Why do we need to refrigerate something after it’s opened? What happens to that thing when it is opened?

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As the question says, why do we need to refrigerate something after it’s opened? I have a bottle of Chipotle Sauce I got from Target, and it was happily sitting in the shelves for who knows how long unrefrigerated. It says on the label to refrigerate after opening. Why? And I don’t think it’s because Target is air conditioned. Some bodegas in nyc for example are un air-conditioned and will sell similar stuff.

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Opening the package exposes the food to oxygen, which oxidizes (degrades) the food.

Most packages contain some method of preservation, such as using nitrogen to displace regular air (which contains oxygen).

Usually foods that are packaged in cans or bottles are sterilized by heating during packaging. The exceptions are things that change in high heat like milk or meat. This sterilization process gives the food long shelf life because bacteria that were in the food were cooked to death and can’t eat the food or harm you.

One you open it, the food is once again exposed to bacteria and mold spores from the air. They will begin to grow in the food, and refrigeration slows their growth down tremendously. Therefore manufacturers recommend refrigeration so that you can keep the food fresher for longer.

Two things that make food go bad are oxygen and tiny little living things, we could call them germs.

For a new bottle of sauce that hasn’t been opened, there’s usually only a tiny bit of air at the top and it’s not enough oxygen to make it taste bad.

The germs are usually killed by heating, or, less often, some other method, like radiation or high pressure.

So a bottle is sitting there with almost no oxygen and no living germs – it can last for a long time without going bad.

But as soon as you open the seal, you allow air in and air has oxygen and germs floating around, so the sauce might go bad in a few days. If you put it in the fridge, the cold makes the germs slow down a lot, so they don’t eat up the sauce and they don’t make new germs as fast. There’s still oxygen, but if you put the cap on, that will also slow down.

You put something in an airtight container, and everything that needs air to live dies (I.e., most problematic bacteria).

You open said container, bacteria can now live inside it because it’s not airtight and is attracted to food because… it’s food.

Packaged food are sterilized when packed, creating a vacuum where no living bacteria are present and cannot get in. This is typically done with heat, either bottling hot produce or heating the can/jar once sealed. Once opened, bacteria can get into the product and begin growing, so refrigeration slows that process.