Evaluation of the effects of blanching and ultrasound pre-treatments on the thin layer drying kinetics of thyme

Abstract

Drying is one of the mostly used preservation methods which involves the removal of moisture from a food product while a simultaneous heat and mass transfer takes place. It is applied to vegetables, fruits, herbs and other agricultural products to minimize microbial spoilage and deterioration reactions thus extending the shelf life. Hot air drying process is widely used in reducing the moisture content of agricultural products due to its low cost and ease of operation, but extended times of exposure to high temperatures causes significant decrease in the product quality. Therefore, it is required to control drying parameters in order to minimize chemical and physical changes in food material, and also to minimize nutritional losses. The drying rate during the operation is affected by the vapor pressures of air and food material, properties of air such as temperature and velocity, and properties of the food material such as thickness, surface area and moisture diffusion within the product. There is an increasing interest in dried vegetables, herbs, and fruits, however recent researches have shown that, application of drying barely has some negative impacts on the quality of the products such as, color loss, structural changes, bioactive content and nutritional loses. Additionally, improving the drying rate and reducing the energy costs are important issues. Thus, there is a tendency for application of pre-treatments before drying. Pre-treatments improve drying efficiency, by reducing drying time and energy consumption and by minimizing nutritional and structural changes of the foods. Mathematical models that describe drying processes are used for improving drying systems or designing new systems. Models provide an improved understanding of the drying process parameters. Information such as temperature and moisture are critical for drying process and equipment design, also for storage and handling procedures of the dried food material. These information can be provided by models that describe drying mechanisms. Thin layer drying models are widely used in describing drying characteristics of food products. These models are classified into three groups namely, theoretical, semi-theoretical and empirical models. Most widely applied thin layer models are semi-theoretical and empirical models as these models consider external resistance to moisture transfer between food material and air, thus providing a better prediction of drying behavior. Empirical models neglect the fundamentals of the process and cannot show a clear view of the drying process, still may express the drying curve for the conditions of the experiment. Semi theoretical models are derived by simplifying the general series solution of Fick's second law and they do not need assumptions of geometry, diffusivity and conductivity of a food material. Widely used semi theoretical thin layer models are the Lewis model, the Page model, the Henderson and Pabis model, and the Logarithmic model. Thyme (Thymus vulgaris) is an herb belonging to the Lamiaceae family, used as culinary and medicinal purposes. The plant has been used for medicinal purposes in ancient times and currently being used for its flavor in cooking. Thyme also has antioxidative effects and its essential oils (thymol and carvacrol) shows antimicrobial and inflammatory properties. Thyme plant blooms in June to July and is cultivated all around the world. Preservation of herbs is performed by two operations in general: drying and irradiation. Drying process is used to store seasonal plants all year round, provides a great level of preservation. The aim of this study was to reduce the adverse effects of conventional drying process on the chemical and physical properties of fruits and vegetables and increase the nutritional quality of dried products. During the studies, it was aimed to demonstrate the drying kinetics of pre-treated products and to determine the thin layer characteristics of the drying operations. The drying process was carried out with two different methods, namely air drying (60, 70 and 80°C) and microwave drying (30% - 390 W, 50% - 650 W, and 70% - 910 W). First of all, for the comparison, drying without pre-treatment was carried out with both methods and the kinetic data were modeled by investigating the compatibility of the moisture change ratios with the Lewis, Page, Henderson and Pabis, Midilli and Logarithmic models. In addition, effective diffusivity and drying activation energy were calculated using the moisture change ratio data. Finally, International comission on illumination (CIE) values were measured using a color measurement device in order to interpret the effect of the drying processes on the color of thyme leaves. In this study, blanching and ultrasonication pre-treatments were applied to improve drying efficiency and increase the quality of the dried product before air and microwave drying. Samples were subjected to blanching pre-treatment for 1, 2 and 3 minutes and ultrasonication pre-treatment for 5, 10 and 15 minutes. The compatibility of moisture change ratios with Lewis, Page, Henderson and Pabis, Midilli and Logarithmic models was investigated and kinetic data were modeled. In addition, effective diffusivity and drying activation energy were calculated using moisture change ratio data. Finally, CIE values were measured using a color measurement device in order to interpret the effect of drying processes on the color of thyme leaves. Consequently, the effects of two different pre-treatment applications on drying time for both drying methods were compared and interpreted. In addition, the color measurement results of un-treated fresh, dried without pre-treatment and dried after pre-treatment thyme leaf samples were compared and the effects of different drying methods and different pre-treatment applications on the quality of the product were interpreted. According to the obtained data, pre-treatment applications before both drying types had an effect in a way that shortened the drying time. In this context, it was concluded that pre-treatment application could contribute to energy saving. For 60°C air drying, 2-minute blanching pretreatment was evaluated as a better option compared to others. Although ultrasonication application at this temperature shortened the drying time, no difference was observed between different application levels. 2-minute blanching application before air drying performed at 70°C emerged as the best option. When the results of samples to which ultrasonication was applied before air drying performed at the same temperature were examined, it was observed that 15-minute application gave better results than the others. Blanching application before air drying at 80°C gave better results than ultrasonication application, but since no difference was observed between application levels, it was concluded that the shortest pre-treatment applications could be preferred in terms of time saving. When microwave drying results considered, it was observed that the drying time was 7 minutes when the boiling process was applied before drying at 30% power level, while it was observed that the drying time was 5 minutes when the ultrasonication process was applied. At 50% and 70% power levels, 15 minutes of ultrasonication application before microwave drying gave significantly better results than other applications. Color values, L*, a* and b* measured after drying operations showed that blanching pre-treatment caused the darkness of samples to increase and greenness to decrease and this may be attributed to the loss of heat sesitive chlophyll content due to the high temperature applications.