How herb extracts could counter food poisons

Nature’s own anti-microbial systems could help consumers avoid food-poisoning from processed foods

WHEN YOU SINK your teeth into a pre-washed salad or a convenient ready-meal from the supermarket, the last thing you want to meet is a bug that will make you sick.

But current approaches to keeping the nasty microbes at bay can leave a bad taste for health-conscious consumers. That’s why researchers at Dublin’s Institute of Technology are looking instead to essential oils derived from familiar herbs, such as oregano and thyme.

The right mix of concentrated extracts from the herbs could kill even hardy microbes while tasting good and winning over consumers, explain Dr Paula Bourke and Dr Catherine Barry-Ryan from DIT’s school of food science and environmental health. They have been looking at how best to use the oils for applications such as washing ready-to-eat salads and extending the shelf-life of ready-meals.

Many essential oils have known anti-microbial properties and could provide an alternative to current methods, such as washing fresh produce with chlorine, says Barry-Ryan.

“Chlorine is cheap and very effective and in most processes it gets rinsed off afterwards, but there’s still some concern about residual chlorine, that it’s linked with possible carcinogenic compounds. In some European countries it has been banned for use with certain products,” she says. “So there is a big big need in the food industry to find an alternative to chlorine, and that’s why there’s research looking at essential oils.”

For consumers, the familiarity factor is also a plus, adds Bourke. “A wide range of herbs and aromatic plants that people would use daily in their culinary activity contain bioactive components, and that is what we are trying to harness in a more concentrated form in the essential oils that have been extracted from the herbs,” she says.

The DIT team looked at a number of plant essential oils with known anti-microbial activity, including basil, lemon balm, marjoram, oregano, rosemary, sage and thyme, screening them against listeria and other food-borne pathogens in the lab and looking at how the oils interact with food ingredients. Using the whole oil rather than individual active compounds is often the most effective way to tackle the microbes, according to Bourke.

“If you take out the main components, sometimes if you analyse them on their own then they may have some activity, but it may not be as good as the whole plant essential oil it originally came from,” she says. “The whole plant essential oil could be composed of maybe a handful or 10 main components but there may be hundreds of smaller components and all together they seem to act synergistically to give us better activity.”

But even the strongest microbe killer won’t win consumers over if it tastes awful.

“This is a big issue,” says Bourke. “Because the essential oils are so concentrated that they can have a high organoleptic [sensory] impact. That’s why we are trying to get the ones with the best anti-microbial activities and combine them at the lowest possible concentration, to reduce the organoleptic impact so that we can expand the range of potential applications in food.”

So not only did the researchers test the anti-microbial strength, they also asked consumers to rate foods treated with various essential oils. Their experiments singled out essential oil superstars such as thyme and oregano, which effectively kill hardy bacteria but have an acceptable taste(consumers were happy to eat oregano-treated carrot and lettuce, and thyme worked for carrot).

The project, which is funded by the Department of Agriculture and Food, is also looking at how the plant oils could boost the safety and shelf-life of cooked ready-meals. And, so far, it seems the plant-oil anti-microbials could be suited to dishes with meat and sauce, such as bolognese.

“With the essential oils of oregano and thyme, their activity was better at higher concentrations of protein and slightly acidic pH conditions, and concentrations of sugars or carbohydrates above 5 per cent didn’t really impact negatively on the usefulness of the anti-microbial activity,” says Bourke.

“So the message is that the essential oils are very suitable for that type of application in those types of foods.”