Cassava is dried by convection in an oven. The temperature is set at 70˚C. Once thermal equilibrium is reached, the sample is introduced into the oven chamber. On each cassava, we have marked with indelible ink three geometric positions where the measurements will be taken. Finally, three measurements were taken to consider the average value. Samples were taken at predetermined intervals throughout the experiment for transverse, longitudinal, and thickness dimensions and mass measurements. We minimize the measurement time so as not to disturb the thermal balance established in the product. Geometric characterization of the sample is carried out by initially measuring dimensions and final values. For this, we use a digital micrometer (MITUTOYO, Japan, precision 2 × 10⁻⁵ m).

During the drying of cassava, its material undergoes physical deformations. The loss of water during convective drying leads to cellular collapse and consequently, the contraction of the solid matrix of the product. The models in the literature are mainly empirical and cannot be transposed from one product to another or from one drying condition to another [9] - [12] . There are nevertheless basic theories in the literature [13] . The multiplicity and diversity of products and their physical properties (density, material concentration, contraction coefficient, collapse, porosity, change in dimensions, etc.) make comparisons very difficult [14] - [20] . From experimental data, contractions are represented by type relationships:

$\frac{l}{l_0}=a_l\frac{X}{X_0}+b_l$ (1)

$\frac{S}{S_0}=a_S\frac{X}{X_0}+b_S$ (2)

where a and b are constants deduced graphically, the indices S, and l being reported respectively the surface and the width. These models have been used by certain authors for different products and applications: for grapes [21] , potato [22] , bananas [23] , gelatin slabs [24] , ocra [25] , mango [26] , tomato [27] .

The difficulty linked to the study of the drying of agri-food products comes from the great diversity in the field. Added to this is the structural factor. The heterogeneity and anisotropicity of the agri-food product give it, during its drying, very complex physical and mechanical characteristics. We can distinguish three main directions:

The isotropicity index makes it possible to characterize and compare the contraction of samples in two directions during drying.

For drying times different from the initial time, the shrinkage isotropicity between X and Y directions was defined as the ratio of the reduction in X divided by the ratio of the reduction in Y.

For these directions we define the isotropicity XY index by the following relation:

$I_{XY}=\frac{\frac{X-X_0}{X_0}}{\frac{Y-Y_0}{Y_0}}$ (3)

Thus, for example, the thickness-length isotropicity index is defined by the following relationship [20] :

$I_{eL}=\frac{\frac{e-e_0}{e_0}}{\frac{L-L_0}{L_0}}$ (4)

where e₀, e are respectively the initial and the current values of the sample thinness and L₀, L respectively the initial and the current values of the sample length.

The principle of drying is to lose the water contained in the product, with the drying time, the product sees its mass decrease as indicated in Figure 2 . This loss of water results in a decrease in the water content of the product with the drying time. The mass of the product decreases from its initial value to m₀. a final value m_f which no longer varies over time. By pushing the drying according to the law [27] AOC, 1995 by staying the sample in an oven at 105˚C for 24 hours, the mass m_f. decreases slightly and reaches the value m_S. The product has therefore lost all traces of water likely to promote biological action. m_s is the mass of the solid skeleton. At the same time, its water content decreases from its initial value X₀ to a final value X_f which can be calculated based on the value of m_f so-called wet basis or value basis m_S called dry base [28] - [30] . The results in Figure 2 show that

m_f/m₀. is 0.347 for cassava. As for the value of X_f/X₀. the product, it takes almost zero value.

During drying, the lateral dimensions of cassava decrease with time, in accordance to other agri-food products [31] . As the product loses its water it undergoes a collapse of the material which compensates for the loss of water. Consequently, its dimensions decrease. Figure 3 tells us that for the dimensions length L, width l and thickness e, they go from 100% to X/X₀ = 1 at around L/L₀ = 0.99, l/l₀ = 0.94 and e/e₀ = 0.93 for X/X₀ = 77%. At X/X₀ = 27% these values are respectively 0.89, 0.86 and 0.87. At the end of drying, i.e. for X/X₀ = 0.07, they all stabilize around 0.82. Let us note an anomaly which occurs at this moment with the appearance of a crack which affects certain measurements.

All variations of the rates L/L₀, l/l₀ and e/e₀ as a function of the rate X/X₀ are quasi-linear and can be put in the form of equion 1.

