The Effect of Storage Temperature on the Outgrowth and Toxin Production of Staphylococcus aureus in Freeze-thawed Pre-cooked Tuna Meat

Introduction: Staphylococcal poisoning is caused by ingestion of enterotoxins produced in food by certain strains of Staphylococcus aureus that can be transmitted during manual handling of food. While staphylococcal poisoning has not been associated with canned tuna, potential contamination is possible when significant manual handling of pre-cooked tuna loins occurs prior to canning. While the canning process will destroy the pathogen, its enterotoxins are thermo-stable. Therefore, to control for this potential hazard, time/temperature control measures should be considered as part of a robust risk management process that also includes effective sanitation and Good Manufacturing Practices.

Purpose:  The aim of this study was to determine the time for a 3-log outgrowth and toxin production of S. aureus on previously frozen, thawed pre-cooked tuna meat (albacore or skipjack) held at either 21 or 27°C.

Methods: A five-strain cocktail of enterotoxin-producing S. aureus was inoculated with the level of ~ 3.0 log CFU/g onto commercially pre-cooked tuna samples defrosted overnight at 4°C. After 3-h incubation at 37°C, inoculated samples were individually vacuum-sealed and stored at -20 ± 2°C for ~4 weeks. Following the frozen storage, samples were thawed to the target temperature and incubated aerobically at 21 or 27°C. Growth of S. aureus in tuna was monitored using Baird-Parker plate; simultaneously, aerobic plate counts and enterotoxin production were determined.

Results: Time for a 3-log CFU/g increase was > 20 h at 21°C and 10 h at 27°C for albacore, and the toxin production was observed at 14 h at 21°C and 8 h at 27°C.  A 3-log CFU/g increase for skipjack occurred in 22 h at 21°C and 10 h at 27°C.  The toxin production in skipjack started at 20 h at 21°C and 8 h at 27°C. 

Significance: The data developed from this study can be used by the tuna industry for modeling the growth and enterotoxin production of S. aureus and to design manufacturing controls that ensure food safety.