Introduction

After Dr.E.G.Irving, R.N., who died at Abeokuta in 1855. The flowers in this small genus resemble those of Klainedoxa but the fruits are quite different. Three species occur in Nigeria, one of which, I. grandifolia, has leaves and stipules very similar to those of Klainedoxa. The Irvingias, however, never have spines.

1a   Leaves cuneate or slightly rounded at base; flowers with distinct stalks: 1. gabonenesis

1b  Leavescordate or broadly rounded at base

2a  Leaves 2.5-12.5 cm long; flowers with distinct stalks: 2. smithii

2b Leaves 10-25 cm long; flowersstalkless: 3. grandifolia

Irvingiagabonenesis (O’ Rorke) Baill.- FWTA (Flora of West Tropical Africa) ed. 2, 1: 693. The Wild Mango or Dika Nut, with mango-like fruits. The tree may be readily recognised by its dense dark green evergreen foliage and characteristic stipules which are similar to those of Klainedoxa but smaller. J.C. Okafor in Bull.Jard. Bot. Nat. Belg. 45: 211-21 (1975) has distinguished two varieties as follows:

Fruits with sweet edible scanty fibrous pulp; bole fluted or cylindrical; lateral branches ascending, making the crown spherical or narrow: var. gabonensis.

Fruits with bitter inedible very fibrous pulp; bole buttressed; lateral branches horizontal, making a wide umbrella-shaped crown: var. excelsa (Mildbr.) Okafor. Names are, Hausa: goronbiri; Nupe: pekpear; Yoruba: oro; Edo: ogwe; Ijaw: ogboin; Igbo: obono; Efik: oyo;Ekoi: osing; Boki: bojep. The plant extends from Senegal to Sudan and south to Angola.

The fruit is widely used as a complement to other foods in most parts of southern Nigeria. Its kernels are a major raw material in the preparation of ogbono soup or as a condiment in the preparation of sauces used with African foods prepared from major staples such as amala, eba, pounded yam, etc2. Some reported pioneering workson the uses were3, ecological studies 4, 5; other works included the nutritional studies of pulp and kernel2 and nutritional value of the pulp6 and most recently the determination of the amino acid contribution of the hull of Irvingiagabonensis seed as food ingredient7.

Materials and Methods

Collection and treatment of samples

The seeds of the fruit were purchased from Ado-Ekiti, Nigeria, market. The seeds were dried, dehulled, separately pulverised, sieved and kept in freezer (-4oC) in McCartney bottles pending analysis.

Crude protein determination and fat extraction

The micro-Kjeldahl method8 was followed to determine the fat-free crude protein. The fat was extracted with a chloroform/methanol (2:1 v/v) mixture using Soxhlet extraction apparatus9 lasting 5-6 h.

Determination of amino acids

Hydrolysis of samples

About 30 mg of defatted sample was weighed into glass ampoules. 7 ml of 6 MHCL was added and oxygen expelled by passing nitrogen gas into the samples. The glass ampoules were scaled with a Bunsen flame and put into an oven at 105±5oC for 22 h. The ampoule was allowed to cool; the content was filtered to remove the humins. The filtrate was then evaporated to dryness at 40oC under vacuum in a rotary evaporator.

Analysis of samples

Amino acid analysis was by ion exchange chronmatography (IEC)10 using the Technicon Sequential Multisample (TSM) Amino Acid Analyser (Technicon Instruments Corporation, New York). The period of analysis was 76 min for each sample. The gas flow rate was 0.50 ml/min at 60oC with reproducibility consistent within ± 3 %. The net height of each peak produced by the chart recorder of the TSM (each representing an amino acid) was measured and calculated. The amino acid determinations were in duplicate. Tryptophan was not determined due to cost. Norleucine was the interval standard.

Estimation of quality of dietary protein

Amino acid score

The amino acid score was calculated in three different ways.

(a) The amino acid score based on the whole hen’s egg11. It was calculated by using the ratio of test protein to the reference protein for each amino acid.

(b) The essential amino acid score based on the provisional amino acid scoring pattern using the following formula12:

Amino acid score = Amount of amino acid per test protein [mg/g]/Amount of amino acid per protein in reference protein [mg/g].

(c) The essential amino acid score based on suggested school child requirement13.

