The Proximate,Mineral,and Toxicant Compositions of Four Possible Food Security Crops from Southeastern Nigeria

Abstract

The proximate, nutritional, and antinutritional compositions of the raw, cooked, and roasted samples of four Nigerian indigenous seeds—Sphenostylis stenocarpa, Pentaclethra macrophylla, Mucuna flagellipes, and Citrullus colocynthis—were evaluated. Also estimated were zinc and divalent cation bioavailability of the seeds using millimolar ratios/kg dry weight of [calcium]/[phytate], [phytate]/[zinc], [calcium][phytate]/[Zn], and [phytate]/[total phosphorus]. The results obtained revealed that the seeds of P. macrophylla and C. colocynthis had high protein and lipid levels. All the seeds were also found to have high energy value and low moisture content. Mineral analysis showed the presence of Na, K, Ca, and Mg in appreciable quantities and Zn, I, Fe, and Se in minute quantities. Antinutritional analyses indicated the presence of traces of tannin, oxalate, phytate, saponin, and cyanide in the samples. The various processing techniques had significant (P ≤ .05) effects on the measured parameters. The calculated [Ca][phytate]/[Zn] molar ratios revealed that these seeds had values above the critical level of 0.5 mL/kg, thus indicating reduced bioavailability of zinc. In view of the high nutrient contents, low antinutritional contents after processing, and their superabundance, these seeds could be cheap nutrient sources. The implications of these findings with regards to food security are enormous.

Introduction

T he importance of plants to humans, directly or indirectly, is demonstrated by the useful by-products from both wild and cultivated species. Plants have numerous uses as sources of food for humans and livestock, as sources of fuel (energy), as sources of medicines and medicinal products, and as structural materials, among others.¹ According to Okigbo,² it is this importance of plants as food sources that links agriculture with civilization.

In Nigeria, the food situation is worsening owing to increasing population and restrictions on the importation of food, among several other factors. This has resulted in a high incidence of hunger and malnutrition, a situation in which children and women are most vulnerable. While every measure is being taken to boost food production by conventional agriculture, a lot of interest is currently focused on the possibilities of exploring the vast numbers of less familiar plant resources existing in the wild.³ Apart from the incidence of hunger posed by the worsened food situation, the widespread prevalence of protein energy malnutrition has resulted in high morbidity and mortality rates, especially among infants and children in developing countries, including Nigeria. The reliance on starchy roots and tubers and protein-deficient cereals as staples results in consumption of stodgy, monotonous, non-nutritious diets. The insufficient availability of animal protein sources and the high cost of the few available plant protein sources have prompted an intense research into the possible exploitation of the nutrient potentials of lesser-known underutilized legumes and seeds.

Some reports^2,4 on some lesser-known and unconventional crops indicate that they could be good sources of nutrients and may have the potential of broadening the present narrow food base for humans. We^5
–7 have also reported that many Nigerian plants in addition to serving as food sources also serve as sources of herbal remedies as well as sources of food supplements capable of addressing several contemporary diseases like dyslipidemia, cardiovascular abnormalities, etc. Among the nutritive and inexpensive indigenous plants in Nigeria are Mucuna flagellipes, Pentaclethra macrophylla, Sphenostylis stenocarpa, and Citrullus colocynthis.

S. stenocarpa, or African yam bean, is a grain legume belonging to the Fabaceae family of flowering plants. It is thought to originate from West Africa. Its other names in various Nigerian languages are “girigiri” (Hausa), “sese” (Yoruba), “okpodudun,” “ijiriji,” or “asama” (Igbo), and “nsama” (Ibibio). It has so many species that vary in seed size, shape, and seed coat color, which varies from white to various shades of cream, brown, gray, and some are mottled with the helium being dark brown in color. Some cultivars produce edible tubers, which resemble that of yam, under the soil which may be up to 0.5 kg in weight.⁸ The cooked seeds may be mixed with palm oil, salt, pepper, and other ingredients, resulting in a pottage that may also be supplemented with yam, cocoyam, plantain, sweet potatoes, tapioca, or cereals such as maize or sorghum.⁸

