## 3 Results and Discussion
### Stage III  
#### 3.3.1 Powder Recovery  
The linear model allowed the significant prediction (\(p\)-value \(<\) 0.05) of \(R_{p}\). A satisfactory fit of the model to the data verified experimentally was noted, which was confirmed by the relatively high value of the determination coefficient (R\({}^{2}\) = 0.7265) and the insignificant result of the lack of fit test (\(p\)-value \(>\) 0.05) (Table 10). Based on the estimated parameter values (Equation (3)) and response surface plots (Figure 7), it can be concluded that increasing the IAT and FC had a positive effect on \(R_{p}\), while increasing IAH had a negative effect. The effect of IAH was larger than IAT and FC; however, as ANOVA showed, all of three tested variables significantly influenced the \(R_{p}\) of MD powders (\(p\)-value \(<\) 0.05).  
As IAT increases, and at the same time drying IAH decreases, it ensures efficient mass and heat transfer that leads to effective and rapid water evaporation, consequently assuring satisfactory powder recovery. Particles that are dried at higher temperatures and additionally use air of decreased humidity become surface-dry quicker; thus, they are less likely to stick to finer particles and to the walls inside the drying chamber during collisions, resulting in the production of powders of a more satisfactory powder recovery. \(R_{p}\) is influenced by a product's collection efficiency, the particles' stickiness, and collector's (cyclone) efficiency [31]. The relationship between drying air temperature and stickiness trend was observed by [PERSON] et al. (2023) who investigated the agglomeration phenomenon during spray drying of MD solutions, and by [PERSON] et al. (2020) who spray dried MD and whey protein mixtures [32; 33]. \(R_{p}\) of produced MD powders varied from 32.89% to 78.98% (Table 10), and generally the lowest values were observed for the variants that considered the ambient IAH and the lowest IAT, while FC had a less significant effect (random order 8, 11, 18), underlining the non-effective water evaporation. According to [PERSON] and [PERSON] (2004), who spray dried tomato pulp with varying solids content,  
Figure 7: Powder recovery (\(R_{p}\)) response surface plots for paired factors: (**A**) inlet air temperature (IAT) and inlet air humidity (IAH); (**B**) inlet air temperature and feed solution concentration (FC); (**C**) feed solution concentration (FC), and inlet air humidity (IAH), when the third factor was adjusted to its midpoint.  
the increase in FC increased the risk of stickiness to the walls of the drying chamber, as the particles become surface-dry later [31]. The authors underlined that the final effect of the solids content in the liquid feed is strongly dependent on the drying medium, which was in accordance with the results of this research. The researchers noted that lowering IAH enabled rapid water evaporation from the particles, thus the particles became surface-dry quicker, lowering the risk of the stickiness between the particles and the walls of the drying chamber. Taken together, these conclusions further support the findings of this research.  
\[R_{p}=14.13401+0.389094\cdot IAT-1.48332\cdot IAH+0.618937\cdot FC \tag{3}\]