Effect of organic acids, hydrogen peroxide and mild heat on inactivation of Escherichia coli O157:H7 on baby spinach

Minimally processed baby spinach contaminated with Escherichia coli O157:H7 has been associated with multiple outbreaks of foodborne illnesses recently. Chlorinated water is widely used to wash vegetables commercially, but this washing procedure has limited efficacy and can lead to the formation of carcinogenic substances. This study was conducted to determine the effects of organic acids and hydrogen peroxide alone and in binary combinations with or without mild heat (40 and 50°C) on the inactivation of Escherichia coli O157:H7 on baby spinach. Baby spinach leaves were dip-inoculated with E. coli O157:H7 to a level of 6log CFU/g and stored at 4°C for 24h before treatment. Individual washing solutions (1% and 2% lactic acid [LA], citric acid [CA], malic acid [MA], tartaric acid [TA], acetic acid [AA], hydrogen peroxide [H₂O₂] as well as binary combinations of LA, CA, MA and H₂O₂ at final concentrations of 1% were used to decontaminate spinach leaves at 22, 40 or 50°C for 2–5min to test their efficacy in reducing E. coli O157:H7. Chlorinated water (200ppm free chlorine) decreased the population of E. coli O157:H7 on baby spinach by only 1.2–1.6log CFU/g, which was not significantly different from DI water washing. Washing with 1% LA at 40°C for 5min was the most effective treatment achieving a 2.7log reduction of E. coli O157:H7 which is significantly higher than chlorine washing. Washing with LA+CA or LA+HP at 40°C for 5min was equally effective against E. coli O157:H7, resulting in a 2.7log reduction of E. coli O157:H7. The application of mild heat significantly enhanced the efficacy of washing solutions on the inactivation of E. coli O157:H7. There was, however, no significant difference between treatments at 40°C for 5min and 50°C for 2min. The results suggested that the use of organic acids in combination with mild heat can be a potential intervention to control E. coli O157:H7 on spinach.