Effect of the drying air temperature variation on the energy consumption and quality preservation of garlic slices

Introduction: Garlic "Allium Sativum L." is a relatively perishable product. Approximately 30% of this product is lost annually in the post-harvest stages due to the lack of suitable storage and convenient transportation facilities. It is rich in medicinal and food nutrition and is widely cultivated throughout the world, including Iran. Recently, dried garlic has been used as one of the ingredients and additives of cooked and semi-finished foods such as sauces and soups. Changing the pattern of consumption from the fresh garlic to the dried one has increased the demand for this product. Understanding the moisture and heat transfer mechanism during the drying is crucial to enhance the quality and reduce energy consumption. The quality changes of a dried product are functions of drying condition as like; drying air temperature, drying time, and the warmth of the product surface. By applying higher drying air temperature, shorter drying time and also in parallel with it, slightly lower energy consumption is achievable; however, it will result in higher quality changes. The objective of this study was developing a simulator for drying with variable air temperature to reduce the drying time and lowering energy consumption as well as maintaining the dried product quality at its highest possible level. 
Materials and methods: In the present study, the mathematical equations of mass and heat transfer during convection drying of garlic have been developed. The following assumptions were used to develop the mathematical model; 3-D transfer of heat and moisture is accrued; the air distribution is uniform inside the dryer; the distribution of initial moisture and temperature throughout the material is uniform; the product shrinkage is negligible during the drying process; heat transfer to the product through radiation is negligible and conductive heat transfer between trays and product was not included. Third type boundary condition was applied to all air-product interfaces and the equations were solved on three-dimensional geometry of garlic slices simultaneously with the Finite Element Method (FEM) using COMSOL MULTIPHYSICS 3.5 software under different drying air temperature. The temperature and moisture profiles were estimated in a garlic slice by the developed simulator. Mesh independence study was performed to establish an optimal mesh density that gives a solution with acceptable accuracy and four meshes, each containing 1430, 3364, 4362 and 12977 elements were used. The results indicated that mesh independence of numerical solution was obtained when the number of elements was above 4362. The developed simulator was validated by experimental study under different drying temperature conditions; 50, 60 and 70 °C; and different thicknesses of the samples; 2, 3 and 4 mm. At the same time, the engineering and qualitative properties associated with thermal processing of garlic slices were studied, such as the effective moisture diffusivity, apparent density and dry material density, and color changes of the samples. Eventually, by using the simulator, the issue of reducing energy consumption by reducing the total time of the drying along with preserving the apparent quality of the product was examined based on the output data from the simulator through the use of the variable temperature of hot air during the process and in different thicknesses. 
Results and discussion: The developed simulator was able to accurately predict changes in the moisture ratio at different temperatures of the hot air and the different thicknesses of the garlic product. The average error in estimating the moisture content at the hot air temperature of 50, 60 and 70 °C was 11%, 6%, and 34%, respectively. Based on the experimental results, the change in hot air temperature and samples thickness did not have a significant effect on the apparent density but affected the final color of garlic slices. The surface temperature of samples had a significant influence on the quality of the dried product; furthermore, the minimum and maximum changes in the color of the samples were observed when the surface temperature of slices was 50°C and 70°C, respectively. The use of drying air at 50 °C and 2 mm thick slices maintains the lightness of the samples and prevents the color changes of the samples during drying.
 However, the use of hot air at a lower temperature can prolong the drying time. Accordingly, by the developed simulator using the drying air with variable temperature for drying garlic slices was evaluated and the predicted surface temperature of sample was monitored during the drying process with the air temperature of 70°C and by the time that the surface temperature of sample was increased and reached 50°C, the hot air temperature was reduced and the drying process lasted from 30 minutes, with drying air at 50 °C. Despite the use of hot air with variable temperature, no noticeable changes were observed during drying compared to drying with an air temperature of 70 °C and drying time was prolonged only 3 minutes. At the same time, during the 25% of the total drying time the drying air with 20 °C lower than the usual has been used, which will reduce energy consumption. The results showed that by using this method, it is possible to dry the garlic slices in a shorter time. Also, by restricting the temperature rise of the product in the final stages of the process, the color change of the samples is prevented. On the other hand, due to the use of the variable temperature of the drying air, the drying time decreased about 43 minutes as compere with the drying of the sample with the air temperature of 50 °C, which had a significant effect on reducing energy consumption. Similar results were obtained in drying garlic slices with a thickness of 3 mm.
  Conclusion: To achieve a high-quality dry product, more transparent color and reduced energy consumption by reducing the drying time, it was necessary to use slices with lower thickness and hot air with variable temperature during the process based on the developed output-model. The proper time to change the air temperature during the drying process was determined. The developed simulator can predict the items mentioned above, with high precision.