From beet molasses to malic acid: holistic development of fermentation and downstream process

The growing demand for sustainable alternatives to fossil-based chemicals has increased interest in platform chemicals derived from renewable biomass sources, such as malic acid. This C4 dicarboxylic acid is valued for its diverse application potential in food, pharmaceuticals, and bioplastics. Sustainable platform chemicals remain commercially uncompetitive primarily due to high production costs driven by high substrate costs. Microbial production using more cost-effective feedstocks like sugar beet molasses shows promise. However, it faces challenges from high osmolality, growth inhibitors, and predetermined substrate composition during fermentation, as well as elevated pigmentation that complicates downstream processing. Moreover, the separation techniques typically used for highly polar carboxylic acids face considerable yield limitations due to the high solubility of malic acid and its salts.

This study developed an all-encompassing production process for malic acid from untreated sugar beet molasses. Fermentative malic acid production with Ustilago trichophora was investigated in batch, fed-batch, and pulsed batch in shake flask scale, followed by a scale-up into 150 L pilot scale. A total of 15.7 kg malic acid was produced in a repeated pulsed batch with membrane-based cell retention with a titer of 108 g/L, a yield of 0.50 g/g, and a space-time yield of 0.66 g/L/h (max. 1.1 g/L/h). In addition, the byproduct succinic acid was detected in concentrations of up to 22.9 g/L. In the subsequent downstream processing, activated carbons were used for two-stage product capture, solvent change, and decolorization, followed by crystallization of the products malic acid and succinic acid. Based on experimental results, an Aspen Plus model was developed to estimate the overall process yields of 0.43 g malic acid (98% purity) and 0.10 g succinic acid per gram sucrose equivalent. A techno-economic analysis suggests production costs within the range of current market prices.

Agricultural residue streams are often proposed as cost-effective alternatives for fermentative platform chemical production, although the challenges addressed hamper the direct transfer of process strategies from established organic acid production. By presenting a holistic approach explicitly tailored to malic acid production from untreated molasses, this work demonstrates the techno-economic feasibility of the developed process at a meaningful scale.