## 5 Conclusions  
A detailed study of the influence of technological parameters on the fermentation process of stems from _S. aromaticum_ L. made it possible to determine the technological parameters at which lactic acid contained in fermented plant extracts is formed with the highest efficiency, namely, with the highest values of AA and TPC at the same time. The results obtained on the influence of the process parameters on the fermentation process of stems in the presence of lactic acid bacteria using beet molasses as a source of six-carbon sugars show that, by conducting fermentations over 9 days using 6.40% of raw plant material and an initial sugar content of 3.20%, it is possible to obtain lactic acid with the highest efficiency while also obtaining high AA and TPC values. The fermentation of stems from _S. aromaticum_ L. in the presence of LAB strains showed modifications in bio-chemical parameters and changes in antioxidant activity, phenolic compound content, and a lactic acid yield of FSEs. Antioxidant activity (AA) increased more than six-fold from 4.12 mmol Tx/L \(\pm\) 0.11 (using process parameters consistent with FSE-1 fermentation) to 26.92 mmol Tx/L \(\pm\) 0. 22 (using process parameters consistent with FSE-17 fermentation); moreover, under these conditions, there was also a more than three-fold increase in phenolic compounds (from 1.91 mmol GA/L \(\pm\) 0.13 to 6.03 mmol GA/L \(\pm\) 0.16), which are desirable for their health benefits, as well as a more than two-fold increase in LA yield (from 40% \(\pm\) 1 to 83% \(\pm\) 1). The fermented plant extract obtained from the stems of _S. aromatic_ L. is a valuable cosmetic raw material with high antioxidant potential and is safe for dermal fibroblasts and keratinocytes. When used in the right concentration, FSE can be used to develop cosmetic products for topical application and skin health, offering an effective delivery of active ingredients with desired properties. Of the two different kinetic models used, the first-order model was particularly suitable for antioxidant activity and phenolic compounds, and the follow-up reaction model was effective in describing the time-dependent yield of lactic acid. However, further research is required to indicate the content of flavonoids that are generally reduced during fermentation in order to understand the mechanisms and metabolism of flavonoid degradation and to identify phenolic acids as metabolic products. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14114900/s1](https://www.mdpi.com/article/10.3390/app14114900/s1)]([https://www.mdpi.com/article/10.3390/app14114900/s1](https://www.mdpi.com/article/10.3390/app14114900/s1)), Figure S1. Example chromatogram of a FSE containing internal standard (octane RT = 4.731 min) and lactic acid (RT = and 5.920 min); Table S1. Kinetic values of lactic acid yield and time for maximum function during stem fermentation in the presence of LAB: FSE-1 (_L. reuteri_ MI_0168); FSE-2 (_L. salivarius_ LY_0652); FSE-3 (_L. brevis_ LY_1120); FSE-4 (_L. acidophilus_ MI-0078); FSE-5 (_L. rhamn_ MI-0272); FSE-6 (_L. plantarum_ MI-0102); Table S2. Reaction rate constants and time for maximum function of lactic acid yield changes during stem fermentation in the presence of LAB; Figure S2. The example chromatogram identifying the major constituents of the FSE: 1: eugenol (RT = 16.58 min), 2: \(\alpha\)-careophyllene (RT = 17.65 min), 3: \(\beta\)-careophyllene (RT = 18.12 min), 4: eugenyl acetate (RT = 18.78 min), and 5: carophyllene oxide (RT = 19.75 min). Conceptualization. E.K. ; methodology. E.K. and M.Z.-D.; software, R.P. ; formal analysis, E.K., M.Z.-D. and R.P. ; investigation, E.K., M.Z.-D. and R.P. ; resources, E.K. ; data curation, E.K. ; writing--original draft preparation, E.K. ; writing--review and editing, E.K. ; visualisation. E.K. ; supervision, P.B. and S.K. ; project administration, E.K. ; funding acquisition, E.K. All authors have read and agreed to the published version of the manuscript.