Stopping salads being deadly

Leafy greens and uncooked vegetables bring with them a high risk of food poisoning. They are implicated in about 20% of reported cases of illness caused by foodborne pathogens in the US. Being eaten raw, the produce preparation does not include a kill step, such as exposure to the high temperatures experienced during cooking, that reduces the risk of live pathogen contamination.

Leafy greens do not naturally contain bacteria that can make consumers sick but the leaves can become contaminated because they are grown close to the ground. Contaminated leafy greens have been linked to cases of food poisoning caused by Salmonella and E. coli, and most commonly, norovirus.

Leafy greens can become contaminated in the field by:

• soil,
• contaminated water,
• animals,
• improperly composted manure.

They can also be contaminated by bacteria:

• during and after harvest from handling, storing, and transporting;
• in the refrigerator or from counters and cutting boards (through cross-contamination with harmful bacteria from raw meat, poultry or seafood).

Despite the frequency of vegetable-related illnesses, they are not the most harmful — this dubious honour belongs to poultry. In the US, the largest proportion of deaths from foodborne illness, one in five, was due to poultry.

So, how can we ensure that the salads and uncooked vegetables, which add freshness and appeal to many ready meals and catering menus, do not end up making our customers sick.

Washing hands thoroughly with warm water and soap for at least 20 seconds, before and after handling leafy greens, and following proper cleaning techniques can help avoid cross-contamination and prevent the spread of food poisoning.

The produce should be washed under fresh, cool running water but should not be left to soak in tubs of water. The bacterial load needed to cause disease is often quite high but consumers can become ill after eating just 10 E. coli. This presents a particular problem for leafy greens grown in soils fertilised with manure which is high in E. coli as splashes during watering can transfer the bacteria onto the leaves.

Reducing the bacterial load can be problematic but researchers at the University of Arizona have discovered natural methods to sanitise these vegetables using ingredients commonly found in the kitchen, such as oregano, cinnamon and vinegar.

Plant extracts, essential oils and organic sanitisers have all proved effective in killing bacteria on leafy greens and extending their shelf life. When emulsified in the water used to wash these leaves, the approach compares to (and sometimes even works better than) bleach or hydrogen peroxide.

“Plant antimicrobials can be used by consumers at home,” said Sadhana Ravishankar, associate professor at the University of Arizona’s School of Animal and Comparative Biomedical Sciences. “Plant extracts and essential oils can be added in the wash water by themselves or combined with vinegar in the wash water for treatment.”

Benefits of using plant antimicrobials and organic sanitisers are that they are natural, environmentally friendly and less energy intensive since they are effective at both room and cold temperatures. They also continue to kill bacteria during storage; their effectiveness is not reduced in the presence of organic matter; and they have added health benefits linked to a reduction in the occurrence of cancer, diabetes and high cholesterol. The wash water containing plant compounds and organic sanitisers can also be recycled and re-used without a loss in effectiveness.

“We have also researched a new way of applying plant antimicrobials to improve salad safety,” said Ravishankar. “We have incorporated plant essential oils into edible films that are added into salad bags and the vapours from the oils kill the bacteria in the salad bags during storage. Edible films are also plant-based sources such as apples, carrots, hibiscus or spinach pulp.”

The outcomes of this project will benefit consumers by reducing and preventing contamination of the leafy greens by foodborne pathogens at the production and harvesting levels, providing a safer product in stores and on their tables.

Moving forward, Ravishankar and her team are testing combinations of plant antimicrobials and the effectiveness of them when the wash water is recycled.  USDA’s National Institute of Food and Agriculture (NIFA) is funding this new research, a four-year, $2,907,354 Organic Agriculture Research and Extension Initiative (OREI) grant.