Oxygen
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What an absorber does
An iron-based sachet that reacts with the oxygen in a sealed package and takes it out of the air inside. It is what lets dry staples keep for years rather than months.
And it removes volume while it does it
Air is about a fifth oxygen. Removing that from a sealed container leaves less gas inside than outside, and the difference presses inward.
Which is why mylar bags suck in
The visible sign that the absorber worked. In a rigid container the same force has to go somewhere else.
A thin-walled jar can deform or draw its lid in
PET in particular is thin and has little column strength under vacuum. 153 of the 287 containers here are PET.
And a container that is not airtight simply fails quietly
The absorber consumes the oxygen, more leaks in, the absorber exhausts itself, and the contents sit in air. Nothing looks wrong.
Which makes the seal the whole specification
9 liners and sealing discs on this shelf, and no gasketed lid. The archetype asks for gasketed and the shelf does not have it.
Absorbers are sized to the headspace
Not to the food. It is the volume of air in the container that has to be consumed, which means a half-full container needs a larger absorber than a full one.
And they start working when the packet is opened
All of them, at once. A bag of absorbers opened on a bench is a bag of absorbers being used up while you fill containers.
They are not for everything
Dry staples, yes. Anything with meaningful moisture, no, because removing oxygen from a moist food creates conditions for organisms that do not need it.
Which is a food safety line this property will not cross
What may safely be packed with an absorber is a food question with real consequences, and it belongs to extension services and food scientists rather than to a container catalog.