Lower pasteurisation temperature may limit spore germination

Even if milk has been pasteurised to kill off microbes that can cause food spoilage and disease, certain bacterial strains can survive the heat shock as spores and cause milk to curdle in storage. Now it seems that higher pasteurisation temperatures may aid spore germination.

Researchers in the Milk Quality Improvement Program at Cornell’s College of Agriculture and Life Sciences have identified the predominant spore-forming bacteria in milk and their unique enzyme activity, knowledge that can now be used to protect the quality and shelf life of dairy products.

“Control of food spoilage is critical in a world that needs to feed 7 billion people,” said Martin Wiedmann, food science professor and study co-author. “Approximately 25% of post-harvest food is spoiled by microbes before it is consumed.”

The study, published earlier this year in Applied Environmental Microbiology by the lab of Wiedmann and Kathryn Boor, the Ronald P Lynch Dean of the College of Agriculture and Life Sciences, identified the predominant strains of spore-forming bacteria, which can foul milk and other food products. The culprits, Paenibacillus bacteria, are ubiquitous in nature and cause off-flavours in a variety of foods and curdling in dairy products.

As spores, the bacteria can survive in dormant form for years despite the best practices in cleaning, processing and packaging.

In fact, the bacteria may be uniquely adapted to overcome the twin tactics of dairy protection: pasteurisation followed by refrigeration. According to co-author and research support specialist Nicole Martin, the spores are not only resistant to heat, the small jolt of heat during pasteurisation may actually stimulate them to germinate. Some can reproduce in refrigerated dairy products at temperatures that would stymy other types of bacteria.

“We studied 1288 bacterial isolates in raw milk, pasteurised milk and the dairy farm environment; however, only a handful of strains accounted for 80% of the spore-formers present,” said Wiedmann. “They grow well in milk - and possibly other foods - at temperatures as low as 6°C, and we can identify Paenibacillus because of their uniquely high galactosidase enzyme activity at 32°C.”

They also investigated how pasteurisation affects the presence of such bacteria.

In the US, concerns about food safety have prompted many dairy processors to increase pasteurisation temperatures above the 72°C minimum set by the government. Anecdotal reports, however, suggested this practice actually led to more spoilage once the products were refrigerated.

Tallying bacterial numbers throughout the refrigerated shelf life of milk pasteurised at two different temperatures - 76°C and 79.5°C - the Wiedmann-Boor lab found that lowering the temperature significantly reduced bacterial growth during refrigerated storage, especially by 21 days after pasteurisation.

The findings are already being applied in the field. The Wiedmann-Boor Lab was enlisted by Upstate Niagara, a cooperative of more than 360 dairy farm families throughout western New York, to further improve the quality of its award-winning milk by assessing milk samples for spore-formers.

Data on samples that contained spore-forming bacteria are now being analysed using DNA fingerprinting to identify the types of organisms present and where they might have come from.

Martin said she hopes the collaborative project will become a model for how to approach spore-forming bacteria in individual dairy processing plants.