When you buy a bag of chips or whatever it will stay fresh for a long time but once you open the bag it will only last for a few days. How is this possible?

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Like there is air in the bag as well so how come it can last for longer period of time when sealed?

In: Earth Science
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Chip bags are filled with nitrogen when processed. So once they are opened, oxygen is reintroduced.

The air in the sealed bag is nitrogen, which makes up some 78% or so of the outside air. When the bag is opened, the chips are exposed to oxygen, which starts the decaying process.

Humidity in the sealed bag is very specific and limited to the captured volume of air. This means the chips don’t absorb moisture from the surrounding air and go stale.

Once you’ve opened the bag, there’s significantly more moisture available for the chips to absorb (from the whole air supply around them in the house, even if you roll the bag tightly.

The “air” in the bag is almost pure nitrogen. By contrast, the air we breathe is only about 80% nitrogen, with a bunch of other things like oxygen, carbon dioxide, and water vapour. When you open the bag, you let in these other gases, and they’re what make the crisps/chips go soft and soggy.

*”…there is air in the bag…”*

Your premise is wrong. There’s no “air” ^oxygen in the bag until you open it. Bags are factory purged/filled with Nitrogen specifically to keep the contents from oxidizing in transit & storage. They’re also filled out fully to protect the contents from damage in shipping. Once you open it you’ve now exposed the contents to both oxygen and moisture, both of which have a negative effect on the freshness of the chips inside.

Humidity is controlled in the bag, the inside of the bags are sterile, so there is no bacteria inside. The air they are filled with isn’t standard “room air” but nitrogen, which is inert.

It can only absorb as much moisture as the air in the bag has which is little, once oppened it has a basically unlimited amount of moisture to be exposed with.

In general foods, have to stay isolated, from other bacteria that could live in the bag. When the cell doesn’t get oxygen or other process components for processes like respiration, it can not survive. It’s like making a ceiling to the rain forest, to kill the trees, because they wouldn’t be able to make photosynthesis. All these processes are essential for all mikro to makro species, getting energy to stay alive. If we open the chips bag, bacteria could potentially live and spread inside. The air is not dangerous for the chips. It’s the bacteria, which could respirate the air