Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 4, 2013
Publication Date: June 2, 2013
Citation: Weimer, P.J., Digman, M.F. 2013. Fermentation of alfalfa wet-fractionation liquids to volatile fatty acids by Streptococcus bovis and Megasphaera elsdenii. Bioresource Technology. 142:88-94. Interpretive Summary: The profitability of alfalfa cultivation can potentially be improved by separating plant parts during harvesting and using each for a different purpose. Stems may be used as a biofuel feedstock, and leaves can be further treated by squeezing out a juice fraction to yield a leaf cake having a moisture content optimal for ensiling for animal feed. The juice fraction that remains is rich in sugars, starch, and protein and could add a third possible use for the separated alfalfa. Fermentation of the sugar and starch in the juice, using several added bacterial species, can yield high concentrations of organic acids that can be chemically converted to hydrocarbon fuels; the acids produced by fermentation aid in the recovery of the proteins for food and feed use. In this study, we identified and tested two bacterial species from the cow rumen that accelerate the fermentation and generate a fermentation product mixture with an improved proportion of longer-chain acids that, upon chemical treatment, will yield liquid fuel hydrocarbons. We further showed that the process is improved by separating the fermentation into two stages to optimize the fermentation conditions at each stage. This research will benefit scientists and engineers who are developing new biofuels processes.
Technical Abstract: A “green juice”, obtained by squeezing freshly harvested alfalfa leaves amended with a commercial lactic acid bacterial inoculant, was readily fermented by 7- to 21-d incubation at room temperature to obtain lactic acid at concentrations of 12-46 g l-1, along with additional acetic and succinic acids and ethanol. Inoculation of the green juice with the ruminal bacterium Streptococcus bovis JB1 and incubation at 39 degrees C shortened the fermentation time to as little as 12 h. The resulting “brown juice” from either fermentation had a pH of 4.2-4.5, and contained a protein precipitate suitable for recovery by filtration. The brown juice, upon adjustment to pH 5.2-6.8 and addition of an inoculum of the anaerobic ruminal bacterium Megasphaera elsdenii T81, was fermented by conversion of lactate and acetate to a mixture of volatile fatty acids (VFA), including propionate, butyrate, and valerate; some C4 and C5 branched-chain VFA were also produced by protein fermentation. Total VFA concentrations in some fermentations exceeded 20 g l-1, but high concentrations of lactate (>19 g l-1) required dilution of the brown juice to prevent inhibition of M. elsdenii. Attempts to conduct the fermentation of green juice to VFA in a single stage were complicated by aggressive growth and lactic acid formation by S. bovis, usually resulting in a low pH that inhibited growth of M. elsdenii before conversion to VFA could occur. Because the fermentations of both green and brown juices could be conducted without addition of any supplemental nutrients and without sterilization of the juices or fermentation vessels, these juice fermentations can potentially add value to an integrated process featuring recovery of precipitated protein and fermentation of alfalfa cell wall-containing fractions (i.e., the press cake containing leaf solids and the alfalfa stem fraction) to VFA and other products.