Listeria monocytogenes is a particular problem for cooked meats, with contamination often occurring after cooking during slicing and packing operations, according to a report in Food Microbiology.
The article, entitled “Effect of high pressure, in combination with antilisterial agents, on the growth of Listeria monocytogenes during extended storage of cooking” looks at ways in which processors can reduce the risk of contamination.
Current methods to tackle the problem include a form of cold pasteurization, known as high pressure processing (HPP) which can help extend shelf life up to 90 days.
However, the report’s authors point out the process is less than perfect, and should any listeria cells survive the process, even if injured and in low numbers, they have the potential to recover and grow during storage, posing a potential food safety risk.
During studies, it was found that the presence of a naturally occurring, pressure-resistant Weissella viridescens (W. viridescens ) strain helped limit listeria growth.
In a series of experiments the scientists used a number of different inoculations to study listeria strains in pressure-treated cooked chicken during storage of up to 105 days at a temperature of 8ºC, the temperature chosen to mimic average consumer fridge levels.
Due to commercial production processes, treatment hold time is generally kept short, which the report points out does not always guarantee a sufficient reduction in numbers of pathogens. To counter this, the scientists introduced either sodium lactate or W. viridescens into the meat prior to cooking.
The experiments showed that with the introduction of two per cent sodium lactate, the level of listeria remained below the level of detection until day 21, and throughout the remainder of the storage period, numbers remained significantly lower than untreated samples.
As a result, the report’s author’s claim W. viridescens could potentially be used as a natural biopreservative during the storage of pressure-treated cooked ready-to-eat meats for up to 30 days, while sodium lactate could extend shelf life to beyond 100 days.
Dr Margaret Patterson, one of the co-authors of the report withAideen Mackle and Mark Linton at the Agri Food & Biosciences Insitute in Belfast, said: “We started the study in response to the fact that in the US, they look for zero tolerance on Listeria, but want a shelf life of 100 days, so we were looking for technology that would give them that extra assurance.”
As a result of the work, she said manufacturers looking for a clean label solution to improving the safety of cooked meat products could use the W. viridescens agent to ensure safe storage up to 30 days.
“Generally, the UK and European producers look for 30 day shelf life so in that case W. viridescens does the job adequately , but for the 100 day storage, you need to add something such as sodium lactate.”
Patterson, M., Mackle, A., Linton, M. Effect of high pressure, in combination
with antilisterial agents, on the growth of Listeria monocytogenes during extended storage of cooked
chicken., Food Microbiology (2011), doi: 10.1016/j.fm.2011.08.006