Home wine makers will be happy to hear that roasting coffee beans is even easier - and the results are often as good as the pros.
A variety of roaster types are available, but even a frying pan or popcorn popper can be used. Be sure to start with ultra-clean equipment, though. Nothing spoils the taste of coffee like left over fish oils or butter.
Dark roasts contain a little less caffeine than lighter roasts, but they lack the acid taste of the latter. Be sure to start with quality beans, of course!
The beans will need to heat to between 460F (223C) and 530F (262C), so be prepared for some smoke. That's easily taken care of with a small room fan or stove top exhaust. Beyond the smoke there will be an odor, so your first experiments should probably be done with the windows open and no one home.
Put the beans in the roaster and turn up the heat! (Take care to be ready to temporarily disable those over-sensitive home fire alarms.)
For some roasters, the thermometer is built-in, but you may want to have an extra for when it's open, or for those frying pan experiments. Candy making thermometers work well for the purpose.
During the process those green beans will turn yellow, then brown. How brown depends on how dark you like your roast, which is always an individual choice.
As they begin to heat up, moisture - both oil and water - will put pressure on the bean surface and you may hear a loud crack when it bursts. Not to worry, this is normal. Stirring every 30 seconds or more, you'll hear this after four to seven minutes of heating.
The sugars inside will begin to caramelize (turn brown and 'burn' slightly) as the roasting continues. Again the degree is a matter of taste. Check the color every 30 seconds or so.
Roast long enough and sometimes a second loud crack will occur. At this stage the beans will be quite dark and for some palates a little overdone. Beyond the second crack you're really just burning the beans and boiling away the sugars. The results will be too harsh for most.
Pour into a metal colander to cool, then agitate. Since the roasting process produces chaff (a fine skin that detaches from the bean as they're agitated), you'll want some method for removing it. Mesh cooking screens are one option.
Try a few batches with varying degrees of time or darkening. Experiment to get the flavor you like. Keep in mind that the heat trapped in the bean will continue to cook it for a short while, so try stopping a little short of your desired end goal.
For the popcorn popper style roasting, be sure to get one that allows you to stir up the beans to keep them moving around and not sticking to the surfaces. For the stove top style, a cast iron skillet works great. Be prepared for lots of stirring and viewing. Roasting happens quickly!
What happens during the roasting process
To achieve a good roast you have to start with beans that have been skillfully selected and dried.
Some bean processors use a wash to remove the fleshy fruit from the bean and to separate different kinds of beans. Density differences in the bean will cause some to float higher, making for easier removal or separation. Others use a slower, more expensive dry-process.
Dry-processed beans will have a more subtle acid profile, while the acidity of wet-processed beans is more striking. Some acidity in coffee is desirable. The alternative is a flat, lifeless cup.
What happens to beans as they heat up during roasting?
During the process aromatics and acids, along with other flavor compounds, are produced in varying concentrations.
During the first stage the beans absorb heat and the green beans are slowly dried to a yellowish tinge. 'Green' doesn't refer to the color, per se, but simply to the beans being unroasted or raw. Properly done, the beans will have an odor reminiscent of toast or popcorn.
From about 170°C-200°C (338°F-392°F) sugars in the bean will begin to caramelize, aided by the increase in temperature of the moisture enclosed by the skin. That's just one reason it's important that beans have the proper moisture content, which comes from correct drying. Caramelized sugars are less sweet, so reaching the proper amount is important for the final brew.
At about 205°C (400°F), beans will expand to about double their original size and become light brown, simultaneously losing about 5% of their original weight. As the temperature rises to about 220°C (428°F), beans will lose about 13% more weight and release some CO2.
When the temperature increases to around 230°C (446°F), the roasting beans become medium-dark brown and take on an oily sheen. Often there will be a loud pop as the beans enter the 'second crack' phase.
Here roasters have to be very cautious not to overdo it. Volatile aromatic compounds are boiled off and the oils on the outside of the bean can combine with oxygen in the air. That process can strip the bean of desirable flavors and lead to a burnt taste.
The goal is to arrive at just the right balance of bitterness, acidity and a host of other attributes making up the final flavor profile.
In tasting guides coffee connoisseurs will sometimes see the term 'body', as if its meaning were self-evident. 'Body' despite what it suggests, does NOT refer to the actual thickness or viscosity of the liquid. That attribute is the result of the kinds of proteins and fibers in the brew.
Used as tasters do, it refers to the feel on the tongue when rubbed on the roof of the mouth. It's the result of the fat content in the drink and that - apart from growing conditions that home roasters can't control - is determined largely by the roasting.
Too light a roast will leave too high a concentration of bitter compounds in the final product. Too dark will produce an excessively chocolatey, burnt taste. Experiment until you find the balance that suits your taste.