M. flagellipes is also a member of the Fabaceae family of flowering plants. It occurs from Sierra Leone to Central African Republic and also from the Democratic Republic of the Congo to Angola in the south. M. flagellipes is called “ukpo” in Igbo, “karangiwa” in Hausa, and “agbaarin” in Yoruba. The plant is a climbing perennial herb and produces long fairly, longitudinal ribbed pods that are covered with brownish to reddish yellow hairs. The pod is green when immature but black when mature and dry. Each pod contains one to four seeds, which are white when immature but black when mature and dry. The seed has a dark helium that is about one-half to one-third of the circumference of the seed. Contained in each seed are two white cotyledons, which are covered by the testa.⁹

P.macrophylla, the African oil bean, is a highly nutritious leguminous crop seed abundant in the rain forest areas of West and Central Africa.¹⁰ It is a perennial tree crop belonging to the Fabaceae family of flowering plants with no known varietal characterization.¹¹ The flat glossy edible seeds, averaging eight (range, six to 10) in number, are contained in a brownish flattened pod, which explodes at maturity to disperse the seeds. The seed cotyledon, which is gray in color, is embedded in a glossy brownish seed coat. The local names include “ugba,” “ukpaka,” or “ukpakala” in Igbo, “Congo acacia” in Congo, and “dualakombola” in Cameroun.¹⁰

C. colocynthis, or colocynth, is an annual herbaceous plant belonging to the Cucurbitaceae or melon family of the flowering plants. It is called “Ogbenwa” or “Ikahu” in the Igbo language. It has other common names in English such as bitter apple, gall, or wild gourd. It is said to have originated from Africa. It is native to dry areas of North Africa, being common throughout the Sahara areas of Morocco, Egypt, and Sudan as well as in West African countries like Nigeria and Ghana. The plant is highly xerophytic and thrives best in a mean annual temperature range of 23–27°C and annual rainfall range of 25–27 cm. The fruit is green and mottled with pale green or yellowish variegated stripes. The pulp is whitish, light in weight, spongy, and easily broken with an intensely bitter taste. The seeds are numerous, oval-shaped and flat, 9–10 mm long and 4 mm wide. They are initially white but turn to brown when mature and dry.¹²

Legumes, whether wild or cultivated species, are known to be good sources of protein, carbohydrates, and other nutrients. However, most of them are characterized by the presence of antinutritional factors like phytates, oxalates, saponins, tannins, and cyanide, which may affect nutrient bioavailability or cause acceptability problems.¹³ Several studies have shown that different processing methods reduce the levels of antinutrients significantly and also improve nutritional quality.^10,14

Because of the increase in the demand of the major food staples it has become absolutely necessary to study, analyze, and elucidate the nutritional potentials of some less familiar seed crops that could serve as viable alternatives. It is also necessary to study the possible processing techniques that will improve or preserve their nutrient quality while eliminating or at least decreasing their antinutrients. As a consequence of this, our study is aimed at evaluating the proximate, mineral, and antinutritional compositions of four indigenous seeds, namely, S. stenocarpa, P. macrophylla, C. colocynthis, and M. flagellipes, before and after two different processing methods (cooking and roasting). The study is also aimed at estimating the zinc bioavailability of the seeds in humans as some antinutrients are known to affect mineral bioavailability.

Materials and Methods

Collection and preparation of samples

Sample collection and preparation followed standard botanical field collection methodology.¹⁵ Samples of the seeds studied were obtained from the villages adjoining the Federal University of Technology, Owerri, Nigeria, and identified by a taxonomist at the Department of Forestry and Wildlife Technology of the University. Prior to processing, infected seeds were separated and discarded. The seeds of C. colocynthis were recovered from the pods, washed, dried, and peeled to recover the whitish cotyledons. The cotyledons of M. flagellipes were recovered from the seeds by cracking off the hard outer covering (testa), whereas the cotyledons of P. macrophylla were recovered from the seeds by peeling off the outer covering.¹⁰ Thereafter, the four seed samples were divided into three distinct groups based on processing.

