A new study, funded by the AMI foundation in the US, has discovered that lactic acid can prove to be a strong listeria inhibitor for food processing environments.
The study, headed by Dr Michael Doyle at the University of Georgia's Center for Food Safety, found that strains of lactic acid bacteria can inhibit the growth of Listeria monocytogenes when in biofilms for extended periods of time at refrigeration temperatures - where listeria is most likely to grow.
In the study, Doyle and his team found that the lactic acid bacteria, which included types often used as starter cultures for meat fermentation, were effective at inhibiting Listeria monocytogenes. Growth even at temperatures as low as 39 degrees F (3.8 degrees C), which is the optimal temperature for refrigerated foods.
The lactic acid bacteria isolated for this study were found in biofilms formed in floor drains from commercial processing facilities with a history of no detectable Listeria monocytogenes. A biofilm is a community of micro-organisms attached to a solid surface that protect bacteria from destruction through sanitation. These organisms would have to adapt to the environmental conditions that occur in food processing facilities including being able to grow or compete in a wide range of temperatures and forming a biofilm that would attach to equipment and drain surfaces.
The Lactococcus lactis subsp. lactis C-1-92 strain produces nisin A and nisin B, which are inhibitory to Listeria monocytogenes. The isolate did not grow at 39 degrees F but apparently produced anti-listerial metabolites at this temperature to keep Listeria monocytogenes populations on biofilms below the detectable level.
Two other isolates, Enterococci durans 141-1 and 152, also proved effective in controlling Listeria monocytogenes in biofilms. Enterococci are sometimes used as starter cultures for meat fermentations when acid production is of primary importance. The E. durans isolates can grow at refrigeration temperatures and also have antagonistic activity toward Listeria monocytogenes under refrigeration conditions. These particular strains would be useful in food processing locations that require a low temperature environment such as a ready-to-eat processing plant, the study concludes.
"Selected lactic acid bacteria isolated in this study appear to be promising candidates for control of Listeria monocytogenes in biofilms in food processing facilities," Doyle concluded.
"Reducing and ultimately eliminating Listeria on ready-to-eat meat and poultry products is one of the top priorities for the AMI Foundation and meat and poultry companies that support our research efforts," said AMIF vice president of scientific affairs Randall D. Huffman. "Dr. Doyle's research is another potential weapon in the war on pathogens."
The AMI Foundation is non-profit organisation dedicated to research, education and information projects that benefit the meat and poultry industry.