Rapid detection and characterization of postpasteurization contaminants in pasteurized fluid milk

Area of Science:

• Food Microbiology
• Dairy Science
• Microbial Detection

Background:

• Microbial spoilage in pasteurized fluid milk is primarily caused by post-pasteurization contamination (PPC) with psychrotolerant gram-negative bacteria or sporeformers surviving pasteurization.
• Current methods for detecting PPC in fluid milk are often time-consuming, hindering efforts to improve milk quality.
• The need for rapid, sensitive, and specific detection methods for PPC is critical for the dairy industry.

Purpose of the Study:

• To evaluate different rapid detection approaches for post-pasteurization contamination (PPC) in commercial fluid milk samples.
• To compare the efficacy of various enrichment protocols and plating media for identifying spoilage bacteria.
• To determine the most sensitive and accelerated method for detecting PPC in pasteurized fluid milk.

Main Methods:

• Microbiological shelf-life characterization of 105 pasteurized fluid milk samples from 20 dairy processing plants.
• Evaluation of six different detection approaches, combining two enrichment protocols (13°C or 21°C for 18 hours) and three plating media (crystal violet tetrazolium agar, Enterobacteriaceae Petrifilm, Coliform Petrifilm).
• Analysis of bacterial counts and identification of contaminating microorganisms, focusing on gram-negative bacteria.

Main Results:

• 60 out of 105 samples exceeded 20,000 cfu/mL due to gram-negative bacteria PPC, with 100% showing contamination by noncoliform, non-Enterobacteriaceae gram-negatives like Pseudomonas.
• Enrichment at 21°C for 18 hours followed by plating on crystal violet tetrazolium agar achieved the highest sensitivity (70%) for detecting samples with PPC.
• Traditional tests like coliform testing were found unsuitable for monitoring PPC, highlighting the need for broader gram-negative bacteria detection.

Conclusions:

• The developed method (21°C enrichment, crystal violet tetrazolium agar plating) offers a more sensitive and accelerated approach for detecting PPC in fluid milk.
• Traditional microbiological tests are insufficient for effective PPC monitoring; methods targeting all gram-negative bacteria are essential.
• Improved detection of PPC is vital for enhancing fluid milk quality and safety throughout its shelf life.