ARS | Publication request: Essential oils reduce Escherichia coli O157:H7 and Salmonella on spinach leaves without affecting produce quality

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 2011
Publication Date: March 1, 2012
Citation: Yossa, N., Patel, J.R., Millner, P.D., Lo, M. 2012. Essential oils reduce Escherichia coli O157:H7 and Salmonella on spinach leaves without affecting produce quality. Journal of Food Protection. 75(3):488-496..

Interpretive Summary: The effect of natural antimicrobials (plant extracts) in killing E. coli O157:H7 and Salmonella on spinach leaves was evaluated. Spinach leaves were inoculated with five strains cocktail of E. coli O157:H7 or Salmonella (5 log CFU/g) and air dried. Inoculated spinach leaves were immerged in cinnamaldehyde and Sporan formulations (800 ppm, 1000 ppm) alone or in combination of acetic acid for 1 min, and then stored at 4 °C. Surviving populations of these pathogens in spinach were determined during storage at day 0, 2, 7, and 14. Quality parameters (color and texture) of treated leaves were determined. E. coli O157:H7 populations were more sensitive to these oils than Salmonella on spinach leaves. Sporan alone (1000S) or in combination of acetic acid (1000SV) and 800 ppm cinnamaldehyde-Tween (800T) significantly reduced E. coli O157:H7 by more than 3 log CFU/g on spinach leaves at day 0 from initial populations (4.8 log CFU/g). Likewise, Salmonella was significantly reduced with 1000SV treatment by 2.5 log CFU/g from their initial populations. The population of background microflora was not affected by cinnamaldehyde or Sporan. The color and texture of Sporan treated spinach leaves were not significantly different from control leaves. This study demonstrates the potential use of GRAS (Generally Recognized As Safe) plant extracts in reducing E. coli O157:H7 and Salmonella on spinach leaves.

Technical Abstract: The efficacy of cinnamaldehyde and Sporan alone, or in combination with acetic acid in reducing E. coli O157:H7 and Salmonella on spinach leaves was investigated. Spinach leaves were inoculated with a cocktail of five-strain Salmonella or E. coli O157:H7, air-dried for 30 min, and then immersed in a treatment solution containing 5 ppm chlorine, cinnamaldehyde or Sporan (800 and 1000 ppm) alone or 800 ppm in combination with 200 ppm acetic acid (20%) for 1 min. Treated leaves were spin-dried and analyzed after washing, and during 14-days storage at 4°C. Inoculated leaves washed with water were used as control. The samples were spiral plated on XLT4 and SMAC agar for Salmonella and E. coli O157:H7 populations, respectively. Samples were also plated on TSA for mesophilic and psychrotrophic bacteria, on MAC for gram-negative bacteria and on DRBC for yeast and mold. Color (Hunter L, a, b color classification) and texture characteristics (force measurements) of treated leaves were analyzed. E. coli O157:H7 populations were more sensitive to the treatments than Salmonella on spinach leaves. Sporane alone (1000S) or in combination of acetic acid (1000SV) and 800 ppm cinnamaldehyde-Tween (800T) significantly reduced E. coli O157:H7 by more than 3 log CFU/g on spinach leaves at day 0 from initial populations (4.8 log CFU/g). Likewise, Salmonella was significantly reduced 1000SV by 2.5 log CFU/g from their initial populations. Pathogen populations reduced during storage at 4°C irrespective of treatment. 1000SV treatment was superior to chlorine and other treatments in reducing E. coli O157:H7 during storage. Background microflora on spinach leaves increased during storage at 4ºC, but remained lower than the control with Sporan (800S) and Sporan-vinegar (1000SV). The quality parameters (color and texture) of Sporan-treated leaves after 14 days were not significantly different from control-treated spinach leaves. Results indicated that not Sporan in combination with acetic acid could be used to reduce E. coli O157:H7 and Salmonella on organic spinach without affecting the produce quality.