The finding that indigenous bacteria in beef are more resistant than E. coli and Salmonella but less resistant to RF energy than Listeria, implies if a thermal process is designed to ensure destruction of indigenous bacteria or coliforms, it would not always provide an adequate degree of protection.
RF cooking heats food directly through conversion of electrical energy to heat, which occurs within the food, according to a Rowley (2001) definition.
The goal was to study the inactivation efficacy of this method of cooking against pathogens in ground meat in comparison to standard convection cooking.
Artificial inoculation
Meat balls were artificially inoculated with green fluorescent protein (GFP) expressing E.coli, Salmonella Typhimurium and Listeria monocytogenes and spores of Bacillus cereus and Bacillus thuringiensis and cooked by convection heating (220°C, 40 min).
This was done by using energy generated from frequencies in the RF bandwidth (RF cooking, 7.5 min) or by combined heating (5.5 min), until the center temperature of each sample reached 73°C.
The mean reductions in total indigenous bacteria obtained by RF and convection were 2.8 and 2.5 log CFU/g, respectively.
Cooking of meat balls with convection reduced the E. coli population (8 log CFU/g) by 5.5 log CFU/g, whilst treatment with RF reduced E. coli population to undetectable levels.
The mean reductions of S. Typhimurium obtained by RF and convection were 5.7 and 6.5 log CFU/g, respectively. The combined treatment reduced the Salmonella population to undetectable levels.
Listeria results
In contrast, L. monocytogenes was poorly affected by RF cooking.
The mean reduction of L. monocytogenes obtained by RF energy was 0.4 log CFU/g, while convection cooking resulted in undetectable levels.
Combined treatment also resulted with undetectable levels of Listeria although time of cooking was reduced by 86%.
One-step cooking had negligible effects on the Bacilluss pores and a two-step treatment of RF or convection was applied, which proved to be efficient with 4.5 log CFU/g reduction for both RF and convection.
The researchers aimed to provide a scientific basis for a comparison of RF cooking technology alone, conventional heating alone, and combination of both technologies in pathogens inactivation.
Source: International Journal of Food Microbiology
Online, ahead of print DOI: 10.1016/j.ijfoodmicro.2012.10.017
“Inactivation of foodborne pathogens in ground beef by cooking with highly controlled radio frequency energy”
Authors: Dov B. Schlisselberg, Edna Kler, Emmanuel Kalily, Guy Kisluk, Ohad Karniel, Sima Yaron