The USDA-NIFA, through NARA, funds research to help expand the array of commercial products produced using post harvest forest residues. Experiments to modify the lignin-rich component of wood residuals into activated carbon, epoxies, cement additives, thermoplastics and dicarboxylic acids are underway. Extensive research has also been applied towards converting the simple sugars in wood residuals into isobutanol. Alternative conversion technologies applied to the simple sugars are being explored to produce volatile fatty acids.
Lactic acid (LA) is another product that can be generated from wood-based sugars. The global demand for LA was 800,000 tons in 2013. LA can be used as a food additive and as a feedstock molecule for solvent and biodegradable container production. NARA researcher Dr. Birgitte Ahring and her team are developing methods to generate LA from cellulosic materials like crop and forest residuals. In a recently published peer-reviewed paper funded by NARA, they evaluate a fermentation system used to generate and remove LA from corn stover material.
Read Performance and stability of AmberliteTM IRA-67 ion exchange resin for product extraction and pH control during homolactic fermentation of corn sugars here.
Reducing product inhibition
Organisms like bacteria and yeast that live in a closed system (like a fermentation tank) can produce molecules that, at high concentration, become toxic. The hypothesis in this study was that high concentrations of LA would reduce bacteria’s ability to produce LA and that the AmberliteTM IRA-67 resin could be used to bind and remove the LA and thus reduce the concentration in the fermentation broth. The authors confirmed that when LA concentration in the fermentation broth exceeded 20 grams per liter, the rate of LA production was reduced dramatically. This result demonstrated a classic case of “product inhibition”.
To reduce product inhibition, the authors designed a fermentation system from which AmberliteTM IRA-67 resin could be inserted and removed. By periodically removing and cleaning the resin from the fermentation broth, the bound LA could be removed from the system and devoted to commercial use while product inhibition is avoided.
The authors found that the AmberliteTM IRA-67 resin bound LA quite effectively when attached in series with a bacterial fermentation system feeding on wet exploded corn stover hydrolysate. Eighty-five percent of the molecules that bound to the resin were LA, while the remaining 15% consisted of acetic acid, ethanol and other organic molecules. In addition, the resin was cleaned and reused 11 times over the experiment’s duration without a loss in binding efficiency for LA. After 25 days of using this system, a LA production rate of between 10 to 13 grams per liter was maintained. Through this continuous extraction of lactic acid from the fermentation broth using the resin, the bacterial productivity was found to increase by 1.3 times when compared to a regular batch fermentation process.
Establishing low cost, highly efficient biological systems to convert wood residuals into valuable chemical products will significantly enhance the economic sustainability in using post harvest forest residuals. This paper demonstrates a potential method that could be commercially viable for LA production.