You’ve sourced a great green coffee from your favorite origin and roasted a delicious batch. But how can you replicate it for your next batch, and the ten or twenty after that? Consistency is key.
When coffee is roasted consistently, baristas and other consumers know what they’re dealing with and can trust they’ll get the same cup every time. Inconsistent batches can’t be sold, so they’re a direct hit to the roaster’s profits.
However, consistency in coffee roasting is not as simple as arriving at the same end temperature within the same time frame for each batch. Read on to find out more.
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Roasted coffee cools. Credit: Juan Pablo Arenas
Understanding Heat Transfer
A key component of coffee roasting is understanding how heat transfer works. In a typical drum roaster, there are two primary sources of heat transfer from the machine to the bean: conductive and convective.
Conductive transfer is the heat transferred by direct contact between the drum and the beans. The heat caused by airflow as the beans are tossed around in the hot drum is convective transfer. Coffee needs both conductive and convective heat transfer to properly develop flavors during roasting.
Be cautious of relying on the thermocouple on the front of the barrel – the reading will only tell you the temperature of the air inside the roaster, not the heat of the spinning drum. This is important because the drum acts as a heat sink, absorbing significant amounts of heat from the flame underneath. This stored energy is then transferred to the beans when they are placed inside the roaster.
A roaster loads green coffee beans. Credit: Neil Soque
Imagine you put two identical pieces of metal in an oven set to 400°F/204°C. If you take one piece out as soon as the oven reaches 400°F, but leave the other ten minutes longer before removing, would you expect both pieces to take the same amount of time to cool down? Even though both pieces were at the same maximum temperature, one had more time to absorb heat.
Something similar happens in coffee roasting. When you roast coffee, there’s a continuous cycle of adding green coffee to the barrel and allowing roasted coffee to drop out. In this process, the drum experiences fluctuations in how much thermal energy it stores. Managing these changes is an important part of achieving consistency with your desired roast curve every time.
A roaster inspects roasted coffee beans. Credit: Devon Barker
Planning For Consistency
There’s no single way to ensure consistency in results when roasting, but there are a few ways to increase your chances of a consistent roasting pattern. Let’s take a look at some tips.
- Organize Your Space & Plan Ahead
A successful roasting schedule starts with a well-organized space. You should be able to move around comfortably and find everything you need easily. Andrew Tolley is a co-founder of Taylor St. Baristas. He tells me that it’s important to have everything prepared before starting to roast.
So before you get started, measure out your green coffee and make sure you have a dedicated area to store roasted coffee as it comes out of the drum. And make sure that you plan for dealing with chaff if your cyclone needs to be emptied during the production run.
You should ideally roast multiple batches of the same profile back-to-back. This will minimize changes in thermal storage due to bean or profile differences. But a long run of the same profile can be its own mental challenge because of the monotony of repeating the same task. This is when avoidable mistakes occur. If you have the flexibility, consider breaking long runs into smaller chunks. And you should definitely schedule regular breaks with water and snacks.
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Bags of green coffee. Credit: Devon Barker
- Preheat Your Roaster
Preheating is essentially loading and storing thermal energy in the barrel of your roaster. If the drum isn’t preheated to the correct temperature, you’ll have less conductive energy to transfer to the coffee. In this situation, you may be forced to apply more flame to compensate. Then, when the barrel is up to temperature, you’ll need to lower the flame. Relying on varying levels of conductive and convective heat from batch to batch, makes it difficult to achieve consistency in results.
How long and at what temperature you should preheat your roaster varies among models. A barrel sample roaster may only need ten minutes to ensure adequate heat distribution throughout, while a 100 kg roaster may need as much as an hour.
However long you preheat for, make sure you’re consistent about it. Andrew says that an adequate warm-up cycle at the beginning of the production run is one of the key checkpoints roasters need to be attentive to when roasting.
A roasting machine. Credit: Devon Barker
- Standardize Your Between-Roast Routine
When you’re roasting in a production run, there isn’t really a series of disconnected S-curves but rather a continuous wave. To keep these curves looking identical and therefore the run consistent, you need to keep the times between roasts identical.
How you reheat your roaster between roasts is vital to how much and what kind of heat is stored in it. The most obvious point to control is the dropping of roasted coffee. When you finish a batch and drop it into your cooling tray, you’re releasing a significant amount of heat. So, it’s important to standardize how long you take to empty a batch before heating up for the next one.
Don’t worry about having the roaster open for as short a time as possible. Instead, focus on making sure that it is open for the same amount of time for each batch.
Roasted coffee beans cooling. Credit: Pete Willis
You should also have consistency in how long you take to load green coffee. The open charge hatch is another point of heat loss. But don’t rush to load in your green beans. Instead, keep it smooth and consistent.
Depending on your individual space and needs, there are likely other between-roast tasks you’ll need to account for. Make these part of a routine so that it becomes natural to perform them in the same way each time.
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A roaster inspects freshly roasted coffee beans. Credit: Devon Barker
- Understand Your Machine’s Convective Heat Flow
Whatever machine you use to roast, it will interact with the air outside it, taking in cool air and expelling hot air and gases. How much and how well your roaster does this relates to your convective heat transfer.
Your machine’s “breathing” is especially important after first crack, when you lower your flame and rely on a (hopefully) consistent hot air environment.
Some machines have valves designed to adjust airflow, but you should identify other factors that may affect it. For example, as your chaff collector fills, the cyclone becomes less efficient. Hot, cold, or humid days can also affect the air being drawn into your roaster. Coffee roasting creates creosote, a black tarry substance, which can build up in pipes and restrict airflow (not to mention become a fire hazard).
Roasted coffee spills out of the roaster. Credit: Yanapi Senaud
- Track & Compare Your Roast Data
Make sure you regularly cup and evaluate the beans you roast. Jordan Dabov is the founder of Dabov Specialty Coffee. He, Andrew, and Dubai-based Q-Grader Maria Eduarda Pavani all emphasize the importance of cupping the previous day’s roasts. Regular cuppings help you stay aware of how your roaster performs over multiple runs and sharpen your palate.
Consider keeping track of this information in a spreadsheet or other format. Platforms such as Cropster allow you to track roast information and cupping feedback. Andrew tells me that he uses a custom form specific to his company’s needs.
Whatever tracking format you use, be sure to include a quantitative measure. This can be as simple as a scale from 1 to 5 of how well the batch matches your target profile. It will help you compare results over time at a glance.
A roaster tracks data during a roasting session. Credit: Devon Barker
You may also want to keep track of the weight loss incurred by roasting. The weight of the roasted batch divided by the original weight of the green coffee will give you a decimal that you can turn into a percentage for easy comparison. Variation in percent weight loss can help identify which batches might need extra scrutiny in quality assurance.
By measuring the color of a ground sample of coffee, you can analyze development – the darker the sample, the more development. You can read up and train your eye to the range of colors or buy a color meter, which will give you instant feedback during production runs. Maria notes that this data can also be passed on to customers as a transparency measure.
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Roasted coffee beans. Credit: Mike Keneally
It can be challenging to achieve consistency in coffee roasting. There are many variables to consider when trying to create a perfect roast profile. And there are many more when you’re trying to repeat it again and again. But with an understanding of the physics of the roasting process, careful attention to detail, and quality control procedures, you can hit that perfect roast every time. So, what are you waiting for?
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