"Best if used by... Three weeks ago?" What now? Ah, but of course: "Mom, can I still eat these crackers?"

"Sure honey, knock yourself out."

Rustle, rustle, munch, munch... "Eww, gross! They're soft and taste like sawdust."

With any luck, you haven't experienced this scenario too frequently, but odds are that you've come face to face with stale or expired food at some point. Expiration dates on foods aren't always exact, but they aren't simply a random number either. The manufacturer or packager has spent a good amount of time studying the shelf life of their product. They have to choose packaging material that lets their product survive long enough to be useful, while keep costs low enough that the consumer can afford to buy it.

Harkening back to the crackers, they were probably stored in a plastic bag inside a cardboard box. This is a fairly common packaging arrangement for dry goods. Why not simply a cardboard box? That would surely serve to hold the crackers in. But then, what happens if the box is set on a puddle? Cardboard doesn't keep moisture out, and that same moisture can help make crackers stale and soggy. (Worse, they might get moldy.) The plastic bag creates a moisture barrier.

Spills aren't the only source of moisture. On any summer day there are likely thousands of people complaining about the humidity at any given moment. Humidity is simply water vapor, and if there is moisture in the air, it can be transmitted to anything it touches - particularly dry, absorbent items like crackers. Any barrier that can keep water vapor out can help preserve freshness.

Air isn't just water vapor, of course. A good 20% of it is oxygen, which is great news for those of us who want to breathe - including aerobic bacteria that would love to grow on your food. Packaging that keeps air out also helps stop them from proliferating. If the food itself can react with oxygen (as when raw meats or apple slices turn brown), preventing exposure to oxygen is also desirable.

Temperature controls are used to slow both bacterial growth and oxidative processes. Chemicals can be added for the same purpose. In short, there are many ways to help protect food freshness, but most people never give it a thought other than to peel off the wrapper and throw it away.

Your mission, should you choose to explore this topic, will be to examine how different packaging configurations affect food preservation and expiration, and to identify best

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