For thousands of years, humans have relied on milk to provide us with a variety of nutrients. Yet the process of getting milk has changed dramatically over the last centuries. The advent of milking machines has allowed us to find milk almost anywhere on the planet, while the discovery of pasteurization helped to keep the product safe for days to weeks of storage.
Unfortunately, milk has become so popular that the need to feed the demand has led to some consequences. One of these happens to be what is known as post-pasteurization contamination. Pathogens can be found in a variety of locations in a processing plant and find their way into the milk, posing a significant threat to health.
One of the more troublesome pathogens associated with contaminated milk happens to be Listeria monocytogenes. The species lives naturally in the environment, including on farms. Over time, the bacteria can end up in different milk processing equipment.
When ingested, the bacterium can cause illness in healthy individuals with symptoms ranging from nausea and vomiting to cramps and diarrhea. But the species is a major health threat to people who are immunocompromised. Infection can lead to a variety of life-threatening conditions including infection of the blood and swelling of the lining around the brain. There also are reports that pregnant women may also lose their unborn children.
Studies to identify Listeria in dairy facilities have revealed this pathogen can make its way into the production line and stay there. The bacteria have an uncanny ability to stick to numerous surfaces such as conveyor belts, drains and distribution tanks. Here, they can grow, resist cleaning efforts and eventually contaminate the final milk product. When this happens, the risk to the public increases as does the potential for recalls.
For an international group of researchers, a better way needed to be found to help control Listeria contamination in milk. But their approach was quite different from conventional bacteria-lowering sanitization and disinfection. For them, the answer to the problem lied not in a chemistry lab, but in a bacterium. As they found, we may one day rely on a microbe to keep milk safe.
The team focused on the species Lactococcus lactis. As the name implies, the bacterium is a milk lover and thrives on milk's primary sugar, lactose. It also is involved in the fermentation of milk products, a practice known to help keep the dairy products over time.
Research into how this bacterium can help preserve food has revealed it sends out into the environment an effective bacteria-killing molecule known as nisin. It's a powerful food preservative and has the ability to deal with Listeria. For the authors, this species was the perfect base to conduct their assessment.
Before performing the tests, the team wanted to give the lactis bacteria a little more killing power as some studies have suggested Listeria can persist in the presence of nisin. The group gave the good bacteria the ability to produce another Listeria-defeating molecule known as leucocin-C. The hope was to see a one-two punch to the pathogen such that it would not be able to survive.
The new, stronger lactis strain was allowed to grow in a liquid for about 8 hours. During this time, the individual cells produced nisin and leucocin-C and sent them out into the environment as usual. At the end of the growth time, the liquid - now filled with these killing molecules - was separated from the bacteria.
The team put the liquid into cultures of Listeria as well as a number of other pathogens such as Escherichia coli O157:H7 and Salmonella. The hope was to see a decent reduction if not total kill. Everything went as expected as the pathogens were easily reduced in number and in some cases, eliminated.
It will be some time before we see bacteria-preserved milk
This set the stage for the ultimate test. The lactis liquid was placed into pasteurized milk containing Listeria and a very small amount of a preservative known simply as EDTA. Once again, it all went according to plan. The levels of Listeria dropped significantly within the first hours and stayed that way for at least a day.
For the authors, this concept of using bacteria to fight bacteria offers a new direction in helping to preserve milk and other dairy products after pasteurization. By keeping the food free of pathogens such as Listeria, the entire food production chain can be considered safer. This could help to reduced numbers of infections and recalls in the future.
It will be some time before we see bacteria-preserved milk but as the process gains greater traction, there's a good likelihood we may see this technique used in all stages of milk production. This could be good for those who insist on drinking raw milk. As the name implies, it has not been treated with heat nor been given any preservatives meaning the potential for Listeria and other infections is high. This technique may be an easy way to keep those who avoid pasteurization some needed relief.
In the meantime, if you drink pasteurized milk, ensure to keep it in the fridge and never drink from the container. Also, follow the Best Before dates to reduce your chances for infection. If you happen to be a raw milk enthusiast, the best advice is to seek out and befriend a farmer. The only way raw milk can be considered safe is when it is consumed directly from the udder to the glass. If this product goes through any kind of processing, you're putting your health, and possibly your life, at risk.