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Storage of Roasted Coffee Beans to Maximize Flavor
by Bob Barraza
It is fairly common to hear from home roasters that the biggest improvement they have every made in their espresso is when they started roasting at home. They state that nothing brews like fresh roasted coffee. As a chemist, this makes a lot of sense to me since roasted coffee is so unstable and continuously deteriorating.
The thing that makes espresso unique amongst all coffee brewing methods is its use of CO2 in the extraction process. Under the conditions of temperature and pressure encountered in pulling a shot, the CO2 from the coffee dissolves in the water forming carbonic acid which lowers the pH (acidity) of the water. This in turn affects the relative solubility of the hundreds of components that reside in the coffee grounds and it changes the profile of the compounds that are extracted. As the extract starts to pour from the basket and it returns to ambient pressure, the dissolved CO2 boils off and creates that honey-like crema which lets us know that we are in the sweet spot. Not too different from popping the cork on a fine bottle of champagne!
So where did the CO2 come from? The coffee roasting process involves the oxidation or controlled burning of the coffee bean. In case you have forgotten, the oxidation of organic compounds results in almost entirely CO2 and water as the byproducts. Since CO2 is a gas at ambient conditions, the gas trapped in the beans will 'boil off' and slowly leave the roasted beans. Fresh roasted beans are loaded with CO2 , and they generally require about a day of 'out gassing' before they make excellent espresso.
I have often read that the best espresso is made from beans that are 2 to 10 days post roasting. I am not surprised that there is a lot of controversy over this range because it will eventually be a subjective preference over the profile of compounds that are extracted. Remember that the age will determine the amount of CO2 remaining in the beans which will not only impact the amount of crema but also the amount of carbonic acid in the extraction water.
So how can we keep the CO2 from escaping? Well, if we have an impermeable container made of either glass or metal with a metal cap and tight seal we can only slow the process down. Very freshly roasted beans in a gas-tight container could cause the container to burst, so some room for escape is needed. However, this type of container would also be an excellent way to protect the roasted beans from some of its other primary enemies: water, oxygen, and light.
The aromatic and flavor components created in the coffee roasting process are very fragile to say the least. They can quickly be degraded in the presence of water and/or oxygen by processes known as hydrolysis and oxidation. However, if kept dry, these delicate compounds are quite stable. The problem is that the roasted beans are super-dried during the roasting at high temperature and low humidity. In fact, the roasted beans finish up in an unnaturally dry condition. Given the chance, they will readily absorb moisture from the surrounding air. Even in an air conditioned environment there is sufficient moisture in the atmosphere for the beans to pick up enough moisture in a day or two to satisfy their thirst. Unfortunately this moisture will destroy the flavors that we are looking for in the cup.
So what kind of container works best? My favorite is a stainless steel canister that has a metal friction fit lid and a glass insert in the lid. This acts as a window to see the contents without having to open it. Keep in mind that every time the canister is opened there is an exchange of gases between the head space in the container and the ambient air which is your source of humidity and oxygen. These lids are not gas tight and they will not allow the CO2 to build up pressure. The good news is that as the CO2 leaves, it is taking some of the oxygen with it. Also, the metal can does not allow light to penetrate which acts as a catalyst for oxidation.
Some of my other favorite but less sexy containers are the Mason type canning jars, spaghetti sauce jars, and salsa jars. All of these have in common that they are made of glass and have metal lids. Warning: they require several cycles through the dishwasher before they are clean and odorless enough for use with your coffee, otherwise you will notice some unique 'oregano' and 'jalapeno' notes in your espresso! In fact, I recommend that you season the jars by storing your outdated coffee in them for a while to completely eliminate foreign odors.
Whether you roast at home, or buy freshly roasted coffee every week like I do, you will want to keep it as fresh as possible. Big containers with a lot of head space are not ideal. Use several small jars that will hold a few days supply and keep them sealed until ready to use. No need to refrigerate. In fact, if you open a jar that just came out of the refrigerator you will get lots of moisture condense on the cold beans just like it does on the milk jug when you pour milk for that cappuccino.
Based on the above, you will probably agree with me that the cute little brown bag with dog ears that we buy our precious beans in is less than ideal for anything other than quickly transporting the beans home. In fact, I take my jars to the shop and my friendly vendor weighs the coffee right into my funky sauce jars! Not quite Folger's 'locked in freshness' or Maxwell's 'good to the last drop', but a noticeable improvement over the paper sack.