What Are Coffee Roasting Defects | Characteristics of Roasting Defects | How to Identify Roasting Defects

When we think of coffee defects, we usually picture defective beans mixed into a batch or problems with brewing. There is another category, though: roasting defects. What exactly are they? FrontStreet Coffee is here to explain.

Roasting is a technical skill. Nobody achieves a perfect roast on the first, second, or even tenth attempt. It takes professional knowledge and the accumulation of many abnormal situations to understand and control your roasting curve. Once you understand the problems that arise and the causes behind them, though, roasting becomes much easier.

1. Baked Coffee

Excessive roasting time before first crack is the main cause of baked coffee. Unfortunately this defect is invisible to the naked eye and can only be identified by tasting. Baked coffee typically carries flat flavors with no sweetness, described as bready or papery.

2. Under Development

Underdeveloped beans often taste grassy and lack caramel aromatics. This sometimes happens when a roaster is aiming for a light roast, and at that point the roast curve needs a slight adjustment.

3. Over Development

Overdevelopment is the opposite of underdevelopment. There is only a subtle difference between a "dark roast" and "overdevelopment." Matt Perger holds that overdevelopment does not really exist. Even so, roasting darker than intended is still an error, producing flavors many professional drinkers dislike. These beans look black and oily, sometimes even charred, and cupping reveals burnt, bitter tastes with smoky and peaty notes.

* Black beans are just one form of overdevelopment.

4. Quaker Beans

Quakers are essentially unripe beans that are hard to spot when sorting green coffee by hand. They are usually caused by poor soil conditions that limit sugar and starch accumulation during growth. Technically this is not a roasting defect, but quakers are typically only noticed once beans are roasted. They are lighter in color than the other beans in the same batch, and if they are not removed they contribute dry, papery, and grainy flavors to the cup.

5. Scorching

Scorching occurs when the roaster drum temperature is too high and the rotation speed is too slow. Scorched beans are very easy to identify: dark, burnt patches appear on the flat surfaces of the beans, in effect burning the whole bean. These beans taste distinctly oily, smoky, and even like burnt meat.

6. Tipping

Tipping is similar to scorching, but the burn marks fall mainly on the edges of the beans. It usually occurs during second crack, though some suggest that excessively high drum temperatures can also cause it.

How to Identify and Prevent Roasting Defects

Roasters can use professional equipment as aids, such as coffee roast analyzers. These devices cannot stop defects from occurring, but they help measure roasting consistency. When roasting, use quality-control charts to record the physical changes the beans undergo, and record the Rate of Rise (ROR) as well; this helps you understand how the beans are developing and make timely adjustments to prevent defects. These basic records also help you find the most suitable roasting curve for each coffee. Roasting should never be judged on color alone. Two beans may look the same shade, yet the curves that produced that color were different, and those curves determine the final cupping flavors.

What Are Roasting Defects?

First: under-baking, where the center of the bean is not fully roasted, which can be understood as an inconsistent roast level between the interior and exterior. In a light roast this shows up in the cup as: 1) weakness, where the interior has not reached the expected roast level and too few soluble compounds have been converted; and 2) a strong grassy, vegetal taste, which is a bit like eating raw meat. Under-baked dark roasts are less common, since most roasting of this kind is done light.

Second: scorching, where the heat is too intense for the beans to handle. This is visible in the beans' appearance:

1. Small black scorch marks at both ends of the bean. Note that small black spots at the ends are not always from excessive heat; they can also indicate moldy green beans, since green-bean defects often appear at the two ends.

2. Large-scale scorching across the bean surface.

3. The entire bean scorched. Coffee with defects like these generally leaves a persistent, obvious bitterness in the throat.

Third: smoke intrusion during drum roasting, where the damper is not managed properly. Smoke intrusion at different stages produces different off-flavors: early in the roast it gives the coffee an obvious salty, astringent taste, while later in the roast it makes the coffee smoky and bitter.

Fourth: uneven roast level. There are many possible causes:

1. Problems with the roaster's agitator blade design.

2. Too few beans in the batch.

3. Excessively high heat.

FrontStreet Coffee Recommendation

Most of the defects above come from a roast curve pushed too far one way or the other, which is why a well-executed medium roast is often the safest way to get balance in the cup. If you dislike both sourness and bitterness, FrontStreet Coffee's Brazil Queen's Estate is a good place to start: a Yellow Bourbon grown in the world's largest coffee-producing country, roasted medium so it offers cream-peanut, chocolate, toast, and sugarcane aromas and sweetness, brewed equally well as pour-over, moka pot, or espresso.

For a sweeter, fruit-driven medium roast, FrontStreet Strawberry Candy blends a Costa Rican honey-process coffee with Panama washed Geisha and is roasted medium to bring out both tangerine-honey sweetness and a rounded body, suited to cold brew, pour-over, and espresso. FrontStreet's 2013 sun-dried Typica, a Chinese single-origin roasted medium to keep its soft, balanced character, adds nut, brown sugar, black tea, and dried apricot notes at very good value; and FrontStreet Jamaica Blue Mountain No. 1, roasted medium for the same reason, unites chocolate, roasted nut, cream, and caramel flavors without any single note dominating.