Growth of lipolytic psychrotrophic pseudomonads in raw and ultra-heat-treated milk

Area of Science:

• Food Microbiology
• Dairy Science
• Enzymology

Background:

• Lipolytic defects in raw milk are a significant concern for the dairy industry.
• Pseudomonads are frequently identified as the dominant microorganisms responsible for these defects.
• Understanding the specific species and their enzymatic activities is crucial for quality control.

Purpose of the Study:

• To identify and characterize the dominant lipolytic bacteria in raw milk exhibiting defects.
• To compare the growth, lipase activity, and lipolytic potential of key Pseudomonas species under varying temperatures.
• To assess the heat stability of lipases produced by different Pseudomonas strains.

Main Methods:

• Analysis of 36 raw milk samples with lipolytic defects to identify dominant bacterial flora.
• Isolation and selection of representative lipolytic bacterial strains.
• In vitro testing of isolate growth, lipase activity, and lipolysis in ultra-heat-treated milk (5-30°C).
• Comparative analysis of Pseudomonas fluorescens and Pseudomonas fragi strains.

Main Results:

• Pseudomonads were the dominant lipolytic bacteria found in the analyzed raw milk samples.
• Pseudomonas fragi exhibited more severe lipolytic defects compared to Pseudomonas fluorescens in milk cultures.
• Ps. fragi strains demonstrated faster multiplication in cold-stored milk and produced more heat-stable lipases than Ps. fluorescens strains.

Conclusions:

• Pseudomonas fragi is a significant contributor to lipolytic defects in raw milk due to its rapid growth at cold temperatures and heat-stable lipases.
• The findings highlight the importance of controlling Ps. fragi contamination for maintaining dairy product quality.
• Further research into lipase inhibition or inactivation strategies for Ps. fragi may be warranted.