The Kent variety used in this study was purchased from a local fruit market of Bobo Dioulasso, town in the west of Burkina Faso. At their arrival to the laboratory, the measures were made on the fruits between the first and seventh day. For measures, good quality fruits were selected and washed into water to which a small quantity of Natrium hypochlorite was added as disinfectant, rinsed with drinking water and peeled. The pulp was separated from the stone and sliced through according to the desired thickness.

Drying kinetics of Kent mango slices were established from samples mass loss kinetics using a laboratory mechanical convection oven dryer with digital controller (universal MEMMERT UFP 600 256L 30˚C - 250˚C) (Figure 1). The drying system allowed controlling temperature and the velocity of drying air. The air velocity is given in percentage of maximal velocity. This maximal velocity was previously measured by an anemometer TESTO 425 (resolution: 0.01 m/s for 0 to 20 m/s). The relative humidity in dryer was measured by digital probe thermohygrometer TESTO 635 (resolution: 0.1% for 0% to 100% RH). Mass measurements according to drying time were carried out at three temperatures (50˚C, 60˚C, and 70˚C) for samples of 5 mm thick. This thickness was the average value of mango slices initial thickness used in dryers in Burkina Faso. To obtain this desired

thickness, the samples thickness were measured using a micrometer Mitutoyo (Japan) of 2 × 10⁻⁵ meter precision at the beginning of the drying. At the end of the drying experiment, the dry mass was evaluated by drying the final sample in an oven-dryer (MEMMERT) at 70˚C for 24 h [7] . From mass loss curves and dry mass, moisture content at each instant of drying and drying kinetics were determined.

The shrinkage curves of Kent mango were established at 40˚C, 50˚C, 60˚C and 70˚C by using parallelepipedic shape samples. The initial dimensions of these samples were presented in Table 1. For shrinkage measurements, the mango slices were dried at 40˚C, 50˚C, 60˚C or 70˚C in an oven-dryer (WTF BINDER). During drying, the dimensions of slices were measured using a micrometer Mitutoyo (Japan) of 2 × 10⁻⁵ meter precision. These measurements were done every 10 min during the first 110 min, then every 15 min until 300 min drying and finally every 30 min until the end of drying determined by constant weight. The dimensions were measured at several places of the mango samples and only their mean values were considered.

Although the sizes changes due to the great water loss during the drying process, it was noticed that the initial shape of the slices was approximately conserved. The results of shrinkage measurements obtained were the means of three trials at every temperature. The volume of the sample at each instant of the drying was thus calculated as length × width × thickness. The shrinkage coefficient was then deduced from experimental data according to the following formula:

where V is the sample volume at an instant of the drying and V₀ its initial volume.

During the drying process, the weight of the sample was regularly measured with a balance (SARTORIUS, 0.001 g precision, France). For each measure, the corresponding weights were noted. An oven-dryer and a balance (SARTORIUS, France) of 0.001 g precision were used to determine samples moisture content. At the end of the drying experiment, the dry mass was evaluated by drying the final sample in an oven-dryer (MEMMERT) at 70˚C for 24 h [7] . Changes in S_b with respect to the moisture content (reported to its initial value) constitute the shrinkage curve. The experimental shrinkage curves obtained were then simulated with a linear model of volume additivity.

Isotherm of Kent mango was established at 40˚C using an activimeter, FA-st (Food Analysis Science and Technology GBX, France) with activity precision of ±0.003. The temperature of this activimeter was regulated by circulating of water under the samples. The pieces of mangoes used in the activimeter were from 14 mangoes. The activimeter was limited by a maximal temperature of 40˚C and did not allowed to obtain desorption isotherms for higher drying air temperatures. A modified Henderson model of isotherms was then used to take drying temperature variation into account. The use of the parameters of this model for the whole drying air temperature range was motivated by the results of Myara and Sablani’s works [8] which showed that mango isotherm does not vary much with temperature in 15˚C to 60˚C range. The experiences were carried out three times to check the repeatability. To prepare samples, the two cheeks of each fruit used were removed and shared into about 5 mm thick slices of about 12 g each. All the pieces were placed at 60˚C in an oven dryer (WTF BINDER, Germany). As drying progressed, a portion of mango was retrieved from the oven at regular time intervals,

chopped into tiny pieces to facilitate the transfer, and then placed in the cupel of the activimeter. The activimeter records the air dew temperature around the product, when equilibrium is reached between the product and the air. From this measurement and that of the dry temperature of air, the relative humidity of air is deducted. At equilibrium, this relative humidity of air equals the water activity of the product [1] . The equilibrium moisture content Xeq, corresponding to the air relative humidity was determined by drying the final sample in an oven-dryer (MEMMERT) at 70˚C for 24 h [7] .

In this current study, the shrinkage was taken into account both in drying rates and mass and energy balances through the surface area shrinkage. The model of shrinkage used is based on the hypothesis of the additivity of volumes of water and solids, assuming that the volume of air in pores is negligible and that cellular tissues are incompressible. Thus, the volume versus the moisture content (on dry basic) X of the product can be expressed as:

where: r_s is the density of solid matrix and r_w the density of water.

Then, the shrinkage coefficient can be stated as:

Then the surface area shrinkage is deduced from the bulk shrinkage as:

u is determined from experimental shrinkage data. Assuming an ideal shrinkage during diffusion period, the value of u is evaluated from a linear fitting of volume ratio versus dimensionless moisture content according to the following formula [2] :

The drying simulation was carried out by using the average data on the drying characteristics of “Kent” mango variety and the thermophysical properties of the drying air showed in Table 2. The water content of mango slices in thin layer was supposed to be uniform. It was obtained from the transfer equations, a simultaneous equations system solved by iterations through an explicit method using a simulation program adapted to software MATLAB version 7.0.1.

The experimental and modelled convective drying kinetics (X/X₀ vs. time) obtained for 5 mm thickness mango slices at 50˚C, 60˚C and 70˚C are presented on Figure 4. The initial water content used was 6.01 d.b. One can

easily notice that the product reached its equilibrium moisture content in 80 min at 70˚C, 150 min at 60˚C and 210 min at 50˚C. According to Figure 4, experimental and simulation results were very close. The correlation coefficient of fitting experimental data by simulation curves was r² = 0.997. Therefore, it can be deduced that experimental drying kinetics of Kent mango can be approached by the model. Thus, the thin layer drying model may be assumed to represent suitably Kent mango slices convective drying kinetics profile. The final water content obtain was about 0.23 X₀ i.e. 13.8% d.b. This final water content of mango slices was in the range 12% - 20% w.b. of previous work reported by Touré and Kibangu-Nkembo [6] and very close to 13% w.b. that obtained by Gbaha et al. [4] .