|Abstract or Summary
- Staphylococcus aureus is a human pathogen capable of producing enterotoxins causing staphylococcal food poisoning, one of the most economically important foodborne diseases in the U.S. This pathogen may be introduced to pre-cooked tuna meat used for the production of canned tuna because significant manual handling occurs after cooking of the raw fish to separate the meat from the skin and bones. This study was conducted to investigate the growth and enterotoxin production of five enterotoxigenic S. aureus strains in pre-cooked albacore and skipjack tuna meat held at 37 and 27°C, the survival of S. aureus in pre-cooked tuna meat stored at -20±2°C for 4 weeks and its ability to grow and produce enterotoxins in the pre-cooked tuna meat at 37°C after frozen storage, and thermal inactivation of enterotoxins in pre-cooked tuna meat at 111 and 121°C. Frozen pre-cooked albacore tuna (loin, chunk and flake) and skipjack tuna (chunk and flake) obtained from a canned tuna producer were inoculated with five S. aureus strains or highly purified enterotoxins A (SEA), B (SEB), and D (SED) for this study. With an initial inoculation levels of 10²⁻⁴ CFU/g, populations of S. aureus in inoculated albacore and skipjack tuna meat increased by >3 log CFU/g after 6 and 8 h of incubation at 37°C, respectively. On the other hand, more than 8 and 10 h of incubation at 27°C were needed to yield the same increase of S. aureus counts (>3 log CFU/g) in albacore and skipjack samples, respectively. However no enterotoxin was produced in any inoculated albacore or skipjack tuna samples exposed to 37°C for 12 h or 27°C for 16 h. S. aureus survived in pre-cooked tuna during frozen storage. With an initial inoculation level of 10³⁻⁴ CFU/g, populations of S. aureus in all pre-cooked tuna samples decreased slightly (<0.7 log CFU/g) after 4 weeks of storage at -20±2°C. However, the populations increased rapidly once the samples were thawed and held at 37°C. Total S. aureus counts in albacore and skipjack samples increased by greater than 3 log CFU/g after 6 and 8 h of exposure to 37°C, respectively. However, no enterotoxin was detected in any samples after 10 h exposure to 37°C. All the samples showed signs of spoilage before enterotoxin was detected. Studies of heat resistances of SEA, SEB, and SED inoculated to pre-cooked albacore and skipjack tuna meat revealed that the heat resistance of SED was stronger than SEA and SEB, particularly in albacore loin, flake and skipjack flake samples. When samples were inoculated with SEA, SEB and SED at levels of 500 ng/g, a thermal treatment at 121°C for 15 min inactivated SEA and SEB in all samples and SED in albacore chunk and skipjack chunk but not in albacore loin, flake and skipjack flake samples. However, a process at 111°C, for 60 min inactivated all SEA, SEB and SED in all tuna samples at levels of 500 ng/g. When the pre-cooked tuna meat samples were inoculated with S. aureus at levels of 10³⁻⁴ CFU/g, SEA, SEB, and SED were detected in all samples after 36 h of incubation at 37°C. However, a thermal process of 121°C for 15 min or 111°C for 40 min destroyed all enterotoxins.