**4. Conclusions**

This work highlighted the valorisation of agro-industrial by-products for the efficient production of ethanol by yeast fermentation. The CCD-RSM methodology was employed as a powerful statistical tool to optimise ethanol productivity while considering the specific techno-economical process constraints such as the cost of *S. cerevisiae* yeast. Experimental results suggested that the raw sugar beet molasses should be diluted between 125–250 g/L of initial sugar and supplemented with a high concentration of nutrient (4 g/L) to achieve high ethanol productivity and yield responses. In addition, the quantity of yeast present in the fermentation media was significantly reduced from 0.70 to 0.27 g/L while achieving high ethanol production performances. Assays performed in 7.5 and 100 L bioreactors suggested that the fermentation process has the potential to be implemented at pilot and industrial scales for the production of bioethanol in the context of the circular economy. In addition, a detailed techno-economic evaluation of the whole fermentation process could provide valuable information for the potential commercialization of this integrated biorefinery approach.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/fermentation7020086/s1, Figure S1: Linear regression between experimental ethanol concentrations calculated by mass loss and quantified by HPLC, Figure S2: "Predicted versus obtained" analysis of the ethanol productivity values generated through the CCD-RSM, Table S1: Fermentation performances of the confirmation experiments designed during the process optimization.

**Author Contributions:** J.-B.B.: conceptualization, methodology, investigation, formal analysis, writing—review and editing; J.M.d.M.D.: conceptualization, methodology, investigation, formal analysis, writing—review and editing; J.-M.L.: supervision, writing—review and editing, funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Conseil de Recherches en Sciences Naturelles et en Génie du Canada (RDCPJ 531896-18) and the Consortium de Recherche et Innovations en Bioprocédés Industriels au Québec (2018-003-PR-C37).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Jean Provencher and Philippe Robert (Gestion P.A.S Inc., Coaticook, QC, Canada) for proving the industrial sugar beet molasses and valuable information regarding its production. Further acknowledgements go to the Analytical Chemistry Laboratory of the Biomass Technology Laboratory for its support with various samples analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
