Summary
The shelf-life of high quality Australian vacuum-packed (VP) primals can exceed 16 weeks when product is stored at -0.5°C. While this is an enviable position for industry in global markets, at present, factors responsible for this extended shelf-life are not well defined. As a result, if product shelf-life falls below expectations, the specific quality control measures needed to correct this problem are not known.
In two previous MLA projects (Predictive Models for Spoilage in Vacuum Packed Primals [A.MFS.0147] and Microbial Communities in Stored Vacuum Packed Primals [A.MFS.0194], we systematically approached this problem. In the first study, collaborating with CSIRO, we modelled the growth of TVC and LAB bacteria in vacuum-packed (VP) beef primals and then compared model predictions with growth profiles for VP striploins and cube rolls produced at six Australian abattoirs. We found that while the model performed well for some abattoirs, in some cases it markedly over-predicted bacterial growth when bacterial numbers only increased 1-2 log CFU/cm2 over 16-30 weeks of storage at -0.5°C.
In the second study, we tested the hypothesis that increases in TVC and LAB on VP primals vary among abattoirs due to different types and levels of bacteria that dominate during refrigerated storage. Using culture-independent methods consisting of Terminal Restriction Length Polymorphism (TRFLP) and clone library, we found that relative proportions of particular species did vary among the abattoirs. However, the inter-abattoir variation in bacterial growth was not associated with unique bacterial species.
As a result, our attention focused on understanding the responses of specific bacterial strains to intrinsic and extrinsic factors of meat. This was done by testing a large collection of bacteria for sensitivity to pH, glucose, organic acids and low temperature, as well as the ability to inhibit other bacteria. The results showed that bacterial strains, on primals from abattoirs with uniquely lower bacterial numbers, were more sensitive to lactic acid and to low concentrations of glucose. More importantly, a higher proportion of these isolates produced inhibitory compounds against other bacteria, with the greatest effect on bacteria of the same species. As a result, these interactions may be important in limiting the overall growth of the bacterial community, resulting in long shelf-life and a higher product quality.
In summary, these studies show that responses of specific bacterial strains to intrinsic properties of VP beef likely define the dominant microbial community, producing a less diverse bacterial flora dominated by LAB. However, bacterial interactions in the early phases of storage may be a critical step in suppressing Enterobacteriacae and Pseudomonas species that would otherwise dominate due to greater resistance to low pH and lactic acid, compared to LAB.
This research benefits the meat industry by providing a more thorough understanding of microbial communities associated with extended shelf-life of VP primals, and potential strategies to maintain these desirable bacteria. Additional studies are needed to validate these findings under controlled industry conditions.
