*3.1. Physical Properties*

The powders obtained from plum juice extracts submitted to different drying techniques differed in terms of moisture content and water activity (Table 1). The moisture content ranged from 2.44% up to 7.34% and was within the range described by Boonyai et al. [25]. The values of moisture content depended on the drying method and the parameters applied. The lowest water content was noted after spray drying during which the highest temperature for water removal was applied [12]. The evaluation of water activity in powders is a key aspect as this parameter informs about their stability, both chemical and microbial. It is connected with the quality of the dried products as the rate for some chemical reactions begins above a water activity of 0.3 [26]. Similarly, a high positive correlation (*r* = 0.789) between moisture content and water activity have been previously described in fruit powders [12]. PEP also differed in color attributes (Table 1).

**Table 1.** Physical properties of the plum extracts powders (PEP) obtained after different drying techniques.


Freeze drying (FD); Vacuum drying 40 ◦C (VD 40 ◦C), 60 ◦C (VD 60 ◦C), 80 ◦C (VD 80 ◦C); Spray drying (SPD); colour parameters (*L*\*, *a*\*, *b*\*), chroma (*C*\*) and hue (*h*). Different letters (a,b,c,d,e) within the same column indicated statistical differences between samples (*p* ≤ 0.05; LSD Tukey).

Among products obtained, lower values of coordinate *L*\* were noted after VD, when compared to FD and SPD, that was in agreement with previously conducted research on chokeberry [11] and apple powders [12]. The *L*\* values were connected with coordinate a\* and b\* pointing a strong influence of the drying technique on the retention of red and yellow pigments. Chroma (*C*\*) is connected with the color intensity and opposite to apple powders [12], the highest values were linked to FD and SPD processes. This could be connected with the fact that, in the current study, the extract of plum juice was dried, and thereby the material differed in terms of the chemical composition, as the obtained extracts were significantly darker when compared to the juice. Taking the above into consideration, the chemical changes that have occurred during drying are strictly connected with the chemical properties of the initial materials. The hue angle (*h*\*) values indicated that the analyzed samples were more reddish as an angle of approximately 0 represents red colour. The highest *h*\* values were obtained for powders gained after FD that was in line with the coordinate *a*\*. Additionally, the lowest values were noted after FD and VD 40 ◦C, pointing to a strong influence on the temperature of the process on this parameter.
