Date of Graduation

7-2026

Document Type

Thesis

Degree Name

Master of Science in Food Science (MS)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Acuff, Jennifer

Committee Member

Bardsley, Cameron

Second Committee Member

Gibson, Kristen

Keywords

Antimicrobial washes; Cross-contamination; Food safety; Pecans; Post-harvest processing; Shiga toxin-producing E. coli (STEC)

Abstract

A study was done to evaluate the survival of Shiga toxin-producing Escherichia coli (STEC) on directly and soil-inoculated in-shell pecans over a 28-day period, determine STEC reductions after treatment with individual and sequential antimicrobial washes on direct and soil-inoculated in-shell pecans, and determine the cross-contamination potential of the spent pecan washes after a treatment. Pecans were directly and soil-inoculated with an STEC cocktail (O157:H7, O157:NM, O121, and O26). Direct inoculation was achieved by spraying the STEC cocktail on the pecans and homogenizing by hand for 2 min. For the soil-inoculation method, autoclaved pecan tree orchard soil was sprayed with the STEC cocktail and homogenized by hand for 2 min before adding 25 in-shell pecans and homogenizing in the same manner. Soil alone was spray-inoculated with the STEC cocktail and homogenized. Direct and soil-inoculated pecans and prepared soil inoculum samples were taken on days 0, 1, 7, and 28 and enumerated on E. coli/Coliform Petrifilm™. STEC persisted on the soil for 28 days and decreased by 0.9 ± 0.4 log CFU/g. The persistence of STEC soil-inoculated pecans was significantly higher than directly inoculated pecans during the 28 days (p< 0.0016). After 24-hr storage at ambient conditions, pecans were washed in treatments of 2% lactic acid (LA), 1000 ppm sodium hypochlorite (NaClO), hot water (HW; 85 ± 2°C), or ambient water (C; 18 ± 2°C) for 2, 5, and 10 min. Pecans were washed sequentially, starting with a pre-wash of ambient water followed by 2% lactic acid or 1000 ppm sodium hypochlorite (C+LA, C+NaClO), for 2 or 10 min. After each treatment, 100 mL spent wash sample was vacuum filtered through a 0.45-µm pore size membrane to enumerate STEC in the spent wash. Overall, five-min treatments were as effective at reducing STEC as 10-min treatments in the individual washes. HW significantly reduced STEC with or without the presence of soil and significantly reduced STEC in the spent wash regardless of treatment time (p< 0.05). No significant differences were found between ambient water control, LA, and NaClO treatments on soil-inoculated pecans (p< 0.05). The sequential wash treatments (C+LA and C+NaClO) for 2 and 10 min did not significantly reduce STEC when compared to the individual wash treatments on soil-inoculated pecans (p>0.05). Spent ambient water, after 2- and 10-min treatments with direct or soil-inoculated pecans, had a significantly higher presence of STEC than the other wash treatments LA, NaClO, HW, C+LA, or C+NaClO (p< 0.05). Antimicrobial treatments, such as LA and NaClO, are most effective at maintaining the quality of the wash water, with STEC levels being generally at or below the limit of detection (1.9 log CFU/100 mL), while HW was the most effective at reducing STEC on pecans and reducing the presence of STEC in the spent wash water at the same time (>5 log CFU/mL reductions).

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