Giovanna M. Aita, Fang Deng, Patrisha Bugayong, Saeed Oladi and Dae-Yeol Cheong

Bagasse is the solid residue that remains after the extraction of juice from crops such as sugarcane, energy cane and sorghum. It is a promising renewable resource that can be used in the production of a syrup feedstock with great potential in the processing of fuels (for example, ethanol, butanol), polymers, microbial lipids and other chemicals. Pretreatment and enzymatic or acid hydrolysis are needed processes to convert the polymeric sugars, cellulose and hemicellulose, already present in the bagasse into monomeric or fermentable sugars, mostly glucose and xylose. These sugars can then be concentrated into a syrup.

In pretreatment, bagasse is treated with chemicals to “soften” the biomass and expose its polymeric sugars. Pretreatment also reduces the lignin content, which is the component that acts as the glue that holds both cellulose and hemicellulose together and gives plants their rigidity. Alkali-based pretreatments, especially ammonia-based, have demonstrated success in removing the lignin and improving enzymatic digestibility. After pretreatment, bagasse that has been softened becomes easier for enzymes to digest. Enzymes aid in breaking down long sugar chains of cellulose and hemicellulose into monomeric sugars, glucose and xylose. This process is known as enzymatic hydrolysis and yields a liquor known as hydrolysate.

A downside to any pretreatment process is the generation of toxic or nonsugar compounds that result from the degration of cellulose, hemicellulose and lignin. These nonsugar compounds include organic acids, phenolic compounds and furaldehydes, which, if present in the hydrolysate, can alter the final quality and purity of the syrup and interfere with downstream processes such as enzymatic hydrolysis and fermentation. For example, organic acids can inhibit microbial growth and reduce fermentation yields; furaldehydes can interfere with cell replication; and lignin-derived phenolic compounds can damage cell membranes, inhibiting cell growth and enzyme activity. However, if recovered from the hydrolysate, these nonsugar compounds can have great potential as platform chemicals in the production of value-added products. For example, acetic acid is widely used in the food industry and in the production of vinyl acetate monomers, acetic anhydride and esters for use in paints, adhesives, paper coatings and textile treatments. Furaldehydes can be converted into building block chemicals used in the manufacture of alternative fuels, polymers, foams and polyesters. Lignin-derived phenolic compounds have antimicrobial, anticarcinogenic and antioxidant properties with applications in the food, pharmaceutical and cosmetic industries. Therefore, the strategy for producing syrup from bagasse or any agricultural residue should be designed not only to remove these nonsugar compounds from the hydrolysate but also recover them as potential value-added products. Some detoxification methods include evaporation, flocculation, overliming and adsorption with activated carbon, ion exchange resins and laccases. Among these methods, activated carbon is one of the most widely applied methods for removing impurities from various sources because of its low cost, high capacity of adsorption and ease of use.

Once the nonsugar compounds and any other impurities have been removed from the hydrolysate, the remaining sugars can then be concentrated into a syrup. Hydrolysates are extremely susceptible to microbial deterioration because of their high water content, and evaporation facilitates the removal of excess water. The final syrup has reduced water activity, which helps control microbial growth and preserve the fermentable sugars, thus improving the logistics associated with long-distance transportation, long-time storage and year-round supply of feedstock materials to processing industries. Once at a processing facility, the syrup can be converted through chemical or biological processes into renewable transportation fuels (ethanol, butanol, gasoline, diesel, jet fuel) and specialty chemicals (polymers, fumaric acid, dyes) for use in the chemical, food, pharmaceutical, leather and textile industries.

Giovanna M. Aita is an associate professor, Patrisha Bugayong and Dae-Yeol Cheong were postdoctoral fellows, and Fang Deng and Saeed Oladi were graduate assistants at the LSU AgCenter Audubon Sugar Institute, St. Gabriel, Louisiana.

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

Alma Plantation in Lakeland, Louisiana, is one of 11 sugar factories in the state. The sugarcane is brought to the mill from the field, where it is processed into sugar. Bagasse is the solid residue that remains after the extraction of juice from sugarcane. Photo by Olivia McClure