Essential amino acid index (EAAI)

It was calculated by using the ratio of test protein to the reference protein for each of the eight essential amino acids plus histidine in the equation that follows14:

Predicted protein efficiency ratio (P-PER)

This was determined using one of the equations developed by Alsmeyeret al.15,

P-PER = -0.468 + 0.454 (Leu)- 0.105 (Tyr)

Determination of other quality parameters

Determination of the total essential amino acid (TEAA) to the total amino acid (TAA) (TEAA/TAA); total sulphur amino acid (TSAA); percentage cystine in TSAA (% Cys/TSAA); total aromatic amino acid (TArAA); etc., the Leu/Ile ratios were also calculated.

Estimation of isoelectric point (pI)

The pI for the mixture of the amino acids was estimated from the equation of the form16:

whereIPm is the isoelectric point of the mixture of amino acids, IPi is the isoelectric point of the ith amino acid in the mixture and Xi is the mass or mole fraction of the ith amino acid in the mixture.

Statistical analysis

Calculations made were the mean, standard deviation (SD), coefficient of variation in percent (CV %), linear correlation coefficient (rxy), coefficient of determination (rxy2), linear regression coefficient (Rxy), coefficient of alienation (CA), index of forecasting efficiency (IFE) and the comparison of r = value (computed from the analytical data) with tabular value at r0.05 with n-2 degrees of freedom17.

Results and Discussion

In Table 1, the amino acid (AA) profiles of the two samples are shown. The protein levels in the two samples had a variation of 76.8 % and the dehulled: hull amino acid being 3.4:1. This is an indication that bulk of the protein had been concentrated in the dehulled sample. With this observation, it would be interesting to look at the distribution of the amino acids on corresponding comparison. Glutamic and aspartic acids were in the highest concentrations among their groups and are both acidic AA. Phenylalanine and tyrosine constituted the highest essential amino acid (EAA) concentration in both samples. The coefficient of variation percent (CV %) values were low with exception of Cys with a value of 52.2 %, whilst the rest ranged from 3.43-38.5 %.

Glu, Asp and Phe +Tyr trends in the present study followed the trend as observed in Gymnarchusniloticus (Trunk fish)18, Clariasanguillaris, Oreochromisniloticus and Cynoglossussenegalensis19 as well as aril and seed of Blighiasapida fruit20. Out of 17 parameters determined, 15 (88.2 %) amino acids were correspondingly higher in the dehulled sample with only 2 (11.8 %; Ala, 4.30 against 2.89 g/100 g crude protein, cp and Tyr, 3.17 against 3.02 g/100 g) being higher in the hull. Arg (4.00-5.10 g/100 g cp) is essential for children and reasonable levels were present here particularly in the dehulled. The Lys content of samples (3.02-3.51 g/100 g) were about half of the content of the reference egg protein (6.3 g/100 g cp), so the samples would only serve complementary roles in nutrition. The Met range was 1.02 (hull) – 1.66 g/100 g cp (dehulled) which compared favourably with 1.25-1.25 g/100 g cp in aril and seed of B. sapida20. Cys had the highest variation of 0.70-1.52 g/100 g cp or 52.2 %; this was also observed in the reference above20.

The contents of TEAA of 39.2 and 29.8 g/100 g cp without tryptophan (which was not determined) (Table 2) were slightly close to the value for egg reference protein (56.6 g/100 g cp)11 particularly for the dehulled sample. The present contents of TEAA are comparable to some literature values (g/100 g cp): 33.6 in Anacardiumoccidentale21; 31.2 in Parkiabiglobosa seeds22; 22.1 in endosperm of ripe coconut 23, 37.6-51.8 in six different varieties of dehulledSphenostylisstenocarpa flour24; values from oil seeds such as 45.2 in pigeon pea25, 53.4 (melon seeds), 38.3 (pumpkin seed), and 53.6 (gourd seed) respectively26; and soy bean with 44.427. The contents of TSAA were generally lower than the 5.8 g/100 g cp recommended for infants12. The TArAA range suggested for ideal infant protein (6.8-11.8 g/100 g)12 has current values greater than the minimum, that is 6.85-8.12 g/100 g cp. The ArAA are precursors of epinephrine and thyroxin28.

