Freeze-dried mixed cultures as reference samples in quantitative and qualitative microbiological examinations of food

Two simulated food samples in the form of mixtures of bacteria, freeze-dried in small glass vials, were tested for their suitability as reference materials in food microbiology. Stability and homogeneity of the samples were assessed during a 1 year period when stored at 2-6 degrees C. Samples were examined for aerobic plate counts, coliform counts, Staphylococcus aureus, Clostridium perfringens, Bacillus cereus, enterococci, yeasts and salmonellas. The samples were homogeneous but the number of colony forming units of some organisms declined during the time period studied. The maximum rate of decline (0.8 log units per year) was observed for the coliforms. Precision estimates (reproducibility and repeatability) for the different testing methods used are given. When compared with these data, the rate of decline was considered to be of less importance.