Photo by Frank Jakobi/Flickr CC
Storage is a complicated subject. This is a quick overview, and there are some specific circumstances that will be neglected. We'll pick those up later or in response to comments.
Once you have your coffee beans at home, the best indicator of freshness is aroma (at room temperature) and taste. A visual indicator is the amount of "bloom" when you pour the water over the coffee. Coffee roasting creates significant amounts of carbon dioxide within the bean. Grinding releases the CO2, which carries the aroma into the room. (Smells great, doesn't it?) The remaining gas will be liberated as foam during brewing. Generally, the more bloom there is, the fresher the beans. (Geek note: the volume of CO2 varies among varieties. The range is three to 20 times the bean volume.)
All coffee is fresh when it comes out of the roaster. What happens later changes the freshness profile profoundly. The very best practice, of course, is to buy your beans weekly at a reputable shop that carefully monitors its inventories and refuses to sell beans past several days out of the roaster.
Oxygen, time, and temperature are the enemies of all food freshness, and oxidation accelerates with higher temperature and slows with lower temperature. I recommend the refrigerator for coffee beans that will be drunk within a week or two. [Curator's note: Unlike Jerry, I'm not a refrigerator guy--ground coffee is an ancestor of baking soda as a refrigerator deodorizer, but if you've got it in an airtight container, you'll at least avoid onion-y coffee.] For longer storage, use the freezer--but observe the caveats below.
N.B. If you are making coffee in an espresso machine, leave the coming day's coffee at room temperature, sealed, away from heat. Cold coffee, directly from the refrigerator, will chill the brew, diminish the crema and inhibit good extraction. For all other methods--assuming proper brewing temperature of 195 to 205° Fahrenheit--brewing the coffee from the refrigerator will not significantly affect the brew temperature adversely because of the high ratio of hot water to coffee grounds.
Unscrupulous roasters have been known to load their coffee with as much as ten percent moisture to increase their profits.
Use the freezer for longer storage. For example, if you bring home a pound of beans, divide it into weekly amounts to store separately. For this week's coffee, leave it at cool room temperature or seal it and put in the refrigerator. The remaining weeks can be put into the freezer to be removed a week at a time.
I don't recommend taking the coffee back and forth from the freezer to the brewer each day because this coffee soon begins to taste flat. I suspect this is the result of condensation on the beans when they are opened in a much warmer atmosphere. The repeated condensation being absorbed into the coffee is what seems to cause the negative effect. [Curator's note: I think it goes flat very soon! The open question is distribution of the flavor-bearing oils in beans and whether after freezing, in which the oil is redistributed, it ever diffuses to where it was in the beans; for more on the discussion, see my chapter on storage in the Baldwin-filled Joy of Coffee.]
The optimal storage vessel should be constructed of aluminum or Mylar foil, backed with a flexible poly sheet. This is similar to the bags that many roasters use. At home I store coffee in such a bag or double--even triple--layers of poly bags. I roll the bag tight to squeeze as much air as possible from the bag to minimize the oxygen available for the coffee to interact with. I close the bag with the wire closure or a rubber band to keep it tight. [Curator's note: I still use what I think of as the Jerry Method: Scrunch that bag just as far down and as airtight as you can get it, a la the Right Way to squeeze a toothpaste tube, and if the wire closure is askew or missing, as by the end of a bag it almost always is, fish out a rubber band.]
I don't use airtight jars because of the larger quantity of air trapped inside the jar. Coffee and oxygen interact in the first few hours of their exposure to one another. In a sealed bag of coffee, oxygen can be measured just after packaging. By several hours later, oxygen is no longer detectable because it has already oxidized some of those aromatic volatiles. But it's a bad idea: the damage is done. For short-term storage, say a week of bean storage, you may not notice the effect. I haven't tried the containers with the pseudo vacuum pump for coffee because I never thought they had much effect for wine. (Lower temperature and full containers work best for preserving opened wine, too.)
The temperatures in a refrigerator and a freezer are geared to the freezing point of water. The only part of the coffee that actually freezes is the moisture content; the rest continues its organic processing, especially in the presence of water or oxygen, though the process slows at lower temperatures.
Few specialty roasters leave significant moisture in their coffee. The residual moisture in a coffee will be about two percent if a water quench is not used at the end of the roast (which is Peet's Coffee's practice). As roasters do use water quench, the moisture level rises. (Unscrupulous roasters have been known to load their coffee with as much as 10 percent moisture to increase their profits.) But using a water quench does diminish the complexity of the flavor of the coffee. Although our dry roast practice makes roasting more challenging, the preservation of flavor is definitely worth the effort.
Geek note: lower MVTR (moisture vapor transmission rate) and O2 permeability rate (standard industry benchmarks) of packaging films and laminations indicate superior packaging. Obviously, a jar has superior barrier characteristics. A collapsible jar would be perfect, but I don't know of one.
And don't even think about buying ground coffee. [Curator's note: I'm not telling Jerry how we order our weekly In Mourning For Sierra Dorada blend--a new combination just yesterday. But he'd approve, at least, of the constant refreshing and experimentation.]
