## 4 Discussion  
Influence of process parameters on the fermentation of stems in the presence of _L. rhamnus_ MI-0272 showed that the highest values of the tested functions (AA, TPC, and LAe) were obtained on day 9 of the process, using a plant material content of 6.40% and an initial sugar content of 3.20%. This produces a fermented plant extract that is characterised by high antioxidant activity (AA) and a high content of polyphenic compounds (TPC). Increasing the content of plant material (above 6.40%) inhibits the hydrolysis of six-carbon sugars (derived from beet molasses) to lactic acid, as a result of which their hydrolysis of sugars is observed [60]. This significantly reduces the efficiency of LA formed during fermentation. Using an optimal initial sugar concentration of 3.20%, lactic acid is obtained with a maximum efficiency of 88% (Table 4). Lactic acid efficiency values first increased during the process and then reached a maximum on day 9 or 10 of fermentation (depending on the strain used). Continuing the fermentation process resulted in a decrease in the efficiency of the lactic acid formed. Fermentation often inhibits LA production, resulting in a gradual decrease in the growth rate of lactic acid bacteria cells. Therefore, a gradual decrease in lactic acid concentration is observed as the process is prolonged. The inhibition of lactic acid is caused by the ability of the undissociated lactic acid to dissolve in the cytoplasmic membrane, while the dissociated lactate remains insoluble. This leads to the acidification of the cytoplasm and the disruption of proton motive forces. This event impacts the pH gradient across the cell membrane and reduces the cellular energy available for growth [61]. Comparing the influences of the type of strain used on the efficiency of the LA obtained, it was found that the use of _L. rhamnus_ MI-0272 as the fermentation strain allowed lactic acid to be obtained with the highest efficiency (88%). Therefore, _L. rhamnus_ MI-0272 was selected as the superior fermentation strain. The results showed that the obtained ferment from _Rhodotorula glutinis_ and _L. casei_ had the highest lactic acid content (15 g/L) [57]. Our study showed that all fermented plant extracts obtained had a higher lactic acid content (about 22 g/L) than the ferment from _Rhodotorula glutinis_ and _L. casei_[57]. When lowering the initial sugar content (from 3.20 to 1.60%), there is a slight decrease in AA values from 4.23 mmol Tx/L \(\pm\) 0.13 (on day 9 of fermentation) to 4.21 mmol Tx/L \(\pm\) 0.18 (on day 9 of fermentation). Reducing the initial sugar content from 3.20 to 1.60% with shorter process times (from 1 to 5 days) results in lower TPC values from 10.74 mmol GA/L \(\pm\) 0.03 to 8.78 mmol GA/L \(\pm\) 0.03 (when fermentation is conducted for 1 day) and from 10.97 mmol GA/L \(\pm\) 0.02 to 9.56 mmol GA/L \(\pm\) 0.01 (when fermentation is conducted for 5 days). This phenomenon is related to the fact that the starting beet molasses contains antioxidants (free phenolic acids), which increase the antioxidant activity in the fermented plant extract [62]. The results correlate with kinetic studies. At the same time, increasing the initial sugar content regardless of the fermentation time decreases the value of the studied function from 75% \(\pm\) 1 to 32% \(\pm\) 1 (Table 3). Lowering the content of plant material, regardless of fermentation time, causes the value of this function to decrease from about 1.73 mmol Tx/L \(\pm\) 0.02 to about 19.94 mmol Tx/L \(\pm\) 0.03. This means that for a plant material content of 6.40% and a fermentation time of 9 to 10 days, it is possible to achieve the highest content of phenolic compounds in FSE. With a fermentation time of 1 to 5 days (at the maximum plant material content), the value of this function gradually increases from 10.74 mmol GA/L \(\pm\) 0.03 to 10.97 mmol GA/L \(\pm\) 0.07. Lower plant material contents (from 0.32 to 3.20%) result in a decrease in TPC from 2.98 mmol GA/L \(\pm\) 0.03 to 8.62 mmol GA/L \(\pm\) 0.03.