## 3 Results and Discussion
### Water Solubility Index and Water Absorption Index  
The reconstitution properties of powders are one of the most important and the best quality indicators commonly used to evaluate the changes occurring during the drying process. These parameters are also essential for the future utilization as food components [49; 50]. For example, higher water absorption index (WAI) values indicate that a powder may retain more moisture, improving its utility and preventing dryness during storage [47].  
The water solubility index (WSI) of freeze-dried foam-mat kiwiberry pulp was significantly influenced by shelf temperature (Table 3). The WSI value increased by 7.8% as the shelf temperature increased from 10 to 40 \({}^{\circ}\)C. A similar trend was also observed by other authors during the convective drying of foamed fruit materials [16; 21; 51]. [PERSON] et al. [16] and [PERSON] et al. [21] were provided a comparable WSI for foam-mat convective-dried lime juice (69.07-69.53%) and banana pulp (57.13-62.09%), respectively.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Drying Temperature** & **Equilibrium Water Content** & **Effective Moisture Diffusivity** & \multirow{2}{*}{\(R^{2}\)} & \multirow{2}{*}{\(\chi^{2}\)} & **RMS** \\ (\({}^{\circ}\)**C)** & **(g water/100 g d.m.)** & **(m\({}^{2}\)/min)** & & & **(\%)** \\ \hline
10 & 4.07 \(\pm\) 0.24 b \({}^{1}\) & 0.97 \(\times\) 10\({}^{-9}\)\(\pm\) 0.10 a & 0.997 & 0.005 & 4.20 \(\pm\) 0.15 \\
25 & 4.37 \(\pm\) 0.20 b & 1.17 \(\times\) 10\({}^{-8}\)\(\pm\) 0.10 b & 0.997 & 0.007 & 7.17 \(\pm\) 0.60 \\
40 & 1.59 \(\pm\) 0.25 a & 1.06 \(\times\) 10\({}^{-8}\)\(\pm\) 0.01 a & 0.993 & 0.001 & 13.11 \(\pm\) 5.29 \\ \hline \hline \end{tabular} \({}^{1}\) Different letters within columns indicate the statistical difference ([PERSON]’s HSD, \(p\) \(<\) 0.05).  
\end{table}
Table 2: Parameters of sorption kinetics of freeze-dried foam-mat kiwiberry pulp.  
Figure 2: Hygroscopic kinetics of freeze-dried foam-mat kiwiberry pulp. Dotted lines represent values obtained from the mathematical modelling.  
The results for the water absorption index (WAI) of freeze-dried foam-mat kiwiberry pulp were in the range of 1.53 to 1.73 (Table 3). A significantly higher WAI value was observed for the sample dried at 40 \({}^{\circ}\)C. The increasing WAI as the temperature increases was also found reported in the literature [21; 47]. In comparison to our results, other researchers for foam-dried materials have presented higher WAI, e.g., from 2.46 to 3.90 for mango pulp [52], 4.18-4.97 for muskmelon pulp [51] and 5.31-5.97 for banana pulp [21] or a comparable WAI, e.g., 1.47-1.73 for lime juice [16] or 1.48-1.81 for yacon juice [47].  
The increased values of WSI and WAI in our study may be due to the total porosity and structural changes, which occurred during drying. In general, freeze-dried products are characterized by a higher porosity which allows the rapid adsorption of water, thus enhancing the solubility of water-soluble compounds. The kiwiberry is rich in these compounds, e.g., sugars, dietary fibre as well as some protein [3; 53]. The samples dried at higher shelf temperatures exhibited a more damaged structure and lower total porosity (see Section 3.6), which may promote enhanced water adsorption and solubility of the components. Moreover, a decrease in the total porosity of the samples may provide better contact between the powder and water, resulting in the enhancement of the WSI and WAI values [21]. Another explanation for the increased solubility (WSI) is linked to the breakage of the molecular bonds in sugars due to a decrease in moisture content [21]. Some other interesting and promising explanations could be related to the statement by [PERSON] et al. [54]. The collapsing of bubbles during drying may promote the compaction of the foam structure and decreased susceptibility to water interaction, thus resulting in a lower water-solubility. Unfortunately, this theory does not hold true for the results obtained.