# The Effect of Freeze-Drying on the Properties of Polish Vegetable Soups
## 1 Introduction
Freeze-drying is a gentle dehydration method, which consists of major steps, such as freezing of the material, sublimation of frozen water during primary drying under vacuum, and desorption of remaining water at secondary drying [36; 37]. The freeze-drying of vegetables is a technique providing products with a quality greater than offered by any other drying method [7]. This drying method enables maintenance of the color, flavor, and nutrients (antioxidants, vitamins) of freshly prepared soup [31]. Freeze-dried products are characterized by high porosity and low density, while shrinkage and structure collapse are negligible [37]. However, freeze-drying of vegetables with \(\beta\)-carotene, lycopene, vitamin E, unsaturated oils may lead to the deterioration of the quality of these biocompatibility due to the acceleration of oxidative reactions at a low water activity of freeze-dried material. The main disadvantage of freeze-drying is the high cost of equipment and maintenance. Freeze-drying also known as lyophilization is an expensive preservation method due to high energy consumption at vacuum conditions and the long drying time. However, this drying method enables the obtaining of products with a low water content and the best protection of nutritional value [36]. Additionally, the freeze-dried product can be more competently reconstituted to the original state by the addition of water in comparison to samples dried by other methods [7]. The porous structure of freeze-dried material is a main factor that influence the reconstitution properties. [PERSON] [38] noted a rehydration ratio up to 4-6 higher for freeze-dried than air-dried materials. Moreover, the minimal color deterioration and the highest lightness were observed for freeze-dried vegetables [10; 39]. Freeze-drying technique by low operating temperature is the best choice to dehydrate vegetables to retain the biocompatibility content in the products [36]. Freeze-drying is a suitable method for dehydration of liquid or semi-liquid products with pieces of biological materials. Even though the production of freeze-dried soups may come at a high cost, these products have great potential as outdoor food for consumers practicing sailing,camping, trekking, backpacking. Due to the specificity of outdoor food, freeze-dried soups can be situated in places at different humidity conditions, which may affect the products' properties and their stability. The moisture sorption isotherms describe the sorption behavior of material stored in an environment of varying humidity. The moisture isotherms represent the relation between the equilibrium water content and the water activity of the product. The different mathematical models are applied to describe the moisture sorption isotherms; these include kinetics models based on monolayer sorption (Brunauer-Emmett-Teller-BET model), models referring to multilayer sorption, and condensed film (Guggenheim-Anderson-de Boer-GAB model, Lewicki model), semi-empirical (Peleg model, Hasley model), and empirical models (Oswin model) [40]. The methods that describe the moisture sorption isotherms can be divided into static and dynamic techniques. The static gravimetric method uses the desiccators for setting the relative humidities. The dynamic vapor sorption system (DVS) automatically generates the different humidity levels for measuring the sorption isotherms. The DVS devise records the change of mass over time (dm/dt) and when dm/dt is close to 0 the next humidity level is automatically selected [41]. The determination of moisture sorption isotherms is crucial for designing and optimizing the drying process and selecting appropriate packaging materials, especially for foods with low moisture content [42; 43; 44]. Numerous studies on the moisture sorption behavior of different freeze-dried foods described by many mathematical models can be found in the literature [40; 45; 46; 47; 48; 49]. However, studies on the moisture sorption properties of freeze-dried soups are very limited.