Development of Survival, Growth, and Death Models for Salmonella during Non-isothermal Time-temperature Profiles in Leafy Green Supply Chain

Introduction: Salmonella is a pathogen of concern for leafy greens contamination, and has been attributed to two leafy green related outbreaks and over 100 illnesses in the US and Canada in the last six years. Most recently, organic baby spinach was recalled by several U.S. spinach producers in 2012 after safety regulators found the presence of Salmonella. 

Purpose: The objective of this study was to develop a survival, growth, and death model for Salmonella in leafy greens in order to predict the growth and decline behaviors of the pathogen under non-isothermal time-temperature profiles.

Methods: Experimental data for the growth of Salmonella under different conditions and storage temperatures were retrieved from published studies, followed by development of three-phase linear model as primary growth model and square-root model as secondary model to calculate growth rate (μ) at different temperatures. Death data was also collected for storage temperature below 5°C. A total of 35 growth and 18 death curves were available from 16 published studies. 

Results: The square-root model was fitted as μ = (0.20*(Temperature+0.57))². Mean death rate at the temperature below 5°C was 0.014 log CFU/h. These models were validated using several dynamic time-temperature profiles for the supply chain of leafy greens. Variability was taken into consideration by calculating 95% confidence intervals. Based on statistical parameters, such as accuracy factor and bias factor, and acceptable prediction zone (APZ) analysis, the model predictions were found to be within acceptable range.

Significance: The results from this study will be useful for future microbial risk analyses and predictions of behavior of Salmonella in leafy green supply chain.