Microwave Drying Kinetics of Thyme

Abstract

Thyme (Thymbra spicata L.) leaves were dried in a domestic microwave oven to determine the effects of microwave output power on the drying time, drying rate, and the colour of the dried product. Seven different microwave output powers ranging from 360 W to 900 W were used in the experiments. Drying of thyme and mint leaves took place mainly in falling rate period with no constant rate period. Increasing the microwave output power resulted in a considerable decrease in drying time. The semiempirical Page’s equation used to describe the drying kinetics of leaf materials gave an excellent fit for all data points with values for the coefficient of determination (R2) greater than 0.997 and the standard error of estimates (SEE) lower than 0.023. The value of the drying constant showed an increasing tendency with the increased microwave output power signifying that with increase in microwave output power, drying curves become steeper indicating faster drying of the products. Both the traditional in-shade drying and microwave drying techniques used in this study caused some undesirable effects on the colour of the thyme and mint leaves. The traditional in-shade drying produced a darker brownish green product. Based on the overall colour evaluation results, compared to traditional in-shade drying, more stable green colours similar to those of the original fresh materials were obtained by using microwave output powers of 810 and 900 W for thyme and 720, 810, and 900 W for mint.

Lišće Thymbra cpicata-e je sušeno u mikrotalasnoj sušari kako bi se odredio uticaj snage sušare na vreme sušenja, stepen sušenja i boju osušenog proizvoda. U eksperimentu je korišćeno sedam tipova mikrotalasnih sušara snage od 360 W do 900 W. Povećanje snage sušare rezultiralo je značajnim smanjenjem vremena sušenja. Poluempirijska Pejdžova formula (Page’s) koja se koristi za opis kinetike sušenja lisnatih materijala, pokazala je dobro uklapanje za sve tačke sa vrednostima koeficijenta determinacija (R2) većeg od 0.997 a standardna greška je bila niža od 0.023. Vrednost konstante sušenja pokazala je tendenciju porasta sa povećanjem snage sušare ukazujući da sa povećanjem snage sušare krive sušenja postaju strmije ukazujući na brže sušenje proizvoda. I tradicionalno sušenje proizvoda u senci i korišćenje mikrotalasne sušare su tokom ovog istraživanja pokazali određene negativne posledice na boju listova timbre i mente. Tradicionalno sušenje u senci je uzrokovalo pojavu tamnije braon boje na proizvodu. Obzirom na izvršenu analizu boje i upoređenjem sa proizvodima sušenim na tradicionalni način, postojanija zelena boja je uočena kod lišća timbre kod sušara snage 810 i 900 W, i 720, 810 i 900 W za mentu.