Reliability of Escherichia coli Counts for Vacuum-Packaged Ground Beef

Area of Science:

• Food Microbiology
• Microbial Detection Methods
• Food Safety

Background:

• Accurate detection of Escherichia coli (E. coli) is crucial for ensuring ground beef safety.
• Irradiation and aseptic processing are methods used to control microbial contamination in food products.
• Refrigerated storage and vacuum packaging extend the shelf life of ground beef while influencing microbial growth.

Purpose of the Study:

• To evaluate the reliability of different methods for detecting E. coli in irradiated and aseptically ground beef.
• To compare the efficacy of selective plating media versus the most probable number (MPN) technique for E. coli enumeration.
• To determine optimal incubation temperatures for accurate E. coli detection in vacuum-packaged ground beef.

Main Methods:

• Ground beef samples were inoculated with E. coli strains.
• Samples underwent irradiation or aseptic grinding, followed by vacuum packaging and storage at 4°C.
• E. coli enumeration was performed using selective plating media and the MPN technique, with incubations at 35°C and 45°C.

Main Results:

• Plate counts on selective media incubated at 35°C or 45°C showed high consistency during storage.
• The standard MPN technique (lauryl tryptose broth at 35°C, followed by EC broth at 45°C) was reliable.
• Direct inoculation into broths incubated solely at 45°C resulted in unreliable and variable E. coli counts.

Conclusions:

• Direct inoculation of selective plating media incubated at 45°C reliably detects E. coli in refrigerated, vacuum-packaged ground beef.
• The standard MPN technique at 45°C was reliable, but direct broth inoculation at this temperature is not recommended due to variability.
• The cause of variability in the 45°C MPN counts was not identified but was not attributed to lactic acid bacteria.