## 3 Results and Discussion  
### Cell Viability  
Chokeberry juice was applied as substrate in lactic acid fermentation by the potential probiotic strain _L. paracasei_ SP5 for the production of a novel functional beverage. Initially, chokeberry juice was fermented for 48 h at three different initial sugar concentrations to find out the optimal sugar content for probiotic growth. The pH value of all fermented juices was monitored at the end of 48 h fermentations and during the 4 weeks (4 \({}^{\circ}\)C). No significant change was observed compared with the initial pH value (4.0). Specifically, the pH value varied between 4.0 \(\pm\) 0.2 for all the periods mentioned, probably due to the high buffering capacity of the fermented juices, as other researchers have reported in similar experiments regarding lactic acid fermentations of various fruit juices [12]. The selected fermented chokeberry juice of initial sugar concentration 40.3 g/L was stored for 4 weeks at 4 \({}^{\circ}\)C. Samples were collected after 48 h of fermentation and after each week of storage (1-4 weeks) for the determination of residual sugar, volatile compounds, lactic and acetic acid contents, total phenolics count, and cell viability. The viability of _L. paracasei_ SP5 was monitored during fermentation and storage. The fermented juice was also monitored for the occurrence of possible spoilage microorganisms. Specifically, yeast, fungi, or coliform bacteria were not detected (nonvisible colony) after juice fermentation as well as during the four weeks of cold (4 \({}^{\circ}\)C) storage (Table 1). Initially, the viability of the potentially probiotic lactic acid bacterial strain _L. paracasei_ SP5 was 9.5 log cfu/mL for all the initial sugar concentrations studied. At the end of fermentation (48 h), a significant decrease was observed in the cases of ISC of 61.0 and 28.8 g/L. A possible explanation of this decrease might be that the cells during the first 48 h were in a phase of adaptation to the new environment. Specifically, various factors such as antioxidant activity or other antimicrobial agents may have caused a limiting effect in the viability of _L. paracasei_ SP5 [31]. Only in the case of ISC of 40.3 g/L was this decrease not observed. It is likely, that the adjusted respective ISC led to proper levels of sugars and maybe other nutrients, that did not affect the viability of _L. paracasei_ SP5. This outcome has been reported in the past and has depended on the strain applied and the fruit juice that was to be fermented. For instance, in a relevant work conducted, the viability of _L. paracasei_ K5 dropped approximately 0.5-fold after fermentation of pomerangate juice at 30 \({}^{\circ}\)C and continued, slightly to be decreased, during its 28 days of fermentation at 4 \({}^{\circ}\)C [31]. In another similar work, the viability of _L. plantarum_ ATCC 14,917 decreased after fermentation of pomerangate juice and suddenly increased 30% at the end of the 14 th day of fermentation at 4 \({}^{\circ}\)C [32]. Likewise, the initial delay in the viability of microbial cultures is not a surprise when the sugar content is not in appropriate concentrations that can be utilized, while after the first shock the microbial culture is expected to overcome this issue. More importantly, cell viability maintained high levels throughout 3 weeks of storage (above 7.5 log cfu/mL) in all fermented juices, providing possible probiotic characteristics to the fermented juice [13]. A significant decrease was observed during the last week of storage in all the samples. However, only in the case of the fermented fruit juice with initial sugar concentration of 28.8 g/L did the viability drop at 5.1 log cfu/mL, while the other