微生物および生化学技術ジャーナル

概要

Brominated Organic Biocides Control Lactic Acid-Producing Bacteria in Bioethanol Fermentation Matrices

Christopher L. Wiatr1*, Julie Bazzell2, Rita deCassia Bortolo Porto3

Over a million barrels of bioethanol are made every day by fermenting plant-based biomass. Since bioethanol manufacturing is not aseptic, addition of antimicrobial agents is of paramount importance to protect ethanol fraction from Lactic Acid Bacteria (LAB) that can grow in the acid environment of the process, contaminate the system, and convert ethanol to unsalable organic acids. Antibiotics are typically applied. Unfortunately, antibiotics are chemically stable and carry through the course of the process and contaminate the solids, called Dried Distiller Grains with Solubles (DDGS) that are collected post-distillation and sold as animal feed for cattle, pigs and poultry. This research evaluated the effectiveness of alternative antimicrobial methods. Quick-killing brominated biocides, 2,2-DiBromo-3-Nitrilo-PropionAmide (DBNPA) and 2-Bromo-2-Nitrilo-Propane-1,3-Diol (BNPD) were investigated as antimicrobial agents to control viable acid-producing bacteria Lactobacillus plantarum and Acetobacter cerevisiae that commonly infect bioethanol fermentation. In a pilot plant study fermenting corn to ethanol using yeast, DBNPA was found effective against these bacteria at a stepwise dose-response from 25 mg/L to 200 mg/L, with an optimal dose reaching 200 mg/L. However, BNPD was not effective at 25 mg/L, but it was effective at 100 mg/L and 200 mg/L. The organic bromicides were then advanced to field trials in a corn-to-ethanol industrial plant. DBNPA killed 3 log₁₀ LAB and nearly 3 log₁₀ total heterotrophic bacteria at a dosage of 100 mg/L, whereas BNPD had a kill that approached 2 log₁₀ for LAB and total heterotrophic bacteria at the same dosage. At a cane sugar plant, the organic bromicides at 100 mg/L were effective in cane syrup with DBNPA outperforming BNPD; however, lower dosages of both biocides were not. During the trials, antibiotics employed at typical application dosages resulted in comparatively unsatisfactory effects, decreasing LAB or total population of bacteria only one log₁₀ (vs. controls). Moreover, during biochemical tests of fermentation corn mashes infected by LAB growing 62 hours, DBNPA doses at ≥ 100 mg/L significantly reduced the final lactic acid level 14-fold, and it completely eliminated the effect of bacterial infection on ethanol yield. DBNPA had no adverse effects on the ethanol production rate, increasing the ethanol yield 2%. An addition of only 0.5% in bioethanol yield is valued at approximately a $4 million additional output at a 50 MGY plant. Furthermore, since DBNPA was previously found analytically to degrade in fermentation co-products and not reach DDGS, this microbicidally effective organic bromicide may also provide a successful alternative to antibiotics for controlling bacterial infections in fuel-ethanol production and thereby help remove antibiotics from the food chain, obviating antibiotic resistance development.