Ever had that awful feeling when you get the milk out of the fridge, and pour it into your coffee only to realise it’s completely gone off. Luckily this isn’t too common, thanks to pasteurisation. In fact, raw or unpasteurised milk is illegal in many countries as it can be contaminated with potentially harmful microbes.
As well as potentially causing illness, these bugs can spoil the milk, making it taste bitter and generally go yucky. But still, some people swear by the health benefits of raw milk, and it can lead to some heated debates.
Now scientists have discovered new species of bacteria that can grow at low temperatures, spoiling raw milk even when it is refrigerated. This tells us that the community of microbes living in unpasteurised milk is more complex than previously thought.
The research was done by Dr Malka Halpern and a team at the University of Haifa in Israel. They looked at the bugs in raw milk, and found that many of them hadn’t been previously discovered. In particular, they pinned down one called Chryseobacterium oranimense, which can grow at cold temperatures and produces enzymes that can spoil milk.
So why is this important, if we usually have our milk heat treated by pasteurisation? The process, in which milk is heated to around 72 degrees centigrade for 20 seconds, kills most enzymes and bugs in the milk, but bacteria that can withstand the cold are sometimes also stable in the heat. So they can still affect the quality of milk, and won’t be affected by subsequent refrigeration.
The researchers say the next step for their research is to develop sensitive tests that can detect these cold-resistant bugs, so that milk can be effectively monitored, to make it safer to drink even when pasteurised, and hopefully avoid that awful moment in the morning when you pour the milk onto your breakfast cereal, only to realise it’s completely off.
The paper “Chryseobacterium oranimense sp. nov., a psychrotolerant proteolytic and lipolytic bacterium isolated from raw cow's milk” was published in the November issue of the Society for General Microbiology’s International Journal of Systematic and Evolutionary Microbiology