What we are protecting against
The same things as in transit, just for longer. Heat, humidity, light and anything aromatic nearby. Green coffee pulls moisture out of the air and it absorbs smells, so a warm damp storeroom is slowly undoing the drying work that happened at origin.
Moisture is the one with a number attached. Drying at origin brings a seed down to somewhere around 10 to 12 percent, which is the range the specialty grading standards specify and test for1. If storage pushes that back up, you are heading toward mould risk. If the room is too dry, the coffee carries on drying and goes flat and papery. Stable matters as much as cool.
Storage does not just slow decline. It changes the score.
Researchers published a study in Scientific Reports where they stored green coffee under a range of different conditions and then cupped it at intervals against the SCA protocol. Samples held at -10°C and at 10°C were still scoring as specialty after three months and after six, and several were still close to that 80 point line even at nine months2.
So this is not housekeeping. Cool and stable storage is the difference between a coffee still being specialty grade when you open the sack and it not being. The farm earned those points. Storage is where you either keep them or hand them back.
Why we do not buy a year of coffee at once
Because of everything above. It is tempting to lock in a price and take a whole harvest, and for some coffees that is the right call. But every extra month a lot sits in storage is a month of slow change, and for the delicate high grown lots that change shows up in the cup faster than it does in a cheaper coffee.
Buying in smaller, more frequent lots costs more per pound and gets you better coffee in the bag. That is a trade we make on purpose.
Try this
Take a bag you have had open for a month and a bag you opened this week, brew them the same way, and taste them next to each other. What you are tasting in the older one is the same slow flattening that happens to green coffee over months. Same process, just further along.