How to tell if stored freeze-dried food has gone bad

Check the bag before you check the food. A properly sealed and absorbed bag goes firm and slightly concave within a day of sealing and stays that way. A bag that has gone soft has either lost its seal or never had a working absorber, and the food inside needs judging rather than trusting.

The bag tells you first

• Firm and slightly sucked-in: the absorber worked and the seal held. This is what you want to see.
• Soft or puffy: air has got in, or the absorber was spent when it went in. Open it and assess.
• Visibly swollen: gas is being produced inside, which means microbial activity. Discard without opening it near food.
• Any moisture inside the bag: the food was not dry enough when sealed. Discard.

Then the food

Sign	What it means	Verdict
Still crisp, snaps cleanly	Correctly dried and stored	Fine
Soft, bendy or chewy	Moisture present	Discard
Clumped or caked together	Moisture got in	Discard
Any visible mould	Moisture, definitively	Discard – do not taste
Rancid or crayon-like smell	Fat oxidation	Discard – it will not hurt you but it is unpleasant
Faded colour, still crisp	Normal ageing	Fine, quality slightly reduced
Flat or cardboard flavour	Normal ageing or oxidation	Edible, use it in cooking rather than plain

The rule when you are unsure

Discard it. Freeze-dried food is cheap relative to the consequences, and the whole point of a long-term store is that you can afford to lose one bag. A stored food that makes you hesitate is not a food you want to be eating in the situation where you actually need your stores.

Do not taste-test suspicious food

This is worth stating plainly because it is a common instinct. If a bag shows any sign of moisture ingress or mould, do not taste a piece to check. Mould in a sealed low-oxygen environment can include species you do not want to sample, and a small taste is not a meaningful test anyway.

Preventing it, which is easier than diagnosing it

Nearly every failure traces back to one of two things: food that was not fully dry when sealed, or a seal that was not checked. Both take two minutes to prevent. Weigh a test tray and confirm the weight has stopped falling; then squeeze every sealed bag and listen for a hiss.