Greenbelt, MD – Consumers love pre-packaged salad mixes for their convenience; you just open the bag, add whatever toppings you like, and presto, you have a salad. However, getting your greens this way can sometimes pose a risk. In fact, researchers recently discovered that bags of prepared salad with broken leaves are far more likely to be contaminated with salmonella than those with whole leaves.

In a study led by Dr. Primrose Freestone and microbiologists from the University of Leicester, the researchers found that damaged leaves release a juice that promotes the growth of bacteria in packaged salad. In fact, broken leaves increased the risk of salmonella by 2,400 times and increased the bacteria’s virulence, making it even more likely to cause an infection in humans.

“Salad leaves are cut during harvesting and we found that even microliters of the juices which leach from the cut ends of the leaves enabled salmonella to grow in water, even when it was refrigerated,” said Freestone. “These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.”

In previous studies, scientists have confirmed that vacuum sealers preserve food three to five times longer than regular plastic bags and containers. However, this new study seems to suggest that contamination can happen prior to packaging, rendering the specific type of packaging irrelevant.

Contamination may occur from non-hygienic packaging or manufacturing equipment and human handling. That is why the conditions inside packaging plants are so carefully regulated. Roughly 70% of all manufacturers use a compressed air system, which are required to comply with the purity regulations set by the Safe Quality Food Institute.

Experts highlight the need for careful storage when it comes to fresh produce. In the recent study, the scientists discovered that as soon as the packaging was opened, the bacteria on the leaves grew at a much faster rate, even when stored in a cold refrigerator. As a result, they recommended that consumers eat pre-packaged produce as soon as possible after opening.

“Avoiding fresh produce is not a solution, but if possible it would be preferable to buy uncut fresh produce over chopped, and to always wash it before you eat – even the ones that are already washed,” said Dr. Kimon Andreas Karatzas, an assistant professor of food microbiology at the University of Reading.