Hot air-drying characteristics of pale-fleshed, white-skinned sweet potato spheres (Ipomoea batatas)

 Corresponding author: Aboubakar Compaore Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Hot air-drying characteristics of pale-fleshed, white-skinned sweet potato spheres (Ipomoea batatas) Aboubakar Compaore 1, 2, 3, *, Salifou Ouedraogo 3, Honoré Kondia Ouoba 3, Fayçal Ilboudo 3 and Bétaboalé Naon 3 1 Lédéa Bernard OUEDRAOGO University (formerly University of Ouahigouya), 01 BP 346 Ouahigouya 01, North Region, Burkina Faso. 2 Laboratory of Environmental Physics and Chemistry (LPCE), Doctoral School of Science and Technology (ED-ST), Joseph KI-ZERBO University, Ouagadougou, Burkina Faso. 3 Laboratory of Materials, Heliophysics and the Environment (La.M.H.E.), Training and Research Unit in Exact and Applied Sciences (UFR/SEA), Nazi BONI University of Bobo Dioulasso, PO Box 1091, Burkina Faso. World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 Publication history: Received on 28 March 2025; revised on 03 May 2025; accepted on 06 May 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.26.2.1707 Abstract Drying of pale-fleshed, white-skinned spherical sweet potato (Ipomoea batatas) (a newly introduced variety grown in Burkina Faso) was carried out at air temperatures of 50°C, 60°C, 70°C, and 80°C using 2 and 3 cm diameter samples to determine its drying characteristics. The results of the analysed drying data indicated that drying occurred during the decreasing rate period. The moisture content of the sample and the drying rate of sweet potato were influenced by the air-drying temperature and the spherical diameter of the samples. The drying time of sweet potato decreased and its drying rate increased with the decrease in the diameter of spherical samples and the increase in air drying temperature. The average effective moisture diffusivity values were obtained in the range of 1.4086 × 10-9 to 3.7214 × 10-9 m2/s and 2.6715 × 10-9 to 6.8775 × 10-9 m2/s for 2 cm diameter samples and 3 cm diameter samples respectively as temperatures increased from 50 °C to 80 °C. This dependence allowed the determination of the activation energy values of 2 and 3 cm diameter sweet potato samples which were found to be 30.50 and 27.71 kJ/mol, respectively. Keywords: Sweet potato spheres; Convective drying; Moisture diffusivity; Activation energy 1 Introduction Food security is one of the major global challenges facing the world today. There are an estimated 795 million people who are food insecure and undernourished. One of the factors leading to food insecurity is the loss of food and agricultural products due to their deterioration throughout the food and agricultural chain and/or during the postproduction period. A major method to enhance food security is to reduce losses due to post-harvest spoilage of these agricultural products. To improve shelf life and reduce spoilage of agricultural products, drying is the most commonly used method [1]. Drying is an elementary process aimed at removing liquid/vapor water from a material and therefore reducing its water activity. Drying food has many advantages, such as: blocking the development and multiplication of microorganisms and food spoilage reactions by reducing water activity as well as reducing transport costs, storage and preservation through reduction of food weight/volume [2]. Hot air convection drying is the process of removing water with air via simultaneous transfer of heat, mass and momentum. The food's need for heat is achieved by contact of the food with a flow of hot air. The energy transmitted to the surface of the food by convection of hot air is transferred inside the food by diffusion and/or convection, depending World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 799 on the physical and biological structure of the food to be dried. This heat flow causes an increase in product temperature and evaporation of surface water. Moisture is transferred from the surface of the product to the air by convection in the form of water vapor and from the interior of the product by diffusion, convection or capillarity. The drying rate and characteristics of dried foods depend on the conditions of the air-drying process such as air temperature, relative humidity of the air, air velocity, orientation/ direction of air flow and vapor pressure in air. The drying rate also depends on the characteristics of the foods to be dried such as geometry, thickness/size, shape and physical/biological configurations of the foods. The complexity of the physic-biological configurations of wet foods, the variety of transport phenomena and biological diversity make food drying a challenge. To meet this challenge, mathematical modelling and simulation can