## 3 Results and Discussion
### Surface Properties of Meringues  
It should be emphasized that the production of meringues without sugar was infeasible. It is interesting to note that there were no quality defects, such as cracks, which is characteristic of confectionery products. The smallest sugar addition (5%) enabled preservation of the foamed structure during solidification. Figure 5a-c illustrates the profilometer images, side profiles and the roughness parameters of the meringues. They indicate that the larger sugar addition resulted in a surface smoothing effect and meringues demonstrated smaller surface roughness, which can be seen in the profilometer images. The WPI/15% sucrose sample was the smoothest (Figure 5c). The similar smoothing effect was also observed by [PERSON] et al. [8], who analyzed high-protein meringues from WPI and erythroid. [PERSON] et al. [40] reported that meringues demonstrated various structures with the different sucrose addition. Structures can become finer with the increasing sugar concentration. [PERSON] et al. [41] described a vitrification phenomenon, i.e., an amorphous glassy matrix was formed by sugars around the proteins on the surface, affecting both the roughness and the foamed structure. [PERSON] et al. [5] emphasized the importance of continuous bubble growth in the foam that can produce a coarse structured cake after solidification. The observed reduction in roughness can be related to the increased viscosity of the pre-foam solutions because more viscous dispersions can retard the migration of bubbles towards the surface of the meringue during baking. The contact angle values define the hydrophobic or hydrophilic character of the surface--the smaller the contact angle, the more hydrophobic the surface. Apparent surface free energy (\(\gamma_{\mathrm{s}}\)) value is the result of topography and surface chemistry [42]. The values of advancing (\(\theta_{\mathrm{a}}\)) and receding (\(\theta_{\mathrm{r}}\)) contact angles of the probe water and diiodomethane and the \(\gamma_{\mathrm{s}}\) values calculated from the CAH (Contact Angle Hysteresis) approach of the meringues are presented in Table 3. For all samples, the values of contact angles decreased as the sucrose addition was larger and the meringue surface became more hydrophilic. For both liquids the largest \(\theta_{\mathrm{a}}\) and \(\theta_{\mathrm{r}}\) values were measured for the WPI/5% sucrose, and the lowest for the WPI/15% sucrose sample. A significant decrease in the contact angle values proves that higher sugar concentrations modify the properties of the tested surfaces. Sucrose is a hydrophilic component and it reduces the contact angle; the probe water and diiodomethane drops are spread on the surface of the meringue more readily. Figure 5: Profilometer images. Z axes are represented by the side profiles on the right and the surface roughness parameters for the samples (surface 0.9 \(\times\) 1.3 mm). The largest roughness values are represented by the red regions and the smallest by the blue ones. \(R_{d}\)-the average roughness. \(R_{d}\)-the quadratic mean of surface roughness. \(R_{l}\)-the maximum roughness height. (a) WPI/5% sucrose; (b) WPI/10% sucrose; (c) WPI/15% sucrose. CAH W (water) represents all interactions including the electrostatic and ionic ones. CAH DM (diiodomethane) represents the dispersive component associated with the charge and these are weak interactions. The \(\gamma_{\mathrm{s}}\) values increased with the increasing sugar concentrations being consistent with the increase in hydrophilic character of the surface, which suggests that the samples were more heterogeneous in terms of topography, polar and dispersive interactions. Dispersive interactions have a great impact on the total value of surface free energy, as can be seen from the surface energy calculated from diiodomethane, which interacts only with the surface in a dispersive way [43]. Higher sugar concentrations produce a number of polar groups on the surface of meringues, which justifies the increase in the \(\gamma_{\mathrm{s}}\) values.