A zap a day keeps listeria away, research shows

Forget chemicals and even refrigerators; Tel Aviv University researchers say they can keep milk safe using pulsed electric fields

Illustrative photo of Tnuva milk products (Nati Shohat/Flash90)
Illustrative photo of Tnuva milk products (Nati Shohat/Flash90)

Listeria, one of the modern era’s most lethal food contaminants, hit home for many American consumers this year when dozens were affected by illness and at least three deaths were reported after individuals ate ice cream made by Bluebell Creameries.

Listeria has proven to be one of the most difficult food-borne scourges in recent years, but a new technique devised by Tel Aviv University researchers could end the threat of listeria in milk, ice cream, and other dairy products.

Even more important, according to lead researcher Dr. Alexander Golberg of TAU’s Porter School of Environmental Studies, the pulsed light pasteurization process his team developed could help farmers and consumers in the developing world who have no way to preserve milk.

“We are on a constant hunt for new, low-cost, chemical-free technologies for milk preservation, especially for small farmers in low-income countries,” said Dr. Golberg. “For 1.5 billion people without adequate access to electricity, refrigeration is simply not a possibility and boiling does not preserve milk’s freshness over time.”

Pulsed electric fields applied to milk could be the solution, the team said in a paper published in the latest edition of the scholarly publication Technology and could allow for “precise control of L. monocytogenes density in contaminated milk, an essential product for small farmers in low-income countries,” the paper’s authors said.

Farmers who produce milk could ship it to local processing plants that do have power, where the electrical pulses could be applied, and then either get it back or sell it.

Listeria has proven to be one of the biggest problems for modern food producers, resulting in repeated recalls of products like cheese, meat, milk, and even vegetables. Listeria has also proven to be one of the hardiest of food-borne pathogens; unlike many other germs, it can grow even in the cold temperature of the refrigerator, and can only be killed by cooking and pasteurization.

The Bluebell Creameries listeria outbreak, first discovered in mid-February, eventually led to one of the largest food recalls in American history, as the company recalled not just affected batches of the ice cream and frozen dessert it distributed in 23 US states and 27 other countries but all its products. Still, dozens of people reported getting sick, with three eventually dying directly from, or from circumstances aggravated by, the germ.

Other notable listeria outbreaks included one in 2011, when cantaloupe contaminated with listeria infected at least 147 people in the US and killed at least 33 – the worst food-borne illness outbreak ever in the US. In Denmark, a listeria outbreak this year killed at least 15 people.

Even with advanced food storage technology, listeria has proven to be a particularly persistent irritant, the result, many scientists believe, of failures on the “safety chain” in food processing and manufacturing that is supposed to cover food from farm to plate.

If that’s the situation in the West, where refrigeration and food handling hygiene is taken for granted by authorities and consumers alike, the situation is far more dire in the developing world. A recent study by the World Health Organization called for “urgent action” in detecting and dealing with listeria, which is responsible for tens of thousands of deaths in the developing world each year.

The technique developed at TAU, using short pulsed electric fields to kill milk-contaminating bacteria, could offer a solution, said Golberg. According to the study, pulsed electric fields – a benign form of food irradiation that uses light energy, not radiation, to kill food-borne microorganisms – could provide an alternative, non-thermal pasteurization process. The stored milk is periodically exposed to high-voltage, short pulsed electric fields that kill the bacteria.

The energy required can come from conventional sources or from the sun. The technology is three times more energy-efficient than boiling and almost twice as energy efficient as refrigeration.

Pulsed light is an emerging technology in the food industry that has been shown to effectively kill multiple pathogens. It received FDA approval in 2001 (as opposed to radiation-based food irradiation, which is still largely banned). The pulsed light used in these techniques is estimated to be as much as 20,000 times as powerful as sunlight, sufficient to kill microorganisms in liquid like milk.

The TAU technique features different pulse levels for various foods, ensuring that only enough power is used to kill listeria, while leaving beneficial components in the milk intact.

For family farms in the developing world, the TAU-developed technique is perfect, said Golberg.

“Our model shows that pulsed electric fields preservation technology does not require a constant electricity supply; it can be powered for only 5.5 hours a day using small, family-scale solar panels,. I believe that this technology can provide a robust, simple, and energy-efficient milk preservation system that would decrease the amount of wasted milk, thus increasing the income of small farmers in developing countries.”

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