What Are the Secrets of Coffee Roasting? What Are the Criteria for Determining Roast Level?

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FrontStreet Coffee's Brief on the Secrets of Coffee Roasting

Coffee roasting is the process where raw coffee beans are heated to a certain temperature to remove excess moisture from the beans, allowing some components to transform into caramelized sugars and flavor oils. Roasting is an essential stage for developing coffee flavor, as the flavor compounds in coffee must undergo roasting to fully express their characteristics.

The chemical reactions in coffee beans during roasting can be assessed through sound, color, and aroma. As continuous heating occurs, coffee beans change from gray-green to golden yellow, then to light brown, followed by slightly oily reddish-brown, and finally to heavily oily, glossy dark brown. If roasting continues further, the surface oils on the coffee beans will dry out, turning black while emitting large amounts of smoke with a burnt aroma—this is a warning sign approaching the combustion point. The color transformation from light to dark is caused by changes in caramelization and acidic substances.

Listening to sounds is also a basis for determining the degree of roast. If the roaster's temperature is sufficiently high, coffee beans will produce two distinct cracking sounds during the roasting process. Taking heat-resistant raw beans like Mandheling and Antigua as examples, the first crack occurs around seven minutes, producing low and sparse sounds, indicating that pyrolysis has begun—this is when starch starts converting to caramel, and moisture escapes along with carbon dioxide from the bean surface, creating these low cracking sounds. After two minutes of calm, the second crack occurs around twelve minutes, representing that the coffee beans are nearly roasted to perfection. The coffee beans are then transferred from the roaster to the cooling tray, instantly filling the air with rich aroma!

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