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Baked defects in Ethiopian Natural Coffee Roasting

By
Sungbin Cho
January 30, 2024
8
min read
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This post continues from "Designing Profiles for Ethiopian Natural Coffee"

In the previous part, we experimented with roasting Ethiopian natural coffee by significantly altering the air flow, conducting two different roasting trials. During these trials, we observed positive flavor development when the roasting was done with a slower air flow.

Once the appropriate air flow that enhances the positive flavors of the coffee is determined, it is crucial to set the roasting points for production roasting. Depending on the desired flavor profile of the coffee, it is necessary to design a profile suitable for light, medium, or dark roasting points.

The roasting profiles provided in this post mostly target a medium-light roast, aiming between light and medium. When roasting Ethiopian natural coffee, even a small change of about 1 degree in the exhaust temperature can result in significant differences in the flavor profile and the final product. Therefore, it's advisable to set the roasting points more precisely. It might be beneficial to initially try designing a medium-light point profile before attempting light or medium profiles.

Light roasting often leads to underdeveloped and baked defects due to its relatively low exhaust temperature. In medium roasting, flavor expression is comparatively easier than in light roasting due to the higher exhaust temperature. However, during sensory evaluation, the acidity might be excessively high, or the coffee might taste roasty, dry, or have a short aftertaste. In such cases, the beans are also considered to have baked defects. Targeting a medium-light point can help avoid roasting defects that are common at light or medium points.

Therefore, it might be easier to first design a medium-light roasting profile and then refine the design for light and medium profiles based on that initial experience.

Here is an example and modification method for roasting profiles, using <Profile 1> and <Profile 2>.

Coffee Information:

Coffee – Ethiopia Sidama Twakok Natural
Region: Oromia, Sidama
Variety: Heirloom
Altitude: 2,000m+
Processing: Natural

Profile 1- Ethiopian Natural Roasting with a Baked Defect

Roasting Machine – Easyster1.8 (2018)
Charge: 1,000g
Yield: 855g
Weight Loss Rate: 14.5%
Roasting Point: Discharge after about an 11°C rise following the start of the first crack.

Profile 1

In <Profile 1>, sensory evaluation reveals a baked defect in the roasting. After the start of the first crack, there is a sufficient temperature rise, allowing for the perception of acidity and sweetness, such as floral and malic notes, during cup testing. However, while the acidity and sweetness are noticeable, the flavors are not distinct or bright. There is a presence of over-extracted black tea notes and roasty aromas, which unfortunately diminish the clarity and cleanliness of the coffee cup.

In such cases, firstly, the airflow inside the machine during roasting is slow. This can reduce the clarity of flavors and lead to a cup that is not bright, dominated by over-extracted black tea notes, resulting in a disappointing quality. When the exhaust is excessively slow, it can easily lead to underdevelopment. The resulting roast at an exhaust temperature of 220°C (about 11°C rise post-crack) was lighter than the expected roasting point. If the exhaust temperature had been slightly lower, not only baked but also underdeveloped defects could have occurred.

Secondly, the heat supply setting is high. In the case of <Profile 1>, it's not that the heat setting is absolutely high in comparison to the characteristics of the green beans, but it's high relative to the exhaust temperature. This scenario can result in bitter or roasty flavors or an over-fermented sensation due to high intensity.

The first step is to balance the heat and exhaust settings. If this balance is not adequate, it becomes challenging to finely adjust the exhaust and charge temperatures. The heat setting should be slightly lowered, and the exhaust sped up a bit. The exhaust temperature can be reduced a little, targeting a medium-light point.

In <Profile 1>, it is necessary to slightly lower the heat setting (by about 5% to 10%) and increase the exhaust speed (by about 1 damper point). While it is advisable to reduce the exhaust temperature, it's important to observe the progress and decide the extent of the reduction based on that.

Profile 2 - Resolving the Baked Issue with Profile 1

Roasting Machine: Easyster1.8 (2018)
Charge: 1,000 g
Yield: 860 g
Weight Loss Rate: 14.0%
Roasting Point: Discharge after about a 7°C rise following the start of the first crack.

Profile 2

The roasting in <Profile 2> was conducted appropriately.To resolve the first baked issue in <Profile 1>, we adjusted the heat to 0.09kPa (about 7% lower) and opened the damper by one notch to speed up the exhaust. The exhaust temperature was lowered by 4 degrees. Flavors such as lavender, peach, blackberry, blueberry, fig, raisin, maltose, ale, and chocolate were perceived as distinct and complex.

