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The aim of the present work was to assess the influence of high-intensity airborne ultrasound application on duration and energetic performance of hot air drying of peppermint leaves. To this end, drying experiments were conducted at constant air flow rate of 1 m s−1 and different temperatures (40, 50, 60 and 70 °C) without and with application of ultrasound power (90, 180, 270 and 360 W). The results showed that the maximum (170.84 MJ kg−1) and the minimum (63.36 MJ kg−1) specific energy consumptions belonged to convective drying at temperature of 40 °C and ultrasonic-assisted drying at 60 °C and 270 W, respectively. Energy efficiency varied from 1.41 to 3.69%. Depending on the power level, application of the ultrasound power at the air temperatures less than 70 °C reduced drying time and improved energetic performance of the process. The obtained findings in this research declare that acoustic power could be effectively used in combination with convective dryers to reduce duration and energy required for drying of heat-sensitive products. • Ultrasound power ability to improve energy efficiency of drying process was studied. • Application of the power reduced the process duration of up to 41%. • At temperatures less than 70 °C, ultrasound power increased the energy efficiency. • The values of energy efficiency ranged from 1.41 to 3.69%. • The power has good ability to reduce the process duration and energy consumption. [ABSTRACT FROM AUTHOR]

Influence of microwave power on dehydration kinetics as well as quantity, chemical composition and antioxidant potential of peppermint essential oil was evaluated. The leaves were dried using microwave powers of 200, 400 and 800 W, and the essential oils extracted by hydro-distillation. The antioxidant potential of the essential oils was quantified by DPPH radical-scavenging activity method. The obtained results showed that increasing microwave power caused significant decrement (p ≤ 0.01) in drying duration. Drying over the considered microwave powers reduced essential oil yield significantly (p ≤ 0.01). In total, 36 compounds were identified in the essential oils belonging mainly to oxygenated monoterpenes. The main constituents namely menthol, menthone and menthyl acetate showed significant variation (p ≤ 0.01 or p ≤ 0.05) with the drying process. Antioxidant assaying indicated significant effect (p ≤ 0.05) on radical scavenging activity of the essential oil by drying and essential oil concentration. Compared to the fresh samples (with an IC50 of 103 μg/ml), the leaves dried at microwave power levels of 200, 400 and 800 W resulted in significant lower antioxidant activities with IC50 values of 201, 291 and 615 μg/ml, respectively. [ABSTRACT FROM AUTHOR]

This paper aims at presenting a comprehensive perusal on hot air and microwave drying of medicinal herbs through investigating the dehydration behaviour, mathematical modelling, energy characteristics and essential oil yield of peppermint leaves drying. For our study the leaves were dried using microwave at power levels of 200, 400, 600, and 800 W, and hot air at temperatures of 50, 60 and 70 °C. The dehydration process of the leaves in microwave drying method occurred in an accelerating rate period at the start followed by a falling rate period while; convective drying took place in the falling rate period entirely. Six mathematical models were fitted into the experimental data and the Midilli and Kucuk model best described the drying curves. The lowest effective moisture diffusivity (1.809×10 −9 m 2 s −1 ) belonged to hot air drying at 50 °C, while the highest (110.552×10 −9 m 2 s −1 ) belonged to microwave drying at 800 W. The activation energy for microwave and hot air drying was determined to be 1.89 W g −1 and 12.46 K J mol −1 , respectively. Specific energy consumption for microwave and convective drying of the peppermint leaves were ranged from 4.581 to 5.890 M J kg w −1 , and from 42.721 to 64.712 M J kg w −1 , respectively. The maximum and minimum oil yields were obtained from the leaves dried at air temperature of 70 °C and microwave power at 800 W, respectively. [ABSTRACT FROM AUTHOR]