The associated growth of Pseudomonas fluorescens, Escherichia coli and / or Lactobacillus plantarum in aseptically-prepared fresh ground beef at 7 ºC or at 4 and 25 ºC of storage
This research was conducted to understand the interactions between normal background microorganisms (Pseudomonas and Lactobacillus) and Escherichia coli on solid food such as fresh ground beef. By using aseptically-obtained fresh ground beef as a model, different levels of background bacteria along with different levels of E. coli were inoculated and applied in three experiments at different storage temperatures. In Experiment I, three levels (zero, 3 logs and 6 logs) of Pseudomonas were combined with three levels (zero, 2 logs and 4 logs) of E. coli and stored at 7 ºC for 7 days. One log increase of VRBA (E. coli) counts was observed for treatments with 2 log E. coli inoculation but no changes were found for treatments with 4 log E. coli inoculation during the 7 days storage at 7 ºC. No effects of Pseudomonas inoculation levels on E. coli counts were observed during the 7 days storage at 7 ºC. The same levels (zero, 2 logs and 4 logs) of Pseudomonas were combined with same levels (zero, 2 logs and 4 logs) of E. coli in Experiment II and stored at 4 ºC for 14 days or 25 ºC for 30 hours for a better understanding of bacteria interaction during storage which occurred in Experiment I when the product was stored at 7 ºC. The results indicated Pseudomonas was still the dominating strain in both storage temperatures (4 and 25 ºC) due to their faster growth rate and short lag time. E. coli counts decreased until Day 10 and then increased at Day 14 when stored at 4 ºC, but E. coli counts increased until Hour 25 and then decreased at Hour 30 when stored at 25 ºC. Higher Pseudomonas inoculation had higher E. coli counts at 25 ºC storage (due to metabiosis; Gram et al., 2002) but no effects of Pseudomonas on E. coli counts was observed when stored at 4 ºC. Different from Experiment II, an additional background bacterium, Lactobacillus was added to Experiment III. A total of three bacteria strains were used and more interactions were observed only with the dominating strains at both 4 ºC for 14 days and 25 ºC for 30 hours. Pseudomonas dominated at both storage temperatures. Lactobacillus counts did increase at 25 ºC storage but not at 4 ºC storage. Lower E. coli counts were observed with higher Pseudomonas inoculation treatments and E. coli counts decreased with storage time when stored at 4 ºC. E. coli counts increased with storage time when stored at 25 ºC. Both Lactobacillus and E. coli showed higher counts while Pseudomonas was co-existing when stored at 25 ºC. More reductions of E. coli by higher Pseudomonas inoculation were observed in Experiment III but not in Experiment II or I when stored at 4 ºC. This implied that the addition of Lactobacillus had a synergistic effect on reducing E. coli at 4 ºC storage. This also suggested the probability that more strains of background bacteria would result in more interactions between species and probably might have a better inhibition effects on the growth of fecal source bacteria with refrigerated storage.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:pseudomonas fluorescens escherichia coli lactobacillus plantarum ground beef
Date of Publication:01/01/2002