How to Roast Coffee

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Reading Roast Curves

HTR 4.03 Roast Defects

Traditionally, most teaching about roasting has focused on visible roast defects such as scorching. In practice, however,  the defects that most roasters need to worry about aren’t visible on the bean itself. They are detectable only by tasting the coffee or by analysing the roast curve. The most important defects are ‘baked’, ‘underdeveloped’, and ‘flicked’.

Baked refers to a roast in which the rate of rise (RoR) crashes at first crack and then flattens or begins to rise. The crash itself has only a small effect on the coffee’s flavour, slightly reducing the sweetness and juiciness. For a light, Nordic-style roast, the crash may not impact the flavour too much, but if you want to roast darker there are no good options for continuing the roast. 

If the RoR continues to decline, then the roast will stall. A stall means the RoR reaches zero and the bean temperature stops increasing before the roast reaches its target bean temperature. On the other hand, if the RoR stops declining and plateaus or increases, then the coffee will taste flat, and it may develop roasty notes or ‘hollow’, cardboard, or straw-like flavours.

The baked defect is particularly apparent to tasters after a coffee cools. On the cupping table, a baked coffee may be perceived to have some sweetness or juiciness when hot, but those attributes ‘fall apart’ at lower temperatures.

Underdeveloped coffees are those that are not fully cooked at the centre of the bean. An underdeveloped coffee bean will be tough and hard to crack open with a fingernail. By comparison, well-developed coffees are more brittle and crack easily. Underdeveloped coffees may taste grassy or like raw peanuts, with savoury, vegetal flavours. Roasters can avoid underdevelopment by roasting darker or adjusting the roast curve so that the beans receive more heat, but it’s usually not possible to spot underdevelopment just by looking at the roast curve. The best way to identify underdeveloped coffee is by taste. Underdeveloped coffee may be less easy to extract, requiring finer grind settings or giving lower extraction yields when brewed.