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When I buy spinach from the supermarket, it comes in a sealed plastic bag. There is always quite a lot of moisture inside the bag on the leaves. I used to find that spinach would last about 3 days in the fridge inside this bag, and then go mushy and bad.

I realised recently that the moisture is the problem. If I open the bag and spread the spinach out to dry for a couple of hours before putting it in the fridge, I find it lasts for up to 2 weeks. Given that the spinach lasts so much better this way, it seems strange to me that the supermarket does not put holes for ventilation in the bag, to keep it from getting so wet.

So, why does spinach from the supermarket come in an airtight bag?

(I live in the UK, and have only ever seen spinach in bags like this. I would also be interested to hear if people have experiences of different types of spinach packaging in other places)

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The airtight bag has an atmosphere of mostly nitrogen. This displaces the oxygen in regular air, which in itself prevents decay but also reduces the moisture content which further prevents decay. Inflating the bag before sale also helps cushion it during transit:

Nitrogen is a popular gas for food packaging because it displaces oxygen. This is important because oxygen can carry moisture. Oxygen is used by bacteria to grow and thrive on organic material.

By removing as much oxygen as possible, bacteria find it more difficult to grow. Additionally, oxygen can cause foods to undergo oxidation, which can cause foods to become foul-smelling and inedible.

In addition to keeping foods from going bad or stale, nitrogen flushing can also help cushion the products during transit (as you see with easily-crushable foods like chips).

Source

Once you open the packaging, regular air is reintroduced. If you seal it again using a clip, then the moisture is trapped inside and the spinach can decay quickly. At this point the most valuable thing to do is to make sure that air can circulate freely so the surface of the spinach is dry, as you have found. Putting a piece of paper towel in with the spinach can also help by absorbing free moisture from the air.

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  • Ah I see. So then I guess the supermarket doesn't write "remove from bag and leave to dry before storage" on the bag because they prefer it to go off so you buy more
    – Jojo
    Mar 10, 2022 at 19:48
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    @Joe I have a bag of rocket in my fridge which says "To keep fresher for longer at home, keep in the original packaging and store in the fridge below 5°C, ideally in the crisper drawer. Adding a piece of kitchen towel to the bag could help to keep your rocket fresher for longer." I think the principle is to go with simple instructions that are easy to follow and hard to misinterpret.
    – dbmag9
    Mar 10, 2022 at 20:07
  • glad to hear that not all supermarkets are crooks! On the bag from my spinach it says only 'Keep refrigerated', and there's a best before date for two days after when I bought it
    – Jojo
    Mar 10, 2022 at 20:39
  • @Joe there's a lesson for you, don't buy grocery items that are past their "best before" date. Mar 11, 2022 at 1:21

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