# The Effect of Pre-Treatment on the Rehydration of Dried Apple Cube
## 1 Introduction  
The majority of dried products (dry mixes, soups, fruits, vegetables, and nuts) are rehydrated during use to restore the properties of the raw food product, most often with water, juice, milk or its derivatives [1, 2, 3], and the speed of this process depends precisely on the type of rehydrating liquid [4, 5]. It can be assumed that during the described process, the following processes take place simultaneously: absorption, swelling of the rehydrated material and leaching of dissolved substances (vitamins, minerals, sugars or acids) from the product into the rehydration medium [6]. A fast and complete rehydration process can result in lower labor costs, storage space requirements and improved production efficiency [7, 8]. Pre-drying treatment, drying and rehydration cause changes in the structure and composition of the product tissue, resulting in a deterioration of the reconstructive properties, so it can be said that the ability of the dried product to absorb water is the main indicator of the correctness of the drying process [9, 10, 11]. As a result, a better understanding of the process in question can lead to an improvement in the quality of both dry and rehydrated products [12, 13]. Analyzing the studies carried out, it can be seen that the rehydration process depends on the drying method and parameters, as well as on the previous pretreatment of the material [13, 14, 15, 16, 17, 18] and the internal damage to the structure of the materials [19, 20]. The chemical composition of the product, its microstructure [21, 22, 23, 24], the way the rehydration is carried out and the external conditions [25, 26] are also not without significance. Rehydration can therefore be regarded as a measure of the degree of change occurring during processing butcannot be regarded as a reverse process to dehydration, as the changes in material structure are irreversible [10; 27; 28]. For a conventional drying process, the mode of heat delivery (convective, contact, sublimation, radiation, dielectric), the nature of the process (intermittent, continuous), the physical form of the material being dried and the operating conditions (atmospheric pressure, vacuum) [29] are important, including the way in which the fruit and vegetables are cut [21; 30; 31]. A number of scientific articles can be found in the literature, in which researchers have presented the results of their research related to the rehydration of dried foods. For example, the work of [PERSON] et al. [32] consisted of a study of the effects of different combined blanching and dehydration systems on the kinetics of mass transfer mechanisms during the rehydration of diced potatoes. Blanching was alternatively carried out in hot distilled water, hot sugar-salt solution, microwaved in distilled water or microwaved in salt solution. The authors stated, that in terms of process speed, color retention and water absorption capacity, the best results were obtained by combining microwave blanching with dehydration on the belt drier. In particular, dehydration on the belt drier leveled eventual negative effects as determined by the blanching treatments. [PERSON] and [PERSON] [13] studied the effect of pretreatment (citric acid solution, blanching by immersion in hot water) on the rehydration characteristics of broccoli, and they found that drying and rehydration characteristics of broccoli slices were influenced by air temperature and pre-treatment. [PERSON] and [PERSON] [33] study rehydration kinetics of open-sun-dried okra, which dried naturally, and pre-treatment was investigated at 25 and 50 \({}^{\circ}\)C. In dehydration experiments, the authors determined that blanching pre-treatment has an influence on the drying time, and okra samples were dried by 18 h. On the contrary, in rehydration experiments maximum equilibrium rehydration values were achieved with the okras dried naturally. [PERSON] et al.