Roasting coffee beans is a crucial part of making them palatable, be it for the brewing of coffee, or for other uses, such as covering them with chocolate and selling them to the unwary. Attempting to brew a cup of coffee with unroasted beans would produce a fluid reminiscent of sour grass clippings, perhaps with a faint hint of boiled sticks.
As with so many other foods, coffee requires the careful application of heat that caramelizes sugars and alters molecular bonds to produce a flavor enjoyable to humans. Tastes vary however, and there is contention as to what manner and degree of roasting will be most pleasing. Added to this debate is the fact that companies may use different names for a certain level of roast, and may toss confusing or misleading claims at the shopper. The virtues of tumble-roasting high altitude beans in clean mountain air aside, there are some things the consumer should know when perusing the selection of beans available to them.
1. The quantity of caffeine in a coffee bean diminishes the more it is roasted, though not to an extreme degree. Roasting does reduce caffeine noticeably, but not to such a degree you should let your addiction scare you away from trying a darker roast.
2. The darker the roast, the stronger the flavor...to a point. As a bean is roasted longer, more flavors are created and sugars caramelized, which helps produce a stronger tasting cup of coffee. If roasting is taken too far however, these flavors will begin to burn off, resulting in a thin cup which lacks body. Or a visit from the fire department, depending on just how long the beans get left in the roaster.
3. Coffee beans lose their "origin flavor" if they are roasted past a certain point. Coffee beans come in many varieties, and are grown in many different places. These factors all lend subtleties to their flavor, usually referred to as origin flavor. Light roasting complements these flavors, but heavy roasting tends to overwhelm them.
Some specific roasts:
Quantifying the complex changes that coffee beans undergo when they are roasted is no easy task. The darkness of the bean is the traditional yardstick by which a roast is sold, however it's an inexact science. Roasters must pay careful attention to aroma, color, temperature, and the noises beans make at various degrees of roasting. Thankfully, there has been some standardization in recent years, so the naming of different roasts is more reliable than it once was. The Specialty Coffee Association of America has divided ground coffee into 8 different shades, beginning with 95 (the lightest) and subtracting 10 for each shade until 25 (the darkest) is reached. Technically the scale goes below 25, but such is more akin to charcoal than anything else. When ground coffee is produced, it is compared to these shades, and the appropriate roast name is assigned to it. This numbering system is known as the Agtron system.
In spite of this system, there is still room for confusion, since some terms, like "French roast" and "Espresso roast" are used so variably they are almost meaningless. That's what we all have to deal with when marketing nitwits won't agree to a standard. Anyway, the names below are as accurate as can be expected.
This roast corresponds to Agtron #95, and is commonly reached at around 375 degrees Fahrenheit. This isn't generally considered to be a 'true' roast, and is not commonly sold, even at specialty shops, as this is simply a stage a coffee bean goes through as it starts releasing moisture and changes from green to a cinnamon-orange color. This roast still doesn't taste very good due to it's high acidity and low caremelization. Probably the only time you'll drink this is when unscrupulous manufacturers mix it in with a darker roast to cut costs.
Light, "City" roast:
This roast corresponds to Agtron #85, and is commonly reached at around 395 degrees Fahrenheit. As steam pressure builds within the beans, their cell walls begin to rupture, producing a noticeable popping sound. This is known as the "first crack", and is an important signpost for roasters. Lighter roasts such as this one seem to be popular in "breakfast blends", possibly due to their high caffeine content and milder flavor.
Medium "Full City" roast:
This roast corresponds to Agtron #55, and is commonly reached at around 430 degrees Fahrenheit. It is at this point that caremelization starts going at full swing, and the beans begin to undergo pyrolysis and emit carbon dioxide. As this gas is produced, it further ruptures the cellular structure of the beans, producing a "second crack". This is a popular roast for those who want to maximize the origin flavor of the beans, since beyond this point it will start to degrade.
Dark "French" roast:
This roast corresponds to Agtron #35, and is commonly reached at around 450 degrees Fahrenheit. By this point, the reactions inside the beans have been producing heat of their own, and the beans have been heavily caramelized. Much of the origin flavors have been consumed, and roasting too far beyond this point will begin to degrade all other flavors.
Very Dark "Italian" roast:
This roast corresponds to Agtron #25. It is commonly reached around 480 degrees Fahrenheit. Italian roasted coffee is generally considered the darkest roast that still tastes good, and at this point is in danger of lapsing into bitterness and char. I'd stress that it's only in danger of that, since I happen to like this level of roasting just fine.
Darkest "Spanish" roast:
Beyond Italian, but before charcoal. Few like this extreme level of roasting, as all of the better flavors in the beans have pretty much gone off to the land of wind and ghosts.
Goodness me, what a nightmare this was to research. I read no less than six separate guides and standards reports, and they all disagreed wildly with each other on nearly every point imaginable, from what makes coffee beans shiny, to what any particular roast is, to what the Agtron standard actually applies to. I should stick to easier things, like which religion is the best one. Needless to say, I am requesting feedback on any inaccuracies that slipped through my grasp until further notice. Professional insights only, thanks, I've already reached the limit of what internet searching can yield. :)