New bacteria discovered in raw milk

Nov 17, 2008

Raw milk is illegal in many countries as it can be contaminated with potentially harmful microbes. Contamination can also spoil the milk, making it taste bitter and turn thick and sticky. Now scientists have discovered new species of bacteria that can grow at low temperatures, spoiling raw milk even when it is refrigerated. According to research published in the November issue of the International Journal of Systematic and Evolutionary Microbiology, the microbial population of raw milk is much 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 Dr Malka Halpern from the University of Haifa, Israel. "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."

New technologies are being developed to reduce the initial bacterial counts of pasteurized milk to very low levels. Most enzymes will be denatured at the high temperatures used during pasteurisation, which means they will stop working. However, the heat-stable enzymes made by cold-tolerant bacteria will still affect the flavour quality of fluid milk and its products. Because of this, research into cold-tolerant bacteria and the spoilage enzymes they produce is vital.

"Milk can be contaminated with many different bacteria from the teat of the cow, the udder, milking equipment and the milking environment," said Dr Halpern. "Milk is refrigerated after collection to limit the growth of microbes. During refrigeration, cold-tolerant, or psychrotolerant, bacteria that can grow at 7°C 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."

There is an ongoing debate about the benefits and risks of drinking unpasteurised milk. Some people believe the health benefits resulting from the extra nutrient content of raw milk outweigh the risk of ingesting potentially dangerous microbes, such as Mycobacterium bovis, which can cause tuberculosis, and Salmonella species. Because of these risks, many countries have made the sale of unpasteurised milk illegal. Pasteurisation involves heating milk to around 72°C for 15-20 seconds in order to reduce the number of microbes in the liquid so they are unlikely to cause disease. Some bacteria produce extracellular enzymes that are remarkably heat tolerant and can resist pasteurisation. Lipase enzymes cause flavour defects and proteases can lead to bitterness and reduced yields of soft cheese.

Raw milk is consumed in rural areas of Europe and is also available in large cities. Distribution of unpasteurised milk is legal in England, Wales and Northern Ireland but illegal in Scotland. There are around 275 establishments in England that are approved by the Food Standards Agency to sell raw milk. However, the green-top bottles must display a warning that indicates the content has not been heat-treated and may contain harmful organisms. Furthermore, farmers are not allowed to sell unpasteurised dairy products if their herd is suspected to be infected with bovine tuberculosis.

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

Source: Society for General Microbiology

Explore further: Bees able to spot which flowers offer best rewards before landing

add to favorites email to friend print save as pdf

Related Stories

Ensuring the safety of artisan cheese

Jun 26, 2014

Buying fresh goat cheese at a local farmer's market or selecting a piece of sharp cheddar from a display of artisanal cheese at the supermarket offers the promise of a nutritious and flavorful snack. Dennis ...

New microfluidic chip for discriminating bacteria

Sep 14, 2010

A new "on-chip" method for sorting and identifying bacteria has been created by biomedical engineers at Taiwan's National Cheng Kung University. The technique, developed by Hsien-Chang Chang, a professor at the Institute ...

Invasion without a stir

Dec 17, 2009

Bacteria of the genus Salmonella cause most food-borne illnesses. The bacteria attach to cells of the intestinal wall and induce their own ingestion by cells of the intestinal epithelium. Up till now, researchers assumed ...

Recommended for you

Why do snakes flick their tongues?

1 hour ago

Many people think a snake's forked tongue is creepy. Every so often, the snake waves it around rapidly, then retracts it. Theories explaining the forked tongues of snakes have been around for thousands of ...

Boat noise impacts development and survival of sea hares

1 hour ago

While previous studies have shown that marine noise can affect animal movement and communication, with unknown ecological consequences, scientists from the Universities of Bristol and Exeter and the École Pratique des Hautes ...

User comments : 5

Adjust slider to filter visible comments by rank

Display comments: newest first

Soylent
not rated yet Nov 17, 2008
That's truly bizarre. Just force full disclosure on the box that raw milk may be harmful(for the thickies who might have thought otherwise) and let the buyer beware.

We don't need a nanny state to hold our hand and steer us away from every single decision that might be remotely harmful.
DUSTEE1
not rated yet Nov 17, 2008
After reading the info about this new bacteria & weighing it against my knowledge from reading the book "The Truth About Raw Milk". I think I'll take my chances with Certified Organic Raw Milk. Lots of scary stuff goes on with pasteurized milk that the Dairy Industry doesn't want you to know.
David6502
not rated yet Nov 17, 2008
I grew up on a farm and was brought up on raw unpasteurised milk. It never gave me any problems. It was very fresh though - it was often still warm from the cow on my morning breakfast cereal. :-)
Too much nanny state protection sometimes.

I wouldn't be surprised if it turns out that lots of such bacteria are actually necessary for a healthy gut flora and modern sanitised food causes other health problems by being too clean.
NOM
not rated yet Nov 17, 2008
In New Zealand the pasteuriser is built into milking machines, so unpasteurised isn't an option.
Milk taken straight from the heating part of the pasteuriser makes the best hot chocolate :)
Keter
not rated yet Nov 17, 2008
"...making it turn thick and sticky" - in other words, turn into yogurt or sour cream. This scary sounding description is merely hos its is SUPPOSED to work. If it's bitter, you know to throw it out; if it's sour, you know to let it set another day or two in the fridge until you have sour cream or sour yogurt that's good for baking, making sauces, putting on potatoes, and making salad dressing. Label and legalize!!!