## 3 Results and Discussion
### Stage II
#### 3.2.3 Moisture Content and Water Activity  
Powders of MC below 5% can be considered as stable, thus MD powders produced at 120 \({}^{\circ}\)C, using the lowest or medium IAH (0.1-1.1, 1.1-1.3 g/m\({}^{3}\)), had the most satisfying results regarding MC (Figure 4A) [23]. Despite increasing FR from 0.16 to 0.58 mL/s, water evaporated effectively leading to the production of powders with MC below 5%. Decreased IAH ensured driving force for water evaporation, despite increased FR. In comparison with the experiments that were conducted using ambient air humidity (9-10 g/m\({}^{3}\)), it can be noted that it was necessary to increase IAT to 120 \({}^{\circ}\)C to produce MD powder of satisfying MC, however at the same time maintaining the lowest FR to ensure enough driving force to evaporate water successfully. According to [PERSON] et al. (2008), who conducted research on the influence of process parameters on MC and \(a_{w}\) of dairy powders, the temperature that the droplet reached during drying was impacted by the temperature of IAT and its absolute humidity, as well as the relative humidity of outlet drying air [24]. Therefore, it directly influences the MC of final product. The results from this study concurs well with the findings of [PERSON] et al. (2008) [24]. However, the FR should be considered as well, as it influences the mass and heat transfer inside the drying chamber and ensures the satisfying water evaporation. Increasing FR reduces the contact time between the droplet and hot drying air, which decreases the mass and heat transfer and results in powders of higher MC. [PERSON] et al. (2017) and [PERSON] et al. (2012) noted similar a relationship when increasing the FR of the mixture of gum Arabic and MD solutions and lactose-rich products, respectively, increased MC of produced powders [17; 25]. Figure 3: SEM images of chosen variants produced in Stage II of the research. The presented samples are those obtained using the lowest and the highest speed rate (0.16-0.83 mL/s) that was possible to produce at each drying temperature. L, M, H signifies the low, medium or high level of drying air humidity; 80, 100, 120 signifies the inlet air temperature; 0.16, 0.58, 0.67, 0.83 signifies the feed rate. When considering product's stability, it is important to also consider \(a_{w}\), as it translates into water availability for microbial growth, biochemical reactions and physical changes; thus, MC cannot be analyzed separately from \(a_{w}\)[24]. In this research, the strong positive correlation was noted between MC and \(a_{w}\), based on the PCA (Figure A2). All of the three variables analyzed influenced \(a_{w}\) (Figure 4B). Powders of \(a_{w}\) below 0.2 were regarded to be stable as the water is bound structurally [26]. MD powders that can be labeled as stable were produced mostly at the first three levels of feed rate (0.16-0.38 mL/s) at all of the tested IAT and IAH, which underlined the FR as the most important variable in producing a powder of satisfying \(a_{w}\). FR influenced the size distribution of droplets and finer droplets indicated large surface area, thus increased heat and mass transfer. For most of the tested variants, regardless of IAT and IAH, increased FR led to the distribution of droplets that were too large in size, which made it impossible to evaporate water effectively, leading to the production of MD powders of \(a_{w}\) pointing to their shelf-stability. Considering IAH, the lowest and the medium levels (0.1-0.3 and 1.1-1.3 g/m\({}^{3}\)) at the highest IAT (120 \({}^{\circ}\)C) made it possible to obtain MD powders of \(a_{w}\) below or equal to 0.2, regardless of FR, which was not likely to be observed using ambient IAH.