The first group consisted of raw seeds of the four samples, each of which was dried to a constant weight in an oven (Gallenkamp model IH-100, Sanyo Gallenkamp plc, Loughborough, UK) at 60°C for 24 hours, after which each was ground separately using a manual grinder, sieved through a number 20 mesh sieve, and stored in airtight, labeled containers in a refrigerator prior to analysis. The second group had the four seeds, each of which was cooked until it became soft as is done traditionally. The cooked seeds were then dried in an oven to a constant weight, ground, sieved, and stored in airtight, labeled containers in a refrigerator before use. The third group consisted of the four seeds, each of which was roasted before grinding, sieving, and storing as in the previous cases. These dried powdered samples were then used for proximate, mineral, and antinutritional analyses.⁴

Proximate analysis

The proximate compositions (moisture, crude fiber, crude fat, and crude protein contents) of the samples were determined by the AOAC methods.¹⁶ The total carbohydrate content of each sample was estimated as the nitrogen free extract using the arithmetic difference method.¹⁶ The caloric value of the sample was estimated by summing the multiplied values for crude protein, crude fat, and carbohydrate by their respective Atwater factors of 4, 9, and 4, respectively.¹⁶

Mineral analysis

Mineral concentration of the samples was determined by ashing 1 g of each sample and dissolving the ash in a final volume of 2 M HCl before filtering and using the filtrate for elemental analyses. Calcium and magnesium were determined by the versenate-EDTA complexiometric titration method. Potassium and sodium were determined using a nonluminous level (blue color) of a self-igniting flame photometer.¹⁷ The selenium, iron, zinc, iodine, and lead contents of the samples were determined using a Perkin-Elmer (Boston, MA, USA) model 372 flame photometer according to the manufacturer's instructions.

Estimation of zinc bioavailability

[Phytate]/[Zn], [Ca]/[phytate], and [Ca][phytate]/[Zn] molar ratios were calculated and used in the prediction of zinc bioavailability.¹⁸

Antinutritional analysis

Tannin levels in the samples were determined using the Folin-Denis spectrophotometric method.⁵ Oxalate content was determined using permanganate titration.¹⁶ The cyanide content of the sample was determined using the alkaline picrate method.⁵ Phytate content of each sample was determined by the spectrophotometric method.^19,20 Saponin content was determined by the double solvent extraction gravimetric method.²¹

Data analysis

Triplicate (duplicate, in some) determinations of each parameter were pooled and expressed as mean ± SD values. Resulting data were subjected to two-way analysis of variance, and differences at P ≤ .05 were accepted as significant.²²

Results and Discussion

Table 1 shows the proximate composition of the seeds studied. It reveals that processing reduced the proximate compositions (except in a few cases where processing either increased or had no significant effect). Table 2 shows the mineral composition of the seeds. The results show that processing (cooking and roasting) also reduced the level of the minerals in most cases. Table 3 shows the calculated [phytate]/[Zn], [Ca]/[phytate], and [Ca][phytate]/[Zn] molar ratios of the seeds studied for possible prediction of zinc bioavailability. The calculated values of [Ca][phytate]/[Zn] molar ratios for the seeds studied followed the pattern: S. stenocarpa > C. colocynthis > P. macrophylla > M. flagellipes. Table 4 shows the results of the antinutrient composition of the seeds and reveals that the seeds studied have low antinutrient levels. The results also show that processing methods reduced the levels of these antinutrients.

Table 1.

