Comparison of Drying Characteristics and Quality of Peppermint Leaves Using Different Drying Methods

Abstract

In this study, we investigated the effects of hot air, infrared and combined hot air-infrared drying methods on the drying time, energy consumption, color, rehydration and oil content of peppermint leaves. Three different air temperatures (30, 40 and 50C) and air velocities (0.5, 1 and 1.5 m/s) were used in the hot air drying. Infrared drying process was implemented using three levels of radiation intensity (1500, 3000 and 4500 W/m2), three distances between emitter and sample (10, 15 and 20 cm) and three air velocities (0.5, 1 and 1.5 m/s). The results showed that the optimum drying period and energy consumption were obtained when infrared- and hot air-drying were applied simultaneously. The results also showed that combined drying resulted in dried leaves of superior quality when compared to the leaves dried separately by either hot air alone or infrared, exhibiting lower total color change, greater rehydration ratio and higher extraction oil yield. Practical Applications Peppermint leaves are highly valuable products with wide range of industrial uses but extremely perishable in normal surrounding temperature-humidity conditions too. This fact makes their preservation important. Drying is a common preservation method that is applied to foods and agricultural products. Hot air has been traditionally used for drying peppermint leaves; however, infrared radiation has been employed for this purpose recently. However, there is very little knowledge, if none, on the use of combination of hot air and infrared drying method on the drying performance as well as the quality of final dried leaves. We treated fresh peppermint leaves by hot air drying, infrared drying and combined hot air-infrared drying, and compared the quality of final products and energy consumption during drying, aiming to find out the optimum drying method and drying parameters; while are beneficial to related industry in drying this valuable product. Our research work confirmed that combined hot air-infrared drying is a good alternative instead of the use of hot air drying or infrared drying separately.