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When diving into the world of coffee roasting, you will find plenty of tips and techniques for perfecting your roast. But when it comes to one of the crucial factors - setting your charge temperature, the advice often feels vague, or even intentionally elusive. Whether it’s overlooked details or guarded trade secrets, clear guidance on using charge temperature effectively can be hard to come by. In this post, I’ll break down the key principles behind charge temperature and how to leverage it for better roasting results.
One common piece of advice you’ll hear is that higher charge temperatures can lead to defects like tipping or scorching. But if you’ve done some roasting yourself, you’ll know it’s not that simple. Tipping and scorching are caused by a combination of factors, including charge temperature, gas pressure, and airflow conditions in the roasting machine. For example, I’ve roasted beans at over 350°C without encountering those issues. This is why it's crucial to experiment and collect your own data. It’s the best way to expand your knowledge as a roaster.
So, how do you set the charge temperature properly? Let’s break it down.
I’ve written about my approach to roasting on the Firescope blog, but here’s a quick overview. In my experience, each coffee and roasting style (end temperature) works best with a specific gas pressure and airflow setup. If I want to adjust something like roast time, I change the charge temperature while keeping the gas pressure and airflow as stable as possible.
I’ve pulled up three roast curves of washed Colombian coffees that I logged using Firescope. These curves represent roasting the same coffee under identical conditions, with the only variable being the charge temperature. In the summary table, you can see the red, blue, and yellow curves representing roasts with charge temperatures of 170°C, 160°C, and 150°C, respectively—a 10-degree difference between each.
As the charge temperature decreases, the turning point occurs at a lower temperature, and the first crack happens later. Although adjusting the charge temperature doesn’t create a perfectly parallel shift in the curves, it generally moves the BT (bean temperature) curve up or down. This allows you to control when the BT reaches key milestones, like the first crack temperature.
You can use this technique in different roasting scenarios. For example, if I’m roasting a washed coffee from Central or South America and aiming for a light roast, I prefer a longer roast time to properly develop the flavors. To achieve this, I lower the charge temperature. Even though these coffees often require strong heat, reducing the charge temperature allows enough time for flavor development. To avoid a sharp drop in the Rate of Rise (RoR) after the first crack (often called the "RoR crash"), it’s important to maintain a high overall RoR. That’s why I use a low-charge temperature and high heat throughout the roast.
On the other hand, when aiming for a darker roast, I need to reach an end temperature of around 230°C, close to the second crack. However, if the roast takes too long, the coffee can taste dull and flat. If I start with a charge temperature that is too low and then try to speed things up by increasing the gas pressure, I risk harsh, burnt, bitter, or astringent flavors.
For dark roasting, what works for me is starting with moderate gas pressure and targeting the second crack within 8 minutes or less. To achieve that, I set the charge temperature to around 280°C. This allows me to create a dark roast that’s smooth, with rich smoky notes and a subtle acidity that keeps it balanced.
Here are two examples of dark roasts where I experimented with charge temperatures. In the blue curve, I started at 270°C, my usual starting point for dark roasting, but it took longer than expected, resulting in a flatter flavor. Keeping the heat and exhaust settings the same, I raised the charge temperature to 280°C and was able to shave off about 50 seconds, hitting my target end temperature of 230°C. The result was much more satisfying.
Dialing in the right profile for your coffee is key. With roasting software like Firescope, you can overlay temperature curves, making it easier to visualize and understand what’s happening during the roast.
If you want to create the perfect roasting profile for your coffee, there are a couple of things to figure out first:
If you were hoping for a step-by-step recipe, I’m sorry to disappoint! My goal here is to help you understand the principles behind setting the charge temperatures. We take a deductive approach to roasting by first determining the desired profile, analyzing the data, and then working backward to adjust the key variables.
This is why using a roasting software like Firescope is so valuable—it helps you fine-tune your roast profiles and make meaningful comparisons between batches.
When diving into the world of coffee roasting, you will find plenty of tips and techniques for perfecting your roast. But when it comes to one of the crucial factors - setting your charge temperature, the advice often feels vague, or even intentionally elusive. Whether it’s overlooked details or guarded trade secrets, clear guidance on using charge temperature effectively can be hard to come by. In this post, I’ll break down the key principles behind charge temperature and how to leverage it for better roasting results.
One common piece of advice you’ll hear is that higher charge temperatures can lead to defects like tipping or scorching. But if you’ve done some roasting yourself, you’ll know it’s not that simple. Tipping and scorching are caused by a combination of factors, including charge temperature, gas pressure, and airflow conditions in the roasting machine. For example, I’ve roasted beans at over 350°C without encountering those issues. This is why it's crucial to experiment and collect your own data. It’s the best way to expand your knowledge as a roaster.
So, how do you set the charge temperature properly? Let’s break it down.
I’ve written about my approach to roasting on the Firescope blog, but here’s a quick overview. In my experience, each coffee and roasting style (end temperature) works best with a specific gas pressure and airflow setup. If I want to adjust something like roast time, I change the charge temperature while keeping the gas pressure and airflow as stable as possible.