Proximate Composition (% Dry Weight) of the Seeds Studied

Seed	Moisture	Crude fiber	Crude fat	Crude protein	Ash	Total carbohydrate	Caloric values (Kcal)
S. stenocarpa
Raw	8.95 ± 0.01	4.95 ± 0.01	1.13 ± 0.05	19.13 ± 0.20	4.99 ± 0.03	60.84 ± 0.20	330
Cooked	7.65 ± 0.01	4.94 ± 0.02	1.07 ± 0.01	16.68 ± 0.20	4.37 ± 0.06	65.29 ± 0.14	338
Roasted	8.05 ± 0.05	4.70 ± 0.10	0.99 ± 0.01	18.33 ± 0.11	4.79 ± 0.04	63.14 ± 0.01	335
C. colocynthis
Raw	6.34 ± 0.01	6.93 ± 0.01	38.09 ± 0.19	28.30 ± 0.28	4.34 ± 0.04	16.07 ± 0.16	520
Cooked	6.16 ± 0.02	6.89 ± 0.03	32.75 ± 0.35	25.08 ± 0.40	3.80 ± 0.22	25.31 ± 0.17	496
Roasted	6.02 ± 0.03	6.77 ± 0.01	37.32 ± 0.02	27.36 ± 0.06	4.20 ± 0.03	18.33 ± 0.09	519
P. macrophylla
Raw	10.19 ± 0.01	5.03 ± 0.01	41.33 ± 0.03	22.63 ± 0.20	2.54 ± 0.03	17.16 ± 0.97	531
Cooked	9.79 ± 0.21	4.99 ± 0.03	36.82 ± 0.07	21.23 ± 0.20	1.77 ± 0.03	25.36 ± 0.38	518
Roasted	10.12 ± 0.01	5.00 ± 0.15	43.95 ± 0.08	24.56 ± 0.02	4.92 ± 0.03	11.45 ± 0.11	540
M. flagellipes
Raw	5.05 ± 0.01	4.16 ± 0.02	8.46 ± 0.02	26.02 ± 0.20	5.76 ± 0.08	50.29 ± 0.56	381
Cooked	5.36 ± 0.52	4.11 ± 0.03	4.67 ± 0.01	23.33 ± 0.54	5.02 ± 0.09	57.83 ± 0.45	367
Roasted	4.98 ± 0.06	4.07 ± 0.03	7.24 ± 0.04	26.67 ± 0.07	5.59 ± 0.01	51.45 ± 0.08	378

Data (except caloric values) are mean ± SD values of triplicate determinations.

Table 2.

Mineral Composition of the Seeds Studied (mg/100 g Dry Weight)

Seed	Na	K	Ca	Mg	I	Zn	Fe	Se	Pb
S. stenocarpa
Raw	23.3 ± 0.1	190.67 ± 1.41	85.51 ± 4.63	27.20 ± 2.77	0.08	0.36	2.90	0.16	0.02
Cooked	18.27 ± 0.1	142.27 ± 2.57	69.47 ± 4.63	16.00 ± 2.77	0.04	0.34	2.85	0.15	0.01
Roasted	21.80 ± 0.1	189.42 ± 1.50	78.10 ± 3.50	20.04 ± 2.77	0.06	0.36	2.87	0.15	0.01
C. colocynthis
Raw	12.77 ± 0.1	138.93 ± 1.22	66.80 ± 3.07	99.20 ± 2.77	0.12	0.52	3.80	0.13	0.04
Cooked	9.80 ± 0.01	111.17 ± 0.72	42.75 ± 4.63	80.00 ± 2.77	0.09	0.46	3.45	0.12	0.03
Roasted	11.11 ± 0.08	136.28 ± 1.41	62.01 ± 4.18	83.75 ± 2.77	0.11	0.50	3.77	0.13	0.03
P. macrophylla
Raw	26.40 ± 0.10	250.13 ± 1.22	109.55 ± 5.04	20.08 ± 2.77	0.03	0.72	3.30	0.09	0.03
Cooked	22.57 ± 0.12	232.00 ± 2.12	93.52 ± 4.63	17.60 ± 2.77	0.02	0.66	3.15	0.08	0.01
Roasted	25.02 ± 0.03	245.27 ± 1.20	105.69 ± 4.96	19.02 ± 2.77	0.02	0.71	3.24	0.09	0.02
M. flagellipes
Raw	21.83 ± 0.06	261.60 ± 1.60	123.58 ± 4.17	36.80 ± 2.77	0.02	0.58	4.30	0.11	0.01
Cooked	17.50 ± 0.10	187.47 ± 1.22	109.56 ± 4.74	25.60 ± 2.77	0.01	0.54	3.90	0.09	ND
Roasted	20.10 ± 0.06	234.12 ± 1.72	120.17 ± 4.01	32.35 ± 2.77	0.10	0.58	4.11	0.11	ND

Data are mean ± SD values of triplicate determinations. Data without SD are values of single determinations.

ND, not detected.

Table 3.

