Airtight Is A Seal, Not A Lid That Fits

A lid that clicks on is not an airtight closure. Genuine sealing requires a compressible material between two rigid surfaces, and most containers do not have one.

What a seal actually needs

Two hard plastic edges pressed together leave microscopic gaps, because neither surface is perfectly flat and neither deforms to fill the other's irregularities.

A rubber or silicone gasket squashes into those gaps, and clips or a screw thread supply the pressure that keeps it compressed.

Without both parts, a container is closed rather than sealed, which is fine for a day and insufficient for months of storage.

What gets through an imperfect closure

Moisture moves in both directions depending on which side is more humid, softening crisp things and hardening moist ones over weeks.

Oxygen drives fats towards rancidity, which affects nuts, seeds, wholemeal flour and anything containing oil long before it looks spoiled.

Odours move too, which is why coffee and spices flavour a cupboard and why butter picks up whatever is beside it in a fridge.

Glass, plastic and metal differ beyond the seal

Glass is impermeable and does not retain odours, but it is heavy, breakable and transparent, which matters because light degrades some foods.

Plastic is light and durable, though it can absorb colour and smell over time, and thin containers flex enough to break a seal when stacked.

Metal tins block light completely and are the traditional choice for tea, biscuits and coffee, but they rarely have a true gasket unless designed for it.

Maintenance of the seal itself

Gaskets take on odour and grease and should be removed and washed, which is a step most people never take because the seal is not obviously dirty.

Silicone hardens with age and repeated dishwasher cycles, and a stiff gasket no longer compresses, so an old container quietly stops being airtight.

Lids warp with heat, so containers washed at high temperature or used for very hot food often lose their fit before anything else fails.

Matching the container to the contents

Crisp dry goods need genuine airtightness, while root vegetables and some produce actively need air movement and rot faster when sealed.

Filling a container as fully as possible reduces the headspace, and less trapped air means less oxygen working on whatever is inside.

Decanting into a smaller container as the contents diminish is the same reasoning, and it does more for shelf life than the quality of the container it started in.