Giovanna M. Aita and Fang Deng

Fumaric acid has been identified by the U.S. Department of Energy as one of the top 12 building block chemicals that can be potentially manufactured using renewable biomass. It can serve as an acidifying agent in food and animal feeds and as the raw material in the production of polymer resins, plasticizers, esters and inks. Currently, fumaric acid is produced from petroleum-derived maleic acid. However, economic and environmental sustainability concerns as well as consumer preference for biobased products have advocated the use of biobased manufacturing methods for renewable feedstocks in the production of fumaric acid.

Pure glucose is the preferred carbon source for fumaric acid fermentation by fungi belonging to the genus Rhizopus. However, using pure sugars as a carbon source significantly increases production costs and sacrifices their use in food supplies. Thus, efforts are underway to achieve ideal fermentation yields using biomass or agricultural residues as a carbon source, including energy cane bagasse and sugarcane bagasse, the solid residue that remains after extracting the juice from the crop stalks. The polymeric sugars (cellulose and hemicellulose) present in the biomass are not easily accessible by enzymes because of the presence of lignin, which acts as a barrier. Therefore, alkali-based pretreatment methods, such as ammonium hydroxide, are needed to disrupt the complex lignin-carbohydrate structure by decreasing the crystallinity of the cellulose and by partially removing the lignin.

Although biomass pretreatment is key in aiding with the access of these sugars (mostly glucose from cellulose and xylose from hemicellulose), the harsh conditions often associated with pretreatment result not only in the release of these sugars but also in the generation of nonsugar compounds such as organic acids, phenolic compounds and furan derivatives, which can inhibit cell growth and fermentation yields. These nonsugar compounds can be removed from the collected mixture of sugars and nonsugar compounds, or hydrolysate, leaving behind the sugars that can then be concentrated into a syrup. This syrup mostly contains glucose and xylose sugars. Furthermore, another challenge in using bagasse as a carbon source is xylose utilization, a sugar that is not readily fermented by most microorganisms as is glucose, which can result in reduced fumaric acid yields.

Fumaric acid can be produced through fungal fermentation in a two-stage process: seed culture and acid production. Optimization of this two-stage process is crucial for maximizing yields. During the seed culture stage, carbon and nutrients (for example, minerals, nitrogen and amino acids) are provided to promote optimum fungal growth; whereas, in the acid production stage, fungal growth must be limited by reducing the nitrogen content in the medium to allow the overproduction of fumaric acid. The addition of a neutralizing agent is key for maintaining an optimum pH, removing any inhibitory end products and providing the required supply of carbon dioxide for producing fumaric acid.

LSU AgCenter researech has demonstrated that syrup from dilute ammonia-pretreated energy cane bagasse can be used as a novel carbon substrate in the fermentation of fumaric acid. Thus, syrup, a renewable feedstock produced from dilute ammonia-pretreated energy cane bagasse, could be used as a substitute for pure sugars and as the carbon source for fumaric acid production.

Giovanna M. Aita is an associate professor and Fang Deng was a graduate assistant at the LSU AgCenter Audubon Sugar Institute, St. Gabriel, Louisiana.

(This article appears in the winter 2019 issue of Louisiana Agriculture.)

Read more about the research at the LSU AgCenter Audubon Sugar Institute in St. Gabriel, Louisiana.

Harvested sugarcane arrives at the Alma Plantation mill in Lakeland, Louisiana, where it will be processed into sugar. LSU AgCenter scientists at the Audubon Sugar Institute in St. Gabriel, Louisiana, are discovering ways that bagasse, a residue produced in the milling process, can be converted into valuable products to be used in the chemical and energy industries. Photo by Olivia McClure