Evaluation of mineral composition of endogenous and improved varieties of maize ( Zea mays ) cultivated in Southern Benin

1 Laboratoire des Sciences des Aliments, Ecole de Nutrition et des Sciences et Technologies Alimentaires, Faculté des Sciences Agronomiques, Université d’Abomey-Calavi, 03 BP 2819 Cotonou, Bénin Republic. 2 Laboratoire de Biologie et de Typage Moléculaire en Microbiologie, Département de Biochimie et de Biologie Cellulaire, Faculté des Sciences et Techniques, Université d’Abomey-Calavi, Cotonou, 01 BP 526, Cotonou, Bénin Republic. 3 Programme Technologie Agricole et Alimentaire, Centre de Recherches Agricoles d’Agonkanmey, Institut National des Recherches Agricoles du Benin, 01 BP 128 Porto-Novo, Benin Republic. 4 Centre de Recherches Agricoles Sud, Institut National des Recherches Agricoles du Bénin, Attogon, Bénin Republic.


INTRODUCTION
Corn (Zea mays L.) is widely cultivated in Benin, and represents about 3/4 of the total country's cereal production (MAEP, 2010).It is cultivated in all the districts with variable importance, and remains the first cereal produced, followed by sorghum, rice and millet.Indeed, corn production increased from 230,000 tons in 1970 to 800,000 tons in 2000 and over to 1 345820 tons in 2013 (DPP, 2013).In this respect corn plays an important role in human diet in Benin, and it is used in various forms according to different destinations (Adégbola et al., 2011;Balogoun, 2012).Over 40% of the country's production is market at national level, and the main customers are the households and the urban food small craft industry (ONS, 2010).Exports to the neighboring countries are not recorded but represent significant amounts (Hell et al., 2000).Maize is mainly used as food and feed (poultry, pigs, cattle) and as raw material for some industries (brewing, soap and oil factories) (Boone et al., 2008).In Benin, corn is consumed in various forms and goes into preparation of several dishes.The average level of maize consumption in Benin is estimated to about 96 kg/capita /year (Gandonou et al., 2010), which places the country in the forefront of major consumers of corn in West Africa.Smith et al. (1997) predicted that corn will become a cash crop and will ensure food security better than any other culture.In Benin, farmers have a large range of corn varieties they cultivate and of which some are often oriented toward specific agro-food processing according to their technological characteristics.These corn varieties include both the local and the improved ones managed by farmers themselves.However, there is a lack of adequate information on the mineral composition (sodium, potassium, calcium, magnesium, zinc, iron, phosphorus, etc.) of most of these varieties of corn mainly consumed by the populations.So, there is a need to characterize these varieties of maize in order to enhance their contribution to food security.The present study aims to investigate the mineral composition of varieties of maize mainly used in southern Benin with the purpose of knowing which of them could be more encouraged for cultivation and consumption.

MATERIALS AND METHODS
Plant material used in the present study was composed of endogenous and improved varieties of maize (Zea mays) cultivated in the southern region of Benin.Grains were collected from farmers who provided information concerning the local name and the type (improved or local) of each variety (Table 1 and Figure 1).

Sampling of maize
The sampling was conducted in 12 municipalities located in the agro-ecological zones (AEZ) V, VI, VII and VIII according to (CIPB, 2007), where maize is mainly cultivated (Figure 1).A total of 30 samples of corn including seven improved and 23 local varieties were collected from producers randomly selected in the sampling zones.The samples were then transported to the laboratory, packaged in canvas bag and stored in cold room at 4°C.

Samples preparation
Samples of each variety of maize were crushed using a crusher (Falling Number, type 3600) and then ground with a laboratory mill (Retsch, Type Z M 1).The maize flours obtained were used for Semassa et al. 3817 the determination of mineral elements.

Analysis of mineral elements
The first step of the determination of mineral elements was the mineralization of flours from each of 30 samples according to Hach (1999).Then, sodium, potassium, calcium and magnesium contents were determined using Atomic Absorption Spectrophotometry method (Atomic Absorption Spectrophotomer, Unicam ATI 929 s.a.a with flame).For the determination of phosphorus, iron and zinc, Molecular Absorption Spectrophotometry (Spectrophotometer DR 2800) was used.

Determination of sodium, potassium, calcium and magnesium contents
For mineralization, two g of corn flour was incinerated in a muffle furnace at 550°C for 24 h.The ash obtained was dissolved in 2 ml of hydrochloric acid solution 6N and then evaporated on a hotplate at 125°C.The viscous residue obtained was dissolved again and recovered in a 100 ml volumetric flask using nitric acid 0.1 M. The solution obtained was then diluted to determine the mineral elements in accordance with the standard EN 14082 (Hach, 1999).

Determination of iron, zinc and phosphorus contents
Known volume of the solution previously obtained was neutralized (pH between 4 and 5) by addition of 5N sodium hydroxide.The final volume was adjusted with distilled water to a known proportion.Then, ferrozine method 8147, zincover method 8009 and ascorbic acid method 8048 were used for the determination of iron, zinc and phosphorus contents respectively (Hach, 1999).

