Compostable tea bags and coffee capsules

Both products are small, single-use, and hold hot water — but their engineering problems are different, and each is instructive about a different failure of compostability claims.

The plastic in a paper tea bag

A conventional tea bag is made of filter paper that must be heat-sealed closed at high speed. Cellulose fibres do not soften and fuse on heating; they char. So heat-sealable filter paper contains a thermoplastic — typically polypropylene fibres at roughly 20–25 per cent — which melts at the seal and bonds the layers.

That is why a bag which looks and feels like paper is neither compostable nor plastic-free. The polypropylene remains after the cellulose has degraded, as fine fibres in the compost.

Pyramid and mesh bags are usually more plastic, not less. Nylon and PET meshes are entirely plastic; PLA meshes are marketed as the compostable answer. The point worth knowing about a PLA mesh is thermal: PLA’s glass transition is roughly 55–60 °C, and brewing water is 90–100 °C. The mesh is therefore well above its glass transition in the cup — softened and rubbery rather than glassy — which is a different physical state from the one it was tested in at room temperature.

The genuinely plastic-free routes remove the requirement instead of substituting for it: unbleached abaca or manila hemp paper closed by folding, stitching or a stapled string rather than by a heat seal.

A capsule has four requirements that fight each other

An espresso capsule must simultaneously:

Exclude oxygen for months. Roasted coffee stales by oxidation of its lipids and loss of volatile aromatics; an unprotected roast degrades noticeably in weeks. This demands a barrier close to absolute, which is why aluminium is the incumbent — a metal foil is effectively impermeable, and no polymer is.

Withstand brewing pressure. Espresso extraction runs at roughly 9 bar at the puck, and machines generate higher upstream. The capsule is a small pressure vessel that is also being pierced.

Survive a hot, wet, acidic extraction without contributing flavour.

Then degrade. Which is a requirement to be structurally weak, in a body that has just been required to be strong and impermeable.

Compostable capsules resolve this with multilayer construction — a PLA or PBS body with an added barrier layer — and generally accept shorter shelf life than aluminium. That trade is the honest description of the product, and it is usually stated in the shelf-life figure rather than in the marketing.

The operational failure is separate from the material one

A capsule can be genuinely certified to EN 13432 and still not be composted. Industrial composting facilities run mechanical pre-screening to remove contamination, and a compostable capsule is not visually or mechanically distinguishable from a conventional one on a sorting line. Many are removed and sent to incineration or landfill regardless of what they are made of.

This matters because it is a different kind of claim from the material one. The material claim is about chemistry and can be certified; the outcome claim depends on collection systems, facility policy and residence time, none of which the manufacturer controls. A facility running a 4–6 week cycle may also simply be faster than the 12-week disintegration window the standard permits.