## 3 Results and Discussion
### Impact of Ingredients and Processing Conditions on Selected Physical Properties
This parameter depends mainly on the intensity of heat and mechanical treatment, as well as on the composition of the raw material, particularly the content of dietary fiber and gelling agents. In this study, the WAI value responded noticeably to all three tested process variables: screw speed, moisture content of the mixture and the level of fresh blueberry addition (Table 5). The highest WAI values were recorded for control sample processed at 100 rpm and 36% moisture, where the index reached 4.07. Such high values are likely related to intense starch gelatinization, which occurs under greater mechanical energy input and in the presence of a sufficient amount of water that promotes swelling of starch structures. These findings illustrate that the combination of high moisture content and intensive mixing contributes to increased water absorption capacity, particularly in systems free from interfering components. A decrease in WAI with increasing levels of fresh blueberry, particularly at the 30% level, was observed in the current study. The presence of fiber and pectin may have disrupted starch swelling, reducing its water-binding capacity. The highest WAI values were recorded for control samples, processed at higher moisture content and screw speed. Similar results were reported in a study by [PERSON] et al. [42] involving goji berry addition, where an increase in mixture moisture and fruit content led to a decrease in WAI. The maximum WAI value of 8.05 g/g was obtained at the lowest moisture content (13%) and the smallest addition of goji (1%), whereas the minimum WAI value of 6.05 g/g was recorded at the highest moisture content (19%) and the highest level of goji addition (5%). This was explained by starch devtrinization and the presence of lipids, which can limit starch gelatinization. Similar results were presented by [PERSON] et al. [43], who reported a gradual decrease in WAI in extruded products with the addition of fruit pomace--from 6.5 g/g in the control sample to 4.6 g/g in samples with the highest fruit content. The authors explained this effect by the reduced gelatinization of starch in the presence of fiber fractions and other components from the fruits. An analogous trend was observed in the present study--an increase in fresh blueberry content led to a systematic decrease in WAI, which may be related to the disruption of starch swelling by pectin and fiber present in the fruit. In both cases, fruit-based plant additives affected the reduction of water absorption capacity in the extrudates. In contrast, the addition of fresh blueberries to the mixtures led to a general decrease in WAI values, especially at the highest addition level. Under these conditions, the index dropped to as low as 2.90 (at 100 rpm and 36% moisture). This reduction can be explainedby the presence of dietary fiber and simple sugars in the fruit, which may limit starch expansion and gelatinization, acting as a barrier to water absorption. Additionally, organic acids naturally present in blueberries may alter the reaction environment, further affecting the material's ability to retain water. At the lowest blueberry addition level (10%), WAI values were similar to those of the control samples and did not show a significant reduction. For example, at 60 rpm and 36% moisture, the WAI was 3.88, suggesting that a small amount of fruit does not noticeably interfere with the swelling process and may even positively influence the water-holding structure of the extrudate under certain conditions. In summary, the highest water absorption capacity was observed in samples without blueberry addition, processed at higher moisture levels and increased screw speeds. The decrease in WAI values with increasing blueberry content may indicate a limiting effect on starch expansion and porous structure formation, which from a technological perspective could significantly influence the rehydration properties of the final product.