Microbial Ecology of Refrigerated Chicken Soup with Naturally Fermented Food Ingredients
In this study, researchers from Eurofins Microbiology and Third Wave Bioactives collaborated to compare the microbial succession of refrigerated chicken noodle soup. The soup was produced with various cultured ingredients, marketed as having some benefit in refrigerated soup, with the purpose of better understanding how these ingredients not only prevent microbial spoilage populations, but also dive deeper into how microbial populations evolve over time.
Introduction
Earlier this year, retail line of fresh refrigerated soups had surpassed $100 million in sales in 2017, making it the first brand to reach this level in the growing refrigerated soup space1. Also, IRI, a Chicago-based market research firm, reported that the whole fresh soup category grew 18% in 2017 as just one of the several retail categories seeing tremendous growth as consumer shoppers reach for fresh and healthy convenience foods2. While some of the refrigerated soups today use High-Pressure Pasteurization or aseptic packaging to maintain quality, this is not the best option for everyone. Hence, some manufactures are turning to naturally fermented food ingredients which are considered as clean label alternatives and can help maintain the quality of their products.
Methods
Chicken noodle soup was cooked from scratch in a commercial kitchen and treatments were added during the end of the cooking process. The treatments included a no preservative control, Cultured Dextrose #1 (0.3%), Cultured Dextrose #2 (0.3%), and Nisin Preparation (400 ppm), a standardized blend of the purified
natural antimicrobial nisin and salt3. Once the treatments were sufficiently incorporated, the soup was chilled, divided into individual sample cups, and shipped overnight on ice to Third Wave Bioactives for microbial analysis. On sampling days, the entire contents of one cup from each treatment was tested in duplicate on De Man, Rogosa and Sharpe agar (MRS) for the recovery of lactic acid bacteria and Tryptic Soy agar (TSA) for enumeration of total bacteria. In addition, samples from the 10-1 dilution were pelleted via centrifugation and were stored for gDNA extraction and downstream analysis at Eurofins Microbiology. After DNA extraction, PCR was conducted to amplify the 16s rDNA genes which were further purified and sequenced on an Illumina MiSeq. Genomic analysis was conducted using standard processes.
Results and Discussion
Microbial plate counts on MRS agar (Figure 1) indicated that, by day 17, the Control sample had begun to support the outgrowth of lactic acid bacteria. The samples containing Nisin also had a spike at day 17, however, the subsequent plate counts declined to ~100 CFU/g by day 27 and remained low for the rest of the testing. Both of the Cultured Dextrose samples sustained the growth on MRS to ~100 CFU/g throughout the testing period. Unlike the microbial findings on MRS, which showed relatively few differences between treatments, there were distinct variances in the total bacteria levels found on TSA (Figure 2). Both of the Nisin and Cultured Dextrose #2 treated samples saw a rapid increase in total bacteria, growing to ~100,000,000 CFU/g by day 17, before leveling off, while these levels remained stable in the Control samples through day 13 and in the Cultured Dextrose #1 treated samples through day 17.
The genomic profiles for all four treatments were similar to each other on day 1, which would be expected, and they showed a predominance of Photobacterium (Figure 3). This psychrotrophic organism has been of increasing interest to food scientists as it has been associated with the spoilage of various refrigerated meat products 4,5,6, however, this microorganism would not have been recovered with the microbiological methods used in this study. The Photobacterium community was quickly replaced, as early as day 9, by Pseudomonas spp. in both the Nisin and Cultured Dextrose #2 samples, and by Leuconostoc spp. by day 13, in the Control samples. These microbial shifts correlate to the increase seen in total bacteria in both the Nisin and Cultured Dextrose #2 samples and the increase seen in LAB in the Control samples. Interestingly, the genomic profile in the Cultured Dextrose #1 samples do not show much change in the relative abundance of Photobacterium for the duration of the trial and found a relatively low predominance of Pseudomonas spp.. In support of this, there was little change in the level of LAB and total bacteria
recovered from the Cultured Dextrose #1 samples.
Conclusion
Psuedomonas is a common spoilage organism, so it was not surprising to see this microbe show up in the genomic profiling of the soup, however, the lack of this microbe in the control sample may be due to the outgrowth of Leuconostoc, creating enough competitive exclusion to delay or prevent the outgrowth of other microorganisms. The Nisin and both of the Cultured Dextrose treatments were selected for their ability to control LAB, which could allow an opening for additional microbes to take residence. The low levels of bacteria and the little change in microbial succession of the Cultured Dextrose #1 ingredient may suggest that it contains an additional component capable of limiting the outgrowth of Pseudomonas spp. which was found in the Nisin and Cultured Dextrose #2 samples. Cultured food ingredients such as Nisin, cultured dextrose, fermented whey, and others can be used effectively as part of a broader food quality system to ensure that consumers get safe and suitable foods. The data from this study further supports this conclusion and demonstrates that, more specifically, these products can be used in refrigerated soups to maintain quality during shelf-life. It also shows that selecting the proper ingredient is critical and the selection and use of these products can change the microbial succession within that food system. The use of genomic ecology can further enhance and support traditional plating techniques and facilitate a better understanding of how these populations are evolving. Overall, understanding how cultured products are influencing microbial populations in foods is an important step in selecting the most suitable protective ingredient for each food application.
Questions on how this information is applicable to your food process?
Additional Resources
Exploring Microbiological Control: Testing Methods for Preservative and Antimicrobial Effectiveness
Food Processing for Safety: What is High-Pressure Processing?
References
- Panera at Home Hits $100 Million in Annual Retail Sales for its Refrigerated Soups. Panera Bread March 22, 2018 https://globenewswire.com/newsrelease/2018/03/22/1444180/0/en/Panera-at-Home-Hits-100-Million-in-Annual-Retail-Sales-for-its-Refrigerated-Soups.html
- What Refrigerated Soup has to do with the Future of Grocery Stores. Maggie McGrath Forbes March 25, 2018
https://www.forbes.com/sites/maggiemcgrath/2018/03/25/what-refrigerated-soup-has-to-do-with-the-future-of-grocery-stores/#6808173c7976 - Nisin Preparation-Prepared at the 68th JECFA (2007), published in FAO JECFA Monographs 4 (2007)
http://www.fao.org/fileadmin/user_upload/jecfa_additives/docs/monograph4/additive-295-m4.pdf - Hilgrath M., Fuertes., Ehrmann M., and Vogel RF., Photobacterium carnosum sp. Nov., isolated from spoiled modified atmosphere packaged poultry meat. System Applied Microbiology, 2018 Jan;41(1):44-55
- Hilgarth M., Fuertes‐Pèrez S., Ehrmann M., and Vogel R.F. (2018). An adapted isolation procedure reveals Photobacterium spp. as common spoilers on modified atmosphere packaged meats. Letters in Applied Microbiology 66, 262–267.
- Hyldgaard M., Meyer R.L., Peng M., Hibberd A.A., Fischer J., Sigmundsson A., and Mygind T. (2015). Binary combination of epsilon-poly-L-lysine and isoeugenol affect progression of spoilage