be a useful tool to examine the drying of foods and the quality of foods obtained after drying. This tool can make it possible to achieve acceptable process conditions or even better than the usual average through a procedure for optimizing the operational variables of drying. Mathematical modelling of food drying involves the use of mathematical equations to predict/capture the physics and/or behavior of drying [3]. Many mathematical models of drying processes are used to design new drying systems or to improve existing drying systems, or even to control the drying process. Among these multiple mathematical models proposed to describe the drying process, thin-layer drying models have been widely used. The term “thin layer” is applied to a single kernel freely suspended in the drying air or one layer of grain kernels. It is also applied to a poly-layer of many grain thicknesses if the temperature and the relative humidity of the drying air can be considered for the purpose of the drying process calculations, as being in the same thermodynamic state at any time of drying [4]. Thin-layer models can be classified as theoretical, semi-theoretical and empirical [5]. Recently, many researchers have focused on the mathematical modelling and experimental drying processes of various tuberous roots such as sweet potato. Sweet potato called scientific name Ipomoea batatas is a plant cultivated mainly for its edible tubers (tuberose roots), rich in starch. These tubers produced are designated by the same name as this plant. Sweet potatoes produce more edible energy on marginal lands than any other major food crop. In addition to this useful property, these tubers can resist unwanted abiotic and biotic stresses and do not require intensive care. They therefore play an important role in the economy of poor households where they constitute a major source of subsistence and are considered a food to combat famine and child malnutrition. Besides this important function, sweet potato cultivation has immense semiindustrial/industrial value for starch extraction and animal feed production. For all these explanations, sweet potato offers great possibilities for achieving food and nutritional security in developing and underdeveloped countries where most agricultural fields belong to vulnerable population categories [6]. Sweet potatoes are important tubers rich in fiber, starch, vitamins, minerals and bioactive compounds. They contain essential carotenoid, phytochemical, anticancer and antimicrobial properties useful for human and animal health. Sweet potato raw or in its processed form can be consumed by humans as a staple food, snack or baked goods. However, sweet potato is susceptible to microbial activities which can lead to degradation and spoilage due to its high moisture content. Furthermore, sweet potato is seasonal and cannot maintain optimal quality level for a long period after harvest. Thus, it is often used shortly after harvest or preserved using the hot air convection drying method [7]. In the literature, several drying processes have been applied to different sweet potato varieties, namely, infrared and fluidized bed drying [8], convective hot air drying [9], microwave drying [10], hybrid microwave and hot air drying [11], spouted bed drying [12], sun and drum drying [13], spray drying [14], freeze-drying [15] and solar drying [16]. Pretreatments before drying have been applied to sweet potatoes including steaming [17], blanching with hot water and steam [13], soaking in sodium metabisulfite solution [18], osmotic dehydration with sucrose and sorbitol [10], immersion in citric acid solution [15], lemon juice and saline solution [19] and soaking in sodium metabisulfite solution [20]. These drying and pretreatment techniques were applied on sweet potato samples with several sliced shapes, varieties, skin and flesh colors including sweet potato cubes [21], white skin and yellow-red flesh sweet potato slices of Kratai cultivar [8], strips [22], chips [23], Nigerian variety slices [24] and Chinese local variety slices [25]. It was found that there are few articles on air drying of spherical sweet potato in this literature. The objective of this paper is to investigate the drying characteristics e.g. moisture ratio, drying rate, moisture diffusivity, activation energy of white skinned and pale fleshed spherical sweet potato, a newly introduced variety cultivated in Burkina Faso. 2 Materials and Methods 2.1 Raw Material and Processing Sweet potato (Ipomoea batatas) was used as drying material in this study. Samples of the local variety of sweet potato with pale flesh and white-skinned, heavily consumed in low-income households, were purchased during the period of July 2023 at the fruit and vegetable