It's hard to argue with directive straight from the source, but it stills begs a couple questions. What is the purpose of foil or plastic seals on food containers in the first place? And how can leaving one on an open container mess with its freshness? Looking for answers, we reached out to two germ experts.
The purpose of the foil or plastic seal on food containers
“There are two reasons for the seal,” says Jason Tetro, microbiologist and author of The Germ Files. “Firstly, it helps to increase shelf life—the container is usually flushed with nitrogen before sealing which reduces the amount of oxygen inside.”
According to Tetro, the second reason is to protect the contents of the container. “It helps to prevent the introduction of chemicals and microbes that could spoil the products and/or cause illnesses," he says.
Microbial ecologist Jack A. Gilbert, PhD, a professor at the University of California San Diego, further explains that the seal is a safety and quality mechanism as it creates a barrier between the product and the external environment, while also indicating to consumers that the product hasn't been tampered with prior to purchase.
Why you want to remove the seal from your food container after you open it
"Breaking the seal," a common term for the first time you go pee while drinking alcohol to indicate that you've opened the floodgates—and there’s no turning back—is equally applicable in this context as well. “Once the seal is broken, the surrounding air and oxygen can get into the product and that will start up the microbial growth,” says Tetro. “Whether the seal is still there or not isn’t a factor. Its sole purpose is to get it from the factory to your kitchen safely.”
What's more, keeping the seal after the product has been opened can lead to a higher risk for contamination from continual touching, according to Tetro, who recommends removing the seal as soon as you open the product. If you must leave it on for some reason (ahem, can't admit defeat), he advises washing your hands before touching it. But your best bet it to just accept that seals were meant to be broken...and discarded.
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