"Since there are no preservatives in our drinks, mold can grow, especially in a leaking pouch," Kraft says on its Capri Sun frequently asked questions website.
During the manufacturing process, the drinks are heated to temperatures that exceed those used for pasteurization. But punctures in the products' package — even microscopic ones — can allow air inside the package, and mold to grow, Kraft says. Fungi need oxygen to grow, Dannelly said.
Capri Sun packages have a shelf life of about a year. The company urges consumers to discard leaking or damaged packages.
In the new study, the researchers filtered Capri Sun through filter paper, and then checked whether any microorganisms were left behind on the paper. The juice contained just a few fungal cells, which grew in laboratory dishes.
Dannelly said if this experiment was done on any juice after it was opened and left in the refrigerator, she would expect both fungus and bacteria to grow.
In a second experiment, the researchers, including Leah Horn, an undergraduate biology major, punctured Capri Sun packages with a sterile needle to mimic damage to the product. When left in a sterile environment for three weeks, fungal mats grew in the juice.
A problem with Capri Sun is that the packages are not see-through, so unlike mold on bread or cheese, consumers can't tell when Capri Sun goes bad.
Kraft said it tried creating clear packages for Capri Sun, but stopped making the packages after it created manufacturing problems.
The company said it will not add preservatives to the product because their customers don't want this. Preservatives give food a longer shelf life, but some, such as the preservative nitrite, have been linked to an increased risk of certain cancers.
Dannelly said there are some natural preservatives, such as citric acid, which are not harmful and could be added to the product (although it would make the product more acidic).