Thermal Resistance of Salmonella during Brewing Related to Length of Storage of Inoculated Green Tea Leaves

Introduction: Salmonella enterica has been implicated in outbreaks linked to brewed tea.  Salmonella enterica is well known for its increased thermal resistance under low moisture conditions, although, it remains unclear if the increased resistance is a physiological response or due to inefficient heat transfer through the low moisture environment.

Purpose: The purpose of this work was to determine if length of storage of tea leaves inoculated with Salmonella resulted in increased thermal resistance during subsequent brewing.

Methods: Sterilized dried gunpowder green tea leaves (1 g) were inoculated with Salmonella enterica serovar Anatum (0.1 ml, approximately 11 log CFU/ml) , then dried to a water activity of approximately 0.2.  The thermal resistance of Salmonella was determined after inoculation of tea, after storage (25°C; ~20% RH) of tea for 24h, and after 28 days storage.  Nine ml  deionized water was added to 1 g inoculated tea leaves in Whirl-pak bags,  then immediately submerged in a circulating water bath (55°C).  At various time points, three bags were removed, and Salmonella enumerated.   Each experiment was conducted three times resulting in 9 samples for each time point.

Results: The  D₅₅-values for Salmonella in tea using freshly inoculated tea leaves,  inoculated tea leaves after 24h storage, and after 28 day storage  were 2.78 ± 0.12, 3.04 ± 0.07, 2.78 ± 0.56 min, respectively.  Although there was no statistical difference in the D-values (P > 0.05), the variability (standard deviation) of the associated D-value increased with storage.

Significance: No change in the thermal resistance of Salmonella in tea was observed related to the length of storage of the inoculated tea leaves. However, an increase in the variability did occur and could reflect attachment/detachment to tea leaves.