BIOTECHNOLOGICAL PRODUCTS AND PROCESS ENGINEERING Propionic acid production by Propionibacterium freudenreichii using sweet sorghum bagasse hydrolysate Ehab M. Ammar 1,2 & Jessica Martin 3 & Luiza Brabo-Catala 1 & George P. Philippidis 1 Received: 13 April 2020 /Revised: 15 September 2020 /Accepted: 7 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Propionic acid, a widely used food preservative and intermediate in the manufacture of various chemicals, is currently produced from petroleum-based chemicals, raising concerns about its long-term sustainability. A key way to make propionic acid more sustainable is through fermentation of low-cost renewable and inedible sugar sources, such as lignocellulosic biomass. To this end, we utilized the cellulosic hydrolysate of sweet sorghum bagasse (SSB), a residue from a promising biomass source that can be cultivated around the world, for fermentative propionic acid production using Propionibacterium freudenreichii. In serum bottles, SSB hydrolysate supported a higher propionic acid yield than glucose (0.51 vs. 0.44 g/g, respectively), which can be attributed to the presence of additional nutrients in the hydrolysate enhancing propionic acid biosynthesis and the pH buffering capacity of the hydrolysate. Additionally, SSB hydrolysate supported better cell growth kinetics and higher tolerance to product inhibition by P. freudenreichii. The yield was further improved by co-fermenting glycerol, a renewable byproduct of the biodiesel industry, reaching up to 0.59 g/g, whereas volumetric productivity was enhanced by running the fermentation with high cell density inoculum. In the bioreactor, although the yield was slightly lower than in serum bottles (0.45 g/g), higher final concentration and overall productivity of propionic acid were achieved. Compared to glucose (this study) and hydrolysates from other biomass species (literature), use of SSB hydrolysate as a renewable glucose source resulted in comparable or even higher propionic acid yields. Key points • Propionic acid yield and cell growth were higher in SSB hydrolysate than glucose. • The yield was enhanced by co-fermenting SSB hydrolysate and glycerol. • The productivity was enhanced under high cell density fermentation conditions. • SSB hydrolysate is equivalent or superior to other reported hydrolysates. Keywords Propionic acid . Propionibacteria . Sweet sorghum bagasse hydrolysate . Lignocellulosic biomass . Glycerol . Fermentation Introduction Propionic acid is commonly used in the food industry as pre- servative and across a wide range of industries as intermediate in the manufacture of polymers, pesticides, perfumes, and pharmaceuticals (Vidra and Németh 2018). The common way to produce propionic acid is via non-sustainable petro- chemical routes. However, as societal concerns about green- house gas emissions and sustainability intensify, it behooves the chemical industry to transition from traditional fossil re- sources to renewable ones (Bozell 2008). Propionibacteria are gram-positive facultative anaerobic bacteria that have been granted GRAS (“generally recognized as safe”) status by the US Food and Drug Administration (Hettinga and Reinbold 1972). They are widely used in the fermentative production of propionic acid (Liu et al. 2012a) and vitamin B12 (Hedayati et al. 2020; Kośmider et al. 2012; Wang et al. 2015a) and in Swiss cheese manufacture for flavor * George P. Philippidis gphilippidis@usf.edu 1 Patel College of Global Sustainability, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL 33620, USA 2 Genetic Engineering and Biotechnology Research Institute, University of Sadat City, El-, Sadat City, Egypt 3 Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL 33620, USA Applied Microbiology and Biotechnology https://doi.org/10.1007/s00253-020-10953-w