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

Results and Discussions

The pH values

There were no three-way interactions for pH values with Pseudomonas, E. coli levels and time (P > 0.05). One two-way interaction (P < 0.05) with Pseudomonas inoculation levels and storage days was found. Pseudomonas inoculation levels also indicated a significant increase in pH (P < 0.05). As the results of the interaction, only the combination effects of storage days and Pseudomonas inoculation levels will be discussed. From the results in Table 2, only the treatments with 10⁶ Pseudomonas inoculation at storage day 7 was significantly different (pH, P < 0.05) from all the other treatments. Thus, the adding of 2 log or 4 log of E. coli did not change the pH of meat. Only the adding of 6 log of Pseudomonas after 7 days of storage shown an increase of pH.

Table 2. The pH values at 0, 4 , and 7 storage days after inoculation. Storage days Pseudomonas inoculation levels (cells/g of meat)   0 103 106   0 5.77 5.72 5.72 4 5.70 5.69 5.79 7 5.60 5.76 6.05* * Mean significant difference (P < 0.05) from other treatments.

Pseudomonas Isolation Agar (PIA) Counts

There were no three-way or two-way interactions with Pseudomonas counts (P > 0.05). Both of main effects of storage days and Pseudomonas inoculation levels as would be expected showed significant differences (P < 0.05) on PIA counts. Thus, as the Pseudomonas inoculation number increased, the PIA counts increased; and as the storage days increased, the PIA counts also increased (Tables 3 and 4).

Table 3. Pseudomonas isolation agar (PIA) counts* after inoculation. Pseudomonas inoculation levels (cells/g of meat)   0 103 106   PIA counts 1.09a 4.64b 6.41c *Counts are illustrated by log10 numbers, CFU/g. abcDifferent letters of superscript mean significant difference (P < 0.05).

Table 4. Pseudomonas isolation agar (PIA) counts* after inoculation during storage days.   Storage days   0 4 7 PIA counts 3.09a 3.91b 5.04c *Counts are illustrated by log10 numbers, CFU/g. abcDifferent letters of superscript mean significant difference (P<0.05).

Violet Red Bile Agar (VRBA) Counts for E. coli

There were no three-way or two-way interactions in VRBA counts (P > 0.05). Also, the storage days and Pseudomonas inoculation levels did not affect the numbers of E. coli. Only the inoculation of different E. coli levels showed significant difference in VRBA counts (P < 0.05) as would be expected. The numbers of E. coli inoculated was 100 times (2 log) different between the two inoculation levels. The data did show a significant difference between the two levels, but this difference was not a magnitude of 100 times. In fact, the VRBA counts of the higher E. coli inoculation did not indicate a tremendously large increase for the 10⁴ inoculation level compared to the 10² inoculation level, even though there was a significant difference (Table 5).

Table 5. Violet red bile agar (VRBA) counts* after inoculation.   E. coli inoculation levels (cells/g of meat)   0 102 104 VRBA counts 1.19a 3.30b 3.58c *Counts are illustrated by log10 numbers, CFU/g. abcDifferent letters of superscript mean significant difference (P<0.05).

Plate Count Agar (PCA) counts (Total plate counts)

There was no three-way interaction in PCA counts (P > 0.05). The storage time main effects did significantly (P < 0.05) influence the PCA counts as expected. Thus, the PCA counts increased as the storage days increased (Table 6).

Table 6. Total plate counts (PCA = plate count agar)* at different storage days after inoculation.   Storage days   0 4 7 PCA counts 4.43a 5.24b 6.17c *Counts are illustrated by log10 numbers, CFU/g. abcDifferent letters of superscript mean significant difference (P<0.05).

The two-way interaction of the inoculation levels of Pseudomonas and E. coli also showed a significant (P < 0.05) difference on the PCA counts. Thus, both Pseudomonas inoculation levels and E. coli inoculation levels had an effect on the PCA counts. As expected, both Pseudomonas and E. coli treatments with higher inoculation numbers had higher PCA counts than treatments with lower inoculation numbers. However, the adding of 2 log or 4 log of E. coli in the treatments with 3 log or 6 log Pseudomonas inoculation did not affect the number of PCA counts (Table 7). When there is no background Pseudomonas, the adding of E. coli resulted in higher PCA counts than the control with no bacteria inoculation as would be expected. The data also suggested that adding of E. coli (10² and 10⁴ levels) did not affect the PCA numbers when there were Pseudomonas (10³ and 10⁶ levels) as the background bacteria. The Control treatment (without any bacteria inoculation) had an average growth of 10^2.2 CFU/g. This indicated that the aseptic procedures used for obtaining meat did not result in completely sterile meat. However, this number was the average combination all the testing conducting during 7 days of storage. Thus, only minor contamination with bacteria growth was experienced.

Table 7. Total plate counts* after inoculation. E. coli inoculation levels (cells/g of meat) Pseudomonas inoculation levels (cells/g of meat)   0 103 106 0 2.22 d 5.10 b 7.03 a 102 3.97 c 5.11 b 7.09 a 104 4.43 bc 5.28 b 7.22 a *Counts are illustrated by log10 numbers, CFU/g. abcDifferent letters of superscript mean significant difference (P<0.05).