market in the town of Bobo Dioulasso (Contact details: 11 ° 11′ 00″ North, 4° 17′ 00″ West), located in the Haut Bassin region of Burkina Faso. Sweet potato samples were transported and stored in refrigerated conditions (4 ± 0.5 °C) before the drying process at the GERME & TI laboratory (Study and Research Group World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 800 in Energy Mechanics and Industrial Techniques) from Nazi Boni University. Before drying, sweet potato samples were placed in laboratory to reach room temperature (25 ± 1 °C). Sweet potato samples were selected, washed, peeled, cut into spheres with diameters from 1 ± 0.002 cm to 3 ± 0.002 cm, measured manually using a digital caliper. Spherical samples are immersed in distilled water to remove excess surface starch film. Excess water on the spherical samples was removed using blotting paper and these sweet potato spheres were arranged in a single layer on a drying tray. The initial moisture content on a dry basis (d.b.) of sweet potato was determined using convective oven method at 105 ± 5 °C for 24 h [26]. Triplicate samples were used for determination of moisture content and the average values were (3.0174±0.01) kgwater/kgdry matter. 2.2 Drying Equipment Drying experiments were carried out in an Air Performance laboratory oven (Froilabo, Model AC Standard Version, France, range 10–250°C with an accuracy of ±0.5°C) installed at GERME & TI laboratory (Study and Research Group in Energy Mechanics and Industrial Techniques) from the Nazi Boni University, Bobo-Dioulasso, Burkina Faso, previously described by Ouoba et al. [27]. Length, height and width of oven were 0.579 m, 0.640 m and 0.526 m respectively. Oven essentially consisted of a centrifugal fan to provide the desired drying air flow, a 1,000-Watt electric heater controlling the temperature of the drying air, an air filter and a proportional-integral-derivative controller (PID controller). Air temperature in convective oven was regulated to ±1∘C using a temperature controller. The oven operated at dry bulb temperatures of 10°C to 250°C. The desired drying air temperature was reached by an electric resistance and controlled by the heating control unit. The air speed was regulated by the centrifugal fan and a fan speed control unit. The air came out of the heating unit and was heated to the desired temperature, then channeled to the drying chamber through ventilation slots located in the rear side wall of the drying chamber. Fan located at the rear of chamber wall produced greater airflow and more intensive horizontal forced air circulation to dry the product samples. The samples were dried on a square perforated stainless-steel tray, having a flow cross section of 0.3 m x 0.3 m. The oven was adjusted to the selected air temperature for approximately 0.5 h before the start of the experiments in order to reach its steady state. 2.3 Drying Procedure Air drying temperatures were 50, 60, 70 and 80°C and air relative humidity was in range from 5 to 20%. Air velocity was kept at a constant value of 2.0 m/s with an accuracy of ±0.03 m/s for all drying experiments. Drying process began when drying conditions reached constant air temperatures. Once the oven reached stable conditions for set points, sweet potato samples were placed on a tray in a single layer and measurement started from that point. Experiments were carried out with 125 ± 0.3 g of sweet potato for all tests. Tray was removed from convective dryer regularly, at 20minute intervals, and weighed with a digital electronic balance, then placed back in oven. The tray was removed from the dryer regularly, at 20-minute intervals, and weighed with a digital electronic balance, then placed back into the oven. The electronic digital balance (model 2102, SARTORIUS, France, range 0–2,100 g with an accuracy of ±0.001 g) was kept less than 1 m from the dryer [28]. Convective hot air drying was continued until there was no longer any significant variation in the evolution of the masses of the spherical sweet potato samples. Drying tests were terminated when masses of samples were stabilized, which assumed that thermodynamic equilibrium was reached. The dried samples were cooled under laboratory conditions after each drying experiment and stored in airtight jars. The mass loss of the samples during drying was converted to moisture content on a dry basis and expressed as kgwater/kgdry matter according to equation (1). For each drying condition, averages of three replicates were taken as drying data. At end of each experiment, sample was heated in an oven at 105 °C for 24 h of drying to obtain the dry matter mass of this sample [29]. 