How to store coffee:
Put coffee in an airtight container and freeze it. Thaw only what you will use in the next week. While thawing frozen beans, keep them in an airtight container so moisture doesn’t condense on them. Once up to room temperature, keep them in an air tight container until use.
What we perceive as “taste” is really mostly smell. With coffee, this is provided by volatile (means easily evaporated, as opposed to flammable) chemicals. Like alcohols, aldehydes, esters, furans… They evaporate easily, forming equilibrium between their gaseous state and the bound state in the beans. The higher the temperature, the more these volatiles will favor the gaseous state. If you store coffee in an unsealed container at room temperature (like the hopper of your coffee grinder) these yummy vapors dissipate, leaving the coffee relatively flavorless. If they are in a sealed container, they can’t go anywhere.
There are lots of non-volatile chemicals which give coffee its flavor as well (carbohydrates, lipids, organic acids, etc). They are not volatile, so they aren’t going to leave the coffee as vapor. But even if they don’t evaporate, they can oxidize. Oxidation means chemical reaction with oxygen. As a rule, chemical reactions double in speed every 10*C increase in temperature. That’s why coffee flavor is preserved longer in the freezer than at room temperature. You can also reduce oxidation by reducing oxygen. Ideally, a coffee container should be purged with an inert gas like Nitrogen, CO2 or Argon before closing it. But not even I am fanatical enough to bother. Fortunately, commercially packaged coffee bags are self-purging. Coffee off-gasses (mostly CO2) after roasting. That little button on your coffee bag is a vent to let gas out so the bag doesn’t blow up like a balloon. This venting displaces most of the oxygen that was trapped in the bag before it was sealed. That’s why commercially packaged coffee has a reasonable shelf life.
- Buy whole coffee beans in sealed multi-layer bags with a vent button. Never buy coffee from an unsealed bin unless you are buying direct from a roaster. If they can’t tell you the day the beans were roasted, don’t buy them.
- Immediately freeze any coffee which will not be used within a week.
- When the multilayer bag is opened, dump the beans into a ZipLoc, expel the air and return to the freezer.
- Thaw beans in a sealed container. Those old fashioned wire-rimmed snap-top glass jars work great.
- When grinding, remove only beans you will use immediately and re-seal the container. Don’t keep beans in the grinder hopper. It isn’t air-tight.
Any Baristas who have been mind-melted by Starbucks will tell you it is bad to freeze coffee. Rubbish. Starbucks doesn’t want to pay for a cold chain or maintain fresh stock, so they store bagged coffee at room temperature for up to 34 weeks, That is over half a year !! https://athome.starbucks.com/helping-keep-coffee-fresh.
There is some buzz about grinding frozen beans. The reason for the buzz is that a study showed that grinding REALLY COLD beans (liquid nitrogen temperature, not deep freeze temperature) produces a smaller, more uniform particle size. This may be relevant for industrial grinders, but not at home.
Bottom line: grinding frozen beans will produce a finer grind for a given setting on your grinder. If you have found the “perfect setting”, tweak it a bit courser if you are grinding frozen beans. The cold coffee will get some moisture condensation and the coffee will be “clumpy”. You may need to disassemble and clean your grinder more often. I think it’s better to wait for the beans to warm to room temperature before grinding.