Effect of combined microwave-hot air under microwave pretreatment on drying kinetics of Myrtus fruit

A decoction of leaves and fruit of Myrtus is used to solve problems such as stomach ailments at the folk medicine. Fruits and vegetables are highly susceptible to microbial spoilage due to their high moisture content. For this reason, drying method have been expanded in order to prevent microbial spoilage of them. The most common method is hot air drying, but combination of this method with other such as microwave drying is developed to solve hot air method disadvantages such as low drying rate and high energy consumption in this method. Therefore, the aim of this study was to investigate effect of combined microwave-hot air method on mass transfer and drying kinetics of Myrtus fruit and to find the best experimental drying conditions in terms of time, rate and mass transfer.

In present study, drying process of the Myrtus fruit by combined microwave-hot air under microwave pretreatment was investigated. Drying experiments were performed at combined powers of 180, 300 and 450W and three temperatures of 60°C, 70°C and 80°C under microwave pretreatment at powers of 300, 450 and 600W in a domestic microwave device. Experimental data were fitted to 10 mathematical models to study drying kinetics of Myrtus fruit. Also, fitting quality of equations was evaluated by using coefficient detemination (R2) and root mean square error (RMSE). The effect of the combined method on mass transfer coefficient and activation energy of samples was also investigated.

kinetic analysis of process showed that among 10 common kinetic models, exponential Two-term model fits well the present combined method in mentioned power range. The results showed that at a certain temperature with increasing microwave power, drying rate constant (k) of exponential two-parameter equation describing drying behavior of the Myrtus fruit increased. According to results, at a certain temperature, with increasing microwave power, drying time decreased and drying rate increased. The highest drying time was obtained 46 minutes at combined power of 180W and temperature of 60 °C. The lowest drying time was obtained at 450W and 80°C equal to 19min, which was 58.7% lower than the maximum drying time. The results revealed that with increasing microwave power at a certain temperature, diffusion coefficient of Myrtus fruit increased and activation energy decreased. The highest effective moisture diffusivity and lowest activation energy was obtained at 450W and 80°C and it was 11.59 ×10-8 (m2/s) and 13.14 (kj/mol), respectively.conclution: Temperature and combined power are two main factors affecting Myrtus fruit drying. Increasing the combined microwave power at a certain temperature as well as increasing the hot air temperature at a specific combined power have dramatically reduced time and significantly increased drying rate. The drying of the Myrtus fruit occured at falling rate. Microwave power also affect drying rate constants and it increased with increasing combined power. In addition, increasing combined microwave power increases mass transfer and thus effective diffusion coefficient of moisture.