Efficacy of radio frequency cooking in the reduction of Escherichia coli and shelf stability of ground beef
Introduction
Escherichia coli O157:H7 has been identified as a major foodborne pathogen. It is responsible for haemorrhagic colitis, haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura (Doyle, 1997). The bacterium was first recognized in 1982 during an investigation of two outbreaks of bloody diarrhoea in Oregon and Michigan (Doyle et al., 2001). Since then, it has been implicated in many other outbreaks. The Center for Disease Control and Prevention has estimated that foodborne diseases caused by E. coli O157:H7 account for 62458 cases of illness, 1843 hospitalizations and 52 deaths in the US each year. Most of the cases were caused by consumption of inadequately cooked contaminated ground beef (Doyle, 1991). The most notable outbreak of 1993 was caused by undercooked hamburger containing E. coli O157:H7 and affected over 500 people and ultimately led to the death of 4 children (Ayres, 1995).
Adequate cooking is the most effective means to eliminate E. coli O157:H7 from meat products and to assure their microbiological safety. Many conventional methods of cooking, including hot air or water, steam, frying with fat or oil, and radiant heating, are applied to cook ground beef. However, such food heating methods require that food is heated externally through conduction, convection or radiation. Because of these heat transfer modes, the conventional cooking methods need longer cooking time and decrease heating uniformity to ensure that the centre is cooked while the surface may overcook (Laycock et al., 2003).
Different from conventional cooking methods, radio frequency (RF) cooking is an innovative heating technique that can save cooking time, and heat food uniformly (Rowley, 2001). RF cooking is a technique based on electro-technologies, such as ohmic heating, or microwave dielectric heating (Piyasena et al., 2003). Unlike other heat transfer modes, RF cooking heats foods directly through energy conversion, from electrical energy to heat (Rowley, 2001). The energy conversion occurs within the food itself, and heat is absorbed directly by the food. Therefore, RF cooking achieves high-energy efficiency and uniform cooking (Orsat et al., 2001).
The application of RF cooking in the food industry can be traced back to the 1940s. RF cooking was initially used to cook meat, bake bread, and dehydrate or blanch vegetables. Later, RF cooking was used to thaw frozen products in the 1960s and to dry cookies in the 1980s. Recently, RF cooking has been evaluated for sterilization and pasteurization of meat products (Piyasena et al., 2003).
RF cooking can be used to reduce high levels of microbial contamination and improve food quality. The effectiveness of RF cooking on the reduction of microbial contamination of fresh carrots (Orsat et al., 2001) and sausage (Houben et al., 1991) has been investigated. Its effect on food quality has also been studied on ground beef (Laycock et al., 2003). They found that RF cooking led to shorter cooking time, lower juice losses, and acceptable colour and texture. However, little research has been reported in the reduction of microbial contamination of ground beef by RF cooking. Furthermore, there is limited information on the shelf-life of RF cooked products.
Thus, the objective of this study was to investigate the effectiveness of RF cooking on the inactivation of E. coli and its effect on the shelf stability of cooked ground beef. The non-pathogenic E. coli K12 strain, possessing the ampicillin resistance, was used as a target bacterium, as RF cooking may not have selectivity against bacterial strains within a species. In addition, the use of E. coli K12 requires less stringent controlled working conditions as compared to the use of E. coli O157:H7 which requires a laboratory licensed for use of human level 2 pathogens.
Section snippets
Equipment
The RF equipment used for this study was similar to the one used by Laycock et al. (2003). A RF heater of 1.5 kW at a frequency of 27.12 MHz (Strayfield Ltd, Reading, UK) was used. The heater included two parts: an RF generator and an applicator circuit. The applicator contained two electrodes, tuning capacitor plates, an inductance coil and an applicator cylinder. The food samples in the cylinder can be dielectrically heated between the two electrodes. The RF power was adjusted by changing the
Relationship between the OD600 values and E. coli K12 counts
The relationship between the OD600 values and E. coli K12 counts were determined using linear regression (Fig. 2). The OD600 values from 0.01 to 0.1 exhibited a linear relationship with the E. coli K12 counts.
Comparison of cooking methods
The average cooking time was from 4.25 to 150.33 min for the central temperature of samples to reach 72 °C, depending on cooking methods (Table 1). There was a significant difference () among cooking methods with respect to the cooking times. The time for RF cooking (4.25±0.25 min) was
Conclusions
RF cooking showed significant effects on reducing E. coli, thus maintaining shelf-life and saving cooking time. E. coli was almost eliminated by RF cooking and the shelf-life could be maintained to 30 days. Although both RF and water-bath cookings had a similar effect on reducing E. coli counts and maintaining of shelf-life, RF cooking significantly reduced cooking time and temperature variation, that implies more uniform heating. RF cooking has high potential as a substitute for hot ware bath
Acknowledgments
Authors appreciate the support and suggestions from Mr. Mike Cottrill, Ms. Cheryl Deflice, Dr. Hai Yu and Mr. Xianhua Yin of Food Research Program, Agriculture & Agri-Food Canada during the experiments of this study.
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