## 3 Results and Discussion
### Impact of Ingredients and Processing Conditions on Selected Physical Properties
The Water Solubility Index (WSI) reflects the degree of degradation of soluble components, primarily starch and soluble fiber fractions, resulting from the combined effects of temperature, mechanical shear and moisture during the extrusion-cooking process. This parameter is particularly sensitive to the intensity of thermomechanical treatment and the presence of water-soluble compounds in the raw material (Table 5). In the control sample, an increase moisture content from 32% to 36% caused a significant rise in WSI, observed at both low and high screw speed. At 60 rpm, WSI increased from 8.23% to 17.22%, and at 100 rpm, from 7.53% to 18.35%. This indicates that higher moisture supports stronger thermohydrolysis and starch fragmentation, leading to a greater amount of water-soluble components in the product. The addition of fresh blueberries to the mixtures resulted in notable changes in WSI values, although the direction of these changes depended on other processing parameters. At low moisture content, blueberry addition led to an increase in WSI. For instance, at 60 rpm with 30% blueberry, WSI reached 12.72%, compared to 8.23% in the control sample. This suggests that the presence of simple sugars and low-molecular-weight fiber fractions in blueberries may have contributed to a higher proportion of water-soluble components, especially under less intense plasticization conditions. In contrast, under conditions of higher moisture content and low screw speed the effect was reversed--the addition of blueberries reduced WSI compared to the control samples. For example, with 10% blueberry, WSI dropped to 8.23%, which may result from increased water retention by fiber and less efficient breakdown of starch fractions in a more viscous environment. A high fruit content may also have altered the thermal profile within the extruder-cooker, potentially limiting complete gelatinization and fragmentation of starch molecules. The highest WSI values were observed under conditions of high moisture content and high screw speed regardless of blueberry addition--for example, 18.52% was recorded for samples with 30% blueberry. This suggests that under such intensive processing conditions, the degradation of structural components was strong enough to override the protective effects of fiber or simple sugars. In the present study authors observed an increase in WSI with higher temperature and screw speed, and a decrease in WSI as the mix moisture was reduced. In [PERSON] et al. [44] research the WSI values for all extrudates ranged from 14.5% to 15.87%, indicating a clear effect of temperature and screw speed on starch solubility. Similar results were reported by [PERSON] et al. [42] who observed that WSI increased with higher temperature and screw speed. The study revealed a maximum WSI value of 25.61% at a feed moisture content of 13% and a 5% addition of goji, while the minimum WSI value of 17.13% was observed at a moisture content of 19% and a 1% addition of goji. Both studies found that a decrease in feed moisture resulted in a decrease in WSI. These results confirm the relationship between extrusion cooking parameters and WSI, although differences in raw material composition and process conditions may lead to some variation in the final values.