Evaluation of temperature and time incubation methods for the enumeration of naturally occurring indicator microorganisms in ground pork and ground beef

Indicator microorganisms are commonly used to assess the microbial quality of beef, pork and poultry products. They are important to determine product safety by testing hygienic conditions. Enumeration of mesophilic, psychrophilic and psychrotroph indicator organisms are important in the shelf life studies. Most shelf life studies primarily focus on aerobic plate counts, that are usually conducted at incubation temperatures of 35°C-37°C. However, testing the psychrotrophic bacteria at 7°C or 10 °C closely resemble real-life meat stored at refrigeration temperatures. In this study, five ground pork purchased from local grocery stores (Lubbock, Texas) and ground beef samples purchased from (Oklahoma City, Oklahoma and Dallas, Texas) were used to assess differences in enumeration of naturally occurring indicators at various time/temperature condition. Each sample was analyzed for Total plate counts (TPC), lactic acid bacteria count (LAB) and Pseudomonas counts. TPC were enumerated on Plate Count agar (PCA) and Aerobic Count PetrifilmTM (AC PetrifilmTM). LAB was tested on Brigg’s media, Man Rogosa and Sharpe (MRS) agar, and lactic acid bacteria PetrifilmTM (LAB PetrifilmTM ). Pseudomonas counts were performed on cetrimide-fucidin-cephaloridine (CFC) media. The plates and PetrifilmTM were incubated at the following treatments: 35°C/48h, 30°C/72h, 25°C/72h, 20°C/72h and 7°C/240h. LAB were additionally tested with treatment at 37°C/48h TPC. The enumeration of TPC, LAB counts, and Pseudomonas counts were performed under both aerobic and anaerobic conditions. Statistical analysis of TPC data, showed no significant difference among treatments, incubation condition, or media in either ground pork or ground beef samples (P > 0.05). However, LAB counts were statistically significant across treatments and media (P < 0.05) in both ground pork and ground beef. Brigg’s media showed significantly higher counts when compared to the MRS and LAB PetrifilmTM. Lower recovery of LAB was observed at 37°C (P < 0.05) than the other temperature and time combination in the ground pork. In ground beef, lower recovery of LAB counts was observed in MRS media compared to LAB PetrifilmTM and Brigg’s media. Incubation conditions for ground pork and ground beef samples were not statistically significant. The Pseudomonas counts showed no significant difference among treatments or incubation condition in the ground pork samples. However, at 35° C/48h in ground beef lower recovery of Pseudomonas count were observed. Correlation analysis were performed between the microorganism counts as a means to demonstrate the relationship of media used in enumeration. Correlation analysis showed a strong association with a coefficient of correlation (r = 0.91) (PCA media with AC PetrifilmTM) in ground pork and (r = 0.90) in ground beef in culture media used for the enumeration of TPC. The coefficient correlation (r = 0.86) (MRS media with LAB PetrifilmTM) in ground pork and (r = 0.87) in ground beef shows counts can be recovered equally well in both culture media used in the enumeration of LAB.

Based on this study it can be concluded for the enumeration of TPC, the choice of temperature and time, incubation conditions and media can be decided based on the bacterial growth requirements used in research work. However, in the case of the spoilage indicators LAB and Pseudomonas based on this study incubating at (25°C /72 h, 20°C/72 h, 7°C/240 h) can give better recovery since most of the spoilage bacteria are psychrotrophs. Predicting accurate shelf life of products is an important parameter for producers to determine for how long it can retain its quality and sensory attributes while reducing losses of unsaleable products.