Why your beans taste flat two weeks after roast day

You bought good beans. The first week was excellent. Now the same recipe tastes muted and slightly dull, and nothing about your routine has changed. Nothing is wrong with your technique. The coffee simply moved.

Roasting fills the bean with gas

Roasting produces carbon dioxide, and a lot of it stays trapped inside the bean afterwards. Over the following weeks it escapes. That process, degassing, is why fresh coffee blooms so dramatically when you wet it, and why very fresh coffee can brew unevenly: the gas physically pushes water away from the grounds.

This is why the one-way valve on a bag of coffee exists. It lets that CO2 out without letting oxygen in.

The window

For most filter roasts, the useful range is roughly 7 to 21 days past roast. Before day seven, the brew is turbulent and the flavours have not settled. After three weeks, the aromatics that made the coffee interesting have largely left.

Espresso tends to want a little longer to settle, often 10 to 28 days, because the pressure of the shot is unforgiving about excess gas.

Check the bag. If it prints a best-before date instead of a roast date, that tells you the roaster is not competing on freshness, and you should judge the coffee accordingly.

Oxygen is the real enemy

Degassing is a clock you cannot stop. Staling is one you can slow down considerably.

• Keep beans in the bag they came in, rolled tight, valve intact.
• Buy quantities you will finish in three weeks. Two 250 g bags beat one 500 g bag.
• Grind immediately before brewing. Ground coffee stales in hours, not weeks, because you have just multiplied its surface area enormously.
• Store somewhere cool and dark, but not the fridge. It is humid, and coffee readily takes on the smell of everything near it.

Freezing genuinely works, but only properly: airtight, in single-dose portions, frozen once, and ground straight from frozen. Repeatedly taking one bag in and out is worse than doing nothing.

What to change when it happens

If your coffee has drifted past its window, do not chase it by grinding finer and finer. You will extract more of what is left, but what is left is the dull part. Adjust for the coffee you have now, and buy a smaller bag next time.