The growth and survival of Listeria monocytogenes in polony preserved with chitosan and rosemary extracts

Abstract

The ubiquitous nature of Listeria monocytogenes has caused major concerns in the processing of ready-to-eat (RTE) foods. Their ability to thrive in environments such as refrigerated conditions and high salt concentration allows L. monocytogenes to persist in RTE foods. This adaptability leads to its persistence and high resistance to synthetic preservatives used in RTE foods such as sodium nitrite and sorbic acid. These factors in the food chain act as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Therefore, this study aimed to investigate and evaluate the growth and survival of L. monocytogenes in polony models preserved with natural preservatives, Chitosan [(1, 4)-2-Amino-2-deoxy-β-D-glucan] and Rosemary (Rosmarinus officinalis L.) extract to inhibit the growth and survival of L. monocytogenes in RTE polonies. Four polony models designated P1 (control sample), P2 (chitosan sample), P3 (rosemary sample), P4 (chitosan and rosemary combination) were developed and investigated at 3 different concentration levels (0.5% g/Kg, 1.0% g/Kg and 2.0% g/Kg) and stored at 4 oC and 10 oC for preservation against coliforms (SANS 4832), yeasts and moulds (SANS 7954), E. coli (SANS 7251), S. aureus (SANS 6888) and L. monocytogenes (SANS 11290). Microbiological tests such as total viable counts (TVC) (SANS 4833), coliforms, yeasts and moulds, E. coli, S. aureus and L. monocytogenes were conducted, and all compared to the standard (control) polony model P1. These microbiological tests were also done to ensure that the models were safe for sensory analysis. The growth and survival of L. monocytogenes was observed and evaluated after injecting 103 log cfu/g of L. monocytogenes into the polony models P2, P3 and P4 preserved with chitosan and rosemary extract compared to the control model P1 preserved with salt (NaCl). The antimicrobial activity of chitosan and rosemary extract assisted with inhibition of S. aureus, E. coli and L. monocytogenes. The mean count (cfu/g) for coliforms, TVC and yeasts and moulds were within the acceptable limit and slightly increased on days 5. Polony models inoculated with 103 log cfu/g of L. monocytogenes indicated a steep decrease from P1 to P2 (p < 0.05), indicating less growth in polony models P2 – P4, thus supporting the hypothesis made in this study. For polony models inoculated with L. monocytogenes at different concentration levels indicated a significance (p < 0.05), indicating that polony models preserved with 1.0% g/Kg showed less growth compared to the models preserved at a 0.5% g/Kg and 2.0% g/Kg concentration. The data obtained for storage temperature indicated that the growth of L. monocytogenes in polony models preserved with chitosan and rosemary extract was higher (p < 0.05) at 10 oC compared to polony models stored at 4 oC. Summarising that storage temperature influence the rate of L. monocytogenes. Little to no growth were found during microbial analysis. Considering Food Safety Management systems (FSMS) polony models used for sensory evaluation were regarded as safe to consume. Sensory analysis showed that the control model P1 and chitosan model P2 were preferred by the participants. In summary, this study found that the use of natural preservatives such as chitosan and rosemary during the production of polony models assisted with the inhibition of L. monocytogenes. This provides alternatives to the synthetic preservatives and helps to promote the production and consumption of polonies preserved with natural preservatives.