Inactivation of E. coli O157:H7 in fresh dairy manure compost by alkaline slow-pyrolysis walnut hull biochar

Authors: Gurtler, Joshua; Mullen, Charles

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2024
Publication Date: 12/18/2024
Citation: Gurtler, J., Mullen, C.A. 2024. Inactivation of E. coli O157:H7 in fresh dairy manure compost by alkaline slow-pyrolysis walnut hull biochar. Journal of Food Protection. 88. https://doi.org/10.1016/j.jfp.2024.100438.
DOI: https://doi.org/10.1016/j.jfp.2024.100438

Interpretive Summary: Animal manure compost has been shown to harbor E. coli O157:H7, which can be transmitted to meat or fresh produce, leading to human illnesses and foodborne outbreaks. A study was conducted to determine if adding biochar to dairy manure compost could inactivate E. coli. Four different types of biochar were added to freshly made dairy manure at 10%. Results showed that one type of biochar, made from walnut hulls, was capable of inactivating over 99.999% of E. coli in dairy manure compost in only 24 hours. The antimicrobial properties of the biochar that inactivated the E. coli might be attributed to the alkaline characteristics of the char, which raised the pH to a value of 10.76 and killed the E. coli.

Technical Abstract: Biochar has been used to accelerate heating profiles during composting by increasing oxygenation, which could also reduce microbial pathogens. Nevertheless, the antimicrobial inactivation of foodborne pathogens in compost, by amending with biochar, has not been evaluated. In this study, we examined the ability of biochar to inactivate E. coli O157:H7 (EC) in fresh dairy manure compost by amending with one of four types of biochar. Two slow pyrolysis biochars (high temperature walnut hull biochar [HTWHB], walnut hull cyclone biochar [WHCB]), and two fast pyrolysis biochars were studied. Compost with 8.1 log CFU/g of EC and 10% biochar was held at 21 C and analyzed for EC weekly. Control treatments sustained ca. 8.7 log CFU of EC through week 7; however, the bacterium was not present in WHCB compost through the entire 49 days (which may be attributed to increased compost alkalinity [i.e., pH 10.76]). Populations of EC in compost supplemented with 10% of the three other biochars sustained EC populations > 9.2 log through week four. The four biochars were further tested in soil at 17% moisture to determine if concentrations as low as 3.5% could inactivate EC. When HTWHB was added to soil, populations were ca. 5 log CFU lower than when compost was amended with 10% of the same biochar. This may indicate that alkaline biochar added to lower moisture soil may be more biocidal than when added to high moisture manure compost. The current study demonstrates that highly alkaline biochar is capable of reducing up to 8 log of EC in high moisture fresh compost in only 24 h, while as little as 3.5% alkaline biochar can reduce 6.7 log of EC in only 14 days. These results may assist farmers in amending compost, manure, cattle feedlots, or soil with biochar to reduce EC, and potentially other pathogens (e.g., Salmonella enterica, Campylobacter jejuni, and Listeria monocytogenes), with the goal of reducing the dissemination of human bacterial pathogens to poultry, meat, and fresh produce.