Results of Computed Zinc Bioavailability Parameters of the Seeds Studied

Seed	[Ca] (mol/kg dry weight)	[Zn] (mol/kg dry weight)	[Phytate] (mol/kg dry weight)	[Phytate]/[Zn]	[Ca]/[phytate]	[Ca][phytate]/[Zn]
S. stenocarpa
Raw	2.14	0.0055	0.64	116.36	3.34	249.01
Cooked	1.74	0.0052	0.58	111.54	3.00	194.08
Roasted	1.93	0.0055	0.58	105.46	3.36	205.64
C. colocynthis
Raw	1.67	0.008	0.98	122.50	1.70	204.58
Cooked	1.07	0.0071	0.72	101.41	1.49	108.51
Roasted	1.55	0.0077	0.80	103.90	1.94	161.04
P. macrophylla
Raw	2.74	0.011	0.55	50.00	4.98	137.00
Cooked	2.34	0.010	0.49	49.00	4.78	114.66
Roasted	2.64	0.0109	0.54	49.10	4.89	129.60
M. flagellipes
Raw	3.09	0.0089	0.51	57.30	0.06	117.08
Cooked	2.74	0.0083	0.38	45.78	7.21	125.45
Roasted	3.00	0.0089	0.46	51.69	6.52	155.06

Table 4.

Results of the Antinutritional Composition of Seeds Studied (Dry Weight)

Seed	Tannin (%)	Oxalate (%)	Cyanide (mg/kg)	Phytate (%)	Saponin (%)
S. stenocarpa
Raw	1.82 ± 1.15	0.29 ± 0.01	11.61 ± 0.05	0.42 ± 0.01	0.19 ± 0.02
Cooked	1.36 ± 1.18	0.20 ± 0.01	4.56 ± 0.03	0.38 ± 0.01	0.07 ± 0.01
Roasted	1.38 ± 0.01	0.25 ± 0.08	7.83 ± 0.04	0.38 ± 0.034	0.11 ± 0.03
C. colocynthis
Raw	2.92 ± 0.96	0.41 ± 0.01	18.18 ± 0.06	0.64 ± 0.02	0.29 ± 0.03
Cooked	1.28 ± 1.23	0.24 ± 0.02	9.73 ± 0.08	0.47 ± 0.01	0.13 ± 0.01
Roasted	1.72 ± 0.01	0.29 ± 0.06	13.15 ± 0.10	0.52 ± 0.02	0.22 ± 0.00
P. macrophylla
Raw	2.69 ± 0.29	0.38 ± 0.03	15.29 ± 0.06	0.36 ± 0.01	0.35 ± 0.02
Cooked	2.37 ± 0.26	0.24 ± 0.01	11.77 ± 0.15	0.32 ± 0.05	0.21 ± 0.02
Roasted	2.45 ± 0.03	0.32 ± 0.01	12.57 ± 0.13	0.33 ± 0.03	0.28 ± 0.05
M. flagellipes
Raw	2.44 ± 0.51	0.30 ± 0.01	14.02 ± 0.06	0.33 ± 0.01	0.35 ± 0.01
Cooked	2.33 ± 0.29	0.19 ± 0.01	9.90 ± 0.08	0.25 ± 0.02	0.23 ± 0.03
Roasted	2.10 ± 0.01	0.18 ± 0.01	10.03 ± 0.06	0.30 ± 0.02	0.25 ± 0.04

Data are mean ± SD values of triplicate determinations.

The results of the proximate analysis (Table 1) of the seeds studied show that they have low moisture contents. This indicates that the seeds could be stored for long periods. This further means that these crops could serve as an insurance against hunger when many staples are out of season. The results also show that the ash and crude fiber contents of the seeds are generally low. The different processing techniques further decreased their levels except for P. macrophylla, where roasting appeared to increase the ash content. The combination of legumes and staples like maize (which are rich in fiber) in Eastern Nigeria is therefore a wise nutritional decision.

