Minimum concentrations of slow pyrolysis paper and walnut hull cyclone biochars needed to inactivate Escherichia coli O157:H7 in soil

Authors: Gurtler, Joshua; Garner, Christina; Mullen, Charles; Vinyard, Bryan

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2023
Publication Date: 12/27/2023
Citation: Gurtler, J., Garner, C.M., Mullen, C.A., Vinyard, B.T. 2023. Minimum concentrations of slow pyrolysis paper and walnut hull cyclone biochars needed to inactivate Escherichia coli O157:H7 in soil. Journal of Food Protection. 87:100210. https://doi.org/10.1016/j.jfp.2023.100210.
DOI: https://doi.org/10.1016/j.jfp.2023.100210

Interpretive Summary: E. coli O157:H7 is known to contaminate crop soil and fresh produce, leading to foodborne illnesses. In the present study, 2 out of 31 biochars were effective at killing E. coli O157:H7 in soil. These were (1) slow pyrolysis paper biochar and (2) walnut hull cyclone biochar. Minimum concentrations to kill at least 5 logs of E. coli O157:H7 in soil were at least 2.5 percent of either of the biochars. The mechanism of inactivating E. coli in soil may be the alkaline nature of the two biochars, which had high pH values of 10.67 and 10.06 at 2.5 percent concentration in soil. These results may provide guidance on the application of biochar in crop soil to inactivate E. coli O157:H7, to help prevent foodborne illnesses.

Technical Abstract: Biochar has been shown to have antimicrobial properties, to varying degrees, for inactivating foodborne pathogens in soil. Previous studies have demonstrated the ability of high concentrations of biochar to reduce E. coli O157:H7 in soil and dairy manure compost, based on alkaline pH. Preliminary studies evaluating 31 different biochars determined that two slow pyrolysis biochars ([1] paper biochar, and [2] walnut hull cyclone biochar) were the most effective at inactivating E. coli in soil. A study was conducted to determine the minimum concentrations of paper and walnut hull cyclone biochar required to reduce E. coli O157:H7 in soil. A model soil was adjusted to 17.75 percent moisture and two types biochar were added at concentrations of 1.0, 1.5, 2.0, 2.5, 3.5, 4.5, 5.5, and 6.5 percent, and mixed thoroughly. Samples were inoculated with 6.84 log CFU/g of a two-strain composite of nontoxigenic E. coli O157:H7 and stored at 21 degrees Celsius for up to 6 weeks. The soil-only positive control samples supported mean E. coli populations of 6.01-6.86 log CFU/g at all weeks between 1 and 6. Populations in all soil with 1.0 and 1.5 percent of either biochar (as well as 2.0 percent walnut hull biochar), resulted in less than or equal to 0.68 log reductions at week 6, when compared with positive controls. All other concentrations (i.e., greater than 1.5 percent paper and greater than 2.0 percent walnut hull) inactivated greater than or equal to 2.7 log at all weeks between 1 and 6. Reductions of E. coli O157:H7 by 2.0 percent paper biochar at the end of 6 weeks were 2.84 log, while 2.5-6.5 percent paper biochar resulted in complete inactivation of E. coli O157:H7, by spiral plating, at weeks 5 and 6. In contrast, 2.0 percent walnut hull biochar effected only a 0.38 log reduction of E. coli O157:H7 at week 6, although 2.5-6.5 percent concentrations of walnut hull biochar resulted in complete inactivation at all weeks between 3 and 6, by spiral plating. In summary, greater than or equal to 2.5 percent paper or walnut hull biochar achieved greater than or equal to 5.0 log reduction of E. coli O157:H7 in soil, based on alkaline pH. At 2.5 percent concentration of paper or walnut hull biochar, pH measurements were 10.67 and 10.06, respectively. These results may provide guidance on the application of biochar in crop soil to inactivate E. coli O157:H7.