# The Effect of Freeze-Drying on the Properties of Polish Vegetable Soups
## 1 Introduction  
Soups are made by combining different ingredients such as vegetables, poultry, meat, or seafood. The selected compounds are boiled in water until the flavors are extracted. Food manufacturers supply a wide range of these products on the market: smooth and particulate soups, clear soups (broth, bouillons), as well as soups thickened with milk, cream, and vegetable pures [1]. The food industry offers soups in the form of powder, liquid, or canned [2]. Powder soups can be prepared by mixing different dry powdered ingredients, such as ground dried vegetables, spices, flavors, and yeast extracts with oil and fat powder [3]. Dried soup powder can be stored for a long time (6-12 months) at room temperature because enzymatic and oxidative spoilage is restricted [4]. Powder soup also has advantages such as being available throughout the year and readiness for rehydration [5]. However, this technology includes the soaking (e.g., lentil, chickpea), blanching, cooking, and drying of different vegetables separately. Then dried vegetables are milled, sieved, and blended to formulate dry soup mixtures [5, 6]. Drying methods and grinding conditions greatly influence the quality of vegetable ingredients [7]. Dehydration methods of vegetables include convective [8, 9, 10], vacuum [11, 12], microwave [13, 14], desicant [15, 16], ultrasonic [17, 18, 19], infrared [20, 21], freeze-drying [22, 23], as well as osmotic dehydration [24, 25] and hybrid (or combination) [10, 26, 27] drying. [PERSON] et al. [28] obtained different instant soupscomposed of corn flour, milk powder, and leafy vegetable powders. Three leafy vegetables namely Kolmou, Jatilao, and Pui were dried separately using the air-drying technique at temperature 50-70 \({}^{\circ}\)C, and then milled. The same drying method was also used to prepare the soup mixtures, which contained green pea, chickpea, lentil, potatoes, tomatoes, carrot. After drying, all vegetables were milled into powdered form and mixed with seasonings [6]. Soy-mushroom-moringa soup powder was prepared by mixing of soy flour mushroom and moringa leaf powder. The mushroom and soybean were dried in a convective-dryer but moringa leaves were dried with the application of a solar-dryer [5]. Spray-drying technique was selected to produce the mushroom (paste)-whey soup powder [29] and powdered product from soup, which consisted of an extract from ham byproducts [30]. The mixtures of instant soup powders were also prepared by mixing the grounded ingredients (particles of chickpea, mushroom, parsley, dill, celery freeze-dried individually) with olive oil and corn starch [22]. The dried soups with the pieces of vegetable can be obtained by preparation of a soup with the vegetable ingredients and drying of the product. Recent studies show that these kinds of soups have been dried by freeze-drying or with application of hybrid drying methods [31; 32; 33; 34]. [PERSON] et al. [31; 35] applied the microwave-freeze-dried technology to obtain the instant vegetable soups. The vegetable soup mix contained cabbage, tomato, carrot, spinach and mushroom, water, salt, sugar, and sodium glutamate, and was cooked, cooled, and frozen. The soups were freeze-dried (at a pressure of 100 Pa) with the microwave heating system at frequency 2450 MHz. [PERSON] et al. [34] explored the hybrid strategy for preparation of dried soups. The frozen soups with pieces of mushroom and bacon were dehydrated by freeze-drying (at a shelf temperature of 30, 50 and 70 \({}^{\circ}\)C) and using microwave drying. The freeze-drying method was applied to obtain different instant vegetable and meat soups [32; 33]. Freeze-dried soups with chopped vegetables exhibited significantly higher rehydration ratios than products with pieces of vegetables, meat, or fish [32]. Presently, consumers expect high-quality meals, which can be prepared in a short time. Instant soup obtained with the application of the freeze-drying process can be a product that meets these requirements [31].