In this work, slices of yellow onions (Allium cape) with a diameter of (6.7 ± 0.05) cm and a thickness of (1 ± 0.05) cm are dehydrated in a drying oven at two temperatures: 40 °C and 90 °C. The average initial mass and water content of the samples are 32.61 g and 92.13%, respectively. The microbial flora in moulds and yeast is also enumerated before and after drying. An experiment plan of two variables, namely drying time and temperature, is followed to optimize the process tests. The moisture content is taken as a response. The interactions between operating variables are shown and a mathematical model is established expressing, thus, the variation of the moisture content of the onion slices as a function of the temperature and the drying time. A correlation of experimental results and calculations is indeed established. The results show that drying duration decreases with increasing temperature and water content of onion slices decreases from 92.13% to 4.5% during 2595 minutes, and from 92.13% to 9.47% during 1245 minutes, at 40 °C and 90 °C, respectively. Also, the experimental results lead to the appropriate choice of the temperature and the drying time in order to reach adequate moisture content. Finally, Oven drying of the yellow onions with determined dimensions eliminates the moulds to desired and encouraging values.

Keywords: Static drying; Oven; Yellow Onion; Experimentation; Microbial Flora; Allium cepa.

This paper presents an experimental study of the shrinkage phenomenon of apple slices during two types of drying, namely: static drying (in an electric regulated oven) and open sun drying (naturally, on one tray) under the meteorological conditions of Ouargla city in the south east of Algeria. In the oven, the experiments are performed with three different temperatures (50, 60 and 70 °C). The effect of the water content loss of the fruit samples on the dimensions (thickness and diameter) and on the volume are considered. During the static and sun drying, the results show that the volume decreases by 80% and 86% of its initial values, respectively. During the static drying, the diameter of each apple slice shrinks by 20% and the thickness by 70 % of the initial values. Not far from these values, the diameter decreases by 25% and the thickness by 76% of the initial values during the open sun drying. In addition, the drying of apple slices shows an anisotropic shrinkage independently of temperature and therefore a unique shrinkage equation is proposed for each of both methods of drying. Finally, the importance of considering the phenomenon of shrinkage of the apple fruit during drying is shown in the Krischer curves.

Keywords: Apple slices, Experimentation, Shrinkage. Static drying, Open sun drying.

In this research, an indirect solar dryer (SSI) was designed and tested for drying tomato in thin layer. It operates without additional energy in the passive and active convective modes. Before realizing the solar drying tests in the SSI, the cover and the absorber of the flat-plate solar collector (CSPA) were optimized. The convective and solar drying of watermelon and tomato occur during the falling drying phase and the Verma and Midilli-Kucuk model are those that perfectly describe the drying of both two fruits, respectively. The follow-up of the shrinkage during the drying of both fruits was realized by X-ray microtomography. In addition to the experimental study, a computer program (Mod_CSPA) for CSPA and a computer program (Mod_CHS) for the drying chamber (CHS) were developed. Mod_CSPA was experimentally validated and it was shown that the optimal inclination of CSPA is variable during the year. The numerical simulation of the mass and heat transfer during the solar drying in thick layer of tomato slices showed that the energy supplied by the drying air is of use to the water evaporation and the temperature rise of the dried fruit.

Keywords: Indirect solar dryer (SSI) - Flat-plate solar collector (CSPA) - Shrinkage - X-ray microtomography - Drying kinetics - Simulation.

Ce papier présente une étude expérimentale menée sur un capteur plan à air intégré à un séchoir solaire de type indirect destiné au séchage des produits agroalimentaires sous les conditions météorologiques de Ouargla (Latitude : 31° 56 '57" N – Longitude : 5° 19' 30" E – Altitude : 138 m) au sud-est de l’Algérie. Le but essentiel des expériences réalisées est de comparer le mode passif (ventilation naturelle) au mode actif (ventilation forcé) du capteur en question. L’étude comparative montre que l’augmentation du rayonnement solaire a plus d'influence sur les températures du capteur solaire en ventilation naturelle qu’en ventilation forcée où les valeurs du rendement thermique sont meilleures. En outre, la valeur de ce dernier paramètre augmente linéairement avec l'augmentation du rayonnement solaire entre 400 et 800 W/m2 pour des vitesses de l’air fixées entre 1 et 1.40 m/s alors que l’effet de la vitesse reste négligeable pour une valeur voisine de 800 W/m2 .

Mots clefs : Capteur thermique solaire, Mode passif, Mode actif, Expérimentation, Rendement thermique.

