## 3 Results and Discussion  
### Ascorbic Acid Content, Firmness, and Mass Gain after VI Process  
Apple fruit is not a rich source of vitamin C. The initial content of this vitamin in the tissue of the investigated apple cultivars ranged from about 5 mg/100 g to 25 mg/100 g, with an average content of 11.1 mg/100 g (Table 1). VI with the solution consisting of 0.5% ascorbic acid, 0.5% citric acid, and 10% sucrose resulted in an increase in ascorbic acid content to range 63-130 mg/100 g of fresh mass. The highest content of vitamin C after the VI process was found in Golden Delicious, Shampion, and Cortland apples. The positive correlation between ascorbic acid content and mass gain after VI was noticed (r = 0.46). Golden Delicious and Cortland cultivars, along with the highest ascorbic acid content (130 mg and 124/100 g, respectively), characterized the highest mass gain (34% and 28%, respectively). The low mass gain after VI (about 15-17%), as in the case of Gala Must, Red Jonorime, or Ligol, was in pair with lower ascorbic acid content after VI (77 mg to 98/100 g). The tissue of these three cultivars is also firmer than other investigated cultivars, but no statistically important relationship between tissue firmness and ascorbic acid content was found. It is possible that other factors may also have influenced the level of saturation of apple tissue with ascorbic acids, such as the porosity of the tissue, the structure of capillaries, or size and shape of the material [6; 9]. VI caused a considerable increase in the level of ascorbic acid in the apple tissue. [PERSON] et al. (2018) also observed an increase in ascorbic acid content during VI of potato slices. After 60 min, the vacuum-impregnated potato had from 9 to even 21 times higher ascorbic acid content than the raw potatoes. The effect strongly depends on the impregnation time. The increase after 15 min was about 2-3 times compared to the fresh sample. In our study, an increase in ascorbic acid from 3 to even 25 times was noticed after 10 min of vacuum pressure. Apple tissue possessed different textural characteristics from potatoes. The major differences can be seen in terms of the porosity of the material and, thus, the space that the impregnation fluid can fill. The porosity of apple tissue is approximately 6.4-31%, while the porosity of potato is only 3 to 8% (19; 20; 31). The shorter time needed to achieve the same effect with apples as with potatoes can be explained by this. The forces values obtained from texture analysis for tested apples ranged from 11.3 kG to 43 kG. Apple cultivars based on their initial firmness could be divided into three groups: Boscoop, Cortland, Gala, Golden Delicious, Jonagold, Jonagord, Lobo, and Pinova, characterized by the softest texture (firmness from 11.3 to 17.5 kG); Idared, Shampion, and Topaz apples, slightly firmer (21.9 and 25.6 kG); Ligol, Gala Must, and Red Jonparince, hard (firmness 32 to 43 kG). The VI process caused softening of the tissue. The results obtained from the texture analysis of the impregnated apple cubes ranged from 6.0 to 34.6 kG. The decrease in firmness after VI may depend on the value of this parameter in fresh apple cubes. Soft samples before (firmness \(\leq\) 17.5 kG) resulted in 1.2 to 2.1 times lower firmness after VI, while initially harder samples (firmness \(\geq\) 30 kG) resulted in 1.1 to 1.2 times lower firmness after impregnation. VI of apple cubes could result in greater deformation in the case of cultivars with softer tissue. VI considerably increases the mass transfer between the product and the surrounding liquid. The amount of solution incorporated into the tissue depends on many factors, such as the working pressure, the time of the process, the composition of the solution, and the characteristics of the tissue (6; 9). The VI process carried out at 15 kPa absolute pressure resulted in the mass gain of the sample in the range of 15 to 34% (Table 1). A connection between initial firmness and the amount of solution incorporated into the tissue could also be found.