Physicochemical Characterization of Nine Cassava ( Manihot esculenta Crantz) Cultivars from Chad

In Chad, despite the multiple culinary uses of cassava leaves and tubers, their nutritional values are untapped. In this study, the physicochemical composi-tions and structure of nine cultivars were assessed. The proteins were obtained by Kjeldahl’s method. Total sugars were determined according to the Luff-Schoorl method. For starch content, the polarimetric method of Earle and Milner was used. Mineral elements were carried out using an atomic absorption spectrophotometry. The cyanide was evaluated by the method of Williams and Edwards. Significant variability has been demonstrated in the leaves and dry tubers except for water content and dry matter. Analysis of the variances of the components of the tubers reveals that the water contents of the cultivars vary from 5.01% to 5.86%. The ash contents vary from 4.23% (cultivar DVA2) to 8.32% (cultivar DVL2). and protein (30.74%) for PG1314, for magnesium for DVA2 (383.41 mg/100g) and Copper for KA0303 (0.0147 mg/100g). The concentrations of hydrocyanic acids are high in both leaves than fresh tubers (85 - 150 ppm). Lowest values are observed in tubers (10 - 15 ppm) for cultivar DVA2. Principal component analysis of the physicochemical characteristics of the leaves revealed four groups: the first very rich in calcium, magnesium and average potassium contents. Groups 2 and 3 are poor in calcium and magnesium but Group 2 has the highest potassium content while Group 3 has an intermediate content. Group 4 is very rich in calcium but low in magnesium and potassium. According to tubers, three groups have been identified which are characterized by low, intermediate and high contents in phosphorus. Cultivars SB1366, DVA2, DVL2, TL0101 and PG1314 show promising nutritional values and chemical constituents even if some have high levels of hydrocyanic acids. They could be recommended for the national selection program and for various applications.


Introduction
Cassava plays an essential role in the nutrition of urban and rural populations in many states. The production is mainly intended for human consumption [1] and it generates monetary income for rural populations [2]. But it is also used in animal feed and in the industrial sector [3]. In terms of nutritional value, [4] has shown that cassava tubers have very varied physicochemical compositions. In the Amazon area, cassava cultivars have a high content of free sugars [5] [6]. According to [7], these types of cultivars are suitable for fermentation, for the industrial production of ethanol and organic acids. Tubers are rich in carbohydrates and contain up to 35% starch but are very poor in protein (around 1.1%) according to [8]. New hybrid varieties with tubers up to 5% protein have been created [9]. Nevertheless, the leaves are richer in protein (more than 25%) and also contain many minerals and vitamins [10] [11] [12]. Unfortunately their level decreases after the processing or cooking processes [13] [14]. In general, the physicochemical composition of the leaves and tubers varies according to the cultivars, their age, cultural practices and climatic conditions [15]. Some dangerous substances are also found in Cassava such as cyanogenic acid which is at the basis of the differentiation of sweet and bitter cassava [16]. Studies carried out on several genotypes have shown their variations in tubers [17] [18] [19]. This variability was among cultivars and across localities [20]. It also depends on the cultural practices, plant age and environmental factors [21] [22] [23]. Cassava plant is characterized by a great diversity of culinary preparations from the leaves and tubers. Various foods produced from cassava have been reported in several countries [24] [25] [26] [27], and also in Chad [28] [29]. For the preparation of different cassava based foods, cultivars are often chosen according to Food and Nutrition Sciences their aptitudes and their physicochemical characteristics. Each cultivar is traditionally associated with a particular way of preparation [30]. In Chad, many cassava cultivars exist in rural areas [31]. Their tubers and leaves are eaten after blanching and sometimes drying in the form of vegetables but their physicochemical composition are not investigated. The present study aims to determine the physicochemical parameters and the structure of nine cassava cultivars in order to understand their technological changes capacities. Those with interesting traits will be included in the national varietal improvement program and for a better industrial uses purposes.

Sample Preparation
The nine cultivars have been planted at the Bebedjia agricultural station. Twelve After drying, the samples were packed and labeled.  (1) Origins determined during the collection phase by pesant; (2) IITA: International Institute of Tropical Agriculture; (3) CAR: Central African Republic; (4)

Qualitative Assessment of Cyanogenic Substances in Fresh Leaves and Tubers
The cyanide presence in the cultivars was evaluated by the method developed by Williams and Edwards [32]. This method allows cyanides to be estimated by sodium picrate in the presence of toluene. The evaluation was made on the first full and fresh apical leaves and on fresh tubers.

