# Some Aspects of the Modelling of Dried Red Beets Rehydration Process
## 1 Introduction
It was stated that this method of pretreatment caused greater tissue shrinkage (the smallest shrinkage for the untreated tissue), and therefore, the rehydration time was longer (the characteristic rehydration time is defined as the time required for the dimensionless ratio of the soluble matter contents of juice to attain one-half of its maximal value). [PERSON] and [PERSON] [21] investigated the effect of the particle shape and size of beetroots and the effect of the initial material load on convective drying kinetics. The drying course was modelled using theoretical and empirical models. [PERSON] [11] dried red beets using the following methods: convective drying, convective predrying and then vacuum microwave drying (VMD), and freeze drying. It was stated that the total time of the combined drying can be considerably shortened by the early use of VMD at a high microwave wattage. It turned out from the research that samples dried via both the second and third methods exhibited lower shrinkage, a better degree of rehydration and higher antioxidant activity than samples dried via convection. [PERSON] and [PERSON] [13] optimized the convective drying of the beetroots pulp with the objective of obtaining the maximum color retention. The drying process was modelled using empirical models. The quality of red beets power was characterized by the rehydration ratio and color. [PERSON] and [PERSON] [22] investigated the effect of drying methods (freeze-drying and heat-drying) on the bioactive compounds of vegetables, including red beets. [PERSON] et al. [23] studied the drying of beetroots via the following methods: sun and shade drying, solar cabinet drying, and hot air tray drying. They stated that increasing the drying air temperature negatively influenced the rehydration ratio, the shrinkage ratio, and the color. [PERSON] et al. [9] showed that the microwave-assisted fluidized bed drying of red beets gave a lower drying time and lower final moisture content than fluidized bed drying. [PERSON] et al. [24] modelled the effective coefficient of moisture diffusion during the drying of beetroots applying finite elements. Freezing pretreatment and the application of ultrasound on convective drying [24] and freezing [25] have also been investigated. [PERSON] et al. [26] studied the effect of microwave power on the drying kinetics of the red beets' puree in the microwave oven, and [PERSON] and [PERSON] [27] studied the vacuum belt dehydration of chopped red beetroots. The drying course was modelled applying theoretical and empirical models. [PERSON] at al. [28] investigated the influence of different drying methods on the quality of dried beetroots. The obtained results showed that microwave vacuum drying and microwave drying were superior to vacuum drying, heat pump drying, and freeze drying in terms of drying time. Beetroots dried via microwave vacuum drying had the largest rehydration ratio, whereas the lowest ratio was obtained via microwave drying. To predict the dried vegetables and fruit behavior of the rehydration process, theoretical models, empirical models, and machine learning techniques are used, and despite the widespread use of computers and their associated software, empirical equations are still widely used in view of their simplicity and ease of computation. Theoretical models are based on the general theory of heat and mass transfer laws and their parameters have physical meaning, but these models are more difficult in application compared to empirical models, and the constants of empirical models can inform us about the described process rate. As can be seen from the literature, there is little information concerning the modelling of the rehydration of dried red beets.