𝑋(𝑡)=𝑚(𝑡)−𝑚𝑠 𝑚𝑠 ……….. (1) Where X(t) is the moisture content on a dry basis (d.b.) expressed in kg water/kg dry matter; m (t), mass of spherical sweet potato samples, expressed in kg at time t in seconds and ms, mass of dry matter of spherical samples (kg). 3 Drying Theory 3.1 Moisture Ratio Moisture ratio (MR) was calculated from moisture content data of spherical sweet potato samples during drying. Equation (2) was used to calculate the moisture ratio [30]: 𝑀𝑅=𝑋−𝑋𝑒 𝑋0−𝑋𝑒 ……… (2) World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 801 Where X, X0 and Xe are respectively the average moisture content at any time of drying (kg water/kg dry matter), the initial average moisture content (kg water/kg dry matter) and the equilibrium moisture content (kg water/kg dry matter). As Xe is much smaller than X0 and X, it is negligible in this study. The moisture ratio then becomes: 𝑀𝑅=𝑋 𝑋0 ………… (3) 3.2 Drying Rate The drying rate (DR) of the spherical sweet potato samples is calculated using equation (4) [31]: 𝐷𝑅=𝑋𝑡+𝑑𝑡−𝑋𝑡 𝑑𝑡 ……….(4) Where Xt and X t+dt are moisture contents at t and t + dt (kgwater/kgdry matter), respectively, DR, drying rate (kgwater/(kgdry matter. s)) and t is the time (s). 3.3 Effective Moisture Diffusivity Drying of most food materials takes place during the falling rate period, and moisture transfer during drying process is controlled by internal diffusion. Fick's second diffusion equation (equation (5)) was widely used to describe drying process during falling rate period of agricultural materials [28]: ∂X ∂t=Deff∇2X ……….. (5) Diffusion equation (equation (5)) is solved for a sphere, assuming one-dimensional volume change of moisture movement, uniform temperature and constant mass diffusivity, and negligible external resistance [32]: 𝑀R= 6 π2∑1 (2n−1)2 ∞ n=1 exp[−(2𝑛−1)2π2Deff 4r2t] …………. (6) Where Deff is the effective moisture diffusivity (m2/s), r is the radius of the spherical sample (m), and n is the positive integer. For long drying times, equation (6) simplifies to a limiting form of the diffusion equation as follows: MR=6 π2exp(−π2Deff 4r2t) ……………. (7) Plotting of ln (MR) versus drying time is expressed with the dimensionless Fourier number F0 according to equation (8). ln𝑀𝑅=−lnπ2 6−π2 4F0 with F0 =Deff r2t …….. (8) Or F0 =0.4053ln(𝑀𝑅)+0.2017 ……. (9) 3.4 Activation Energy Effective diffusivity can be linked to air temperature by Arrhenius type expression [30], such as: Deff =D0exp[− Ea R(T+273.15)] ……. (10) Where D0 is the constant of the Arrhenius type equation (m2/s), Ea is the activation energy (J/mol), T is the uniform temperature of the sweet potato (°C) and R=8, 3145 is the universal gas constant (J/mol K). Equation (10) can be rearranged into the form: World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 802 ln(Deff)=ln(D0)− Ea R(T+273.15) ……….. (11) 3.5 Statistical Analysis For the adjustment of the drying data of the spherical sweet potato samples, a regression analysis by the least squares method was carried out using MATLAB 8.0 software. Four statistical parameters were used to determine the ability of the tested model to represent the experimental data, namely: the coefficient of determination (R2), the root mean square error (RMSE), the reduced chi-square (χ2) and the sum of squared errors (SSE). • The coefficient of determination(R2) The main goal of using R2 in the context of statistical models is to predict future outcomes based on associated experimental data. R2 helps capture the amount of dispersion in a drying data set which is accounted for by a mathematical model. It measures how likely future outcomes are to be predicted by this mathematical model. R2 is rarely equal to 0 or 1, but rather somewhere between these limiting values. The closer it is to 1, the more the experimental and predicted values agree. This value is used for the comparison rule and shows the level of good fit between the measured and predicted values. This was one of the first coefficients used to select the appropriate drying model to describe food drying behavior [4]. 𝑅2=1−∑(𝑃𝑒𝑥𝑝,𝑖−𝑃𝑝𝑟𝑒,𝑖)2 𝑁 𝑖=1 ∑(𝑃 𝑒𝑥𝑝−𝑃𝑒𝑥𝑝,𝑖)2 𝑁 𝑖=1 …………….. (12) • Root Mean Square Error (RMSE) The root mean square deviation, RMSD, or root mean square error, RMSE, is a tool for measuring the differences between the values predicted by a mathematical model and the values actually observed from drying experiments. RMSD measures the accuracy of this model well and is used to group the residuals into a single measurement of the predictive tool. It must evolve towards a zero value for a good adjustment of the drying model and can be expressed as follows [29]: 𝑅𝑀𝑆𝐸=[∑(𝑃𝑒𝑥𝑝,𝑖−𝑃𝑝𝑟𝑒,𝑖)2 𝑁 𝑖=1 𝑁]12 ⁄ …………… (13) • Chi-square reduced (χ2) This statistical coefficient of chi-square reduced (χ2) is a tool to measure the mean square of the differences between the experimental values and those predicted