Shelf life is more than pathogen control

A food product can be microbiologically safe and yet no longer sensorially acceptable. Conversely, it may still smell completely normal even though a health risk already exists. A reliable shelf-life study must therefore answer two different questions: Until when is the product safe? And until when does it still possess the characteristics that consumers expect?

Pathogenic microorganisms determine the non-negotiable safety limit. However, the actual usability of a food product is often limited by spoilage organisms and their metabolic products. Especially with chilled fresh poultry, Salmonella and Campylobacter are not the primary drivers of spoilage under standard refrigeration conditions. During storage, however, cold-tolerant spoilage bacteria can multiply. They break down components of the meat, producing substances such as organic acids, sulfur compounds, amines, alcohols, and ketones. As soon as these metabolic products exceed their sensory perception threshold, sour, sulfurous, fermented, or putrid odors develop.

In other words, humans do not smell a total bacterial count. They smell microbial metabolism.

This is demonstrated by the recent work of Langsrud and colleagues. The study examined industrially packaged chicken breast products from Hungary, Norway, and Portugal. The products differed in terms of packaging atmosphere, initial bacterial count, and the composition of the spoilage flora, among other factors. Storage took place both continuously at 4 °C and under a second scenario where the temperature was increased to 8 °C over time. In each country, around 120 consumers evaluated whether they would still prepare the meat or discard it based on its smell [1].

A rejection rate of 25 percent was reached at very different total bacterial counts: at 6.4 log CFU/g for the Hungarian product, 7.4 log CFU/g for the Norwegian product, and 7.3 log CFU/g for the Portuguese product.

This shows that the frequently used DGHM total bacterial count guideline of 5x10⁶ CFU/g (in many studies also 10⁷ CFU/g) can be a helpful reference value. However, it is not a universal proof that a product is sensorially spoiled. The decisive factor is not just how many microorganisms are present, but which organisms are growing, which metabolic pathways they use, and what changes they cause in the product.

Microbiome and total bacterial count

At the end of the storage period, different bacterial groups dominated in the three products examined. Despite great similarities, all products have a different microbiome. In the Hungarian product, Photobacterium was primarily detectable; in the Norwegian product, lactic acid bacteria dominated; and in the Portuguese product, organisms including Brochothrix and representatives of the Yersiniaceae were found. Spoilage flora is product-specific [1].

Not every species produces the same sensory changes at the same cell count. Even strains of the same species can differ in their spoilage potential. A total bacterial count merely summarizes these biologically very different organisms into a single number. It neither reliably predicts which organism will dominate nor what sensory effect it will have.

Furthermore, it must be taken into account that a total bacterial count does not necessarily show the entire microbial load, but only those germs that grow under the conditions of the specific test. This is shown by the findings regarding Photobacterium. In a preliminary test, this organism dominated the Hungarian product but did not grow sufficiently using the standard method on Plate Count Agar. Therefore, the researchers additionally used a more specialized culture medium. Without this supplementary investigation, a significant component of the spoilage flora would likely have been underestimated [1].

If the total bacterial count and sensory results do not align, a more detailed investigation into which microorganisms are actually dominant should be conducted. Culture-independent methods such as 16S sequencing can be a useful supplement to classical cultivation, but they do not replace it. 16S sequencing primarily provides information about the relative composition of the microbial community. It does not provide reliable absolute bacterial counts or direct evidence of which metabolic pathways are actually active. Functional methods like metatranscriptomics go further here, but are currently only useful for routine shelf-life testing in specific cases [4].

Now, not every shelf-life study necessarily needs to include sequencing or other molecular biological methods. However, the investigation method must be appropriate for the expected spoilage flora.

Quality and safety

Previous studies show that Photobacterium can be associated with the formation of various volatile metabolic products and sensory deviations. These include, among others, acetoin, diacetyl, and other odor-active compounds [3]. Metatranscriptomic studies on poultry meat also confirm that Photobacteria can be metabolically active in the product and involved in the breakdown of product components [4].

A reliable determination of shelf life should therefore evaluate safety and quality separately and then consider them together. First, the relevant pathogenic microorganisms must be assessed based on the product, process, packaging, storage temperature, and method of consumption. Sensory normalcy must never be interpreted as proof of safety.

However, a clearly defined sensory endpoint is also required. Before the study begins, it should be established which sensory changes or rejection rates are still acceptable at the end of the shelf life. Langsrud’s work suggests that microbiological thresholds should not be set as blanket values, but rather derived from a pre-determined level of consumer acceptance [1]. This does not mean abandoning microbiological assessment; instead, it links it to the target group's actual perception in a product-specific manner.

A shelf-life study that only tests for pathogens answers the question of safety, but not fully the question of product quality. A study that relies exclusively on total viable counts may identify the number of cultivable microorganisms, but it may fail to identify the specific spoilage organism or its sensory impact.

Conclusion

For quality management, this leads to a clear consequence: shelf life should not be based solely on a general total viable count. A robust assessment combines food safety, product-specific spoilage flora, and sensory acceptance.

For all decisions, however, the number and distribution of the batches tested must be of a statistically significant scale. Initial contamination levels and growth rates can fluctuate significantly between batches and seasons and should be taken into account. A currently non-peer-reviewed annual study from two European facilities suggests that the dominant spoilage community within a facility can be relatively stable, while initial microbial counts and the timing of spoilage vary [2]. Due to its preprint status, this finding should still be interpreted with caution. Nevertheless, for practical purposes, it underscores that a single batch is rarely sufficient to establish a robust shelf life.

A general limit value does not account for the specific product. A single culture medium does not capture every relevant spoilage organism. And a cell count does not automatically predict what the consumer will smell.

References

[1] Langsrud, S. et al. (2027): Using consumer acceptance to guide microbial thresholds for the shelf life of fresh chicken breasts. Food Microbiology 141, 105226. DOI:10.1016/j.fm.2026.105226. Published online 2026.

[2] Moen, B. et al. (2026): Microbial variation and shelf life of chicken breast fillets: a year-long study across two European producers. SSRN-Preprint, not yet peer-reviewed. DOI:10.2139/ssrn.6869331.

[3] Nieminen, T. T., Dalgaard, P., Björkroth, J. (2016): Volatile organic compounds and Photobacterium phosphoreum associated with spoilage of modified-atmosphere-packaged raw pork. International Journal of Food Microbiology 218, 86–95. DOI: 10.1016/j.ijfoodmicro.2015.11.003.

[4] Höll, L. et al. (2019): Prediction of in situ metabolism of photobacteria in modified atmosphere packaged poultry meat using metatranscriptomic data. Microbiological Research 222, 52–59. DOI: 10.1016/j.micres.2019.03.002.

[5] Yi, J. et al. (2025): Decoding spoilage metabolism in chicken breast: a functional microbial perspective on off-flavor formation. International Journal of Food Microbiology 442, 111367. DOI: 10.1016/j.ijfoodmicro.2025.111367.

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