Detection of Salmonella in Fresh Cheese, Poultry Products, and Dried Egg Products by the ISO 6579 Salmonella Culture Procedure and the AOAC Official Method: Collaborative Study

Three food types were analyzed for the presence of Salmonella by the AOAC culture method and by the International Organization for Standardization (ISO 6579:2002) culture method. Paired test portions of each food type were simultaneously analyzed by both methods. A total of 21 laboratories representing federal government agencies and private industry, in the United States and Europe, participated in this interlaboratory study. Foods were artificially contaminated with Salmonella and competing microflora if naturally contaminated sources were not available. No statistical differences (p < 0.05) were observed between the AOAC and ISO culture methods for fresh cheese and dried egg products. A statistically significant difference was observed for one of the 2 lots of poultry from the first trial. The poultry meat used in this run was radiation sterilized, artificially contaminated with Salmonella and competitive flora, and then lyophilized. A second trial was conducted with 2 separate lots of raw ground chicken that were naturally contaminated. The results from the second trial showed no statistical difference between the 2 culture methods. A third trial involving 4 laboratories was conducted on 2 separate lots of naturally contaminated raw poultry. Again, no statistically significant differences occurred. It is recommended that ISO 6579:2002 culture method for Salmonella be adopted Official First Action for the analysis of fresh cheese, fresh chilled and frozen poultry, and dried egg products.

Enumeration of Total Aerobic Microorganisms in Foods by SimPlate® Total Plate Count–Color Indicator Methods and Conventional Culture Methods: Collaborative Study

The relative efficacy of the SimPlate® Total Plate Count–Color Indicator (TPC–CI) method (SimPlate 35°C) was compared with the AOAC Official Method 966.23 (AOAC 35°C) for enumeration of total aerobic microorganisms in foods. The SimPlate TPC–CI method, incubated at 30°C (SimPlate 30°C), was also compared with the International Organization for Standardization (ISO) 4833 method (ISO 30°C). Six food types were analyzed: ground black pepper, flour, nut meats, frozen hamburger patties, frozen fruits, and fresh vegetables. All foods tested were naturally contaminated. Nineteen laboratories throughout North America and Europe participated in the study. Three method comparisons were conducted. In general, there was <0.3 mean log count difference in recovery among the SimPlate methods and their corresponding reference methods. Mean log counts between the 2 reference methods were also very similar. Repeatability (sr) and reproducibility (sR) standard deviations were similar among the 3 method comparisons. The SimPlate method (35°C) and the AOAC method were comparable for enumerating total aerobic microorganisms in foods. Similarly, the SimPlate method (30°C) was comparable to the ISO method when samples were prepared and incubated according to the ISO method.

Growth suppression of Listeria monocytogenes in a meat product

Bologna-type sausages were manufactured using standard industry processes and four different formulations: (a) standard commercial formulation (control), (b) with 2% sodium lactate, (c) with 2% lactate and 0.25% glucono-delta-lactone (GDL), (d) with 2% lactate and 0.5% GDL. The sliced sausages were inoculated with a mixture of L. monocytogenes serotype 1 and 4 (approximately 100 cfu/g). The samples were vacuum-packaged and stored at 5 degrees C and 10 degrees C for 35 days. Rapid growth of L. monocytogenes was observed in control samples stored at 5 degrees C and in control samples and lactate treated samples (without GDL) stored at 10 degrees C. In sausages containing GDL (both levels) growth did not occur within 35 days at either 5 degrees C or 10 degrees C. Thus the presence of GDL at low level seemed sufficient to prevent growth of L. monocytogenes, although pH in the product was as high as 6.3. In samples with 2% lactate and without GDL, growth was suppressed for 28 days at 5 degrees C. Results indicate that it is possible to suppress growth of L. monocytogenes in chilled cooked meat products by using suitable amounts of sodium lactate combined with lowering of pH. However, when using GDL as a pH lowering agent, off-flavours may be registered by some individuals at high levels of GDL.

ICMSF methods studies. XIII. An international comparative study of the MPN procedure and the Anderson-Baird-Parker direct plating method for the enumeration of Escherichia coli biotype I in raw meats

The most probable number (MPN) and a direct membrane-plating (DP) method were compared for enumeration of Escherichia coli biotype I in raw meats by 11 laboratories. The DP method yielded higher counts of E. coli than the MPN method for frozen samples but neither method consistently gave higher counts for non-frozen samples. The DP method was less variable and gave a higher rate of detection of low numbers of E. coli in frozen samples. Despite the inability of the DP method to enumerate E. coli biotype II and intermediate types, which comprise only 3–5% of the Escherichia strains (Ewing 1972), the method is preferable to the MPN method for enumerating E. coli in raw meats because of its lower variability, better recovery from frozen samples, rapidity, decreased requirement for media, and decreased costs for analysts' time.