Six months. Take that number home with you. It is the time it takes a bare green bean to lose the power to germinate. A coffee bean is a seed, and alive. And living things age. Here is "old beans taste bad", taken apart from the origin side.
The clock readers know is the roasted-bean clock. Weeks since the roast date, days since opening. But before that, another clock is already running: the green-bean clock. Between harvest and processing and roasting, green coffee ages quietly in warehouses, on ships, on the roaster's shelf. The freshness of your cup has started to be decided by this clock before the roast date.
(Research block: from here on, public data.)
The lead actor in green-bean ageing is lipid oxidation. In accelerated tests, acid value, peroxide value and free fatty acids kept rising during storage, and predicted shelf life was calculated at about 57 days at 40 °C and about 23 days at 60 °C (Molecules, 2020). The hotter, the faster the clock runs. Packaging has been confirmed to make a difference too (a demonstration of barrier packaging, 2022).
Another important finding comes from a German research group. The germination capacity (viability) of green coffee was lost in about six months in bare storage, and maintained for over a year in parchment. And in parallel with the loss of viability, a "flattening" appears in the cup (Selmar et al., 2008). Before off-notes appear, the outline disappears first. "Not bad, but nothing moving about it": that cup is, in most cases, this.
With roasting the clock switches to a different mechanism. Roasted-bean deterioration is three phenomena overlapping: volatilisation of aroma compounds (escaping with the CO2), lipid oxidation (rancid notes: secondary oxidation products such as hexanal), and moisture uptake. A trained panel has been reported to detect rancidity in air-packed roasted beans after about four months (SCA review).
And the greatest accelerator is the grinder. Grinding multiplies the surface area hundreds of times, and oxidation and volatilisation race ahead. The timing of grinding matters far more than any debate about storage containers.
So we take pains to slow the green-bean clock. We finish drying to the targets for moisture and water activity before storage. We store in a cool warehouse out of direct sun, never directly on the floor. We ship the oldest lots first. Modest, but each of these makes the freshness of a cup on the other side of the sea.
Three things for the buyer. Buy beans with a known roast date. Grind just before brewing. And buy only as much as you can finish in two weeks. Freezing and sealed containers I will write about elsewhere, but beside these three they are small refinements.
As a guide, after a few days of degassing, aroma peaks until around three weeks. Sealed, dark and cool storage extends it, but ground coffee has hundreds of times the surface area and deteriorates at once.
It depends on conditions. Research found bare green beans lost germination capacity (viability) in about six months, while parchment kept it for over a year. Higher temperatures accelerate lipid oxidation; cool, sealed packaging slows the clock.
Beans that have lost flavour under normal storage almost never cause harm. Deterioration is mainly a matter of flavour. Mouldy beans from hot, humid storage are another matter: do not drink them.
For the chemistry of green-bean deterioration see How green coffee degrades; for the foundation of drying and shelf life, The science of drying; for the link with sourness, The other reason your coffee is sour.
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