The fat contents of P. macrophylla and C. colocynthis were exceptionally higher than those of S. stenocarpa and M. flagellipes. This indicates that these seeds might serve as sources of oil for diverse uses, including biofuel/biodiesel production, as earlier observed for some other Nigerian crops.²³ The different processing steps decreased significantly (P < .05) the fat content of the seeds except in P. macrophylla, where roasting increased significantly (P < .05) the fat content. The seed of P. macrophylla is quite popular in the preparation of snacks in Eastern Nigeria. Our result compares favorably with the value of 41.50% reported earlier.¹⁰ This value and that of C. colocynthis are higher than the values reported for other common Nigerian legumes.²⁴

The results also show that the studied seeds have high protein levels. Legumes are usually rich in proteins, and they also enhance soil fertility because of their symbiotic relationship with nitrogen-fixing bacteria.²⁴ However, processing techniques significantly (P < .05) reduced the protein content of the seeds in most cases even though the levels remained appreciable. The protein content of these seeds compared favorably with values reported for some other legumes like Vigna radiata (24.50%), Vigna aconitifolia (25.30%), Cicer arietinum (20.70%), Vigna mungo (23.60%), Phaseolus vulgaris (25.10%), and Prosporis africana (24.40%).²⁴ The protein contents of the seeds were lower compared to the value of 27.30% reported for Mucuna pruriens, but higher than the values reported for some staple foods in Nigeria such as yam (7.80%), rice (8.66%), and maize (7.80%).²⁵ Because these seeds are good sources of proteins, they can be combined with low-protein carbohydrate staples like cassava, maize, etc., especially for children who are the worst hit by protein-deficient diets.

The results also show that the carbohydrate contents of S. stenocarpa and M. flagellipes are higher than those of C. colocynthis and P. macrophylla. The carbohydrate content of S. stenocarpa, from our results, compares favorably with the value earlier reported for the same seed.²⁵ The seeds are also high energy yielders, as seen from the results of their energy values. The high energy values are in consonance with the high concentrations of fat, protein, and carbohydrate in the seeds in most cases. This further confirms the usefulness of these seeds as good sources of nutritional energy to humans.

The results of mineral analysis (Table 2) reveals that the seeds contain appreciable levels of the major minerals (Na, K, Ca, and Mg), with M. flagellipes having the highest levels of K and Ca, whereas P. macrophylla had the highest level of Na, and C. colocynthis had the highest Mg content. Zinc levels in all the samples were generally low. The Ca in each of these seeds satisfies the recommended dietary allowance of 800 mg for both adults and children. Also, the Mg, Fe, and P levels are adequate and meet the recommended dietary allowances of 350, 10, and 800 mg, respectively.²⁶ The Mg levels are higher than those reported by Udosen²⁷ for some Nigerian vegetables. The mineral levels of the seeds are also higher than what Ola and Oboh²⁸ reported for some Nigeria mushrooms gathred from the wild. However, the Fe, Zn, and K levels are lower than the values reported for some commonly consumed tropical yam tubers.²⁹ The combination of these seeds with yam or cocoyam is therefore nutritionally wise and ensures the intake of these essential minerals. M. flagellipes showed the presence of the antinutrient Pb, even though roasting, which is one of the methods used for processing some of these seeds before consumption by Nigerians, was sufficient to eliminate it. This trace of lead may have arisen in the first place as an artifact. These findings make these seeds important because many enzymes require these ions contained in them as cofactors. The presence of iodine (even though in trace amounts) further accentuates the importance of the seeds especially now that goiter arising from iodine deficiency is a global problem. The seeds can therefore be readily incorporated into diets like gruels prepared from maize, millet, or guinea corn, which are known to be nutritionally deficient.

As shown in Table 3, the calculated [phytate]/[Zn] molar ratios for the processed and unprocessed studied seeds were above 15, the critical value for predicting a negative effect of phytate on zinc bioavailability. Also, the calculated [Ca]/[phytate] molar ratios for the seeds studied were below 6.0 (except for M. flagellipes), the positive predictive critical value. These results show that zinc bioavailability is depressed.¹⁸ Calcium has a sparing effect on Zn, and at the critical [Ca]/[phytate] molar ratio of 6:1, phytate is completely precipitated from the solution. Zinc is then spared in the solution and so is available for absorption. Below this ratio, phytate precipitation is incomplete, so that some of the dietary Zn remains in solution.³⁰ Evidently, the solubility of phytate and the availability of Zn in the intestine depend on dietary calcium levels. Oberlease²⁰ has taken the argument further by recommending the [Ca][phytate]/[Zn] molar ratio as a better index for predicting Zn bioavailability, with a critical value of 0.50 mol/kg, in place of [phytate]/[Zn] molar ratios, positing that because of a kinetic synergism that exists between [Ca] and [Zn] ions, a ternary complex Ca: Zn:phytate is formed. This complex is less soluble than the binary phytate complex formed with either Zn or Ca alone. The calculated [Ca][phytate]/[Zn] molar ratios obtained in this study were generally higher than the proposed critical value of 0.50 mol/kg. This then reinforces the observation that the phytate contents of the seeds are capable of reducing their Zn bioavailability. This, however, should not give much cause to worry because these molar ratios are superior to the values reported for the more commonly consumed cassava (0.02–0.14) mol/kg and mushrooms (0.01–0.22 mol/kg)²⁸ Nigerian staples. Furthermore, some processing methods like boiling are known to enhance the quality of foods¹⁰ and so can improve the bioavailability of nutrients.

