Ultrasound assisted convective drying of selected herbs – Process kinetics and product's quality analysis
Grzegorz Musielak*, Dominik Mierzwa, Joanna Zembrzuska, Marta Jachna, Agata Kwiecińska,
Faculty of Chemical Technology, Poznań University of Technology
Berdychowo St. 4, 60-965 Poznań, Poland
All over the world more than 1000 types of herbs are used as medicines, spices and flavors . The herbs are usually very delicate plants. Their final quality depends on many factors. One of them is the applied drying method. Usually herbs should be dried at low temperature in order to avoid thermal damage of material. Therefore, usually herbs are dried convectively . Convective drying at low temperature is a slow process. It can be accelerated using ultrasound .
Thyme (Thymus vulgaris) and linden flour (Tilia cordata Mill.) were selected as tested material. Thyme is used as medicinal plant due to its bactericidal and fungicidal activity. Thyme contains from 0.75 to 3.5% of essential oil, the main ingredients of which are thymol and carvacrol. The essential oil contains also some phenolic acids . Linden flours contains as the main active ingredients flavonoid compounds (about 1%), mucus (about 3%) and essential oil. So far, more than 20 flavonoid compounds found in linden inflorescence have been known. Thanks to these ingredients, the linden blossom has antioxidant properties .
Both herbs has been dried in two ways: by convection (CV) and by US-assisted convection (CVUS). Air inlet temperature was equal to 50°C (drying of thyme) or 60°C (linden flours). In all processes the air speed was 2 m/s. Ultrasound with 200 W power were used. The condition for completing drying for all processes was the final moisture content 0.2 kg/kg dry basis.
In the case of thyme the retention of thymol, carvacol as well as selected phenolic acids was measured. In the case of linden flour the retention of selected phenolic acids and antioxidant properties was measured. The tested isomers and phenolic acids were isolated from the dry material by extraction and analyzed by the LC-ESI MS/MS technique. The antioxidant properties were measured by spectrophotometric methods.
The use of ultrasound accelerated the drying process about 2.5 times in the case of thyme and about 40% in the case of linden flour. In the thyme dried using ultrasound, a significantly higher contents of thymol and carvacol have been obtained. Application of ultrasound significantly improved retention of feluric, chlorogenic and caffeic acids in both materials, and slightly improved/worsened retention of gallic acid in thyme/linden flour, respectively. Ultrasound did not significantly affect the antioxidant properties of linden flour.
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This work was supported by the Polish Ministry of Science and Higher Education, projects No 0911/SBAD/0402 and 03/32/SBAD/905.