Purpose: This study developed dynamic models to describe the growth pattern of Staphylococcus aureus and pathogenic Escherichia coli in lettuce, sprout, and water celery under fluctuating temperature.
Methods: Five-strain mixtures of S. aureus and E. coli were inoculated in lettuce, sprout, and water celery at 3 log CFU/g. Bacterial populations were enumerated on mannitol salt agar (S. aureus) and E. coli/Coliform count plate (E. coli) at periodic intervals during storage at 4°C (336 h), 10°C (288 h), 15°C (288 h), 25°C (96 h), and 30°C (96 h). The Baranyi model was then fitted to the S. aureus and E. coli growth data to calculate maximum specific growth rate (µmax; log CFU/g/h) and lag phase duration (λ; h). Temperature effects on the parameters were analyzed by the square root model (µmax) and a polynomial equation (λ). S. aureus and E. coli growth in vegetables were simulated under dynamic temperature. The predicted values from dynamic temperature were compared with observed values, and root mean square error (RMSE) was calculated to evaluate the model performance.
Results: E. coli and S. aureus growth were observed only on lettuce and water celery at 10 - 30°C. The µmax values of the pathogens on lettuce and water celery increased (P < 0.05) from 0.002 to 0.428 log CFU/g/h as temperature increased, and λ for the bacteria ranged 1.49 to 52.16 h, depending on product and temperature. h0 (µmax·λ) values were used to simulate E. coli and S. aureus growth under changing temperature. RMSE values were 0.3 - 0.5.
Significance: The developed models should be useful in describing E. coli and S. aureus growth patterns on lettuce and water celery under dynamic temperature.