## 3 Results and Discussion
### Moisture Content, Water Activity, and Particle Size D\({}_{4,3}\) Parameter of Powders
casei_ 431 by spray drying in either whey protein isolate, gum Arabic (WPI-GA) system or whey protein isolate, tuna oil, gum Arabic (WPI-O-GA), that the water activity of final powder 0.23 \(\pm\) 0.004 and 0.26 \(\pm\) 0.014%, respectively with final conclusion of positive effect of tuna oil on final number of active bacteria cells [27]. [PERSON] et al. (2017) have reported that the best expected \(a_{w}\) values to store probiotic powders were between 0.07 and 0.11 [46]. According to [PERSON] et al. [47], lower \(a_{w}\) (0.2-0.3) caused better cell viability during storage [47]. In the case when powders are stored at room temperature, water activity is a key factor for the high viability of lactic acid bacteria [48]. Another example would be spray drying of three _Lactobacillus_ isolates in 10% w/v sodium caseinate (NaCas) with added LMF or vegetable oil, with a ratio of oil or LMF to NaCas of 1:1. In both types of microcapsules the moisture content of the powders was approximately 3.3 and 3.7% wt., and the water activity was 0.19 and 0.20 for oil + NaCas and LMF + NaCas, respectively [38]. In addition, it is known that the high \(a_{w}\) value contributes to the growth of other microorganisms including bacteria and fungi, causing undesirable chemical reactions and as a consequence the reduction of the number o of probiotic bacteria and shorter shelf-life [49]. In this case, the selection of proper protective substances is important, because different substances have different capabilities of absorbing water molecules, both during the drying and post-storage process. These properties help to keep the dormant state of probiotics acquired in the drying process during the whole storage [50]. After 4 weeks of storage at room temperature, in the case of powders with the addition of more oils, one could observe a more rapid decrease in the percentage of live _Lbs. rhamns_ in powders with higher \(a_{w}\) values. In the case of powders with an \(a_{w}\) in the range of 0.225-0.258, the reduction in viability was at the level of approx. 70-81.5%, while in the case of water activity of 0.118 and 0.128, it was approx. 64.5% and approx. 52%, respectively. No relationship was observed for powders with 20% wt. content of oil stored for 4 weeks at 20 \({}^{\circ}\)C.  
The average size of powder particles may affect their final properties, including their stability and efficiency of microencapsulation. Microcapsules with bigger mean diameters due to a lower surface-to-volume ratio offer better protection against external factors than smaller ones, but at the same time, they have lower dispersibility in final products. However, sometimes too small particles may be so-called \"hollow microcapsules\" that have no core materials embedded [51]. The particle size parameter of the microcapsules presented as D\({}_{4,3}\) values are displayed in Table 4. The particle size D\({}_{4,3}\) parameter in all powders, both with the addition of lower (20%) and higher (30%) content of oils, significantly differs from the reference sample (\(p<0.05\)). The highest average particle size (506.8 \(\pm\) 2.0 \(\upmu\)m) was observed for powder with the addition of milk thistle oil in the higher ratio (30%) to dry mass of solution, whereas the lowest (54.5 \(\pm\) 2.7 \(\upmu\)m) was noticed for powder with the same oil but in a ratio of 20%. The average particle size of the reference sample was 29.4 \(\pm\) 2.1 \(\upmu\)m. It was observed that the average particle size of the powder rises with increasing the oil concentration. Most of the diameters of microcapsules in this study (Table 4) are very high, which may be due to the fact that the powder particles have the tendency to form aggregates, especially in the case of higher oil load. [PERSON] et al. [51] spray drying _L. acidophilus_ using as wall materials \(\beta\)-cyclodextrintrin and acacia gum in a ratio of 9:1 (w/w), and microcapsules were prepared at four different concentrations of wall materials (15, 20, 25, and 30% (w/v)).