## 3 Results and Discussion
### Dry Matter Content and Water Activity  
The dry matter content of dried materials is one of the main factors determining their overall quality, nutritional value and storage stability. The dry matter content of freeze-died foam-mat kiwiberry pulp is summarized in Table 1. The average values ranged from 97.2 to 99.6%, with the lowest value for the sample dried at 10 \({}^{\circ}\)C. The differences in dry matter content may be related to the capillary diffusion of the water molecules within the material. At a higher temperature the higher driving force of the heat exchange process occurs, caused by the higher difference in temperature between the material and the heat source [23; 43].  
The dry matter content determined in our study is consistent with the results of other studies. [PERSON] et al. [43] showed that under a constant thickness of layer, the dry matter  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Drying Temperature (\({}^{\circ}\)C)** & **Dry Matter Content (\%)** & **Water Activity (-)** \\ \hline
10 & 97.2 \(\pm\) 0.1 a \({}^{\dagger}\) & 0.221 \(\pm\) 0.003 c \\
25 & 98.2 \(\pm\) 0.1 b & 0.180 \(\pm\) 0.003 b \\
40 & 99.6 \(\pm\) 0.1 c & 0.159 \(\pm\) 0.005 a \\ \hline \hline \end{tabular} \({}^{\dagger}\) Different letters within columns indicate the statistical difference ([PERSON]’s HSD, \(p\) < 0.05).  
\end{table}
Table 1: Dry matter content and water activity of freeze-dried foam-mat kiwiberry pulp.  
Figure 1: Drying kinetics of freeze-dried foam-mat kiwiberry pulp. \({}^{\dagger}\) Different letters within column for effective water diffusion coefficient (\(D_{\text{eff}}\)) indicate the statistical difference ([PERSON]’s HSD, \(p\) < 0.05).  
content of powders increased with the increase in air-drying temperature. For instance, foam-mat cantaloupe powder dried at 40, 55 and 70 \({}^{\circ}\)C at the layer of 3 mm in thickness resulted in a dry matter content, respectively, from 91.9 to 95.4%. In another study, the dry matter content of the cocoa powder increased from 95.7 to 98.5% with an increase in the air-drying temperature from 50 to 70 \({}^{\circ}\)C [11]. In turn, [PERSON] et al. [44] noted the dry matter content of 92.7% for foam-mat freeze-dried bacaba powder.  
The water activity affects the factors that determine the sustainability of food such as microbial growth, enzymatic and chemical activity, and the course of the non-enzymatic browning reaction [43]. The values of water activity were in the range of 0.159 to 0.221 (Table 1) with a descending trend with an increasing shelf temperature. All the samples had water activity values below 0.6, which is important for microbial stability during storage [8]. The water activity corresponding to the monolayer moisture content (a\({}_{w}\) < 0.25) of the material ensures its durability and high-quality properties, while exceeding this value causes undesirable changes in the product [45; 46]. According to [PERSON] et al. [13], the low water activity values of the foam-mat dried samples were probably related to the loss of water due to their porous structure. The water activity determined in the current study is in line with the results of other studies, for instance, from 0.147 to 0.288 for foam-mat dried cantaloupe powder dried at temperatures of 40, 55 and 70 \({}^{\circ}\)C [43], from 0.120 to 0.190 for foam-mat yacon juice dried at temperatures of 50, 60 and 70 \({}^{\circ}\)C [47], from 0.110 to 0.789 for foam-mat bacaba powder dried at temperatures of 50, 60 and 70 \({}^{\circ}\)C and 0.533 for foam-mat freeze-dried powder [44].