## 4 Conclusions  
This study successfully demonstrates that carrot discard is a viable feedstock for the co-production of 2,3-butanediol and acetoin using non-pathogenic _Bacillus_ strains. The separate hydrolysis and fermentation (SHF) configuration proved unequivocally superior to the simultaneous saccharification and fermentation (SSF) strategy for both strains, mitigating the inhibitory effects observed. A key finding is the distinct product specialization of the two strains: _B. amyloliquefaciens_ emerged as the superior producer of 2,3-BDO, while _B. licheniformis_ showed a remarkable capacity for acetoin synthesis. Under the optimal SHF process, _B. amyloliquefaciens_ yielded 16.7 g of 2,3-BDO and 8.0 g of acetoin per 100 g of dry carrot discard. In contrast, _B. licheniformis_ produced 10.9 g of 2,3-BDO and a substantial 24.2 g of acetoin under the same conditions. Future research should focus on overcoming the limitations of the SSF process, potentially through fed-batch strategies, to alleviate the inhibition caused by high osmotic pressure and thereby develop a more integrated and cost-effective single-vessel bioprocess.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15147808/s1](https://www.mdpi.com/article/10.3390/app15147808/s1)]([https://www.mdpi.com/article/10.3390/app15147808/s1](https://www.mdpi.com/article/10.3390/app15147808/s1)), Figure S1: Fermentation kinetics (pH) in: semi-defined media of glucose by _B. licheniformis_ (**a**); semi-defined media of glucose by _B. amyloliquefaciens_ (**b**); carrot discard fermentations in SHF and SSF configurations by _B. licheniformis_ (continuous lines) and _B. amyloliquefaciens_ (dashed lines) (**c**); Figure S2: Fermentation kinetics in semi-defined media of glucose by _B. licheniformis_: lactic acid (**a**) and acetic acid (**b**); Figure S3: Fermentation kinetics in semi-defined media of glucose by _B. amyloliquefaciens_: lactic acid (**a**) and acetic acid (**b**); Figure S4: Kinetics of carrot discard fermentations in SHF and SSF configurations by _B. licheniformis_ (continuous lines) and _B. amyloliquefaciens_ (dashed lines): consumption of glucose (**a**), fructose + galactose (**b**), and arabinose (**c**); Figure S5: Kinetics of carrot discard fermentations in SHFand SSF configurations by _B. licheniformis_ (continuous lines) and _B. amyloiduefaciens_ (dashed lines): lactic acid (**a**) and acetic acid (**b**).  
Conceptualization, M.T.G.-C., M.C., S.L., and J.C.L.-L.; methodology, [PERSON]-L. and A.M.G.-G.; formal analysis, M.C. and S.L.; investigation, J.C.L.-L. and A.M.G.-G.; resources, J.C.L.-L.; writing--original draft preparation, J.C.L.-L.; writing--review and editing, M.T.G.-C. and J.C.L.-L.; supervision, M.T.G.-C., M.C., S.L., and J.C.L.-L.; project administration, M.T.G.-C. and J.C.L.-L.; funding acquisition, M.T.G.-C. and J.C.L.-L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Spanish Ministry of Science and Innovation (project PID2020-115110 RB-I00/AEI/10.13039/501100011033 and project PID2023-147967 OB-I00/MCIU/AEI/10.13039/501100011033/FEDER, EU) and by the Junta de Castilla y Leon (UIC 320).  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
[PERSON] gratefully acknowledges the research grant received from the University of Valladolid (UVa postdoctoral). The authors would like to thank the company Horcaol for supplying the carrot discards.  
The authors declare no conflicts of interest.