1. Introduction
Carrots (Daucus carota L. subsp. sativus) are one of the most widely cultivated and consumed vegetables throughout the world [1], [2] and [3]. In recent years, the consumption of carrot and its related products has increased steadily based on their higher concentration of carotene compared to other foods [1], [2] and [3]. Carrots are a high-moisture food with moisture content of 90 g/100 g on a wet basis [2]. The dehydration of carrots seems to be a convenient alternative for long-term storage or usage. Dried carrots are used as an ingredient in various food products, such as soups, sauces, ready-meals as well as healthy snacks [4] and [5]. The high content of carotene is a unique property that makes dried carrot slices an excellent candidate for developing oil free, healthy snack foods. However, the nutritional value must be well preserved and a puffed texture should be generated in the drying process.
Dehydration, or drying, involves transient heat and mass transfer accompanied by physical, chemical, and phase change transformations. Unfortunately, these transformations may cause changes in the product’s quality as well as the mechanisms of heat and mass transfer [6]. Presently, drying with hot air is the most widely used method. However, long drying times and overheating of surface during hot air drying consequently result in color darkening, loss of flavor, and decrease in rehydration ability [7]. Freeze drying produces a high quality product, but its application for vegetable drying is limited as it is an expensive process. Vacuum drying is another alternative method and is especially suitable for products that are prone to heat damage like fruits and vegetables. However, in vacuum processes that require heat, the transfer of heat energy to the workload becomes difficult since convection is ineffective at low pressure [7].
In recent years, attempts have been made to shorten the drying period and to improve the energy efficiency of the drying process and quality of the dried products [8]. It is possible to enhance the quality of the dried products by decreasing the drying temperature or shortening the drying time [8].
Ultrasonically assisted drying has been a topic of interest for many years. Ultrasound can be employed as a pre-treatment before drying to improve the drying kinetics and to reduce the energy costs involved in the unit operation [9] and [10]. On the other hand, many studies on the direct use of ultrasound during drying have been reported [10], [11], [12] and [13]. Applying ultrasound interrupts the continuity of the cytomembrane, and thus increases the mass transfer rate between the cell and its extracellular surroundings [10]. Ultrasound applied during dehydration process has proven to improve the quality of dried food due to its non-thermal character [10]. Moreover, the drying time can be reduced, and the process can be performed at lower temperature, which is notably significant for products containing thermolabile substances [11], [12] and [13].
Although previous research suggests that ultrasound treatment can be used in many ways to accelerate drying processes, it is rarely connected with vacuum drying. Therefore, this work presents a new drying method by combining ultrasound and vacuum processes. In this method, the advantages (e.g., increase efficiency of heat energy transfer and accelerate dehydration) of both combined processes are used to shorten the drying period and improve the quality of dried food. The influences of drying technology on drying kinetics, dried material quality, and energy efficiency were investigated. To our knowledge, no studies have yet to be conducted on the combined use of ultrasound and vacuum to dry carrot slices or any other fruits and vegetables products.