Statistical Analyses
Analyzes were carried out using software XLSTAT-Pro version 2013.5.01. Data collected were analyzed using descriptive statistics and then subjected to analysis of variances. Duncan's tests at the 5% level have been performed. Principal Component Analysis (PCA) and Ascending Hierarchical Clustering (AHC) were used to identify the different group of cultivars formed.

Descriptive Analysis of Biochemical Constituents of Dry Tubers
Results coming from nine samples analyzed in duplicate reveal small differences between the minimum and maximum values of the moisture, dry matter, ash, total sugar, fiber, starch, zinc and manganese contents. In opposite, these differences are significant for calcium, magnesium, iron, potassium and phosphorus.

Descriptive Analysis of Physicochemical Constituents of Dried Cassava Leaves
Significant variations in the content of mineral constituents have been highlighted in the leaves of the cultivars studied ( Table 3). The differences observed between the minimum and maximum values are very high for calcium, potassium and copper which are respectively 1360.60 mg/100g, 1350.56 mg/100g and 0.072 mg/kg. The coefficient of variation ranged from 15.77% to 54.18%. High variability was found in the physicochemical constituents of leaves.

Analysis of Variances of Physicochemical Characteristics of Cassava's Dry Tubers
Highly significant differences were observed for four (4) of the parameters analyzed (Table 4). These characteristics enable to distinguish at least three (3) groups of cultivars. The water content and dry matter are not significant according to the Duncan test at 5%. The DVA2 cultivar has the lowest ash rate (4.23 g/100g) of all the cultivars studied. On the other hand, DVL2 has the highest ash content (8.32 g/100g) but the lowest in carbohydrates (53.63 g/100g). Cultivars PG1314 and BA0909 do not differ significantly from their carbohydrate contents. This is also the case for DVL12 and KA0303. The lowest levels of fiber (1.74 g/100g) and starch (28.93 g/100g) were observed in the cultivar DVL12. On the other hand, SB1366 has the highest starch content of the cultivars studied (31.05 g/100g). Likewise, highly significant differences were observed for all the mineral elements (

Analysis of Variances of the Chemical Constituents of Dried Cultivar Leaves
Highly significant differences were observed for all components (

Variation in the Content of Hydrocyanic Acids in Fresh Leaves and Tubers
These contents are variable according to the cultivars (

Structuring of Variability from the Physicochemical Constituents of the Leaves
Seven physicochemical characteristics were used to perform the principal component analysis (PCA) of the cultivars. The first three axes explain 83.09% of the overall variability. Six of the characteristics contribute for more than 18% to a given axe. The phosphorus and potassium contents are strongly represented on the first factorial axis. Calcium contributes to both axes 1 and 2. The protein and magnesium contents contribute to the second axe. Iron is strongly represented on the third factorial axe (Table 8).

Variability of Cultivars Based on Chemical Characteristics of Tubers
The principal component analysis of the dried tubers of the cultivars was carried out using 11 mineral elements. Significant contributions were observed on the three factorial axes for all the contents studied except calcium. The biplot ( Figure  1) highlighted the relationships between the cultivars and the mineral constituents. PG1314 is very associated with fibers. The starch content is strongly associated with SB1366 like iron with DVA2 and manganese with KA0303. In contrary DLV12 is relatively linked to manganese. The cultivar DVL22 is very linked to phosphorus. This bond is less between BA0909 and phosphorus. DVL2 is strongly associated with potassium, with ash contents but moderately with phosphorus. As for TL0101, it is only relatively associated with magnesium. In Table 10, the ascending hierarchical classification analysis of cultivars based on seven physicochemical constituents of tubers, allowed them to be divided into three groups. Except for the phosphorus content, significant differences were not observed between the different groups of cultivars. Group 1, composed of SB1366, KA0303, DVA2, TL0101, BA0909 and DVL2 are characterized by intermediate contents (246.46 ± 21.99 mg/100g). The cultivars DVL12 and PG1314 belong to group 2 whose phosphorus concentrations are the lowest (155.26 ± 38.09 mg/100g). On the contrary, the cultivar DVL22 belongs to group 3 and whose phosphorus contents are the highest (360.78 ± 53.87 mg/100g) of all the cultivars studied.

