Volatile fatty acids identified in this article.
Volatile fatty acids identified in this article.

Fermentation is a biological process that can convert the simple sugars from wood and other cellulosic-based feedstocks into chemical products and represents a key step in the conversion processes evaluated by NARA to convert wood residuals into biojet fuel. Gevo Inc, a NARA member, relies on specialized yeast to ferment simple sugars into isobutanol. The isobutanol is then used as a feedstock in the alcohol-to-jet fuel conversion process.

Alcohols, however, are not the only commercially valuable products generated from the fermentation process. Volatile fatty acids (VFAs) like acetic acid, propionic acid and butyric acid are produced from fermentation in specific bacteria. World demand for acetic acid has steadily increased from 5.4 million tons per year in 1997 to 10.2 million tons per year in 2010. NARA member Birgitte Ahring and her team at Washington State University’s Bioproducts, Sciences and Engineering Laboratory (BSEL) are optimizing a process called BioChemCat that uses bacteria to convert cellulosic feedstock into volatile fatty acids.

Creating the optimal conditions for yeast and bacteria fermentation is one challenge taken up by Gevo and Dr. Ahring’s group; removing the valuable products from the fermentation broth is another challenge.

In a recent paper published in the Journal of Supercritical Fluids and funded by NARA, researchers at BSEL describe a novel method used to extract volatile fatty acids from the fermentation broth.

Read Design and optimization of a semi-continuous high pressure carbon dioxide extraction system for acetic acid here.

Supercritical carbon dioxide

According to the authors, separation of organic acids, including VFAs, from the fermentation broth represents between 20% to 50% of the total capital and operating costs involved with the fermentation process. Also, these processes generally involve the use of organic solvents that are toxic to human health and hard to clean or dispose. To help reduce these costs, the authors evaluated the use of a “green” solvent such as supercritical carbon dioxide (carbon dioxide under high pressure and low temperature) to carry the VFAs away from the fermentation broth. The advantages of using supercritical carbon dioxide as opposed to other organic solvents are that supercritical carbon dioxide is non-toxic, abundant and its solvent properties can be tuned based on requirements through varying temperature and pressure conditions. A disadvantage to supercritical carbon dioxide is that it usually does not extract VFAs as well as the organic solvents.

To study the effectiveness of supercritical carbon dioxide as a solvent for VFA extraction, the authors conducted a series of extraction experiments on acetic acid that varied temperature, pressure, flow rate, initial concentration and time. Their first set of experiments tested the effectiveness of supercritical carbon dioxide to extract acetic acid from solution. Depending on the conditions, the amount of acetic acid removed from solution ranged from 10.1% to 85.6%. Higher initial concentrations of acetic acid, faster flow rate and longer time contributed to a high extraction rate.

The researchers then applied the optimized extraction parameters to acetic acid in cow rumen solution. Rumen solution contains many of the ingredients found in a typical bacterial fermentation system. It was found that the extraction rate was lower than with pure acetic acid solution because competing organic acids like propionic and butyric acids were also carried by the supercritical carbon dioxide. Extraction rates in cow rumen were most affected by time. Sixty percent of the acetic acid was extracted in one hour, whereas only 80% was extracted at the end of five hours. It is believed that the additional time allows for the nearly complete extraction of all the organic acids from the fermentation broth.

Future work

Their work expands opportunities to generate valuable bio-based products from wood residuals. The significant output from this paper is an optimized protocol for using supercritical carbon dioxide to extract acetic acid from cow rumen fluids. Additional papers from the Ahring lab that describe the production of VFA’s from fermented cellulosic feedstocks have been submitted to journals and will be published soon. In addition, alternative methods are being evaluated to extract larger VFA’s from the fermentation broth.