## 1 Introduction  
The kiwiberry (_Actinidia arguta_), also known as mini kiwi, baby kiwi or hardy kiwifruit, is a small, grape-sized fruit with a thin, edible smooth and hairless skin and therefore may be eaten whole without peeling [1, 2]. This fruit is rich in over 20 bioactive components, e.g., vitamin C, minerals, dietary fibre, phenolic acids, flavonoids, anthocyanins and pigments [3, 4]. The shelf life of kiwiberry fruits is limited; that is, under refrigerated conditions they may be stored for up to six weeks at the harvesting maturity stage and for up to one week after reaching the eating maturity stage [5]. During storage, their quality decreases due to a reduction in their content of the bioactive components, loss of firmness and hardness, and also changes in colour [2, 4]. The results of these changes contribute to consumers' lower acceptability and direct consumption of kiwiberry fruits. Therefore, some processing methods are required, and drying seems to be the most promising option. Drying is one of the oldest and most common methods of food preservation, during which water is removed from the dried material via evaporation to the surrounding environment. The effect is a product with reduced water activity and an elongated shelf life [6; 7]. However, drying may result in undesirable changes in the physical and chemical properties of dried products, e.g., shrinkage, reduction in rehydration capacity, loss of bioactive compounds or non-enzymatic browning [8; 9; 10]. The food industry is therefore seeking to solve these problems by using new drying methods or optimizing the known ones to achieve minimum nutrient loss, reduce adverse texture changes and retain the best sensory properties at the lowest possible processing cost [11; 12]. The combination of foaming and freeze drying seems to be a promising method for powdered food production. Foam-mat drying consists of foaming the material with foaming and stabilizing agents and then drying it [13; 14]. The obtained porous structure provides more efficiency in heat and mass transfer, and so lower temperatures can be used, and better properties of the resulting product can be maintained [13; 15]. In addition, the dried product may be crushed into a powder form, which makes it much easier to transport, store as well as manage in other branches of the food industry. For example, fruit and vegetable powders can be used as an ingredient in baby food, confectionery fillings, ice cream, soups, cakes, extruded cereal products, yogurt, dietary supplements and many other food products [16; 17; 18]. Therefore, some increasing trends in the foaming of fruit and vegetable pulps, pureses, concentrates and juices are observed. This is due to their low-molecular-weight substances and low glass transition temperature (\(T_{g}\)), e.g., 33.1 \({}^{\circ}\)C for freeze-dried apple puree powder [17], freeze-dried mang puree 48.9 \({}^{\circ}\)C [19] or \(-18.3\)\({}^{\circ}\)C for freeze-dried kiwi powder [20]. Drying this type of product in an unchanged form by employing commonly used techniques can lead to a gel or gum layer and, as a result, makes further processing and industrial application impossible [17; 21]. Some unfavourable changes occurring during the traditional drying methods promote the application of other drying methods such as, e.g., freeze drying. Freeze drying, also known as lyophilization, is a technique that dehydrates a product by removing water from it through sublimation and desorption [22; 23]. It is a time-consuming and uneconomical method of drying [24]; however, it allows the obtaining of a high-quality product. Thanks to the use of low temperatures and vacuum conditions during the process, the obtained product retains its colour, aroma, flavour and valuable nutrients, and has good reconstitution properties and a long shelf life [25; 26]. Due to its high energy consumption and thus expensiveness, some modifications are used for obtaining high-quality food powders, e.g., foaming before the freeze-drying process [24].