The ratios of TEAA to the TAA in the samples were 46.6 % (hull) and 47.4 % (dehulled) which were well above the 39 % considered to be adequate for ideal protein food for infants, 26 % for children and 11 % for adults12. The TEAA/TAA percentage contents were strongly comparable to that of egg (50 %)29, 43.6 % reported for pigeon pea flour27. The percentage of total neutral AA (TNAA) ranged from 55.0-58.0 indicating that these formed the bulk of the AA; total acidic AA (TAAA) ranged from 28.7-31.0 which was lower than the % TNAA, whilst the percent range in total basic AA (TBAA) was 13.2-13.9, which made them the third largest group among the samples. The present % TNAA was better than in B. sapida fruit: 50.9-54.5; close to % TAAA, 29.5-29.9 but lower than % TBAA, 16.0-19.220.

Most animal proteins are low in Cys and hence in Cys in TSAA. For example, (Cys/TSAA) % were 35.5 % in Archachatinamarginata, 38.8 % in Archatinaarchatina and 21.0 % in Limicolaria sp., respectively30; 29.8 % in G. niloticus18; 23.8 % in C. anguillaris, 28.4 % in O. niloticus and 30.1 in C. senegalensis, respectively19. In contract, many vegetable proteins contain substantially more Cys than Met, for examples, 62.9 % in coconut endosperm23; 44.4 % in P. biglobosa22; 44.3 % in cola acuminate., 37.8 % in Garcinia kola; 50.5 % in A. occidentale21; 40.8 % in aril and 66.8 % in seed, both from B. sapida fruit20. Our present results of 40.7-47.8 % were within the group of the values mostly prevailent in plant samples. Although FAO/WHO/UNU13 did not give any indication of the proportion of TSAA which can be met by Cys in man, for rats, chicks and pigs, the proportion is about 50 %13. Information on the agronomic advantages of increasing the concentration of suphur-containing amino acids instaple foods shows that Cyshas positive effects on mineral absorption, particularly zinc31, 32.

The P-PER values were higher than 1.21 (cowpea), 1.82 (pigeon pea); 1.62 (millet ogi) and 0.27 (sorghum ogi)33 and close to 2.0 (P. biglobosa)22; reference casein with PER of 2.5033; 1.89-2.22 in three different fish samples19; but much lower than 4.06 in modified corn ogi33 (see Table 3).

In the consumption of maize and sorghum, it has been suggested that an amino acid imbalance from excess Leu might be a factor in the development of pellagra34. The present Leu/Ile ratio range was 1.95-2.24 with a difference of 3.43-3.54 g/100 g cp or 48.7 – 55.4 % (Table 3).

Clinical, biochemical and pathological observations in human and rat experiments showed that high Leu in the diet impairs the metabolism of Try and niacin and is responsible for the niacin deficiency in sorghum eaters. High Leu is also a factor contributing to the pellagra-genic properties of maize. Excess Leu could be counteracted by increasing the intake of niacin or Try and also with supplementation with Ile. These studies suggested that the Leu/Ile balance is more important than dietary excess of Leu alone in regulating the metabolism of Try and niacin and hence the disease process34. The present Leu/Ile ratios were low in value. Also all of the present Leu values were less than 11.0 g/100 g cp; with actual range of 6.39-7.05 g/100 g cp, and could be beneficially exploited to prevent pellagra in endemic areas.

The essential amino acid index (EAAI) ranged between 0.87-1.21 (Table 3). The EAAI method can be useful as a rapid tool to evaluate food formulations for protein quality. However, it does not account for differences in protein quality due to various processing methods or certain chemical reactions35. Essential amino acid index for defatted soy flour is 1.2635. The EAAI values here were close to the values in B. sapida aril and seed with values of 1.08-1.62.

The isoelectric points (pI) as calculated for the AA were 3.56 (hull) and 4.53 (dehulled) (Table 3). The total neutral AA has pI 5.0-6.3, the TAAA has pI of 3.0-3.1 whilst pI for TBAA is 7.6-10.8. Olaofe and Akintayo16 used this method to predict pI of legume and oilseed proteins from their AA in which the overall average percentage deviation was 23.3 %. This method is, therefore, a good starting point in order to enhance a quick precipitation of protein isolate from a biological sample.

