less familiar plant resources existing in the wild [8-10]. Many such plants have been identified, but the lack of data on their chemical composition has limited the prospects for their utilization [11, 12]. Most reports on some lesser-known and unconventional crops indicate that they could be good sources of nutrients, and many have the potential of broadening the present narrow food base of the human species [13-16].

This study provides data on the nutrient composition of nine species of wild-gathered plant seeds (Afzelia bella, Albizia zygia, Daneillia ogea, Diospyros mespiliformis, Entandrophragma angolense, Gliricidia septum, Lonchocarpus sericeus, Millettia thonningii, and Pterocarpus santalinoides) in comparison with soya bean (Clycine max) grown in Nigeria. The present uses of these species vary with location and there is little information on these uses. However, there are reports on the use of these species as human food (D. mespiliformis, D. ogea, A. bella, A. zygia) or animal fodder (A. bella, M. thonningii, G. septum), or of their toxicity to humans or animals (G. septum, L. sericeus). Use of these species for fuel or timber (E. angolense, M. thonningii) and medicine (D. mespiliformis, M. thonningii, E. angolense) has also been reported [17,18]. Most of these species are presently being screened for agroforestry systems (Akinside BA. Personal communication. ICRAF/IITA Project, Ibadan, Nigeria, 1995).

Materials and methods

Mature seed samples were harvested from villages around the city of Ibadan with the help of the local people. The samples were identified at the Forestry Research Institute in Ibadan. The soya bean sample (Glycine max. TGX 1660-15F) was obtained from the International Institute of Tropical Agriculture in Ibadan. The seeds were milled to flour in a Wiley mill to pass a 0.5-mm mesh sieve and stored at 4°C until use.

Moisture content was determined by drying at 110°C for at least 24 hours until the weight became constant. Nitrogen was determined by the standard micro-Kjeldahl method [19] using a digestion apparatus (Kjeldatherm System KT 40, Gerhardt Laboratory Instruments, Bonn, Germany) and a titration system (T110-TR160-TA10-TM120, Schott-Gerate GmbH, Hofheim, Germany). Crude protein content was calculated for the present by multiplying by 6.25. Crude lipid content was assayed by extraction with petroleum ether (boiling point 40°-60°C) in a soxhlet extractor. Gross energy was determined by the use of an adiabatic bomb calorimeter (IKA-Calorimeter C4000, Janke & Kunkel, IKA Analysentechnik, Heitersheim, Germany). Total soluble sugars and starch were determined by the combined methods of Duboise et al. [20] and Kalenga et al. [21]. Soluble sugars were extracted with ethanol (95%) and residual starch was then hydrolysed with perchloric acid into monosaccharides. The sugar was then colorimetrically determined with phenol-sulphuric acid by means of a UV-VIS spectrometer (UVIKON 932, Kontron Instruments S.p.A., Milan, Italy).

Results and discussion

Table 1 shows the proximate composition of the seeds. As usual in plants, there is wide variation in composition [22]. Crude protein contents ranged from 43.52% in G. septum to 5.46% in D. mespiliformis. The crude protein contents of G. septum, L. sericeus, and A. zygia were found to be higher than those of soya bean and commonly cultivated legumes such as cowpea, pigeon pea, and lima beans [15, 23]. The legumes are generally high in protein and thus have the greatest promise as concentrated sources of low-cost plant protein [23]. Comparably high crude protein levels have been reported in some lesser-known tropical legumes [24, 25]. D. mespiliformis, D. ogea, A. bella, and A. zygia, with high contents of carbohydrates (77%, 74%, 54%, and 50%, respectively) could be good supplements to scarce cereal grains as sources of energy in feed formulations. A high fat content was found in M. thonningii (30.66%), G. septum (23.9%), A. hella (23.17%), and E. angolense (59.30%), resulting in high energy contents as well. The oil content of E. angolense was particularly high and compared well to that reported for groundnut (46%) [26]. It promises to be a good source of edible or industrial oil. The ash content was lowest in D. ogea (1.9%) and highest in G. sepium (3.9%). The soya bean sample contained more ash than any of the seeds, and therefore soya bean may not be a good source of minerals.

TABLE 1. Proximate composition of plant seeds

NO = not determined.

The total soluble sugars, starch, and gross energy contents are shown in table 2. A substantial amount of starch was found in D. ogea, A. hella, A. zygra, and D. mespiliformis (69.62%, 51.43%, 40.46%, and 37.81 %, respectively). Except for A. zygia and D. mespiliformis, with sugar contents of 10.04% and 14.61 %, respectively, the sugar contents are low, but they compare favourably with the range of 4.99% to 7.22% reported for some legumes and cereals [27]. The seeds of M. thonningii L. sericeus, A. bella, and E. angolense had high fat contents and hence high gross energy contents. In view of the data presented here, these tropical crop seeds could play a substantial role in alleviating food shortages in Nigeria and elsewhere if given adequate attention [28 30]. However, further research into possible toxic constituents, amino acid patterns, and digestibility will be necessary before they can be used as human or animal food.

TABLE 2. Total soluble sugars, starch, and energy contents of plant seeds

ND = not determined.

Acknowledgements

I. Ezeagu was a scholar of the Deutscher Akademischer Austauschdienst, Bonn, Germany, at the German Institute for Human Nutrition, whose financial support is gratefully acknowledged.

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