Bacterial Diversity in Laboratory Heat-treated and Commercially Pasteurized Fluid Milk Along the Milk Chain

Introduction: The dairy industry needs to control spoilage microorganisms, such as spore-forming bacteria (Bacillus spp. and related genera) and Post-Pasteurization Contaminants (PPC), to improve the quality and shelf life of fluid milk. PPC can be controlled with pasteurization and sanitization procedures, but spore-forming bacteria can survive pasteurization and persist in the plant environment by forming biofilms. Strategies to better control these organisms require an assessment of the bacterial ecology throughout the fluid milk chain. 

Purpose: Evaluate the bacterial diversity of laboratory heat-treated and commercially pasteurized fluid milk along the milk chain, and identify common genera present in both set of samples. 

Methods: Raw and pasteurized milk samples were collected at different locations (farm, trucks, processing plant, and packaged products) from a medium-size milk chain in the Midwest during three periods (Spring-2012, Fall-2012 and Spring-2013). All raw milk samples and aliquots of some pasteurized milk samples (Spring-2013) were heat-treated in the laboratory (80°C, 12 minutes) to eliminate vegetative cells. Milk samples (heat-treated and commercially pasteurized) were stored at <7°C for 21 days. Bacterial isolates were collected from samples at different time points and characterized using rpoB and/or 16S rRNA sequencing. 

Results: A total of 220 bacterial isolates were collected from all milk samples. The isolates from laboratory heat-treated milk samples were identified as Bacillus spp. (79%, 109/138), Paenibacillus spp. (14%, 20/138) and other spore-forming bacteria (7%, 9/138); while the ones from commercially pasteurized milk samples were characterized as Bacillus spp. (56%, 46/82), PPC (28%, 23/82), Paenibacillus spp. (11%, 9/82), and other spore-forming bacteria (5%, 4/82). 

Significance: Although most of the isolates were identified as spore-forming bacteria, PPC is still a concern in commercially pasteurized milk, especially during the filling stage. Spore-forming bacteria were detected in samples from different locations, thus strategies for control would need to encompass the entire milk chain.