from mathematical models. This makes it possible to evaluate the adequacy of this model with the experimental data. The smaller the values of χ2, the better the quality of the fit and could be expressed as follows [4]: 𝜒2=∑(𝑃𝑒𝑥𝑝,𝑖−𝑝𝑝𝑟𝑒,𝑖)2 𝑁 𝑖=1 𝑁−𝑧 …………… (14) • Sum of Squared Errors, SSE The sum of squared errors (SSE) gives the measure of the deviation of the data of an experimental sample from its theoretical values predicted by a mathematical model. This parameter is explained as the difference between the experimental data and those predicted by the drying model and expressed as [29]: 𝑆𝑆𝐸=∑(𝑃𝑒𝑥𝑝,𝑖−𝑃𝑝𝑟𝑒,𝑖)2 𝑁 𝑖=1 …………… (15) Where P is the hot air-drying parameter, Pexp,i is the experimental value of the parameter, Ppre,i is the value of the parameter P predicted by the statistical model, P exp,i is the average value of the parameter P, N is the number of World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 803 experimental observations and z is the number of constant coefficients in the model regression. A good fit of the drying model is found for the highest values of R2 and for the lowest values of RMSE, χ2 and SSE [29]. 4 Results and Discussion 4.1 Moisture Ratio From the moisture ratio evolutions in (a) (b) Figure 1a, it can be deduced that the sweet potatoes were completely dried for 2000, 1376, 840 and 580 minutes at 50 °C, 60 °C, 70 °C and 80 °C respectively for the spherical samples of 2 cm in diameter. For the 3 cm diameter samples, the drying durations were found to be 2080, 1600, 1419, and 915 minutes at 50 °C, 60 °C, 70 °C and 80 °C respectively ( (a) (b) Figure 1b). Therefore, the moisture content of the spherical sweet potato samples decreased to a constant point in a time that depended on the air-drying temperature and the diameter of the spherical samples, with the lowest time being at 80 °C (i.e. 580 minutes) and the highest time at 50°C (i.e. 2000 minutes) during the drying of spherical samples of 2 cm in diameter. For the 3 cm diameter spherical samples, the lowest time at 80°C was 915 minutes and the highest time at 50°C was 2080 minutes. Thus, an increase in drying air temperature results in a reduction in drying time. For spherical samples of a given diameter, oven temperature levels had a significant influence on the moisture content of sweet potatoes. (a) (b) World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 804 Figure 1 Drying kinetics of spherical sweet potato samples dried with hot air at temperature levels (50, 60, 70 and 80°C) with spherical samples of (a) d=2 cm and (b) d= 3 cm in diameter The lower moisture content of dried spherical sweet potato samples showed a decrease in the volume and transportability of these dried samples during processing, preservation, and storage. The reduction in moisture content reduced their water activities, which also minimized the microbial deterioration and spoilage reaction during their storage. Our drying results of spherical sweet potatoes were in agreement with those of other researchers for various foods such as onion [29], Kiwi [33], okra [34], cassava [35], apple [36], carrot [37] and tomato [38]. In (a) (b) Figure 1, regarding the physical mechanism of drying, convective drying of spherical sweet potato samples did not show a constant rate period. Moisture content decreased with increasing drying time of the spherical samples during the decreasing rate period at the four temperature levels for both 2 and 3 cm diameter spherical sweet potato samples. This showed that moisture diffusion was the dominant physical mechanism of water movement in the spherical sweet potato samples during the drying process [39]. Water diffusion during the drying process could induce a decrease in absorption due to the reduction in the water content of our product to be dried [40]. Some research has been conducted on the influence of air temperature on drying kinetics. For drying pear slices in a convective dryer, Doymaz [41] investigated the influence of drying air temperature. He noted that the reduction in total drying time with increasing temperature may be due to the increase in vapor pressure inside the product with increasing temperature, which resulted in faster migration of moisture to the product surface. 