We see that the size of the dimension has little influence on contraction. Slightly, the smaller the dimension, the more quickly it decreases. Thus the line relating to e/e₀ has a relatively steeper slope than that of l/l_0. The curve of the largest dimension, which is the length L/L₀, has the smallest slope.

The change in dimensions during drying results in variations in the volumes and surfaces of the samples. However, in the assessment of the finished product, the state of these parameters affects its quality. In the local market, players sell products in bags. Buyers visually choose by volume and not mass.

This study shows us, in Figure 4 , the trends in the surface areas and volumes of the cassava samples submitted to our study. As shown in Figure 4 , the variation of tau S/S₀ and V/V₀ as a function of tau X/X₀ is quasi-linear. V/V₀ and S/S₀ go respectively from 100% for X/X₀ = 1, to respectively 0.85 and 0.89 when X/X₀ = 0.72. For a value of X/X₀ = 0.29, V/V₀ and S/S₀ reach 0.69 and 0.78 respectively. The end of drying is marked by X_f/X₀ = 0.08 or S_f/S₀ and V_f/V₀ stabilize at 0.67 and 0.55 respectively. The linearity of the linear dimensions leads to a linearity of the surface S and the volume V of the samples in their evolution with convective drying.

We examine the change experienced by the samples from the start of drying to its end. We can see, from Table 1 , that the largest dimension goes from 3.21 cm at the start of drying to 2.32 cm at the end of drying, a reduction of 27.73% in its value. Likewise, the width and thickness increase respectively from 2.57 cm and 1.03 cm at the start of drying to 1.59 and 0.67 cm at the end of drying. They reach a reduction of 38.13% and 34.95%.

The surface S and the volume V increase respectively from 14.20 m² and 8.50 m³ at the initial instant to their final value of 6.31 m² and 2.47 m³. They therefore reach 44.41% and 29.08%.

Monitoring the contractions of the dimensions of the cassava during its convective drying seems to show a progressive evolution of its density. This situation has advantages because it allows easy transport of the product. Figure 5 gives us an idea of the evolution of the density of a cassava sample as it loses its water during convective drying. This evolution is almost linear with $R^2=0.9599$ if the equation of the line describing the evolution of this density is $\rho =375.58\left(X/X_0\right)+790.57$. This line does not take into account the start of drying which has thermal and water turbulence before reaching the steady state.

Thus, the density goes from $\rho =1093.71\text{g}/{\text{cm}}^{\text{3}}$ at the initial instant, increases slightly because of the turbulence, and falls to 1063.33 for a standardized water content of 0.69. For $X/X_0=0.39$, the density falls to $\rho =949.81\text{g}/{\text{cm}}^{\text{3}}$. At the end of drying, $X_f/X_0=0.39$ the density drops to ${\rho }_f=949.81\text{g}/{\text{cm}}^{\text{3}}$.

The examination of the contractions of the dimensions of the cassava during its convective drying seems to show a difference in behavior depending on its directions. Generally speaking, smaller sizes have a higher contraction rate compared to larger sizes. We obtain for I_eL, I_el and I_lL index curves above unity. We notice a large difference at the start of drying where the index I is clearly above 1. Towards the end of drying, the index approaches unity, showing a slowdown in contraction on all dimensions ( Figure 6 ). Ouoba et al. [26] show in the case of okra that the direction of the fibers slows down its contraction compared to the direction

orthogonal to the x fibers. This is also to intervene in the case of cassava when we know that all the directions are not visibly isotropic. In addition to other Ouoba studies [27] [28] have shown that sizes play a considerable role in the behavior of drying samples. This can also be a cause of the anisotropicity of the samples, when we notice that the behavioral difference is linked to the size of the dimensions considered.

As we saw in paragraphs 3.2 and 3.3, the loss of water from the product leads to a proportional collapse both in its linear dimensions which are the length L, the width l and the thickness e, but also in its dimensions surface S and volume V.

This linearity leads us to find mathematical models that will allow actors to predict the behavior of samples. Equation (1) adapted to the width and thickness, as well as Equation (2) applied to the surface allowing us to braid Table 2 .

If these models are practical for prediction, note that an error given by the value of R² is committed. These different models give acceptable satisfaction, seen from R² is quite close to unity.