Inactivation of Escherichia coli in non-intact beef steaks using radio frequency heating and its quality evaluation
Rincon, Angela Maria
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Adequate cooking is one of the most effective means to eliminate foodborne pathogens from foods including Shiga toxin-producing Escherichia coli (STEC). The efficacy of radio frequency (RF) heating on inactivation of pathogens and quality studies on meat products has been previously reported. The effect of thickness (1.2 cm, 1.9 cm and 2.5 cm) and endpoint temperature (55°C, 65°C and 75°C) on quality of non-intact beefsteaks cooked in a RF oven was evaluated. Physicochemical characteristics were mainly affected by degree of doneness; however thickness had an effect on cooking time, heating rate, cooking and drip losses. The use of antibiotic resistant microorganisms as selective markers for meat products is a common approach in studies of new interventions. Validation of nalidixic (Nal) acid for STEC and nonpathogenic Escherichia coli using RF heating in a model solution was conducted. Buffers were heated to three different endpoint temperatures (55°C, 60°C and 65°C). STEC strains O157:H7, O26:H11 and O111 as well as nonpathogenic E. coli showed no significant differences compared to parent strains either before treatment or after heating therefore validating the use of Nal acid resistance as selective marker in RF studies. Results also showed RF heating being more effective at temperatures ≥ 65°C. Thermal inactivation of STEC and nonpathogenic E. coli in packaged non-intact steaks heated up to 60°C or 65°C was also carried out. Results indicated a log reduction of 0.99, 3.08, 2.85 and 5.02 for O157:H7, O26:H11, O111 and non-pathogenic E. coli respectively at 60°C and a 5.0 log reduction at 65°C. A study for the USDA-FSIS cooking guidelines for mechanically tenderized product at 63°C holding at room temperature for five minutes was also performed for STEC strains. Validation of the protocol was effective for E. coli O157:H7 and E. coli O111 but not for E. coli O26:H11. Therefore, either adjustments to the present protocol or more heat treatment needs to be conducted for E. coli O26:H11 strains. Heating steaks to 65°C and holding at room temperature for five minutes resulted in a 5 log reduction for all strains; therefore, showing a potential validation temperature for non-intact steaks cooked with RF. The cooking protocol developed for the present study has a practical relevance for the industry since the experiments were carried on a pilot-scale RF oven and also the pathogens were tested under realistic processing conditions.