Investigating the impact of natural anti-microbial agents on responses of key pathogens and microbial communities in beef burgers
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Date
2025-12-01
Authors
Papadochristopoulos, Angelos
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Publisher
University College Cork
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Abstract
As demand for processed food products free from synthetic preservatives is rising, natural anti-microbials have emerged as potential alternatives to enhance safety and extend the shelf-life of processed meat products, including beef burgers. This thesis aimed to evaluate the use of natural anti-microbial agents in fresh beef burgers to enhance safety against key meatborne pathogens (Listeria monocytogenes, Escherichia coli O157) and to control spoilage bacteria during chilled vacuum packed storage.
Initially, the anti-microbial activity of several agents (n=11) was assessed in vitro by determining their minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) against multiple strains of L. monocytogenes (n=5), Shiga-toxigenic E. coli (STEC) (n=8) and lactic acid bacteria (n=2), under various growth conditions. The agents with the most promising results against L. monocytogenes and STEC were separately selected and added to beef burgers (20% fat) at varying concentrations. Burgers were inoculated with a cocktail of L. monocytogenes (n=5) or E. coli O157 (n=3) strains, vacuum packed, and stored at 3±1˚C for up to 16 days. Chitosan from both shrimp and mushroom sources exhibited the highest efficacy against both pathogens. Specifically, shrimp- and mushroom-derived chitosan (2.5%) significantly reduced (P < 0.05) L. monocytogenes by 1.21 and 1.55 log10 CFU g-1, respectively, compared to the control at the end of chilled storage. The E. coli O157 population was also significantly lower (P < 0.05) by 1.94 and 1.66 log10 CFU g-1, respectively, compared to the control after 16 days of storage. Cranberry extract (2.5%) also proved effective, significantly lowering (P < 0.05) L. monocytogenes and E. coli O157 by 0.39 and 0.72 log10 CFU g-1, respectively, compared to the control at the end of storage.
Additionally, the mode of action of mushroom-derived chitosan and cranberry extract against E. coli O157:H7 was investigated. A subinhibitory concentration (1.25%) of cranberry extract activated stress responses, including membrane, acid, and oxidative stress, while causing severe iron deficiency in the bacterial cells. Similarly, a sublethal concentration (0.063%) of mushroom chitosan likely disrupted the pathogen’s electron transport chain, resulting in oxidative stress.
Apart from its activity against pathogens, cranberry extract (2.5%) was also effective against spoilage bacteria in vacuum packed beef burgers significantly lowering (P < 0.05) counts of total viable bacteria, lactic acid bacteria, and Enterobacteriaceae by 1.82, 1.53, and 2.5 log10 CFU g-1, respectively, compared to the control at the end of chilled storage. B. thermosphacta was below the detection limit after only 8 days of storage. In contrast, chitosan (2.5%) from both sources was ineffective at reducing spoilage bacteria. Moreover, the anti-microbial agents influenced the beef burger’s microbiome; Carnobacterium sp. dominated in chitosan-treated burgers, while Lactobacillus sp. dominated in those with cranberry extract. The challenges posed by the ground meat and selected agents to the extraction of high-quality RNA, preventing metatranscriptomic analysis in vacuum packed beef burgers are also discussed.
The impact of chitosan (both sources) and cranberry extract on the quality attributes of raw beef burgers during chilled storage was also evaluated. Chitosan (2.5%), regardless of source, had no significant impact (P > 0.05) on colour (L*, a*, b*) and overall quality rating was acceptable throughout storage. However, cranberry extract (2.5%) significantly (P < 0.05) affected the burgers’ colour (L*, a*, b*), and the overall quality rating was marginally above the minimum threshold.
Overall, this thesis demonstrated that certain natural agents have potential to exhibit anti-microbial effectiveness in vacuum packed beef burgers. Key insights into the mode of action of selected agents and their impact on the burger microbiome were also gained. Further research regarding the application of natural anti-microbials in beef burgers and other food products is warranted.
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Keywords
Natural anti-microbials , Beef burgers , Shelf-life , Microbial communities , Mushroom chitosan , Listeria monocytogenes , Shiga-toxigenic Escherichia coli , Shrimp chitosan , Cranberry extract , Foodborne pathogens
Citation
Papadochristopoulos, A. 2025. Investigating the impact of natural anti-microbial agents on responses of key pathogens and microbial communities in beef burgers . PhD Thesis, University College Cork.