Coffee – Roasting Beans
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.