Statistical analysis
Cluster analysis (agglomerative hierarchical cluster; Ward's method) was computed using SAS software (version 9.2) in order to classify all the varieties in a more limited number of groups of relatively homogeneous items.Corn varieties groups obtained were subjected to analysis of variance (ANOVA).Means difference were determined using Student Newman Keuls (SNK) test, and significance of difference was established at p < 0.05.Clusters of corn varieties were then associated with different mineral elements through principal component analysis (PCA) using XLSTAT software (version 2011, Addinsoft, Paris, France).

RESULTS
The mineral contents of maize varieties analysed are summarized in Table 2.The data showed that phosphorus (P) and potassium (K) contents of samples varied from 51.04 to 4860.98 mg/kg and from 117 to 11949 mg/kg respectively.Sodium (Na), magnesium (Mg) and zinc (Zn) contents ranged between 104 -*Corresponding author.E-mail: victor.anihouvi@gmail.com.
The principal component analysis (PCA) on corn groups and mineral contents resulted in two axes accounting for 100% of the total variation of which 70.1% was explained by the first axe (Axe 1) and 28.9% by the second axe (Axe 2) (Table 4 and Figure 3).The first axe opposes Na and Fe contents to Mg, Zn and Ca contents of maize varieties (Table 5 and Figure 3).This means that any maize varieties having high contents in Fe and Na, had low contents in Mg, Ca and Zn and vice-versa.Regarding the second axe, it opposes Zn content to Na and Fe contents (Table 4 and Figure 3).It's also appeared that corn samples having high content in Zn, had low contents in Fe and Na and vice-versa.

DISCUSSION
Several essential trace elements have effects on organisms functions (Sodipo et al., 2012).In addition, information of food composition data and its chemical components is important in nutritional planning and source of data for epidemiological studies (Ali et al., 2008).This study allowed to classify different corn varieties into three clusters consisting of relatively homogeneous items.Among the three clusters obtained, samples of cluster 3 contained higher level of P, K, Na, Mg, Ca and Zn, those of cluster 1 contained higher amount of Fe, while samples of cluster 2 were poor in Fe (Table 3).The average of phosphorus content of cluster 3 samples was 2310 ± 0.04 mg/kg.This results agree with the findings of Nago (1997) (2360-3490 mg/kg), but comparatively higher than the value of 0.1 mg/kg found by Adeoti et al. (2013) and lower than 200 000 mg/kg reported by Sule Enyisi et al (2014).The recommended daily intake of phosphorus for adults and children is 8000 mg/kg per day (Pillai and Nair, 2013).Phosphorus associated with calcium, helps to strengthen bones and teeth, especially with children and nursing mothers (Andzouana and Mombouli, 2012).The average phosphorus content obtained for varieties of cluster 3 was lower than the recommended standard.Therefore the varieties of cluster 3 were not the best corn for phosphorus intake despite being richer in phosphorus than the varieties of clusters 1 and 2. The average potassium content in samples of cluster 3 (7958 ± 0.09 mg/kg) was 4.7 times higher than that of cluster 1 (2033 ± 0.02 mg/kg) and three times higher than that of cluster 2 (1883 ± 0.02 mg/kg).However, this level of potassium in cluster 3 samples is lower than the value of 90 000 mg/kg  reported by Sule Enyisi et al. (2014).Moreover, when comparing the potassium content of varieties under study to the mineral contents of other cereal such as millet (3300-3700 mg/kg) (Békoye, 2011), the cluster 3 varieties could be considered as very good sources of potassium.
Sodium is an important mineral that contributes to the regulation of blood flow and holding potential of electrons in body tissues (Alinnor and Oze, 2011).The average sodium contents in maize varieties of clusters 1, 2 and 3 were 261 ± 0.00 mg/kg, 213.4 ± 0.00 mg/kg and 266 ± 0.00 mg/kg respectively.These levels of sodium contents were lower than the reported values of 594 mg/kg (FAO, 1993) and 1000 mg/kg (Enyisi et al., 2014), but higher than 178.6 mg/kg found by Adeoti et al. (2013).

