## 1 Introduction  
Apple trees (_Malus domestic_) are cultivated throughout the world, and apples are the second fruit crop in the world. The largest apple producers are China, the United States, Turkey, and Poland [1]. Apples are popularly consumed fresh, but also as processed products. For instance, juices, nectars, chips, snacks, or applesace are appreciated by customers. A very popular form is minimally processed apples, in the form of cubes, which could be in demand by the bakery and confectionary industry. Apples are also important from a nutritional point of view. They are a rich source of monosaccharides, minerals, dietary fiber, and phenolic compounds [2, 3, 4, 5]. The enrichment of the raw material with active ingredients can be achieved, among others, by applying the vacuum impregnation (VI) technique. Incorporation of a suitable composed solution into apple tissue could enhance both sensory and nutritional characteristics [6, 7, 8, 9]. The main cultivars produced in the European country are Golden Delicious, Gala, Idared, Jonagold, and Jonagored [10]. Different apple cultivars represent different technological properties. It is very important to select and identify cultivars and their properties that predispose them to a specific processing method. VI is a technology that accelerates the mass transfer of an adequate solution to a solid matrix. Internal porous matrices, which contain air and fluids, such as tissue from fruit or vegetables, are well-suitable for VI. VI has by now found many applications in the food industry, for instance, for salting, degreasing, or fortification of food. Depending on the composition of the solution incorporated into food matrices, different effects could be achieved. In the literature, examples of increasing nutritional values can be found, by the addition of antioxidants or minerals, changing physicochemical properties by the addition of pH-lowering agents or cryoprotectants, protecting from browning by the addition of ascorbic acid or improving texture by the addition of calcium ions [6; 9; 11; 12; 13; 14; 15; 16]. Tissue impregnation by the solution during the VI process has been theoretically described by the hydrodynamic mechanism (HDM) and the deformation-relaxation phenomena (DRP). In the proposed model, two phases of the process are distinguished: vacuum and impregnation. In the first vacuum phase, food (usually cut into particles of uniform shape and size) is immersed in the impregnation solution, and pressure is reduced. It causes the internal gas and fluids trapped in the product pores to be expanded and partially evacuated. In the second impregnation phase, pressure is restored to the atmosphere. During that, the external solution penetrates the pores of the product, and residual gas is compressed until the equilibrium pressure is reached [6; 9; 17; 18]. The effectiveness of the process is determined by synergistic external (process conditions, among others, pressure and temperature, time of vacuum maintenance and relaxation, the size and shape of the material, and others) and internal factors (properties of the impregnated material and its three-dimensional architecture). Apple tissue has properties such as high porosity, the presence of large intercellular spaces, and quite firm texture that predispose it to VI [19; 20; 18]. Many studies were conducted to optimize VI conditions, including vacuum level, restoration times, solution composition, and temperature [21; 22; 23; 24; 25; 26]. The vacuum level that allows the removal of native fluids from the tissue is related to the morphological characteristics and porosity of the fruits [27]. [PERSON] et al. [28] evaluated the effect of vacuum pressure (135-674 mbar) and its application time (3-45 min) on the volume of isotonic solution impregnated in slices of apple, peach, and other fruits. The authors reported the vacuum pressure had a significant effect on the volume of the impregnation solution in all fruit slices; furthermore, the impregnation also depended on the VI time, except for the apple.