A review on the production of C4 platform chemicals from biochemical conversion of sugar crop processing products and by-products

Authors: Bruni, Gillian; Terrell, Evan

Submitted to: Fermentation
Publication Type: Literature Review
Publication Acceptance Date: 5/6/2022
Publication Date: 5/10/2022
Citation: Bruni, G.O., Terrell, E. 2022. A review on the production of C4 platform chemicals from biochemical conversion of sugar crop processing products and by-products. Fermentation. 8(5):216. https://doi.org/10.3390/fermentation8050216.
DOI: https://doi.org/10.3390/fermentation8050216

Interpretive Summary: The sugar industry produces large volumes of wastes and/or by-products, other than raw sucrose (i.e., table sugar), from the processing of sugar crops (e.g., sugarcane, sugar beet, sweet sorghum). Some of these non-sucrose products include molasses and bagasse, a woody/grassy residue that is left over from crop processing. In order to improve the efficiency and economy of raw sugar production, it is important that things like molasses and bagasse be more effectively managed and utilized. One promising route for using these by-products is through biochemical conversions, where bacteria and yeast consume the sugar crop by-products and generate valuable chemicals in return. Specifically, this review paper highlights the production of "C4" chemicals (from bacteria and yeast), which are molecules that contain four carbon atoms (specifically: acetoin; 2,3-butanediol; succinic acid; malic acid; fumaric acid). These types of C4 molecules are valuable because they can either be used directly, or can be further transformed with additional processing into a wide variety of alternative final products. Applications of these products include biofuels, biochemicals, high-value materials, and flavor/fragrance additives for foods and cosmetics. Directions and opportunities for future research and industrial biotechnology development relevant to the processing of sugar crops are also discussed.

Technical Abstract: The development and commercialization of sustainable chemicals from agricultural products and by-products is necessary for a circular economy built on renewable natural resources. Among the largest contributors to the final cost of a biomass conversion product is the cost of the initial biomass feedstock, representing a significant challenge in effective biomass utilization. Another major challenge is in identifying the correct products for development, which must be able to satisfy the need for both drop-in fossil fuel replacements and novel, high-value fine chemicals. Both challenges can be met by utilizing wastes or by-products from biomass processing, which have very limited starting cost, to yield platform chemicals. Specifically, sugar crop processing (e.g., sugarcane, sugar beet) is a mature industry that produces high volumes of by-products with significant potential for valorization. This review focuses specifically on the production of acetoin (3-hydroxybutanone), 2,3-butanediol, and C4 dicarboxylic (succinic, malic, and fumaric) acids with emphasis on biochemical conversion and targeted upgrading of sugar crop products/by-products. These C4 molecules are easily derived from fermentations and can be converted into many different final products, including food, fragrance, and cosmetic additives, as well as sustainable biofuels and other chemicals. State-of-the-art literature pertaining to optimization strategies for microbial conversion of sugar crop byproducts to C4 chemicals (e.g., bagasse, molasses) is reviewed, along with potential routes for upgrading and valorization. Directions and opportunities for future research and industrial biotechnology development are discussed.