Pouch guide
Coffee Bag Valves and Degassing: What Roasters Need to Know
How much gas roasted coffee releases, why a one-way valve matters, where it sits and how it affects the bag.
Why does roasted coffee give off gas?
Roasting creates carbon dioxide inside the bean. Once the beans cool, that gas works its way out over the next one to three weeks. Roasters call this degassing.
The amount is larger than most people expect. A kilo of fresh roasted beans releases roughly 10 to 20 g of carbon dioxide, which is 5 to 10 litres of gas at room temperature. Around 40 percent of it comes out in the first 24 hours, and dark roasts give off more than light roasts.
Ground coffee degasses faster, because the gas has less distance to travel. Most of its gas is gone within a day or two, which is one reason ground coffee goes stale sooner.
What happens if you seal coffee in a bag with no valve?
The gas has nowhere to go. A sealed bag of fresh beans swells like a balloon, the seals come under pressure and, in a warm van, the weakest seal lets go.
The older fix was to let the coffee rest for a week before bagging, so most of the gas had gone. That costs a week of freshness and shelf space. The other fix was to leave the bag unsealed, which lets oxygen in.
Oxygen is the real enemy. Within days of exposure, the aromatic compounds that make coffee taste of something start to oxidise. A bag that lets gas out but no air in solves both problems at once.
How does a one-way degassing valve work?
The valve is a small plastic disc, about 20 mm across, welded to the inside of the bag over a hole of 2 to 3 mm in the film. Inside the disc, a rubber membrane sits over a small outlet.
When the pressure inside the bag rises above the pressure outside by a few millibars, the membrane lifts and gas escapes. When the pressure equalises, the membrane closes again. Air cannot push its way in, because the pressure outside is never enough to lift the membrane the wrong way.
A thin film of silicone oil under the membrane makes the seal airtight and stops coffee dust holding it open. A filter on the inside keeps fines out of the mechanism. The coffee pouches with a valve on this site are fitted with that type.
Where should the valve sit on the bag?
The valve goes on the back or a side gusset, 30 to 50 mm below the top seal, where it is above the fill line once the bag is full. Below the fill line, coffee presses against the membrane and can hold it open.
On a stand up pouch it is usually on the back panel, offset to one side so it clears the zipper and the tear notch. On a flat bottom bag it often sits on a gusset, which keeps both faces clean for print. On a side gusset bag it sits on the back, centred.
It is punched and welded before printing is finished, so the proof shows a circle where it will land. Keep text, barcodes and the window at least 15 mm from that circle. A flat bottom coffee bag gives the most freedom, because the gusset is otherwise plain.
- Stand up pouch: back panel, one side, 30 to 50 mm below the top seal
- Flat bottom bag: on a side gusset or the back panel
- Side gusset bag: back panel, centred
- Never below the fill line, and never across a seal or fold
Which coffee bags need a valve and which do not?
The rule is about timing. If you bag coffee within about ten days of roasting, you need a valve. If you rest the coffee longer than that, the pressure is low enough that a plain bag survives, though you lose freshness.
Whole beans need the valve more than ground coffee, because they hold their gas longer. Ground coffee packed within a day or two of grinding still needs one.
Coffee that is not roasted does not need a valve. Green beans, instant coffee, coffee bags that steep like tea and cold brew grounds all go in a plain barrier pouch.
| Coffee | Packed when | Valve | Why |
|---|---|---|---|
| Whole beans | Within 10 days of roast | Yes | 5 to 10 litres of gas per kilo still to come |
| Whole beans | Rested over 2 weeks | Optional | Little gas left, but a valve still protects a long shelf life |
| Ground coffee | Within 2 days of grinding | Yes | Fast, heavy degassing in the first day |
| Ground coffee | Rested over a week | Optional | Most gas gone, oxygen is the main risk |
| Nitrogen flushed beans | Any time | Yes | The valve lets the flush gas and CO2 escape without air returning |
| Green beans, instant, pods | Any time | No | No degassing, a plain barrier pouch is enough |
How does a valve change the bag film?
The valve lets gas out, so the bag relies on its film to keep oxygen from coming in anywhere else. That means a foil or metallised laminate. A kraft / PET / PE pouch with a valve is a half measure: the valve works, the film does not.
A foil laminate of about 100 to 120 microns is the usual build. For a 12 month shelf life, aluminium foil. For 6 to 9 months, metallised PET is acceptable.
A paper look is still possible. A kraft / foil / PE laminate puts paper outside and foil inside, and the valve welds through both. The kraft coffee pouches with foil behind the paper are offered with a valve in exactly that build.
How does a valve affect filling and sealing?
The bag arrives with the valve already welded in and its top open. You fill, settle the coffee, and heat seal the top. Nothing about the valve changes that, as long as the valve sits above the fill line.
Settle the beans by tapping the bag before sealing. If beans are heaped up against the valve, fines can lodge under the membrane. Keep the seal area clean of grounds, because a single ground in the seal is a leak.
A bench or line sealer set between 130 and 160 °C, held for one to two seconds, seals a standard PE sealant. If you nitrogen flush, the flush gas leaves through the valve over the first hours and the bag flattens. That is normal.
- Fill by weight through the open top.
- Tap the bag to settle the coffee below the valve.
- Wipe the inside of the seal area if any grounds have caught.
- Seal across the full width at 130 to 160 °C, 1 to 2 seconds.
- Stand the bags upright for the first day while they degas.
What goes wrong with coffee bag valves?
Most valve complaints are not valve faults. A valve below the fill line, a bag overfilled so the seal is wrinkled, or a kraft bag with no barrier film behind it account for nearly all of them.
A genuine valve fault shows as a bag that swells hard and stays swollen. A bag that flattens slowly over a day or two is working.
Two more to watch. A window cut into a valve bag weakens the barrier the valve relies on. And a bag stored flat on its back, valve down, can let coffee rest on the membrane.
- Valve placed below the fill line
- Overfilling, leaving no clean film for the top seal
- Kraft or clear film with a valve and no barrier behind it
- A large window on a bag that needs full barrier
- Expecting a bag to stay puffed up: a valve bag should relax, not swell
How do you order coffee bags with a valve?
Send the fill weights, the bag style, the laminate and the quantity, and say whether you nitrogen flush. If you want the valve on a particular panel, say so and the proof will show it there.
You get the price in writing, starting at £0.25 for each bag at the quantity you name. A free design proof marks the valve, the zipper and the seals. Bags are made in 4 to 6 business days once you approve, then sent free and tracked to your UK address, which takes 2 to 3 business days. The stand up pouch sizes guide covers the dimensions for 250 g, 500 g and 1 kg.