(177n) Modelling the Growth of Listeria in Novel Viscoelastic Biphasic Systems Rich in Fat with/without the Presence of Nisin or Nisin-Producing Lactococcus Lactis

Introduction:

To date, most studies investigating and modelling the growth of pathogenic bacteria in solid food are based on monophasic systems (e.g. agar) while food is a complex multicomponent and multiphase system¹. Although informative the results obtained from monophasic systems, they do not reflect the real situation of most food products since they lack complexity. Moreover, the increase in physicochemical and rheological complexity (e.g. by adding fat) may affect the growth of bacteria in food². Listeria monocytogenes is an important pathogen that has the ability to grow over a wide range of temperatures (2–45°C)³. At the same time, there is an increasing demand for effective decontamination of food with minimal processing techniques such as the use of natural antimicrobials. Nisin is the only bacteriocin that has been permitted to be applied in food as affirmed by the FDA and has been used to control the growth of Gram-positives including L. monocytogenes⁴. However, the presence of fat may affect the growth of L. monocytogenes during storage and may also have an impact on the efficacy of nisin and nisin-producing Lactococcus lactis in controlling the growth of L. monocytogenes. The aim of this study was to investigate the effect of varying the fat concentration in the presence of nisin and nisin-producing L. lactis on the growth of L. monocytogenes in viscoelastic systems that mimic the structure of real food.

Methodology:

L. monocytogenes (10³CFU/mL) were grown at temperatures of 10, 25 and 30^oC in biphasic Alginate-gel/oil-based viscoelastic systemswith varying concentrations of oil (0, 10, 20, 40, 60%) with or without the presence of nisin (100 IU) or nisin-producing L. lactis (10³CFU/mL) in the continuous phase. Viability of L. monocytogenes was assessed by plate counts and growth kinetics has been modelled using Baranyi-Roberts growth model⁵.Flow cytometric analysis was employed to assess the physiological status of L. monocytogenes cells by quantifying the percentage of live/injured cells while confocal fluorescent microscopy and Scanning Electron Microscopy were used to observe the growth behaviour (growth form and colony size) and spatial distribution of the cells within the viscoelastic systems. The effect of bacterial growth and activity on the stability of the viscoelastic systems was monitored by measuring viscoelasticity (Rheometer) and oil droplet size (Mastersizer) before and after incubation.

Results:

The growth rates of L. monocytogenes were reduced with increasing fat concentrations with all temperatures. Furthermore, in the presence of nisin or nisin-producing L. lactis there was a decrease in viability and an increase in percentage of dead/injured L. monocytogenes cells in the presence of nisin or nisin-producing L. lactis at higher concentrations of fat. In the viscoelastic systems L. monocytogenes grew as colonies that showed smaller size at higher fat concentrations with or without nisin or nisin-producing L. lactis. The growth of L. monocytogenes showed no effect on the structure or stability of the visco-elastic system throughout the incubation period at all temperatures.

Conclusions and relevance:

This is the first study that demonstrates the effect of fat on growth of L. monocytogenes. Our findings show that the presence of fat reduces the growth rate of L. monocytogenes but increases the efficacy of nisin and nisin-producing L. lactis in controlling the growth of L. monocytogenes. These results demonstrate the importance of taking into consideration food complexity when modelling growth of pathogens with different temperatures. Therefore, our findings are of significance to food safety and predictive microbiology in understanding the growth of pathogenic bacteria in food.

Acknowledgements:

This work was supported by the National Biofilm Innovation Centre, the Royal Society, the EPSRC and the Department of Chemical and Process Engineering of the University of Surrey.

References:

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