The amino acid scores (AAS) based on whole hen’s egg are shown in Table 4. Histidine (His) is a semi-essential AA particularly useful for children growth. This same characteristic also applies to Arg; both His and Arg had high scores in comparison to hen’s whole egg. Ser had the lowest score (0.35 or 35.0 %) in dehulled sample whilst met had the least score (0.32 or 32.o %) in hull. The correction ratio36 for the whole AA in dehulled would be 100/35 x dehulled protein and 100/32 x hull protein or 2.86 xdehulled protein and 3.13 x hull protein respectively in order to bring all the EAA to the required standards when they serve as sole sources of protein. Our results of AAS in whole egg comparison followed the trend also observed in B. sapida aril an seeds20. Table 5 shows the EAA scores (EAAS) based on provisional amino acid scoring pattern. The limiting EAA (LEAA) was Lys (0.64 or 64.0 %) in dehulled sample and Met + Cys (0.49 or 49.0 %) in the hull. Corrections here would, therefore, be 100/64 x dehulled protein and 100/49 x hull protein or 1.56 x dehulled protein and 2.04 x hull protein respectively in order to bring all the EAA to the required standards when they serve as sole sources of protein. Table 6 shows the EAAS based on the suggested pre-school child requirements. The LEAA for both samples was Lys: 0.61 (dehulled seed) and 0.52 (hull). For correction, each would require 100/61 or 1.64 x dehulled protein and 100/52 or 1.92 x hull protein to satisfy requirement when each serves as the sole source of dietary protein. On the overall scoring pattern, Gly was best in Table 4 (1.35) for dehulled sample, Phe + Tyr was best in Table 5 (1.14-1.35) as we have in B. sapida (1.02-1.19)20 and Val was best in Table 6 (1.26) in hull just as we have it in B. sapida fruit (1.15-1.24)20 respectively.

Table 7 shows the summary of statistical analysis from Tables 1, 3, 4, 5 and 6. The correlation coefficient (rxy) for Tables 1, 3, 4 and 5 values were positively high and significant at r = 0.05 and n-2 degrees of freedom since r calc. > r table. From Table 6, the value for rxy was low and positive but not significant. The Rxy values were low and positive except in Table 1 (TEAA) where Rxy was negative. The index of forecasting efficiency (IFE) was high in Tables 1 and 3 thereby making prediction easy because IFE is actually a reduction in the error of prediction. For example error of prediction in Table 1 (whole sample) would be 100-66.8 = 33.2 %. On the other hand prediction of relationship between dehulled samples and hull would be difficult in Tables 4, 5 and 6 where coefficient of alienation was high (67.1-85.4 %) since none relationship was high in Tables 4, 5 and 6.

Figure 1   Click here to View figure

Table 1: Amino acid composition of the hull and dehulled seeds parts of Irvingiagabonensis (g/100 g crude protein dry weight).

*Essential amino acids.

Table 2: Summary of some amino acid quality parameters of Irvingiagabonensis hull and dehulled seeds parts (g/100 g crude protein).

Table 3: Summary of some amino acid quality parameters of Irvingiagabonensis hull and dehulled seeds parts.

Table 4: Amino acid scores of Irvingiagabonensis samples based on whole hen’s egg.

*Try was not determined.

Table 5: Essential amino acid scores of Irvingiagabonensis samples based on provisional amino acid scoring pattern.

Table 6: Essential amino acid scores of Irvingiagabonensis samples based on suggested pre-school child requirement.

Table 7: Summary of statistical analysis from Tables 1, 3, 4, 5 and 6

rxy = correlation coeificient; Rxy = regression coefficient;  = mean of dehulled sample;  = mean of hull sample; CA = coefficient of alienation; IFE = index of forecasting efficiency; n-degrees of freedom. Results significantly different at r 0.05 and n-2 degrees of freedom.

Table 8: Summary of the amino acid profiles into factors A and B.

Conclusion

In summary, this study indicates that the amino acid profiles of Irvingiagabonenesis hull and dehulled samples have close composition (Table 8 particularly the EAA under both factors A and B means). Both are good sources of many of the essential amino acids. Hence, both samples would complement each other when they are used as soup ingredients.

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