# The Effect of Hybrid Drying Methods on the Quality of Dried Carrot
## 4 Discussion
It is possible to successfully use this method (freeze-drying-convection) as an alternative to convection and freeze-drying because the obtained dried material has a water activity similar to that of freeze-drying. Inverse results were obtained in drying, in which freeze-drying was used as the last step of drying. In the case of products obtained by these combined drying methods, the water activity value is higher in comparison to methods in which freeze-drying has been used as the first drying step. The results for convective-drying and combined convection-freeze-drying as well as microwave-freeze-drying are similar. The reason for this may be the order of the drying methods used. Microwaves and convection-drying causes the formation of a crust on the surface of the dried material, which hinders the evaporation of water [33]. All dried samples obtained dry matter content exceeding 94%. Similar values (95%) were obtained in the studies by [PERSON] et al. [34], which concerned the influence of drying methods on the quality of convective-dried carrots. Studies carried out by [PERSON] and [PERSON] [35] on the influence of drying methods on the quality of dried apples showed that the microwave-dried samples obtained a dry matter content of 94.3%. In the presented investigations the highest dry matter content was observed for freeze-dried carrots (-98%). The freeze-drying process is considered to be the best drying method in terms of removing water. The complicated mechanism of this process, as well as the freezing process before drying, caused the water to freeze, thanks to which during drying under reduced pressure, the frozen water, omitting the liquid phase, was transformed into water vapor and removed from the raw material in a quantity higher than in other drying methods [36]. For example, the value of this quality factor for freeze-dried pumpkin in the research conducted by [PERSON] et al. [18] was equal to 95.87%. The combination of convection and freeze-drying (K-F, F-K) made it possible to obtain dried material with very similar dry matter content values, 94.11% and 95.68%, respectively, but lower than for freeze-dried samples (F). The freeze-dried carrot with microwave-drying (F-M) is characterized by a similar level of dry matter content to the freeze-dried (F), therefore it can be used as an alternative to freeze-drying and the duration of the process can be reduced. The apparent density and porosity of plant material have an effect on changes during dehydration due to the loss of moisture and formation of air pores [37]. Apparent density is defined as the result of water, dry matter, and air density [38]. The increase in apparent density in plant tissue causes high shrinkage. The type of raw material also influences the apparent density value. The density of dried products depends on changes in porosity and contraction of the tissue. The more free intercellular spaces, the lower the density of the dried material. Freeze-dried carrots obtained the lowest density, whereas the convective and microwave-dried samples characterized the highest value. Additionally, the combined drying method was less favorable than the freeze-drying method but better than convective and microwave-drying. [PERSON] et al. [39] obtained the lowest value of bulk density for freeze-dried black mulberries and a similar result for samples hybrid dried in freeze-dryingwith explosion puffing drying. The authors explained that in freeze-dried products the frozen water in the material sublimated directly from the solid phase to the gas phase and the porous structure of the samples was well maintained. [PERSON] et al. [40] showed that freeze-dried apples had lower density with higher shrinkage. However, in the case of dried carrots, this relationship is not confirmed, which may be related to the difference in the structure and tissue of carrots and apples. A similar relationship was obtained by [PERSON] et al. [41] for convective-dried carrots and showed that the higher the shrinkage, the greater its density. [PERSON] et al.