## 3 Results and Discussion
### Viscoelastic Properties  
Because the high-protein fat-free dairy dessert structure can be described as gel-like, we can measure G' and G'' parameters. The storage modulus (G') represents energy that is retained by the system as a result of elastic deformation during the temporal application of stress, and the loss modulus (G'') reflects the ratio of the viscous stress amplitude to the strain amplitude and indicates how much energy during deformation has been dissipated as heat [38]. The ratio of loss to storage modulus is represented by tan(\(\delta\)), which provides information about the number of elastic and viscous components contained in the sample [39]. Increased G' values along with the addition of WPC80 (5-11%) testify to the strengthening of the dessert gel structure (\(p<0.05\)). Only in a product with 13% WPC80 was a decrease in the value of the measured feature recorded (Table 3). In all tested samples, the values of the storage (G') modulus were always higher than those of the loss (G'') modulus. It means that the high-protein fat-free dairy desserts exhibited elastic (gel) properties throughout the measurement. Samples with 13% WPC80 had lower G' and G'' values than the product with 11% WPC80, indicating that the gel structure of the tested product had weakened and formed a less elastic system. A correlation of storage (G') and loss (G'') moduli with hardness values indicates that the structure of the obtained product has become harder and more compact with the addition of WPC80 (5-11%). A sample of the dairy dessert with 13% WPC80 presented lower values than the product with 11% WPC80. When the loss value is tan(\(\delta\)) \(<1\) the tested dairy dessert exhibits elastic (gel) properties; otherwise (i.e., tan(\(\delta\)) \(>1\)) the product shows viscous properties. In every tested product, the value of tan(\(\delta\)) was lower than 1. The highest values of loss modulus were noticed for the 13% WPC80 concentration. Our results are consistent with the conclusions of other researchers who confirm the relationship between the amount of protein and values of storage (G') and loss (G'') moduli [40]. Figure 1: Effect of different WPC80 contents on the apparent viscosity of high-protein fat-free dairy desserts. Letters (a–d) indicate significant differences at \(p<0.05\) ([PERSON]’s HSD test). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Content of WPC80 [\%]** & **G’(Pa) \(\pm\) SD** & **G′′(Pa) \(\pm\) SD** & **tan(\(\delta\)) \(\pm\) SD** \\ \hline
**5** & 262.81 \({}^{\mathrm{a}}\)\(\pm\) 3.88 & 73.12 \({}^{\mathrm{a}}\)\(\pm\) 0.05 & 0.28 \({}^{\mathrm{a}}\)\(\pm\) 0.05 \\
**7** & 502.2 \({}^{\mathrm{b}}\)\(\pm\) 6.77 & 148.95 \({}^{\mathrm{b}}\)\(\pm\) 22.77 & 0.3 \({}^{\mathrm{a}}\)\(\pm\) 0.046 \\
**9** & 867.07 \({}^{\mathrm{c}}\)\(\pm\) 40.73 & 342.99 \({}^{\mathrm{c}}\)\(\pm\) 8.94 & 0.4 \({}^{\mathrm{b}}\)\(\pm\) 0.02 \\
**11** & 1608.6 \({}^{\mathrm{e}}\)\(\pm\) 75.33 & 488.15 \({}^{\mathrm{d}}\)\(\pm\) 67.56 & 0.31 \({}^{\mathrm{a}}\)\(\pm\) 0.06 \\
**13** & 1119.15 \({}^{\mathrm{d}}\)\(\pm\) 76.99 & 557.86 \({}^{\mathrm{e}}\)\(\pm\) 29.93 & 0.5 \({}^{\mathrm{c}}\)\(\pm\) 0.06 \\ \hline \hline \end{tabular} Data is presented as means \(\pm\)SD (standard deviation). \({}^{\mathrm{a}}\)* Means in the same column with different superscripts are significantly different (\(p<0.05\), [PERSON]’s HSD test).  
\end{table} 
Table 3: Effect of different WPC80 contents on storage (G’) and loss (G′′) moduli, and tan(\(\delta\)) of high-protein fat-free dairy desserts. This is because, with a low lactose content in concentrates and isolates, when the whey proteins are denatured, their gelation temperature is shifted [41]. We also noticed a significant difference in the values between 13% and 11% WPC80 (\(p<0.05\)). The decrease in this value may be related to the foamability of the whey proteins.