Coffee roasting is both an art and a science, demanding precision, experience, and care to unlock the best flavors and aromas in every batch. Yet, even the most skilled roasters face challenges along the way, leading to what are known as roasting defects. These issues can significantly impact the flavor, aroma, and overall quality of the final product. Let’s take a closer look at some of the most common coffee roasting defects, their causes, and how to recognize and prevent them.
Scorching: causes and prevention
Scorching is identified by burnt spots on the flat surfaces of coffee beans, resulting in a smoky or ashy flavor in the cup. This defect often occurs when beans are exposed to a higher-than-ideal charge temperature at the start of the roast. Other factors, such as using lower-density beans (which are softer and more heat-sensitive) or roasting natural process coffees, can also increase the risk of scorching. Natural process beans typically require a longer and slower drying and development phase compared to washed coffees.
Additionally, a roaster with a slow drum speed or an overloaded drum can contribute to scorching. Overfilling restricts bean movement and reduces airflow, both of which can lead to uneven heat exposure.
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Tipping: identifying and addressing burn marks
Tipping is characterized by small burn marks located at the tips or edges of coffee beans, and it imparts a flavor profile similar to scorched coffee. Tipping is typically caused by a combination of a too-high charge temperature and elevated heat levels during the roast. The shape of the coffee beans also plays a role, as moisture tends to evaporate more quickly at the tips, making them more vulnerable to burning from the hot surfaces of the roasting drum.
Internal scorching: managing moisture distribution
Internal scorching occurs when the moisture inside the coffee bean isn’t evenly distributed. Green coffee beans have a complex cellular structure, with over half a million individual cells. If the beans are improperly dried during production, it can create areas with uneven moisture content. When these beans are roasted at high temperatures, the drier areas are more prone to scorching from the inside out.
To prevent internal scorching, start the roast with a lower initial charge temperature. A gradual and controlled temperature increase allows for more even heat distribution and minimizes the risk of damaging the bean’s internal structure.
Over-roasting: recognizing and avoiding bitterness
Over-roasted coffee is often intensely bitter and loses the delicate balance of flavors typical of well-roasted specialty coffee. Excessive heat breaks down the natural sugars and oils, leading to a burnt, bitter taste, muted acidity, and the loss of sweet, clean aftertastes. Moreover, over-roasting can mask the bean’s natural origin characteristics, leaving the coffee tasting flat and overly uniform.
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Quakers: unveiling hidden defects post-roast
Quakers are underdeveloped beans that are almost impossible to detect before roasting. Although not technically a roasting defect, they do impact cup quality. Quakers are typically caused by poor soil conditions or insufficient nutrients during coffee cultivation, resulting in beans that fail to develop adequate sugars. After roasting, Quakers appear lighter in color and can contribute to undesirable flavors.
Baked beans: preventing underdevelopment
Baking occurs when the roast progresses too slowly — particularly when the Rate of Rise (RoR) falls below 2°C per 30 seconds, or when the period between the start of the first crack and the end of the roast exceeds two minutes. Baked coffee often tastes dull and flat.
To avoid baking, roast with sufficient energy to maintain an appropriate RoR and aim for a more dynamic roast curve, especially after first crack.
Conclusion: avoiding underdevelopment
When coffee has a green, grassy taste and lacks vibrant acidity, it’s often a sign of underdevelopment. Underdeveloped beans may show a lighter color on the inside compared to the exterior, indicating uneven heat penetration during roasting. This issue often arises when roasters attempt to achieve lighter roasts but stop the roast too early to avoid over-roasting.
To prevent underdevelopment, it’s essential to continue roasting for at least 45 to 60 seconds after the start of the first crack, ensuring full development of the coffee’s flavors without losing its bright, lively characteristics.
At Giesen, we understand that roasting is both an art and a science. We’re here to support you at every step. Whether you're upgrading your equipment or troubleshooting your roast profile, we’ve got solutions tailored to your needs.
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