Predicting the growth of Pseudomonas in minced beef

Predictive microbiology offers an alternative to traditional microbiological methods of assessing the quality and safety of foods. The development of mathematical models enables prediction of the rate of growth (or decline) of microorganisms under a given set of environmental conditions. The spoilage of minced beef at refrigeration temperatures is mainly due to Pseudomonas spp. Strains of Pseudomonas isolated from minced beef purchased from retail outlets in Tasmania were studied together with strains isolated from pork in Victoria. Growth rates of all strains were compared at -10°C and the fastest growing strains selected for further study to represent the 'worst-case' scenario. The growth characteristics of these strains were modelled in nutrient broth in the temperature range -0° to 30°C, using turbidimetric measurements. Data were analysed and fitted to square root - type models (Ratkowsky et al. 1982). Models were calibrated by comparing the turbidimetric data with data obtained by viable counting, the latter being the standard method of enumerating microorganisms in foods. The relationship between growth rates obtained from turbidimetric methods and those obtained from viable counting was found to be constant, therefore models constructed by turbidimetry were calibrated using a simple factor. Models developed in the laboratory were validated in minced beef. Mince from retail outlets had initial levels of pseudomonads ranging from 10⁵ to 10⁷ cfu/g. These levels were too high to enable the true growth rates to be monitored. To achieve the desired starting level (10⁴ cfu/g or less) mince, aseptically prepared in the laboratory, was inoculated with individual strains or blended with retail products. Evaluation of the model, using the bias and precision indices (Ross 1993a), revealed a difference of less than 10% on average between predicted and observed values. At temperatures below 10°C the observed rates of pseudomonad proliferation in meat were as predicted by the model in broth. However, above 10°C rates were slower than predicted indicating that some other factor may be involved. No lag phase was detected when sterile meat was inoculated with pseudomonad broth suspension pre-incubated at 25°C for 48 h, or with minced beef purchased from a local butcher. The effect of pH on the rate of growth of several Pseudomonas strains was also examined, using lactic acid as the acidulant. The strains grew at similar rate between pH 5.4 and 8.6, with no growth below 5.0. As the initial pH of fresh meat is generally -5.5 - 6, pH will not be a critical factor affecting the growth rate of pseudomonads. At refrigeration temperatures, the fastest growing Pseudomonas strains will dominate the meat microbiota and outcompete all the other organisms. To examine if the growth of other organisms can inhibit pseudomonads, Pseudomonas strains were grown (-4 - 20°C) in mixed cultures with microorganisms isolated from mince. Isolates tested included Enterobacteriaceae, -Lactobacillus spp., Brochothrix thennosphacta, and Staphylococcus aureus. No interactions were evident as Pseudomonas grew at the rate predicted in a cocktail containing the above organisms