Can the Adsorption-Desorption State Affect Salmonella Thermal Inactivation Kinetics in Low-moisture Foods?

Introduction: To comply with FSMA Preventive Controls Rules, processors of low moisture foods must establish and validate pathogen reduction steps. Validation can be accomplished in several ways, including predictive microbiology. However, predictive models must be valid for the product and process.

Purpose: The objective of this work was to evaluate the effect of adsorption-desorption hysteresis on Salmonella thermal inactivation on almonds.

Methods: Three batches of almonds (initially 0.36 a_w) were inoculated (eight log CFU/g) with Salmonella Enteritidis PT30 and subjected to one of three adsorption-desorption treatments in controlled humidity chambers (10 to 30 days): (1) equilibrated by adsorption to 0.42 a_w and 3.9% MC (moisture content, dry basis) , (2) equilibrated by adsorption to 0.62 a_w and 5.9% MC, and then equilibrated by desorption back to 0.42 a_w and 4.3% MC, or (3) equilibrated by adsorption to 0.46 a_w and 4.2% MC. This yielded two samples at the same a_w but different % MC (1 and 2), and two at the same % MC but different a_w (2 and 3). Subsequently, samples were heated in a water bath (80°C, 1 h) and sampled every 10 min. Surviving Salmonella was enumerated on modified trypticase soy agar. The survival curves were analyzed using analysis of covariance.

Results:  The inactivation rates were not significantly different (P>0.05). D-values ranged from 15-18 min, with a 0.3 min difference between treatments 1 and 2, a 2.3 min difference between treatments 1 and 3, and a 2.0 min difference between treatments 2 and 3.

Significance:  Sorption and desorption state were not a significant factor in Salmonella thermal inactivation on almonds. There were similar trends for the D-values as a function of % MC and a_w. Further research is needed to test the effect of sorption state on other products with potentially larger hysteresis and to reduce the uncertainty of the inactivation parameters.