## 3 Results and Discussion  
### Properties of Pre-Foam Solutions  
The properties of pre-foam solutions (surface tension and viscosity) relate to their foamability and rheology of the resulting foams ([PERSON] et al., 2016; Polska, 2016). Figure 1 illustrates the effect of sucrose on the viscosity curves of WPI solutions. For all the analyzed solutions, the viscosity decreased with the increasing shear rate (0.1-250 s\({}^{-1}\)) and increased as the concentration of sugar was higher. For instance, at the shear rate of 100 s\({}^{-1}\), the highest viscosity was observed for the WPI/15% sucrose solution and the WPI/0% sucrose sample proved to be the least viscous. [PERSON] and [PERSON] ([PERSON] and [PERSON], 2016) also observed that viscosity of the WPI solutions increased gradually with the increasing sucrose concentrations.  
Table 1 shows the surface tension (\(\gamma\)) of the pre-foam solutions. The smallest \(\gamma\) was recorded for the WPI/0% sucrose sample. Higher concentrations of sucrose (up to 10%) resulted in a surface tension increase. It can be concluded that there is a certain concentration of sucrose between (10-15%), for which the surface tension achieves the maximum value and its further increase does not change this property. [PERSON] and [PERSON] ([PERSON] and [PERSON], 2016) found that larger additions of sucrose resulted in an increase in the surface tension of WPI solutions as they reduce the surface activity of globular proteins due to their lower hydrophobicity. [PERSON] et al. (2016) reported that the sucrose addition can diminish the interactions between proteins as sucrose is recognized to facilitate less favorable conditions for protein unfolding. [PERSON] et al. (2016) pointed out to the relation of pre-foam solution viscosity with its surface tension. Sugars increase the viscosity and thus the surface tension in the presence of sugars is greater. Therefore, protein molecules tend to remain in a continuous phase, and they do not adsorb at the air-water interface, which can affect the protein foamability. This can be confirmed by the values of the zeta potential (\(\zeta\)) recorded for the WPI solutions presented in Figure 2.  
The \(\zeta\) is one of the most useful parameters to study electric interactions of dispersed particles. This can markedly affect the food properties, such as stability, surface hydrophobicity and rheology [29]. It can be observed that the zeta potential of WPI solutions increased with the addition of sucrose, which indicates that sucrose reduced the charge of molecules and electrostatic repulsion among whey proteins. [PERSON] [30] classified the solutions with the zeta potential values of \(\pm\)0-10 mV as greatly unstable. However,  
\begin{table}
\begin{tabular}{c c} \hline
**Sample** & \(\boldsymbol{\gamma}\) **(mN/m2)** \\ \hline
20\% WPI/0\% sucrose & 44.68 \({}^{\mathrm{c}}\)\(\pm\) 0.02 \\
20\% WPI/5\% sucrose & 48.83 \({}^{\mathrm{b}}\)\(\pm\) 0.26 \\
20\% WPI/10\% sucrose & 49.79 \({}^{\mathrm{a}}\)\(\pm\) 0.19 \\
20\% WPI/15\% sucrose & 49.67 \({}^{\mathrm{a}}\)\(\pm\) 0.14 \\ \hline \end{tabular} The differences among the mean values in the column designated with different letters are statistically significant (\(p<0.05\)).  
\end{table}
Table 1: Surface tension values of the pre-foam solutions.  
Figure 2: Zeta potential (\(\zeta\)) values of the pre-foam solutions. The differences among the mean values in the column designated with different letters are statistically significant (\(p<0.05\)).  
it can be stated that higher sugar concentrations resulted in the decrease in stability of nanoparticles of the analyzed solutions. These results indicate that the sucrose was soluble in the protein solutions over the entire range of the tested concentrations.