eli5 coffee bag shrinking on plane

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Hey guys, last week I was traveling and grabbed a bag of coffee beans from a cafe. The bag was closed and it did not appear to be “tight” or vacuum sealed to the beans. After the flight home, the bag appeared to have shrunk or “tighten” to the beans inside. Why did this happen?

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Air pressure is lower in a plane than at sea level, there’s less air. The bag of coffee beans lost air whilst on the flight until it had the same air pressure as its surroundings.

If you left it long enough at sea level the reverse would happen and it’d go back to normal.

Coffee bags often have valves that allow gas to escape, as recently roasted beans are still “degassing”. When you went up to high altitude, the air pressure inside the cabin dropped, leaving the inside of the bag at a higher pressure. This high pressure caused air to be pushed out through the valve. And since the valve is (approximately) one-way, the reverse didn’t happen when you returned to lower altitude. Even though the air pressure outside was now higher than inside the bag, air could not (easily) enter the bag through the valve.

These aren’t super high-quality valves so if you left it for a while I bet that the bag would slowly refill with air until the pressure was equalized again. But they’re probably good enough that it takes a while, and so for some time after landing the bag would have stayed in this “shrunken” state.

This is really fascinating to me! Thank you guys for the info!