4.2 Drying Rate Drying rate is defined as mass of water removed per mass of dry matter and time [kg water/ (kg dry solid s)] under the experimental conditions. We estimated it based on equation (4) and its variations as a function of drying time at air temperatures of 50, 60, 70 and 80°C for spherical sweet potato samples of 2 cm in diameter were as shown in Figure 2. It could be observed on the drying kinetics a significant influence of the air-drying temperature on the drying rate of the spherical sweet potato samples. This could show that the drying rate continuously decreased with increasing drying time, except for the drying rate curve at 60°C air temperature which showed a small initial heating of the sweet potatoes. World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 805 There was no constant rate drying period in these curves and most of the drying process took place during the decreasing rate period. These results were in good agreement with previous observations of various products such as onion slices [29], grape leaves [42], pomegranate arils [43] and pomegranate peels [44]. Moisture removal inside the spherical sweet potato samples at an air temperature of 80 °C was greater and faster than at other drying temperatures studied due to the rapid movement of the moisture to the surface of the sweet potato and the high evaporation rate from the surface to the surrounding warm air. The high level of air temperature could facilitate these two physical phenomena by rapid activation of molecular diffusion of water from the sweet potato solid matrix. The bond of water molecules with the solid matrix was easily broken, allowing water molecules to exit the sweet potato samples. The removal of moisture inside the sweet potato decreased with decreasing humidity level and hence the drying rate also decreased. Additionally, shorter drying time was observed in Figure 2 at higher temperatures due to increasing drying rate. This increase in drying rate was due to the increased heat transfer potential between the drying air and the spherical sweet potato samples, which accelerated the evaporation of water from the spherical sweet potato samples. Since the relative humidity of hot air at a higher temperature was lower than that at a lower temperature, the difference in partial vapor pressure between thin-layer sweet potato spheres and their surroundings was greater for the case of drying sweet potato at high air temperature. Similar results had earlier been reported in the others works suck as the convective drying of tomato slices [45], convective air drying of sweet potato cube [21] and the hot air drying of pear slices [46]. Torki-Harchegani et al. [47] reported that food drying occurred in the period of declining rate. During the period of decreasing rate, water migrated from areas of the food to be dried with a higher moisture content to areas of the same food where the moisture content was lower. The induced movement of water from the interior of the food to its surface was controlled by molecular diffusion of moisture. This physical phenomenon could be explained by Fick’s second law. The investigation by Jiang et al. [48] explained our findings that the average moisture migration rate inside wheat grains was lower than the average evaporation rate outside the wheat grains. He et al. [49] also explained this migration by the fact that promoting the moisture transfer movement of sea cucumbers during the drying process at decreasing rate could shorten the drying time of their cucumber samples. Figure 2 Drying rate curves versus time (min) for different temperature at diameter 2 cm of sweet potato spheres The kinetics of the drying rate of spherical sweet potato samples 2 cm in diameter as a function of moisture ratio was presented in Figure 3. A constant rate period was not observed in all cases of air-drying temperature. Examination of Figure 3 revealed that, in general, two distinct drying periods were identifiable, namely the period of warming to 60 °C and the falling-rate period at all air-drying temperatures. The short initial warming period corresponding to spherical samples heated to 60 °C was due to non-isothermal drying conditions of the sweet potatoes at the start of the process. The presence of decreasing drying behavior was indicative of a progressive increase in internal resistance to mass and heat transfer [29]. A continuous decreased in drying rate as the moisture ratio decreased could be observed in Figure 3 . At the same time, the increase in drying rate was observed as the air temperature increased, as shown Figure 2. Drying rates were higher at the beginning of the process and then decreased with decreasing moisture content in the spherical sweet potato samples during the drying process. The reduction in drying rate could be due to the reduced porosity of the spherical samples. This reduced porosity originated from matrix removal from the spherical samples as the drying process progressed. This resulted in an increase in massive resistance to water migration, leading to a further decline in sweet potato drying rates [50]. The results were consistent with observations made by other researchers on drying foods such as bay leaves [51], purslane [52], by-products of pomegranate [53] and apple pomace [54]. Mrad et al. [55] reported that, upon the pears drying, the falling drying rate period resulted from the predominance of internal diffusion mechanism due to bound water at the surface and shrinkage of the product. World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 806 Figure 3 Drying rate curves versus moisture ratio for different temperature at diameter 2 cm of sweet potato spheres The evolutions of drying rates as a function of humidity level for two spherical sweet potato samples with diameters (2 cm and 3 cm) at an air-drying temperature of 50 °C, are illustrated in Figure 4. The results of others air temperatures (60, 70, 80°C) observed similarly. Drying rate at 2 cm diameter was higher and faster than the 3 cm diameter. Shorter drying time was obtained for spherical sweet potato samples with smaller diameter, which increased their drying rate. Thus, the drying rate increased with decrease in sample diameters. The spherical diameters of the samples could influence the rate of convective drying for the same moisture content of the sweet potato: the smaller the diameter of the sample, the higher the drying rate of the sweet potato. This increase is due to the decreased mass transfer resistance of the spherical sweet potato samples, which facilitates the movement of water from the interior of the spherical sweet potato samples to their surfaces. Similar results were reported by others researches on foods. Jiang et al. [56] found when hot air-drying crabapple slices that the thickness of crabapple slices influenced the hot air-drying time. With the same moisture content, the thinner the thickness of the product slices, the higher the drying rate and the shorter the drying time of crabapple slices. Jongyingcharoen et al. [57] explained that the thinner the thickness of coconut dregs, the shorter the drying time of the process to 70 minutes. Ndisya et al. [58] found that as the thickness of their sample slices to be dried increased, the drying time of the purple-speckled coconut slices also increased. In thin layer drying of onion varieties, Sobowale et al. [59] noticed that the drying rate of white and red onion slices with increasing thickness illustrated the rate at which liquid is migrated inside-out of the bulb scales, simply through mass-transfer bound over time. Similar results on the influence of the thickness have been reported for hot air drying of banana [60], convective drying of apple slices [61], hybrid convective drying of tomato slices [62], hot air drying of “Violet de Galmi” onion slices [29], hot air convective drying of tomato slices [63] and convective hot air drying of potato, garlic and cantaloupe [64]. Compaore et al. [29] was found that the drying rate leek slices was higher at thin slices, and the total drying time reduced substantially with the decrease in slices thickness. Sadin et al. [65] reported that infrared drying rate of tomato slices was increased with increasing temperature and reduction thickness so that maximum drying time to temperature and thickness 60°C, 7 mm respectively and that minimum drying time to temperature and thickness 80 °C, 3 mm respectively. Ouoba et al. [66] indicated that, during convective drying of different sizes of sweet potato, the smaller the sample size was, the faster the drying rate was. The drying was found more efficient for small samples. World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 813 [52] Doymaz İ (2013) Hot-air drying of purslane (Portulaca oleracea L.) Heat and Mass Transfer. 49 835–41. https://doi.org/10.1007/s00231-013-1128-9 [53] Kara C and Doymaz I (2015) Thin Layer Drying Kinetics of By-Products from Pomegranate Juice Processing Journal of Food Processing and Preservation. 39 480–7. https://doi.org/10.1111/jfpp.12253 [54] Kara C and Doymaz İ (2015) Effective moisture diffusivity determination and mathematical modelling of drying curves of apple pomace Heat and Mass Transfer. 51 983–9. https://doi.org/10.1007/s00231-014-1470-6 [55] Djendoubi Mrad N, Boudhrioua N, Kechaou N, Courtois F and Bonazzi C (2012) Influence of air drying temperature on kinetics, physicochemical properties, total phenolic content and ascorbic acid of pears Food and Bioproducts Processing. 90 433–41. https://doi.org/10.1016/j.fbp.2011.11.009 [56] Jiang N, Ma J, Ma R, Zhang Y, Chen P, Ren M and Wang C (2022) Effect of slice thickness and hot-air temperature on the kinetics of hot-air drying of Crabapple slices Food Science and Technology. 43 e100422. https://doi.org/10.1590/fst.100422 [57] Jongyingcharoen J S, Wuttigarn P and Assawarachan R (2019) Hot air drying of coconut residue: shelf life, drying characteristics, and product quality IOP Conference Series: Earth and Environmental Science. 