First Component
Second Component  Therefore, the flours from maize varieties of clusters 3 are significant sources of sodium and could be recommended for pregnant women and those with hypertension and kidney diseases of whom the direct consumption of salt should be minimized (Emebu and Anyika, 2011).In addition, cluster 3 samples showed the higher content in zinc, followed by cluster 2 and cluster 1 varieties (Table 3).According to Sandstead et al. (1998), zinc is very useful for protein synthesis, cell division, cell maturation, immunity and sexual function.In the same way, the samples of cluster 3 had the higher content in magnesium, followed by samples of clusters 1 and 2. The recommended dietary intake of magnesium is 3500 mg/kg for adults and 1700 mg/kg for children (Alinnor and Oze, 2011).Based on the results concerning the magnesium content of corn varieties analysed, we can conclude that only the samples of cluster 3 had magnesium content that meet daily needs of children.
According to Alinnor and Oze (2011), magnesium plays a vital role in calcium metabolism and bone formation and is also involved in the prevention of diseases related to the circulatory system.It also helps to regulate blood pressure and secretion of insulin.The average calcium contents of samples of clusters 1, 2 and 3 were 113, 93 and 166 mg/kg respectively (Table 3).These values are in agreement with the findings of Nago (1997) (49-159 mg/kg) but were comparatively higher than 10 and 100 mg/kg found respectively by Adeoti et al. (2013) and Sule Enyisi et al. (2014), and lower than data reported by FAO (1993) (483 mg/kg) for maize and millet (300-400 mg/kg) (Békoye, 2011).The recommended daily intake of calcium stipulated by the World Health Organization of the United Nations (WHO) is 8000 mg/kg for adults and children.This study showed that the calcium contents in all the varieties of corn studied were below the standard recommended by WHO.Calcium is the most abundant mineral in the human body and is involved in blood clotting, muscle contraction, neurological function, formation of bones and teeth (Senga Kitumbe et al., 2013).It is also an important factor in the enzymatic metabolic processes (Karau et al., 2012).The iron contents in corn varieties of three clusters were largely above the daily level recommended by WHO which is 100 to 150 mg/kg (Senga Kitumbe et al., 2013).
According to Andzouana and Monbouli (2012), iron as oligo-element, plays many biochemical roles and constitutes a fundamental element in the metabolism of all living organisms.In human, iron is an essential component of many types of proteins and enzymes (Andzouana and Monbouli, 2012).Therefore, corn varieties investigated meet the nutritional requirement in iron if consumed reasonably.The ratios of calcium to phosphorus (Ca/P) and sodium to potassium (Na/K) were presented in Table 3.According to Shills and Young (1988) diets rich in protein and phosphorus may promote the loss of calcium in the urine; this had led to the concept of the Ca/P ratio (Adeoti al., 2013).The Ca/P values were 0.38, 0.21 and 0.07 for clusters 1, 2 and 3 samples respectively.Ca/P ratio greater than 2 contributes to increase the absorption of calcium in small intestine (Adeyeye and Aye, 2005;Alinnor and Oze, 2011).Furthermore, food is considered as good if Ca/P ratio is greater than 1 and poor if this ratio is less than 0.5 (Alinnor et Oze, 2011).Consequently, the 30 maize varieties cannot be considered as good source of mineral since the Ca/P ratios were lower than 0.5 for all of them (Table 3).The Na/K ratios of cluster 1, cluster 2 and cluster 3 were 0.13, 0.11 and 0.03 respectively (Table 3).According to Alinnor and Oze (2011), Na/K ratio is of great importance for prevention of high blood pressure.Indeed, Na/K ratio helps to control blood pressure and food having a Na/K ratio less than 1 lowers blood pressure.Thus, flour obtained from the 30 maize varieties investigated would probably reduce the risk of high blood pressure.
The 30 samples analysed were collected in four agroecological zones representative of all agro-ecological zones of maize production in Benin (Figure 1).All the four zones have the same climate (sudano-guinean) with an annual rainfall ranging between 800 and 1400 mm/year (MEPN, 2008).However, it appeared through the analytical data that maize varieties of cluster 3 collected from agro-ecological zones V (cotton culture zone),VI (bar ground zone) and VII (depression zone) showed the higher contents in phosphorus, potassium, magnesium, zinc and calcium.In addition, in these three areas corn is usually cultivated in association with other crops such as legume plants, and tuber and to a lesser extent cotton (MEPN, 2008).Based on these observations we can conclude that ecological conditions of production areas may have an impact on the mineral composition of corn varieties as reported by Hussaini et al. (2008) and Saïdou et al. (2012).

Conclusion
This study revealed the mineral potential of 30 corn varieties produced in Southern Benin.The results showed that the 30 corn varieties can be divided into three groups based on their mineral composition profile.With the exception of iron, cluster 3 corn varieties contained the highest mineral contents whereas cluster 2 samples showed the lowest levels of potassium, magnesium, zinc and iron.In return, cluster 1 varieties contained the highest amount of iron but very poor in zinc.Based on the results obtained, some of the studied maize varieties could be considered as complementary sources of mineral in spite of their ratios of calcium to phosphorus were lower than 1.Thus, when consumed frequently, they can help to meet the daily recommendations of essential mineral nutrients and improve the nutritional status of rural and urban populations.

Figure 1 .
Figure 1.Map showing the sampling areas in southern Benin.

Table 1 .
Maize varieties (endogenous and improved) collected in study zones.

Table 2 .
Mineral elements contents of maize varieties samples collected.
Figure 2. Agglomerative cluster analysis dendrogram for clustering maize varieties samples (n=30) into groups of similar mineral profile.

Table 3 .
Quantitative contents (mean ± standard deviation) of mineral elements associated with different groups of maize varieties.

Table 4 .
Eigen value of the first three principal components.

Table 5 .
Variables associated to the first two components.