Why do vacuum seal box containers lose their seal?
May 24, 2020 11:08 AM   Subscribe

If only I did better in science class.

I took one of those vacuum seal containers where you put the pump on top and it sucks all the air out. It sucks the air out so well that I literally cannot open the container afterwards and a big WHOOSH sound happens when I pull the silicone valve.

So after that test, I vacuum it again, PLUS I put suran wrap over the container as an extra precaution. The next day the half banana (no peel) that I put in there still turns brown. Why?
In case it matters I also had in the container the following: Spinach, pinnapple, jalepeno, lime and cashews. (I was going to blend it all into a smoothie the next day).
posted by fantasticness to Food & Drink (8 answers total)
I think it's the cold temperature, not exposure to air, that makes bananas brown. They turn brown in the fridge with their peel on, as well.
posted by bbqturtle at 11:11 AM on May 24 [1 favorite]

So the brown banana is still safe to eat?
posted by fantasticness at 11:17 AM on May 24

I believe the cold temperature is why banana peels turn black, but AFAICT the inside browning is indeed related to atmospheric oxygen. It may affect the texture in an unpleasant way but it is not a sign of microbial activity and doesn't indicate spoilage.

Even at maximum vacuum, a food saver vacuum still leaves 20% of the air there. That's a pretty respectable partial vacuum, but it's not going to halt all oxidation completely.

Also, was it easy to open the next day? Because if not, the seal* was functioning properly.

posted by aubilenon at 11:22 AM on May 24 [2 favorites]

So the brown banana is still safe to eat?

After less than a day? It would still be safe even if you hadn't vacuum sealed it at all. The browning is purely a cosmetic problem.
posted by showbiz_liz at 1:07 PM on May 24 [3 favorites]

You seem to have multiple questions here. Is your banana safe to eat? Undoubtedly yes. Why is it brown? Enzymatic browning caused by exposure to oxygen.

Why did it do this even though you pulled a vacuum on it? As mentioned, home-grade vacuum sealers don't actually pull a perfect vacuum, so any air in the container will continue to allow browning to occur. Wholesalers and distributors maintain ripeness through a different method - usually storing produce in 100% dry nitrogen environments, which prevents the enzymes from activating. This is an interesting article on how bananas get to New York bodegas perfectly ripe.

As far as your title question - why do vacuum containers lose their seal? Well, nature abhors a vacuum. Seals are not perfect, materials are not perfect, and eventually air will get in. It's inevitable, especially with stuff not built to very tight tolerances. Putting plastic wrap over your vacuum container is not going to help anything. By way of example, we just recently were trying to figure out how gas was getting in to a vacuum chamber at work and messing up our very sensitive electronics; turns out, it wasn't seeping in through the seals - the smaller air particles were simply transpiring through the glass because the atoms are much smaller than the gaps in the lattice structure.
posted by backseatpilot at 1:20 PM on May 24 [8 favorites]

home-grade vacuum sealers don't actually pull a perfect vacuum

Just like the vacuum wine savers and vacuum coffee savers, these devices change the air pressure inside the container by a very small percentage and will only keep that small pressure difference for a few days at most, often only a few hours.
To generate a genuine vacuum would require a contraption at least the size of a large washing machine. These things are just marketing BS.
posted by Lanark at 2:57 PM on May 24 [1 favorite]

Wholesalers and distributors maintain ripeness through a different method - usually storing produce in 100% dry nitrogen environments,

Broadly speaking, this is how oxygenation prevention works in most circumstances. Consumer-grade or even industrial-grade large-scale vacuum-creation isn't actually the most effective way to keep most things from contacting oxygen, because by and large removing all the gas from a rigid container is extremely difficult, and even removing it from a soft container is rarely perfect. Also, the product in question might not react to a low-pressure environment at all well. What's much easier than evacuating air from a container is flushing it out with something that's not oxygen: depending on the specifics of the reactions you want to prevent, nitrogen, carbon dioxide, and argon are all moderately popular.
posted by jackbishop at 3:19 PM on May 24 [1 favorite]

In addition to air coming in from outside you also have what's called outgassing (in this case, probably mostly water vapor). One of the garage the vacuum sealer is pulling out is water, and so the bananas and other elements will start to dry out faster and all that water goes into the air of the chamber.
posted by Lady Li at 3:35 AM on May 25

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