The results of the antinutrient composition of the studied seeds (Table 4) reveal the presence of cyanide in the seeds, with C. colocynthis having the highest level, whereas S. stenocarpa had the lowest level. Processing, however, decreased the cyanide levels significantly (P < .05). The cyanide content of these seeds was higher than what Oboh and Ekperigin²⁴ reported for some seeds gathered in the wild, but lower than the value of 21.30 mg/kg reported for cassava flour, a major Nigerian staple. The seeds could therefore be considered safe with regard to cyanide poisoning because the cyanide levels are far below the detrimental level of 30 mg/kg reported in the cassava study.³¹

Phytate was found to be present in the seeds studied, although in very low concentrations. Phytic acid (inositol hexaphosphoric acid) is widely distributed in vegetables, and it is considered an antinutrient because of the possibility of its interference with proteolytic digestion, in addition to the fact that the phosphorus in it is not nutritionally available to monogastric animals.³² It is also considered an antinutritional factor because it complexes with nutritionally essential divalent cations like Ca, Fe, Mg, and Zn, thus reducing their absorption from the diet. Cooking was found to significantly (P < .05) decrease the phytate content of the seeds in most cases, while roasting decreased significantly the phytate content of C. colocynthis only. In view of this, cooking is a better means of reducing the phytate content of these seeds. The values of phytate reported in this study were higher than the reported values for some East African foods but lower than the values reported for some Nigerian seeds gathered in the wild.^24,28 The presence of phytate in popular Nigerian foodstuffs is not strange because it has been copiously documented.^10,33

The oxalate levels of the studied seeds were generally low compared to the values reported for other seeds in Nigeria.³⁴ This makes the seeds safe relative to oxalate as oxalate consumption brings about a significant reduction in calcium level by forming an insoluble calcium oxalate.³⁵ Each processing step in this study further decreased the oxalate content of the seeds. Processing is therefore recommended in order to checkmate possible oxalate toxicity, especially if the seeds are to be consumed in large quantities or over a long period.

Tannin and saponin levels were also found to be present in low quantities in the seeds studied. The successive processing techniques were also found to decrease their levels in the studied seeds except in S. stenocarpa and P. macrophylla, where roasting had no significant (P > .05) effect on their levels. Roasting ordinarily is not a traditional method of processing P. macrophylla seeds. The seeds are boiled, sliced, and left to ferment before being consumed as a snack. These antinutrients will therefore be taken care of. The saponin and tannin contents of these seeds were higher than what was reported³⁵ for the seeds of Solanum nigrum. The possible toxic effects of tannin and saponin may be extremely reduced on consumption of these seeds because of their low levels, their further reduction by processing methods, and their joint presence since the presence of saponins reduces the toxic effect of tannins.³⁶

In conclusion, the results of the study show that the seeds studied are good nutrient sources even though they contain traces of antinutrients, the levels of which can easily be reduced by conventional processing techniques. Further processing may be necessary, however, to improve the cation bioavailability and eliminate or reduce antinutrients as recommended in some recent studies.³⁷ Considering the availability of the seeds and the relatively little or no cost of some of them, they could be integrated fully into human nutrition, especially into the weaning diet of growing children in this period of acute food insecurity and global economic meltdown.

Footnotes

Author Disclosure Statement

We confirm that no competing financial interests exist.

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