Drying of peppermint leaves (Mentha piperita L.) in a combined hot air-ultrasonic dryer: Influences of temperature and ultrasound power on duration, energy consumption,and essential oil yield

Over the last decade, the use of ultrasound power has been attended as one of the most novel research fields in processing of food and agricultural products. In the present research, influences of the air temperature and ultrasound power were investigated on some important indices of peppermint leaves drying process. The experiments were conducted by using a combined hot air-ultrasonic dryer at different levels of air temperature (40, 50, 60 and 70°C) and ultrasound power (0, 90, 180, 270 and 360 W). The obtained results showed that although, increasing temperature improved the process time and energy consumption, but decreased the essential oil yield of the samples. In addition to decreasing the process duration and consumed energy, ultrasound power at temperature levels of 40, 50 and 60°C increased the extracted essential oilyield. At air temperature of 70°C, using of the ultrasound power led to more energy consumption without any effect on the two other indices. Generally, results of this research indicated that application of ultrasound power could be a useful method to improve the performance of hot air dryers in drying of heat-sensitive products such as medicinal and aromatic plants.