Discussion
The importance of the variability of the physicochemical constituents highlighted in this study, testifies to the richness in nutritive elements of the cultivars. In similar studies, many authors have also shown the existence of such a variation in the nutritional quality of cassava tubers [4]. The descriptive analysis showed either small or high differences in the contents of the constituents. The coefficients of variation are between 1.8% and 75.38% for tubers and from 14.57% to 54.18% at the level of the leaf constituents.   Analysis of the variances of the physicochemical constituents of the tubers of the nine cultivars reveals that the water contents are very low (5.01% to 5.86%) and similar to the water contents of the flours of certain cassava cultivars (4.21% to 5.85%) of Côte d'Ivoire [33]. And they are significantly lower than 13%, the standards recommended by the codex [34]. Ash contents of the cultivars analyzed are between 4.23 to 8.32 g/100g and are higher than those of [36] which vary between 2.29 and 2.67 g/100g. Likewise, total sugar concentrations are higher than those reported by many authors [7] [36] [38]. However, their fiber contents are very low compared to the work of Gil and Buitrago [39]. Average potassium values are significantly higher than those reported by Richardson [40]. Compared to the results of Chávez et al., (2005) [4], the contents of calcium, phosphorus, magnesium and potassium are lower. In contrast, the average zinc concentrations of certain tubers are similar to those studied by Burns et al., (2012) [20].
In general, at the level of the tubers, the analysis of the results shows that three cultivars have at least two constituents with high concentrations. SB1366 is rich in potassium and zinc. High levels of zinc and manganese are observed in cultivar KA0303. The DVL2 cultivar contains the greatest number of constituents, calcium, magnesium, phosphorus and potassium with high concentrations. Similar work done on tubers of Dioscorea alata showed that the ash and total carbohydrate contents on the basis of dry weights [41] are low compared to the cassava tubers evaluated in this study. However, the starch and fiber are higher. Phosphorus, calcium, magnesium, potassium and manganese are also so while the zinc content is low.
From the analysis of the variances of the physicochemical constituents of the leaves, it appears that the calcium and copper contents are low while the phosphorus and potassium are high compared to the results of Cereda [42]. About 55.56% of the cultivars have high protein contents which are similar to those obtained by [43]. However, these are lower than the minimum of 32% reported by Nassar and Ortiz (2010) [9]. Compared to the cyanide concentrations of fresh leaves and tubers, the study revealed that they are above 10 ppm, a value recommended by the FAO and WHO [45]. These contents vary according to the cultivars. The differentiation of sweet and bitter cassava depends on the cyanogenic glucoside content of their tubers [16] [46]. Analysis of these contents in tubers has shown that cultivars SB1366, TL0101, DVL12 and DVL3 have high concentrations of cyanide. The concentrations of cultivars DVA2, BA0909, DVL2, KA0303 and DVL22 which are classified as sweet by farmers, except DVA2, are varying between 25 and 40 ppm in tubers. In addition, they exceed the standards of the Codex Alimentarius. Indeed, for reasons of health security, the United Nations systems through FAO and WHO have set up standards for cassava flour intended for human consumption [34] [45]. Consequently, the consumption of these cultivars in fresh form or not presents risks of toxicity. The peasant practices of classification of cassava into sweet or bitter type is close to that suggested by two authors [16] DOI: 10.4236/fns.2020.117053 752 Food and Nutrition Sciences [47]. According to these authors, the cultivars having less than 100 ppm of hydrocyanic acid are considered to be sweet and bitter for the concentration higher than 100 ppm. In other way, similarly studies done by [48] shows also that sweet and bitter manioc landraces are differentiated in South America but not in Africa. In fact, the same also found that some clones of cassava are classified by some farmers as sweet and by others as bitter.
As Mehouenou et al., (2016) [49] showed, cyanide content does not affect much the choice of cultivars but it help pesant to select the appropriate method to eliminate it. Finally, the results showed high toxicity (>100 ppm HCN) in some fresh tubers and leaves of Chadian cassava varieties was also found in Senegalese cassava varieties [50].

Conclusion
The study reveals a significant variability in the physicochemical constituents.