For <Profile 2>, if targeting a lighter point by reducing the exhaust temperature by 3-4 degrees, you should slightly increase the initial heat setting (by about 5%) and somewhat lower the charge temperature (by about 20 degrees). The exhaust may need to be slightly increased in this case.If aiming for a medium point in <Profile 2> by increasing the exhaust temperature by 4-5 degrees, the initial heat setting should be slightly lowered (by about 5%) and the charge temperature slightly increased (by about 10 degrees).

In Profiles 3 and 4, we used a coffee called Ethiopia Guji Shakiso Natural for roasting.

Green Bean Information of Ethiopia Guji Shakiso Natural
Region: Guji, Shakiso
Variety: Heirloom
Growing Altitude: 1,800m
Processing: Natural

Profile 3 - Ethiopian Natural Roasting with a Baked Defect

Roasting Machine Information – Easyster1.8 (2018)
Charge: 1,000 g
Yield: 860 g
Weight Loss Rate: 14.0%
Roasting Point: About 8°C rise after the start of the first crack

In <Profile 3>, the roasting results in a baked quality. Most of the coffee character is perceivable, with substantial development of acidity and sweetness. However, slight dryness or bitterness reduces the quality of the mouthfeel. This issue arises from a high heat supply setting, similar to the second baked reason in <Profile 1>. The heat setting is not absolutely high compared to the green beans' characteristics but is high relative to the exhaust temperature, leading to potential bitter or roasty flavors or high intensity, giving an over-fermented sensation.

To modify the profile targeting the same exhaust temperature as in <Profile 3>, the heat setting should be slightly reduced (by about 5%).To increase the exhaust temperature in <Profile 3> by 2-3 degrees, the heat setting should be somewhat lowered (by about 10%), and the charge temperature slightly increased (by about 10 degrees).To reduce the exhaust temperature in <Profile 3> by 2-3 degrees, only the charge temperature should be slightly lowered (by about 10 degrees).

Profile 4 - Resolving the Baked Issue with Profile 3

Roasting Machine: Easyster1.8 (2018)
Charge: 1,000 g
Yield: 855 g
Weight Loss Rate: 14.5%
Roasting Point: About 8°C rise after the start of the first crack

The roasting in <Profile 4> was conducted appropriately.Using the baked resolution method from <Profile 3>, we aimed for the same exhaust temperature and adjusted the heat setting to 0.04kPa (about 5% lower).The quality significantly improved with flavors like peach, berry-jam, cane sugar, Earl Grey tea, pulpy, sweet, and aromatic flavors, and cleanliness.

For <Profile 4>, if targeting a lighter point by reducing the exhaust temperature by 3-4 degrees, the initial heat setting should be slightly increased (by about 5%), and the charge temperature somewhat lowered (by about 20 degrees). The exhaust may need to be slightly increased in this case.If aiming for a medium point in <Profile 4> by increasing the exhaust temperature by 4-5 degrees, the initial heat setting should be slightly lowered (by about 5%), and the charge temperature slightly increased (by about 10 degrees).

In light roasting profiles, it is necessary to slightly increase the heat and exhaust speed from the medium-light roasting profile that produced good results, gradually forming a faster airflow. Then, the charge and exhaust temperatures should be lowered.For medium point profiles, slightly decrease the heat and exhaust speed from the successful medium-light point profile, gradually forming a slower airflow. Then, increase the charge and exhaust temperatures.

The roasting processes discussed in this post are not extreme examples that lead to defects. Therefore, when comparing temperature graphs, a baked profile might not appear significantly different from an appropriate one. Hence, roasters should record feedback on taste along with log graphs when initially designing profiles for different types of green beans. If you do not conclude testing with just one good profile but collect data statistically, even in similar graph shapes, you can discover subtle differences like those in the profiles discussed.

In your roasting result chart, check and compare the following figures.

  1. Charge temperature
  2. Turning point of bean temperature(BT) and time
  3. Turning point of exhaust temperature(ET) and time
  4. Maximum RoR(Rate of Rise) of BT and ET
  5. BT RoR shape just before the start of the first crack
  6. Temperature and time of the first crack start
  7. BT RoR value at the start of the first crack
  8. BT RoR shape after the first crack starts
  9. Minimum value of BT RoR
  10. BT RoR rise after the end of the first crack
  11. Exhaust temperature. Finally, after roasting is completed
  12. Weight loss rate
  13. Notable points in RoR are checked.

There is no set standard for what constitutes good values for these figures. Depending on each environment, compile and compare data from many positive and negative cases statistically. Also, we are adjusting airflow speed, charge temperature, and drum speed when deviating from these norms, alongside sensory evaluations.

There are several instances in Ethiopian Natural coffee roasting where the characteristics of the green beans create peculiarities in the roasting chart's profile. In future articles, we intend to discuss techniques for adjusting these instances to align with statistical norms.




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Sungbin Cho
CMG Coffee Bar Head Roaster, Wecoffee Education Manager
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