Microbiological shelf-life testing

Why are we undertaking this project?

Establishing an accurate shelf-life is key to a product’s success. The shelf-life should allow the product to be economically viable and minimise waste whilst maintaining key sensory, chemical and microbiological characteristics. It is therefore vital that the correct procedure is used when assessing shelf-life.

What are we doing as part of this project?

A working group was set up that advised and helped scope and edit the revised Guideline 46 document. This document is now published. Further areas investigated was the impact on shelf life of using various time and temperature storage regimes, open shelf life and the impact of storing in consumer fridge’s, plus the effect on shelf life if opened at various points throughout shelf life. The shelf life of ingredients can have an impact on the shelf life of a final product and this another area covered within this project. The final area of consideration is the impact of storage at increased temperatures when chilled/frozen products are delivered via non chilled courier.

Current status of the project

The project is now in its final year and has recently been focussing on open life and impact of ingredient life on shelf life of final product. A trail was carried out where consumers were given packs of cooked chicken were asked to store one opened and one unopened in their fridges. The fridge temperature was also monitored. The results of this study have shown that the Enterobacteriaceae level in most cases was slightly higher in the open packs and that fridge temperature did not seem to impact on the levels present. They also showed that the packs stored at a higher temperature did not always have the highest levels present. The Aerobic Plate Count (APC) in most of the open packs was greater than that of the closed packs; the packs stored at the higher end of the temperature range tended to have the highest levels present. The dominant microflora varied between closed and open packs. The closed packs had a mix of Lactococcus and Streptococcus present compared with the open packs where the non-pathogenic Yersinia Interrnedia and Yersinia Frederikseniae appeared to be the dominant population. There also appeared to be more Brochothrix present in open packs than closed ones. This study has shown care must be applied when interpreting the results of open life studies. There can be large differences in the types and levels of organisms present which are not always dependent on storage temperature. When designing this type of study, consideration should be given to the storage of samples in a range of incubators/fridges to allow for some of this variation.

A further study was carried out where packs of ham were opened at various points throughout shelf life and stored in 3 domestic style fridges. The results showed that open life varied depending on the point in shelf life packs are open. This should be factored into any open life shelf life tests with tests being carried out on packs at the end of life. According to the HPA (2009) for cooked sliced meat (group 5) the APC is unsatisfactory at >10⁷ cfu/g. The end of shelf life is therefore the last timepoint a level below this is observed. All unopened samples met this criteria as did all open samples. However, it was noted that there was an approximate 2 log difference between the maximum APC counts obtained for samples opened on day 0 and those opened on day 6. So for other product types this may well impact on whether product was within acceptable guidelines and shows the importance of open shelf life testing.

The shelf-life of a product will be influenced by the age of the ingredients used in its production. Ingredients at the start of their shelf-life are likely to have a much lower microbial loading than those at the end of shelf-life. As a consequence, this will impact on the potential shelf-life of the final product. Coleslaw was produced in the lab and was made from ingredients that were fresh or had been stored chilled for up to 2 days. The results showed that the levels present in onion, carrot and cabbage stored for up to two days at 5°C were higher than the levels present in fresh ingredients. The results also showed that the total aerobic count, Lactics, Enterobacteriaceae, and yeast levels were higher in coleslaw produced with aged ingredients. For coleslaw, the HPA (2009) microbiological guidance gives a total aerobic count level of >10⁷ cfu/g as being unacceptable. These results have shown that coleslaw produced with aged ingredients reach this level quicker than fresh ingredients.