Sugarcane contains many valuable substances other than sugar. Could they be used to produce polylactic acid (PLA)? Paulien Harmsen, senior scientist at Wageningen Food & Biobased Research, worked on the isolation of sugars from bagasse (sugar cane residues) for fermentation into lactic acid (a C3 acid) within the C3 acids project of the BE-Basic programme.
Lactic acid is a building block of the bioplastic PLA. It is currently produced from potato starch and tapioca starch – however, this results in the loss of the potatoes and tapioca as food. A more elegant solution would be to make PLA from agricultural waste streams. This led the chemical companies Corbion and DSM to explore the suitability of bagasse as a raw material. The study took place within BE-Basic, a research programme which focuses on fundamental scientific questions related to the biobased economy.
Developing a procedure
Bagasse is traditionally burned, often to power sugar cane mills. It consists mainly of cellulose, hemicellulose and lignin, the first two of which can be converted into sugar. The challenge was to find the least expensive way to carry out this transformation, and then convert the sugar into lactic acid and eventually PLA. “The matrix of cellulose-hemicellulose-lignin make woody plants recalcitrant, so they are difficult to break down,” Harmsen explains. “We investigated how best to do this – whether or not to use hydrothermal methods, and whether to use acids or bases such as soda.”
The scientists ultimately chose soda due to the possibilities it offers in terms of recycling the waste streams. However, this resulted in other challenges, such as lower sugar yield due to the fact that much of the cellulose remained untouched. The best process they found was to first break down the lignin in bagasse using a strong base and then convert the cellulose and hemicellulose enzymatically into sugars, which in turn, via fermentation, yielded the lactic acid around which the project revolved.
Disposing of lignin
The research provided new insights into various processes. Bagasse appeared to be a recalcitrant lignocellulosic biomass, more than for example wheat straw. With sugarcane bagasse, using a base to break down the lignin was found to be the best option. “Lignin inhibits the conversion of cellulose to sugars,” Harmsen continues. “Removing it from the raw material first greatly speeds up the subsequent processing steps. However, the base still needs to be removed, however, and we have to find an application for the lignin too.”
This complexity made it clear to the scientists that the process had to be entirely optimised for each raw material. “To do this, the procedure has to be perfectly customised and completely integrated within the whole process, from sugar isolation to PLA,” Harmsen concludes. “What works well for sugar cane bagasse would not obviously translate to corn stalks.”