Hey ya'll,
My friend Greg - a great home roaster, and chemist - wrote this up for me regarding chemical changes that occur to coffee shortly after roasting.
I've always pointed out that the flavor changes for several days, often fairly significantly. This post attempts to shed some light on the chemistry behind it. Yay!
There are several complex processes at work after roasting that result in the development, and eventual degradation of flavor. It is not only oxidation, far from it, in fact. Oxidation is (mostly) responsible for the eventual degradation of flavor. Many processes result in the development of flavor. I will try to describe these a bit, but I will have to simplify things a lot to do so... A reasonably stable green coffee bean is roasted, which is kind of like having a little revolution within the bean. All kinds of irreversible oxidations take place, as well as some reductions and some dehydrations. Some compounds bond together, as in the caramelization of sugars, while other larger molecules are fractured. The pieces of the fractured molecules are typically quite a bit more reactive/unstable than they were in their original state. After this turmoil, it takes time for things to settle down. It is these reactions, that occur between these newly-formed, less stable compounds that cause the development of most coffee flavor.
1) Carbon dioxide (CO2) is gradually released from the beans. This predominantly comes from the decarboxylation of chlorogenic and other organic acids. Some is created during the roast, but trapped within the cellular structure until it slowly leaches out. Some of the decarboxylation occurs over time. The pH changes as CO2 is lost, which causes other reactions to commence (or cease), as well as changing their relative reaction rates. The amount of CO2 coming from each process, roasting and post-roast reactions, will proportionally vary with the degree of roast. This is a facet that favors some aging, as the flavor improves as the CO2 goes away.
2) Many of the fruity flavors that 'grow in' come from organic acids and esters. Some of these form from aldehyde-like compounds that were created during the roast, and take a bit of time to develop. Some form from decarboxylated acids. Sometimes you will even see this happen in the cup when very freshly roasted coffee is brewed, and the cup gets fruitier as it cools. Some of these acids and esters cannot form while there is a lot of CO2 or associated pressure present, so again, this process requires some aging.
3) I think of the body growing in as analogous to Jello or gravy solidifying. There are lots of little pieces and tidbits of hydrophobic molecules that gradually combine to form 'fatty' and 'oily' compounds that give coffee its body. This also happens over time and is dependent on the amount of initial reactants available as well as their environment. This is a process that favors a little aging, and is one more reason that the degree of roast influences the amount of time it takes for coffee to 'peak'.
4) Many of the compounds that are the most aromatic and flavorful are also volatile or semi-volatile. Like CO2, these will gradually leave the bean by evaporation. So this is a process that mostly favors freshness of roast, although some of these do form after roasting so there is a small aging factor, too.
5) Oils and fatty compounds begin to taste rancid as they get oxidized. While keeping oxygen away from your roasted coffee is the goal, there is some oxygen and other oxidizing material still entrained in the roasted beans, so gradually the oils will deteriorate even if you practice good prophylaxis and isolate your roasted beans from air using vacuum and valve bags. So this is a process that, again, favors freshness of roast.
6) Oxygen is a very small molecule, and no matter how rigorous your attempts to isolate your beans from it, it is going to gradually get in there. Reactions occur based on 'molar ratios', where a 'mole' is a specific number of molecules. A mole of oxygen gas molecules weighs about 32 grams, and each molecule can oxidize two other molecules (which, themselves, may go on to oxidize further molecules). A mole of of tasty coffee flavorant will be mostly be between 110 and 240 grams, depending on the specific flavor. So a relatively small amount of oxygen can ruin a much larger amount of coffee. Not only that, but the human palate is rather sensitive to some of the oxidized flavors, so in some cases a few parts per million can produce a significant off-flavor.
All that drivel was just basically to say, 'your coffee is eventually going to go South, drink it before it does'.When you have hundreds of competing reactions, things like the rate of CO2 loss can make a difference in the eventual flavor profile. Also the storage temperature of roasted coffee can make a difference in the rate of these reactions. I don't know that this has been extensively studied, at least in the primary literature. Many compounds in roasted coffee are light-sensitive, as well, so if you have them in a mason jar on the counter they are going to taste different than if you keep them in a dark cabinet.
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