Celery (Apium graveolens) is a well-known herbaceous vegetable from the family Umbelliferae (Marongiu et al., 2013; Sowbhagya, Srinivas, & Krishnamurthy, 2010). Parts of celery such as roots, leaf stalks, green leaves, and seeds are grown for different purposes (Orton, Durgan, & Hulbert, 1984). Celery constitutes phenolic compounds, micronutrients, and vitamins, often utilized as herbs to suppress hypertension and prevent cancer (Sapei, Tarigan, Sugiarto, & Gianluca, 2019). It is also used due to its characteristic aroma which it possesses as a result of the phthalide found in its essential oil (Kurobayashi, Kouno, Fujita, Morimitsu, & Kubota, 2006; Sellami et al., 2012). Furthermore, dried celery leaves with high nitrate content and are also used as curing agents in the meat industry (Sebranek, Jackson-Davis, Myers, & Lavieri, 2012). Herbs such as celery constituent higher moisture contents which increases their susceptibility to physiological deterioration and microbial development. An efficient drying approach to enable the reduction of the moisture content and thus maximize the quality and shelf life of celery is necessary (Sapei et al., 2019).

Although the drying and preservation of fruits and vegetables first started with sun drying, today the drying process has taken its place as an industrial process (Bennion, 1990; Doymaz, 2004). The term drying means removing almost all the moisture content present in the food (Ratti, 2001). Drying reduces the amount of food moisture to prevent the growth of microorganisms, also enzymatic activity and chemical reaction rates are significantly reduced (Lewicki, 2006; Mayor & Sereno, 2004). Reducing the mass and volume of the product reduces transport and packaging costs. Dried products with longer shelf life, more intense nutritional value, and a wider range of use are obtained. As a result, energy is saved with this approach, which is mostly preferred in fruit and vegetable products (Kara, Ulgen, & Hepbaslı, 2008).

Tray drying is an inexpensive approach commonly used for vegetable drying. Besides its low cost, it is very easy to use and manage with control systems (Nindo, Sun, Wang, Tang, & Powers, 2003). In the tray drying method, heated air is used as the drying medium. The product is exposed to the hot air stream and the heat is transferred to food by convection. Examination of the drying mechanism of the tray dryer indicates that drying is a simultaneous heat and mass transfer process (Hernández, Pavón, & Garcı́a, M. A., 2000). During the drying process, the heat required for the evaporation of the water in the product is transferred to the product, and as a result of the heat transmitted; mass transfer is realized from the product to the environment as water vapor. The original temperature of the product, the physical properties of the product as well as the conditions of the drying environment affect the speed of mass transfer (Simal, Femenia, Garcia-Pascual, & Rosselló, 2003; Toledo, Singh, & Kong, 2018). The rate of heat and mass transfer as well as energy cost and savings are directly effective.

Besides changes in the attractiveness and quality of food after drying, another major disadvantage of drying in the food industry is its high cost. In most of the industrial experimented countries, 7–15% of the total energy used in the industry is used for drying (Gunhan, Demir, Hancioglu, & Hepbasli, 2005). This high energy use has led to research of different approaches in the industry. In order to increase energy efficiency, an intermittent drying (ID) approach has been applied and it has been seen that product quality and energy efficiency have increased (Jumah & Mujumdar, 1996: Silva et al. 2016). Yang, Zhu, Zhu, Wang, and Li (2013) showed that the rate of drying increased, the quality of the product improved and energy efficiency increased. Chin and Law (2010) emphasized that intermittent drying (ID) improves the colour of food. Also as investigated by Zhu, Pan, McHugh, and Barrett (2010) positive changes in several quality characteristics by utilization of intermittent drying of apples, bananas (Chua, Mujumdar, Hawlader, Chou and Ho, 2001a, Chua, Mujumdar, Hawlader, Chou and Ho, 2001b; Nishiyama, Cao, & Li, 2006), potatoes (Chua, Mujumdar, Chou, Hawlader, & Ho, 2000), rice (Aquerreta, Iguaz, Arroqui, & Virseda, 2007), yerba mate (Ramallo, Lovera, & Schmalko, 2010), guava (Chua, Hawlader, Chou, & Ho, 2002; Ho, Chou, Chua, Mujumdar, & Hawlader, 2002), Ganoderma tsugae (Chin & Law, 2010). During intermittent drying (ID), controlling of thermal energy is supplied when variation of airflow rate, air temperature, humidity, or vacuum pressure is controlled (Jumah & Mujumdar, 1996).

The application of the intermittent drying method in food products has positive effects on energy efficiency and product quality. In order to further increase this increased energy efficiency, the drying rates of food products at constant temperature were first examined. Examinations have been made on the drying rate values where the change in drying speeds creates differences. It is thought that changing the drying temperature at intervals determined by examining the changes in these drying speeds will lead to an increase in the energy efficiency of the drying system. In other words; the idea of controlled temperature drying based on moisture content was introduced. Food products drying, which have high drying rate, can be achieved at relatively higher temperatures. However, as a result of rapid drying, the quality of food products generally decreases. Energy efficiencies also increase with intermittent drying. It is known that free water in food products will disappear faster at higher temperatures. In light of this information, it is aimed to prevent quality losses in celery by heating it at high temperatures up to a certain moisture content and then gradually decreasing the temperature.

In this study, celery drying experiments were carried out using 3 different drying approaches: constant drying temperature (CDT); controlled temperature drying based on moisture ratio (CTD-MR), and intermittent drying (ID). The effects of the drying experiments carried out with each different approach and the drying conditions belonging to the effective mechanism specific to each approach on the physical and chemical properties of the product, with energy efficiency and drying rate were examined.