## 3 Results and Discussion
Hardness is a specific force required to deform food between the teeth or between the tongue and palate [23]. Hardness was the dominant feature among other texture parameters and was positively associated with them. A uniform increase in product hardness was observed in the range between 5 and 11% of the added product (21.56-94.2 N). [PERSON] and [PERSON] [24] examined changes in yogurt hardness caused by the addition of selected milk preparations. They observed that the highest values of hardness were found in the product with added WPC85 among other tested ingredients (OMP, WPC35, WPC65, degreased WPC (WPCO)). They also suggested that the increase in hardness of tested yogurts can be explained by the correlation between the content of casein and whey proteins. The structure of the final products became more compact and therefore harder [24]. The results obtained in our research confirm this conclusion, but a decrease in hardness was noted in the product with 13% WPC80. This might be caused by the type of whey concentrate used in our experiment. WPC80, among other whey-protein concentrates (WPC35, WPC50 and WPC65), has the lowest moisture and lactose content and the highest protein content [25]. This composition together with higher amounts of added WPC80 caused a substantial increase in the product hardness with 5, 7, 9 and 11%, but had no influence on the product with 13% WPC80. The decrease in hardness of the dairy dessert with 13% WPC80 can be ascribed to phase separation between whey proteins and k-carrageenan. Thermodynamic incompatibility (phase separation) is the most common phenomenon occurring between proteins and hydrocolloids in a system where pH surpasses the protein isoelectric point [26; 27], which is in line with our study, where the pH was 6.2. A greater amount of added WPC80 increased the surface-active material in the system and the number of protein interfacial shells [28]. [PERSON] et al. [18] tested the effect of WPC80 on processed cheese sauces. They also noticed that in a product with added coconut oil, not only were the values of hardness lower in samples with the highest amount of added whey-protein concentrate compared to a product with a lower amount of WPC80, but also the adhesiveness of the samples. Adhesiveness is a force that is required to remove food from the palate or teeth while chewing [23]. The quantity and hydrophobicity of proteins are not always the factors that determine the highest adsorption at fat-water interfaces during emulsification [18]. Moreover, whey proteins perform in the same way as fat globules, thus, the production of high-protein desserts without fat can be achieved. This, however, increases product adhesiveness [22]. As other studies show, the incorporation of whey-protein concentrate increases not only the hardness in yogurts [29], but also the adhesiveness [30]. [PERSON] and [PERSON] [30] investigated the effect of supplementing the set-type yogurt made from goat's milk with whey protein concentrate. They reported that WPC35 enhanced the firmness, hardness and adhesiveness during a 28-day storage period. Cohesiveness is a representation of the forces of internal bonds holding the product as a whole together [31; 32]. The lowest cohesion was measured in a product with a 5% protein addition of WPC80, while the values of products containing 7, 9 and 11% concentrations of whey remained at the same level (\(p>0.05\)). Only the highest level of WPC80 (13%) caused a significant increase in the cohesiveness of the tested product. These results may suggest that increasing the protein concentration in a product has little effect on the increase in cohesion values. In research into the storage stability of the texture