Research and Reviews: Meat 2001

Special Circular 183-02

The Growth of Pseudomonas Flourences and Non-Pathogenic E. Coli in Aseptically Obtained Fresh Ground Beef Under 7^OC Refrigerated Storage

Materials and Methods

To non-denatured, aseptically obtained fresh lean ground beef were added two different levels of a specific background bacteria (Pseudomonas) and two different levels of a non-pathogenic E. coli. Non-denatured, aseptically obtained fresh beef was used to eliminate unknown microflora and the tissue was then inoculated with Pseudomonas as background microorganisms and E. coli as coliform to play the role of fecal contamination. Pseudomonas is about 90 % of the aerobic spoilage psychrotrophs on meat (Seymour et al., 1994) and had been reported as an inhibitor to some foodborne pathogens (Gram, 1993 and Cheng et al., 1995). However, the Pseudomonas used in those investigations were found on fish, pork or chickens and applied to agar assays. No antimicrobial effects of Pseudomonas on ground beef have been examined. In the current research, the antimicrobial effects of Pseudomonas on ground beef were examined to investigate the possible interaction with non-pathogenic E. coli. The growth of Pseudomonas and E. coli were evaluated and recorded by the total plate counts, Pseudomonas counts and total E. coli counts of meat samples on days 0, 4, and 7 utilizing 7^oC (45^oF) refrigerated temperature. The changes of pH values were also recorded. Pseudomonas levels were fixed in all the experiments to represent low to high levels (103 and 106 CFU/g) of background bacteria. Two E. coli inoculation levels were used to represent low and high levels (102 and 104 CFU/g) of fecal contaminations. The proper E. coli level will be selected in Experiment I and will be applied in Experiment II for further testing.

Objectives

1. To evaluate the competition of growth between different levels of a pre-inoculated background microorganism strain (Pseudomonas flourences) to two levels of non-pathogenic E. coli on non-denatured aseptically obtained fresh ground beef.
2. To determine the proper E. coli levels for the following experiment II.

Experimental Procedure

1. Aseptic beef tissue were obtained from beef rounds by heating the beef surface with an electric skillet [temperature of 232^oC (450^oF for 30 seconds)] to destroy the surface microflora on the meat and this was followed by the aseptic removal of the cooked exterior.
2. Aseptically obtained non-denatured beef tissue were aseptically ground and weighed, then randomly assigned to the nine different treatment groups.
3. The groups of ground beef samples were inoculated with different levels of background microorganisms and different levels of E. coli as shown in Table 1.

   Table 1. Treatment Descriptions for Experiment I. Treatment Inoculation Description A) Control Maintained in a sterile state B) E2 E. coli (102 cells/g of meat) C) E4 E. coli (104 cells/g of meat) D) P3 Pseudomonas (103 cells/g of meat) E) P3/E2 Pseudomonas (103 cells/g of meat) +E. coli (102 cells/g of meat) F) P3/E4 Pseudomonas (103 cells/g of meat) +E. coli (104 cells/g of meat) G) P6 Pseudomonas (106 cells/g of meat) H) P6/E2 Pseudomonas (106 cells/g of meat) +E. coli (102 cells/g of meat) I) P6/E4 Pseudomonas (106 cells/g of meat) +E. coli (104 cells/g of meat)

4. All nine ground beef samples were tested for pH , total plate counts, Pseudomonas counts, and total E. coli counts on day 0, 4, and 7 during storage at 7oC (45oF). Details were shown in Figure 1.
   
   Figure 1. Experimental design of Experiment I.