## 2 Materials and Methods
### Experimental Plan  
The experimental plan was divided into three stages (Figure 11, Appendix A):  
**Stage I**: Spray drying/evaporation of distilled water at variable feed rate (FR), inlet air humidity (IAH) and inlet air temperature (IAT) (54 experiments) (Table 1). During 10 min of the experiment, the outlet air temperature, the relative and absolute humidity of outlet air were measured every 1 min and documented (AZ9871 Data logging/Printing Anemometer, AZ Instrument Corp., Taichung, Taiwan). The process performance was also observed visually, and the time at which water evaporation stopped (the droplets appeared on the internal surface of a spray dryer's window) was noted. Before running every experiment and during the experiment, the ambient temperature and humidity were monitored using thermo-hygrometer (testo 622 Scientific Ambient Monitor, Testo SE & Co. KGaA, Titisee (Neustadt, Germany)).  
**Stage II**: Spray drying of 30% (solids, _w/w_) MD solutions at the same levels of independent variables and procedure of monitoring drying air parameters and ambient air parameters as in Stage I (54 experiments) (Table 1). Additionally, powder recovery (\(R_{p}\)), its moisture content (MC), water activity (\(a_{w}\)), loose bulk and tapped densities (\(d_{L}\) and \(d_{T}\)), flowability (HR) were determined. The morphology of selected variants was analyzed as well. * **Stage III**: Optimization of MD solutions spray drying process variables (inlet air temperature (IAT), inlet air humidity (IA), feed solution concentration (FC)) for maximum powder recovery and powders of low moisture content and activity using response surface methodology (RSM) with random run order and center points runs added, with three coded levels for each variable: low (\(-1\)), standard (0), and high (\(+1\)) (Table 2). A total of 19 combinations of variables were considered: 8 factorial points in 2 replications and 3 center points runs placed in the beginning, middle and at the end of the experiment (Table 3). The levels of drying process variables were chosen based on the preliminary research and technical specification of air-dehumidification system. Additionally, particle morphology and particle size (\(D_{50}\)), loose bulk and tapped densities (\(d_{L}\) and \(d_{T}\)), and flowability (HR) of obtained powders were determined. The procedure of monitoring drying air and ambient air parameters was the same as in Stage I and II of the research.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Feed Rate** & **Inlet Air Humidity [g/m\({}^{3}\)]** & **Inlet Air Temperature** [\({}^{\circ}\)**C]** \\ \hline \multirow{4}{*}{0.16} & \multirow{4}{*}{0.1–0.3} & 80 \\  & & 100 \\  & & 120 \\ \cline{1-1}  & & 80 \\ \cline{1-1}  & & 100 \\ \cline{1-1}  & & 120 \\ \cline{1-1} \cline{2-3} \end{tabular}
\end{table}
Table 1: Experimental plan for spray drying/evaporation of distilled water (Stage I) and spray drying of 30% (solids, \(w\)/\(w\)) maltodextrin solution (Stage II).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Independent Variables** & **Symbol** & \multicolumn{4}{c}{**Coded Values**} \\ \cline{3-5}  & & \(-\)**1** & **0** & **+1** \\ \hline Inlet air absolute & \multirow{2}{*}{X\({}_{1}\)} & \multirow{2}{*}{0.1–0.3} & \multirow{2}{*}{1.1–1.3} & \multirow{2}{*}{9–10} \\ humidity [g/m\({}^{3}\)] & & & & \\ Inlet air temperature [\({}^{\circ}\)C] & & & & \\ Feed solution & & & & \\ concentration [\%] & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: The independent variables and representative coded levels in Stage III.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Random Order** & \(\mathbf{X_{1}}\) & \(\mathbf{X_{2}}\) & \(\mathbf{X_{3}}\) \\ \hline
1 & 0 & 0 & 0 \\
2 & \(-1\) & \(-1\) & \(+1\) \\
3 & \(-1\) & \(+1\) & \(+1\) \\
4 & \(-1\) & \(-1\) & \(-1\) \\
5 & \(-1\) & \(+1\) & \(+1\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: The experimental plan—Stage III; the full factorial design (FFD) experiment matrix with randomization and center points.