One of the reasons I left Coffeed -- every topic turns into ten posts by the usual suspects talking up their "craft experience," rather than answering the question, as if it were an advertising bulletin board from the 15th century. By the time that's done, everyone's eyes have glazed over in boredom and anything interesting posted to the thread is lost. If these usual suspects bothered to pick up any of the works on coffee research and perhaps a food science text to help decode what they are reading, they would be able to give a fairly precise answer to the question, since a lot of the required information is well known.
There are three basic things happening when you roast coffee: 1) water evaporates, 2) organic acids and aromatics break down or are boiled off, and 3) water, sugars, and amino acids combine in a chain of chemical reactions collectively called the Maillard reaction.
- Water Evaporating This happens from the moment the beans are heated up to the first crack, when the remaining free water escapes. As the beans exceed 300F, a steam wave moves out of the bean from its center. This wave also starts the Maillard reactions (see below). Below 300F bean temperature, not much is happening except water evaporating. If the bean is too moist going above this, the bitter organic acids will not break down as quickly (see next section). If too much water evaporates, the Maillard reactions are starved, and the roast will be dominated by dry distillates. The drop-in temperatures or early bean heating should be adjusted to achieve this balance
- Organic Acids and Aromatics Breaking Down or Boiling Off Roasts have to go to partway into the first crack, since this is where the chlorogenic acids that make unroasted coffee intensely bitter finish their breakdown. The smaller acid molecules and aromatics, which are responsible for the fruity aromas and acidic tastes start breaking down around around here. The longer the beans stay at temperatures above about 390F, the lower the acidity of the result. For instance, it is said that Italian espresso roasters stall the beans around the first crack to reduce acidity without reaching temperatures that caramelize off the sugars. Too much of this is a roasting flaw called baking, which overly flattens the flavor of the coffee.
- Maillard Reaction Chains This is where the complexity of coffee is created, since these reaction chains are hugely complicated. However, there are some overall guidelines. The early Maillard reaction, from 300F to the 1st crack, creates nutty, toasty, and woody flavors. At higher temperatures, sugars stop reacting with amino acids and start caramelizing on their own, creating caramel, vanilla and chocolate flavors. Finally, also at higher temperatures, and when the water required for early Maillard reactions and caramelization runs low, the Strecker degradation changes the compounds created earlier in the roast to dry distillates: smoky, spicy and peaty flavors. The simplest lesson here is that these processes compete for water, so that taking longer in the ramp to the first will and less time thereafter will favor the woody, toasty, nutty flavors and reduce the caramel ones. Also, if the roast is to get very dark, the only way to avoid overwhelming distillate flavors is to dry the beans less (go as fast as possible earlier in th roast), so the Strecker degradation creating these flavors is controlled.
There is also the inconvenient fact that beans are 3 dimensional objects. Roasts faster than about 6 minutes will have significant differences between the inside and outside of the bean, and the inside can be under roasted; slower roasts will be more even. However, the slower a roast, the more the aromatics are cooked off.
So, after making sure one has dried the bean to the right degee in the roast below 300F, and balanced the flavor developments that takes place below and above 400F, one should roast as fast as possible.