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

FrontStreet Coffee's Brief on the Secrets of Coffee Roasting

Coffee roasting is the process of heating raw coffee beans to a certain temperature to remove excess moisture, allowing some components to transform into caramelized sugars and flavor oils. Roasting is an essential stage in developing coffee flavor, since the flavor compounds must undergo roasting to fully express their character.

The chemical reactions in the beans during roasting can be assessed through sound, color and aroma. As heating continues, 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 dry out and the beans turn black while emitting large amounts of smoke with a burnt aroma—a warning sign that the beans are approaching the combustion point. The color transformation from light to dark is caused by changes in caramelization and acidic substances.

Listening to sounds is another basis for determining the degree of roast. If the roaster's temperature is high enough, coffee beans produce two distinct cracking sounds during roasting. Taking heat-resistant raw beans like Mandheling and Antigua as examples, the first crack occurs around seven minutes, producing low, sparse sounds that indicate 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 cracks. After two minutes of calm, the second crack occurs around twelve minutes, meaning the beans are nearly roasted to perfection. The beans are then transferred from the roaster to the cooling tray, instantly filling the air with rich aroma.

FrontStreet Coffee Recommendation

The Mandheling used as an example above is one of the classic heat-resistant beans that roasters learn on, and FrontStreet Coffee's PWN Golden Mandheling is a strong reference for the style. It comes from Indonesia's PWN company, which applies exceptionally strict standards: beans of 18 mesh and up, fewer than 3 defects in every 300-gram green-bean sample to meet the top G1 grade, then three rounds of hand sorting to remove every remaining defect, which raises the cup's cleanliness and the intensity of its sweetness. The result is uniform, plump, bright beans with a low defect rate; roasted medium-dark, they carry a rich toasty aroma, and the brewed black coffee is clean and full-bodied, ideal for anyone who dislikes acidity. Seen through the roasting lens this article describes, Mandheling is forgiving enough to help you hear both cracks clearly. For a livelier, brighter expression, FrontStreet Coffee's Lintong Mandheling also uses medium-dark roasting to soften the acidity while keeping the wild herbal character that wet-hulling gives Sumatra—clove and woody aromas with a minty, low-acid, strongly sweet finish. If you would rather drink Mandheling through milk, FrontStreet Coffee's Classic Blend builds on the creamy nuttiness of Brazilian coffee with medium-dark Mandheling—butter, roasted chestnut and caramel, plus sandalwood, herbs and spice—and a small share of Robusta for a fuller crema.