A team of scientists from the University of Pennsylvania found that using pullulan film active packaging containing the bacteriocin sakacin A can cut or even stop growth of LM on the surface of RTE food.
LM represents a significant worldwide health problem, with RTE foods recognised as being at high risk from the potentially deadly foodborne pathogen, said the research group. They evaluated ways of controlling the growth of LM through the use of antimicrobial active packaging in a bid to extend RTE shelf-life and reduce the risk from pathogens.
In a paper published in the Journal of Food Safety, Valentina Trinetta et al used the bacteriocin sakacin A – a naturally occurring compound that can act as a biopreservative – in conjunction with the biopolymer pullulan to inhibit LM growth in turkey deli meat.
"We demonstrated the effectiveness of sakacin-A containing pullulan films to control Listeria monocytogenes growth and the applicability of active pullulan films as a means of delivering a bacteriocin directly to the food surface," said the research.
Advantages of pullulan film and sakacin
Use of pullulan film demonstrated a number of advantages over direct application of sakacin A to RTE meats. The biofilm required less antimicrobial, showed longer antimicrobial activity and allowed for controlled migration of the molecule from the film to the food, added the group.
Previous research had shown that bactreriocins such as nisin and pediocin could control LM growth in meats. Sakacin A – produced by Lactobacillus sakei DSMZ 6333 isolated from meat products – is able to inhibit LM and lactic acid.
Pullulan responds to consumer and industry demand for greener packaging. This biopolymer produces a film that is colourless, tasteless, resistant to oil, as well as exhibiting very low oxygen permeability and appropriate barrier properties, said the research authors.
In order to evaluate the effectiveness of sakacin A and pullulan film in controlling LM growth, turkey deli meat was exposed to two different types of epidemic clones – the small number of LM isolates believed to cause "most listeriosis outbreaks", said the paper.
The epidemic clones - one resistant and the other sensitive to sakacin A – were chosen to simulate best and worst case scenarios with inoculated turkey deli meat for up to three weeks at refrigerated temperatures of 4°C.
Nine LM strains representative of the four epidemic LM clones were selected. The study found that the above meat product treated by the direct addition of sakacin A – with the highest concentration at 50mg/ml – showed a reduction of between 1.5 and 2.5 log10 cfu/g. No interaction between sakacin A and the food components was apparent.
Some 800µL of sakacin A was then incorporated into the pullulan film for a final concentration of 1mg/cm².
"The formulated films reduced the population of L.monocytogenes-sensitive strains by 3 log10 cfu/g as compared with the control film, but with a lower dose of sakacin A used, when added directly," said the research.
The most resistant strain, LM J1-123, was reduced by around 1 log10 cfu/g compared to the control.
The group said its results "clearly demonstrate" that sakacin A can migrate from the biopolymer into food and inhabit the pathogen over three weeks of refrigerated storage. They described the use of packaging films containing antimicrobials to be a "promising delivery method" even if the active agent was limited to the surface of the food.
The team said more research was needed to "better understand the release kinetics of sakacin A from pullulan films and how to maximise the effectiveness on other RTE foods under long-term refrigerated storage".
Source: Journal of Food Safety
Title: Sakacin A-containing pullulan film: an active packaging system to control epidemic cones of Listeria monocytogenes in ready-to-eat foods
Authors: V. Trinetta, J. Floros, C Cutter;