Purpose: The objective was to develop a mathematical model that describes the reduction of E. coli O157:H7 as a function of chlorine level (ppm).
Methods: Iceberg lettuce pieces of 1 x 4 cm were inoculated with 4 log CFU E. coli O157:H7. Sodium hypochlorite solutions were prepared in sterilized water to obtain different concentrations of free chlorine (0, 25, 50, 100, 150, and 200 ppm). Inoculated samples were then introduced into 20-ml tubes with different chlorine concentrations and analyzed at different treatment times (0, 10, 30, 60, 150 and 300 s). The surviving E. coli O157:H7 cells were enumerated by using Sorbitol MacConkey Agar (Oxoid). Counts were log-transformed and statistical modeling and analysis was performed by using Matlab™ software (Mathwork).
Results: Results indicated that E. coli O157 was able to survive at all assayed free Cl levels, with a maximum log-decrease corresponding to 2 logarithms observed in replicates treated with 150 and 200 ppm free Cl. The greatest log-decrease rate was found in the first 30 seconds, and followed by a gradual reduction until the end of the treatment (5 min). Low levels of free Cl (0-50 ppm) yielded similar log-decrease values (0.5 log). The survival pattern was described by a two-phase log-linear model (R2>0.7) which considered the two observed survival rates. The mathematical relationship between survival rates and Cl levels could be modeled using an exponential family function.
Significance: Disinfection models for E. coli O157:H7 on lettuce are valuable tools for the validation of control measures in the fresh-cut vegetable industry and contribute to the improvement of quantitative risk assessments.