عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Introduction Almond (Amygdales Communist L.) is a perennial plant growing in the cold and xeric environments of Iran. The kernels of almond form an important source of energy and protein. An infrared- vacuum dryer with microwave pretreatment benefits includes high mass transfer coefficients and high quality and the appropriate control on dryer conditions. The aim of this study was to evaluate the effect of air temperature, microwave power and vacuum pressure in drying process of almond kernels and calculate the effective moisture diffusivity, activation energy, energy consumption, shrinkage and color changes.
Materials and Methods Fresh almond kernels were obtained from a field located in Asadabad (Hamedan Province), Iran and stored in a refrigerator at 4±1˚C for experiments. The initial moisture content of almond kernels was determined by drying of 10 g of sample in an oven at 105±1°C until constant weight was attained. In this study, the drying properties of almond kernels with moisture content of 47% (d.b.) in an infrared- vacuum dryer with microwave pretreatment were investigated. Three levels of air temperatures (45, 60, 75 °C), three levels of microwave powers (270, 450 and 630 W) and three levels of vacuum pressures (20, 40 and 60 kPa) were applied to perform the experiments. Seven mathematical models were fitted to the experimental drying data of almond kernels. Weight loss of samples was measured and recorded every 20 seconds in microwave dryer and every 300 seconds in infrared-vacuum dryer, respectively. Drying time was defined as the time required to reduce moisture content of samples to 0.1 g of water per g of dry mass.
Results and Discussion It was observed that increasing the air temperature and microwave power decreased the time required to reach a certain level of moisture ratio. Also, by reducing the vacuum pressure, drying time for almond kernels was decreased. The results showed that the highest values of coefficient of determination were obtained with the Midilli et al. model. The Midilli et al. model gives higher R2 and lower RMSE and . Therefore, the Midilli et al. model may be supposed to demonstrate the drying behavior of the almond kernels in an infrared-vacuum dryer with microwave pretreatment. The maximum value of Deff (5.33×10-9 m2/s) during the experiments was depending on the air temperature of 75˚C, vacuum pressure of 20 kPa and microwave power of 630W. The minimum value of Deff (8.03×10-10 m2/s) depended on the air temperature of 45˚C, vacuum pressure of 60 kPa and microwave power of 270W. Air temperature had a larger effect on the Deff values of almond kernels drying. Minimum and maximum values of activation energy (Ea) for almond kernels were 28.73 and 51.84 kJ/mol, respectively. The highest and lowest values of energy consumption were 0.26 at air temperature of 45˚C, vacuum pressure of 20 kPa and microwave power of 270W and 0.07 kWh at air temperature of 75˚C, vacuum pressure of 60 kPa and microwave power of 630W, respectively. Increase in inlet air temperature demonstrated an exponential decrease in energy consumption. It was also observed that increase of inlet air temperature, vacuum pressure and microwave power decreased specific energy consumption.
Maximum and minimum values of shrinkage were 14.14% at air temperature of 75˚C, vacuum pressure of 60 kPa and microwave power of 630W and 7.78% computed at air temperature of 45˚C, vacuum pressure of 20 kPa and microwave power of 270W, respectively. The results indicated that the shrinkage increased with increasing air temperature, vacuum pressure and microwave power but the effect of air temperature was more than other parameters. Raising the drying temperature increased the movement of water molecules and made increasing the distance between the molecules in the structure of the sample.
The highest and lowest values of total color change were 8.85 at air temperature of 75˚C, vacuum pressure of 60 kPa and microwave power of 630W and 2.61 at air temperature of 45˚C, vacuum pressure of 20 kPa and microwave power of 270W, respectively. Results showed that total color change increased with increasing air temperature, microwave and vacuum pressure.
Conclusion With respect to the quality indices of shrinkage and color changes, the recommendation is to dry the almond kernels under air temperature of 45 °C, microwave power of 270 W and vacuum pressure of 20 kPa.