In this paper, an indirect solar dryer was developed and tested for drying food. This unit operation is an interesting technique for preservation in agricultural applications. The dryer was designed at industrial scale in order to carry out experiments in real applications and to get concrete results. It operated without additional energy in the passive and active modes, allowing the comparison between the two. The relationship between the solar collector air temperature and the relevant ambience parameters was deduced. The collector efficiency reached 66.56 % with forced ventilation and 46.32 % with natural ventilation. It also increased linearly with the solar radiation varying between 400-800 W/m2 for air velocities ranging between 1 and 1.40 m/s. The effect of the air velocity on the collector efficiency was negligible for solar intensities close to 800 W/m2 .The tomato drying occurred mainly in the falling drying rate period. On average, the moisture content in dry basis was reduced from 14.32 kg water /kg dry matter to 0.14 kg water/kg dry matter. The equilibrium moisture content of tomatoes was reached after 12 h when the system was used with one layer. In the case it was used with four layers, the difference among the four durations of drying was one hour between each tray.

Keywords: Flat-plate solar collector, Drying chamber, Convection, Efficiency, Drying kinetics.

A whole single cherry tomato was dried in a forced convective micro-dryer. The experiments were carried out at constant air velocity and humidity and temperatures of 50, 60, 70 °C. In order to study the effect of the skin, two sets of experiments were performed using a tomato with and without skin (easily removed). Shorter drying times were obtained when increasing drying temperatures as well as when removing sample skin. X-ray microtomography, a non-destructive 3D imaging technique was used to follow shrinkage of the samples. This phenomenon was introduced in the modelling part of this study. Analytical solutions of the Fick’law were used to determine the diffusion coefficient at the three temperatures studied, and then the activation energy was obtained through fitting the Arrhenius equation. The skin effect was clearly evidenced by showing that the mass transfer parameter values of an original tomato with skin were largely smaller than the one without skin. Indeed, the moisture effective diffusivity ranged from 2.56×10-11 to 7.67×10-11 m2 ·s-1 with activation energy of 50430 J·mol-1 for tomato with skin and ranged from 4.59×10-10 m2 ·s-1 to 6.73×10-10 m2 ·s-1 with activation energy of 17640 J.mol-1 for tomato without skin.

Keywords: Micro dryer, Skin, Cherry tomato, Diffusion model, Shrinkage effect.

The behavior of peeled and unpeeled cherry tomatoes was investigated during forced convective drying. The study showed that the drying process highly alters the shape of the samples. This alteration (shrinkage) was examined using a non-destructive X-ray microtomography imaging technique. For both cases (peeled and unpeeled tomatoes), the volume of the sample decreased linearly with its moisture content. Furthermore, the effects of the operating air temperature as well as the peel on the drying curves were explored. Accordingly, increasing the air temperature deceased drying time from 1200 ks at 50 °C to 500 ks at 70 °C for the unpeeled sample and from 80 ks at 60 °C to 50 ks at 70 °C for the peeled sample. The effect of the peel was substantial as the drying time of the unpeeled sample was 5–10 times higher than that of the peeled sample. Based on the analytical solution of the diffusion model, a moisture diffusion coefficient was determined using two approaches. The first approach used a graphical representation and the moisture diffusion coefficient was directly deduced from the trend line of the curves. For the second approach, a correction factor was introduced into the analytical solution and the modeling results showed that the moisture diffusion coefficient was varying with the moisture content of the tested material. The comparison between the experimental data and the modeling results using the two approaches showed that the second approach, which included the effect of shrinkage, was more suitable for predicting the variations of the drying curves for the different operating conditions and for both peeled and unpeeled tomatoes. Using this second approach, the moisture diffusion coefficient for the unpeeled tomato was 2.0 × 10⁻¹¹ m²/s at 50 °C and 3.5 × 10⁻¹¹ m²/s at 70 °C. Similarly, the maximum values of the moisture diffusion coefficient for the peeled tomato varied from 3.0 × 10⁻¹⁰ m²/s at 50 °C to 5.0 × 10⁻¹⁰ m²/s at 70 °C. Moreover, performing modeling while neglecting shrinkage resulted in an over estimation of the moisture diffusion coefficient. In addition, operating conditions, dimensions of the samples and shrinkage had a direct effect on the external mass transfer coefficient.

Keywords : Forced convection; Moisture diffusivity; Mass transfer coefficient; Drying temperature; Shrinkage; Diffusion model.