## 3 Results and Discussion
### Microstructure of Meringues  
Figure 6 shows the optical images of surfaces (a-c) and cross-sections (d-f) of the samples. Sections (a-c) confirm the previous observations from the profilometer that the surface roughness of the meringues decreases with the increasing sugar concentration. It can be also seen that the increased sugar concentration up to 10% results in an increased bubble size. The sample with 15% sugar addition is characterized by a smaller number of larger bubbles that were most likely disrupted during the solidification. This indicates that the 10% sugar concentration is optimal for maintaining the air bubbles of this system (Figure 6b,e), which is additionally confirmed by the rheological properties of the corresponding foam, its overrun and stability. In this case, a smaller viscosity of the pre-foam solution is also of special significance when compared to that containing 15% sucrose. Larger viscosity of the pre-foam solution can create a barrier to protein adsorption and thus affect the aerated structure of the meringue. [PERSON] et al. [11] indicate that the increased viscosity can affect the final cake volume by stabilizing the wet foam before turning into the solid one. [PERSON] and [PERSON] [20] reported that the sugar addition up to 10% kept a hold of meringue matrix more effectively. The increased sucrose concentration results in an increased final volume of the meringues (Figure 7a-c). The smallest cake volume for the sample containing 5% sugar (Figure 7a) can be due to a collapse of foam matrix during solidification, which suggests that the bulking and stabilizing effect of sugar on WPI was insufficient. The meringue with the 15% sugar content demonstrated the largest volume (Figure 7c); however, this does not correspond to the smaller overrun and stability of the liquid foam. This relation is rather hard to determine because according to [PERSON] et al. [11] air bubbles expand and/or coalesce during baking. The authors emphasize that the sucrose addition can change the bubble size and thickness of lamella film resulting in the increased meringue volume due to the bulking effect. At the larger sugar additions the meringue structure can change from coarse to collapsed due to the protein film rupture and bubble collapse. During solidification air bubbles in the meringues expand as temperature rises and collapse due to \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Probe Liquid** & \begin{tabular}{c} **Contact** \\ \(\theta_{\mathrm{a}}\) \\ \end{tabular} & \
\begin{tabular}{c} **Angles** \\ \(\theta_{\mathrm{r}}\) \\ \end{tabular} &
\begin{tabular}{c} **\(\gamma_{\mathrm{s}}\) (m/m\({}^{2}\))** \\ \end{tabular} \\ \hline
20\% WPI/5\% sucrose & Water & 120.6 \({}^{\mathrm{a}}\)\(\pm\) 8.9 & 110.1 \({}^{\mathrm{a}}\)\(\pm\) 8.0 & 33.4 \({}^{\mathrm{c}}\)\(\pm\) 1.2 \\
20\% WPI/5\% sucrose & Diiodomethane & 74.8 \({}^{\mathrm{a}}\)\(\pm\) 4.6 & 68.3 \({}^{\mathrm{a}}\)\(\pm\) 2.4 & 21.2 \({}^{\mathrm{c}}\)\(\pm\) 3.9 \\
20\% WPI/10\% sucrose & Water & 83.2 \({}^{\mathrm{b}}\)\(\pm\) 5.5 & 65.6 \({}^{\mathrm{b}}\)\(\pm\) 4.2 & 46.4 \({}^{\mathrm{b}}\)\(\pm\) 2.8 \\
20\% WPI/10\% sucrose & Diiodomethane & 52.7 \({}^{\mathrm{b}}\)\(\pm\) 5.0 & 45.5 \({}^{\mathrm{b}}\)\(\pm\) 2.8 & 42.0 \({}^{\mathrm{b}}\)\(\pm\) 1.0 \\
20\% WPI/15\% sucrose & Water & 63.1 \({}^{\mathrm{c}}\)\(\pm\) 5.1 & 35.9 \({}^{\mathrm{c}}\)\(\pm\) 3.2 & 60.1 \({}^{\mathrm{a}}\)\(\pm\) 1.6 \\
20\% WPI/15\% sucrose & Diiodomethane & 51.5 \({}^{\mathrm{b}}\)\(\pm\) 3.3 & 43.8 \({}^{\mathrm{b}}\)\(\pm\) 2.7 & 42.4 \({}^{\mathrm{b}}\)\(\pm\) 1.1 \\ \hline \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 3: The advancing (\(\theta_{\mathrm{a}}\)), receding (\(\theta_{\mathrm{r}}\)) contact angles and apparent surface free energy measured for the samples. the bubble film rupture. The final volume and its structure is a resultant of both processes.