301 012033. https://doi.org/10.1088/1755-1315/301/1/012033 [58] Ndisya J, Mbuge D, Kulig B, Gitau A, Hensel O and Sturm B (2020) Hot air drying of purple-speckled Cocoyam (Colocasia esculenta (L.) Schott) slices: Optimisation of drying conditions for improved product quality and energy savings Thermal Science and Engineering Progress. 18 100557. https://doi.org/10.1016/j.tsep.2020.100557 [59] Sobowale S S, Omotoso O B, Kewuyemi Y O and Olatidoye O P (2020) Influence of temperature and thickness on thin layer drying characteristics of onion (Allium cepa L.) varieties and rehydration capacity Croatian Journal of Food Science and Technology. 12 165–76. https://doi.org/10.17508/CJFST.2020.12.2.04 [60] Thuwapanichayanan R, Prachayawarakorn S, Kunwisawa J and Soponronnarit S (2011) Determination of effective moisture diffusivity and assessment of quality attributes of banana slices during drying LWT - Food Science and Technology. 44 1502–10. https://doi.org/10.1016/j.lwt.2011.01.003 [61] Beigi M (2016) Energy efficiency and moisture diffusivity of apple slices during convective drying Food Science and Technology. 36 145–50. https://doi.org/10.1590/1678-457X.0068 [62] Nwakuba N R, Chukwuezie O C, Asoegwu S N, Nwandikom G I and Okereke N . A A (2018) Energy requirements and effective moisture diffusivity of tomato slices in a hybrid convective dryer An ASABE Meeting Presentation; 2018 Detroit, Michigan July 29 - August 1, 2018 2018 Detroit, Michigan July 29 - August 1, 2018 . https://doi.org/10.13031/aim.201800038 [63] Obajemihi O I, Olaoye J O, Cheng J-H, Ojediran J O and Sun D-W (2021) Optimization of process conditions for moisture ratio and effective moisture diffusivity of tomato during convective hot-air drying using response surface methodology Journal of Food Processing and Preservation. 45 e15287. https://doi.org/10.1111/jfpp.15287 [64] Kaveh M, Rasooli Sharabiani V, Amiri Chayjan R, Taghinezhad E, Abbaspour-Gilandeh Y and Golpour I (2018) ANFIS and ANNs model for prediction of moisture diffusivity and specific energy consumption potato, garlic and cantaloupe drying under convective hot air dryer Information Processing in Agriculture. 5 372–87. https://doi.org/10.1016/j.inpa.2018.05.003 [65] Sadin R, Chegini G-R and Sadin H (2014) The effect of temperature and slice thickness on drying kinetics tomato in the infrared dryer Heat and Mass Transfer. 50 501–7. https://doi.org/10.1007/s00231-013-1255-3 [66] Ouoba K H, Zougmore F and Desmorieux H (2018) Effect of Initial Size and Shape Importance on Masse Transfer during Convective Drying Food and Nutrition Sciences. 09 1514. https://doi.org/10.4236/fns.2018.912109 [67] Samimi. Akhijani H, Arabhosseini A and Kianmehr M H (2016) Effective moisture diffusivity during hot air solar drying of tomato slices Research in Agricultural Engineering. 62 15–23. https://doi.org/10.17221/33/2014-RAE [68] Demiray E, Seker A and Tulek Y (2016) Drying kinetics of onion (Allium cepa L.) slices with convective and microwave drying Heat and Mass Transfer. 53, 1817–1827. https://doi.org/10.1007/s00231-016-1943-x [69] Mitra J, Shrivastava S L and Srinivasa Rao P (2011) Vacuum dehydration kinetics of onion slices Food and Bioproducts Processing. 89 1–9. https://doi.org/10.1016/j.fbp.2010.03.009 World Journal of Advanced Research and Reviews, 2025, 26(02), 798-813 814 [70] Chen C, Venkitasamy C, Zhang W, Khir R, Upadhyaya S and Pan Z (2020) Effective moisture diffusivity and drying simulation of walnuts under hot air International Journal of Heat and Mass Transfer. 150 119283. https://doi.org/10.1016/j.ijheatmasstransfer.2019.119283 [71] Onyenwigwe D I, Ndukwu M C, Igbojionu D O, Ugwu E C, Nwakuba N R and Mbanaso J (2023) Mathematical modelling of drying kinetics, economic and environmental analysis of natural convection mix-mode solar and sun drying of pre-treated potato slices International Journal of Ambient Energy 44 1721–32. https://doi.org/10.1080/01430750.2023.2182359 [72] Mbegbu N N, Nwajinka C O and Amaefule D O (2021) Thin layer drying models and characteristics of scent leaves (Ocimum gratissimum) and lemon basil leaves (Ocimum africanum) Heliyon 7 (1) . https://doi.org/10.1016/j.heliyon.2021.e05945 [73] Moura R L, Figueirêdo R M F de, Queiroz A J de M, Santos F S dos, Lima A G B de, Rego Junior P F do, Gomes J P, Silva W P da, Paiva Y F, Moura H V, Silva E T de V, Costa C C and Gregório M G (2023) Smelling Peppers and Pout Submitted to Convective Drying: Mathematical Modeling, Thermodynamic Properties and Proximal Composition Foods 12 2106. https://doi.org/10.3390/foods12112106 [74] Malik A, Shimpy and Kumar M (2023) Advancements in ginger drying technologies Journal of Stored Products Research. 100 102058. https://doi.org/10.1016/j.jspr.2022.102058