New bacteria species may spoil refrigerated raw milk

By Stephen Daniells

- Last updated on GMT

Related tags Raw milk Milk Bacteria

Scientists have discovered new species of bacteria that can grow at low temperatures, and may even spoil raw milk during refrigeration.

According to findings published in the International Journal of Systematic and Evolutionary Microbiology, researchers from the University of Haifa in Israel report that the microbial population of raw milk is more complex than previously thought.

"When we looked at the bacteria living in raw milk, we found that many of them had not been identified before,"​ said researcher Dr Malka Halpern. "We have now identified and described one of these bacteria, ​Chryseobacterium oranimense, which can grow at cold temperatures and secretes enzymes that have the potential to spoil milk."

The research is important for dairy processors since the cold-tolerant bacteria can reportedly produce heat-stable enzymes that are not denatured during pasteurization and may affect flavour quality of fluid milk and its products.

"Milk can be contaminated with many different bacteria from the teat of the cow, the udder, milking equipment and the milking environment,"​ explained Dr Halpern. "Milk is refrigerated after collection to limit the growth of microbes.

“During refrigeration, cold-tolerant, or psychrotolerant, bacteria that can grow at seven Celsius dominate the milk flora and play a leading role in milk spoilage. Although we have not yet determined the impact on milk quality of ​C. oranimense and two other novel species (​C. haifense and ​C. bovis) that were also identified from raw milk samples, the discovery will contribute to our understanding the physiology of these organisms and of the complex environmental processes in which they are involved.

“There is still a lot to learn about the psychrotolerant bacterial flora of raw milk," ​added Dr Halpern.


Heating milk to around 72 degrees Celsius for 15-20 seconds – pasteurisation - reduces the microbial load of the liquid. However, some bacteria produce enzymes that can resist pasteurisation. For example, lipase enzymes cause flavour defects and proteases can lead to bitterness and reduced yields of soft cheese.

"In Israel, dairy companies estimate that cold-tolerant bacteria can cause a 10 per cent loss of milk fats and proteins. When researchers looked at these bacterial communities, they found that 20 per cent of the bacteria isolated were found to be novel species and 5 per cent of these were members of the genus ​Chryseobacterium,"​ said Dr Halpern.

"Because of their effect on milk quality, it is important that we develop sensitive and efficient tools to monitor the presence of these cold-tolerant bacteria."

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