Survival of Fecal Coliforms in Frozen Vegetable Homogenates

Area of Science:

• Food Microbiology
• Microbial Ecology
• Food Safety

Background:

• Frozen vegetables can be a source of microbial contamination.
• Fecal coliforms are often detected in food safety testing, but their survival mechanisms, especially in frozen foods, require further investigation.
• Understanding the differential survival of various bacterial groups under freezing conditions is crucial for accurate risk assessment.

Purpose of the Study:

• To compare the freezing resistance of fecal coliforms isolated from frozen vegetables with that of Escherichia coli from fecal sources.
• To investigate whether lower freezing resistance of E. coli could explain the presence of fecal coliforms without E. coli in frozen vegetable samples.

Main Methods:

• Isolation and characterization of fecal coliforms from frozen vegetables and E. coli from fecal sources.
• Assessment of bacterial survival after 200 days of storage at -10°C in various vegetable homogenates.
• Evaluation of resuscitation methods, including direct plating versus pre-enrichment on Trypticase Soy Agar followed by Violet Red Bile Agar (VRBA) overlay, to enhance bacterial recovery.

Main Results:

• Fecal coliforms isolated from frozen vegetables exhibited significantly higher survival rates (0.014% to 75%) after 200 days at -10°C compared to E. coli from fecal sources (0.49% to 18%).
• A 1-hour resuscitation step on Trypticase Soy Agar before plating on VRBA increased bacterial recovery by approximately 11-fold.
• Mixed vegetable homogenates supported higher bacterial survival rates compared to homogenates from specific vegetables like snap beans, broccoli, or mustard greens.

Conclusions:

• Fecal coliforms indigenous to frozen vegetables possess greater resistance to freezing temperatures than typical fecal E. coli.
• The enhanced resuscitation method significantly improves the detection of viable but stressed bacteria in frozen food matrices.
• These findings highlight the importance of considering bacterial source and environmental adaptation when assessing microbial safety in frozen produce.