How to Control Charge Temperature: A Coffee Roasting Guide
Don’t underestimate the power of knowing how to control charge temperature. If you want to really control your roast, highlighting the coffee’s best features for espresso or filter, then understanding how to manipulate this variable will be a huge benefit.
I reached out to several roasters and coffee experts to find out their advice for controlling charge temperature, whether you’re roasting a fruity and natural processed single origin or a chocolatey low-altitude espresso blend. Here’s what I discovered.
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Freshly roasted coffee cascades out of the roaster. Credit: Damian Krzywoń
WHAT IS CHARGE TEMPERATURE?
Carlos Juárez, Master Roaster at Impetus Casa Tostadora de Café, Mexico, explains that charge temperature is the temperature of your drum just before you add the coffee. If you’ve ever seen a roasting line graph, you’ll know that the temperature line looks a bit like a tick mark. As soon as the coffee beans are added, the temperature plummets until it reaches the turning point and then starts increasing. But the charge temperature is the reading before the drop.
Since roasting is the process of heat spreading – both through your roaster and through the coffee beans – this initial temperature has the power to affect the entire roast. And like all elements of roasting, there are many aspects to consider when you want to control charge temperature.
Recording roast profiles, including charge temperature. Credit: Padre Coffee for Roast IQ
THE DANGERS OF THE WRONG CHARGE TEMPERATURE
Carlos tells me that too low a charge temperature will prevent proper flavour development. This is because it will take too long to generate the energy needed for the roast. Stretching the roast out too long can lead to flat or even baked coffee. The only exception is if you’re trying to control or reduce a coffee’s acidity: in this case, a lower charge temperature will help.
Too high a charge temperature, on the other hand, can burn the bean from the outside (something called scorching). This will create dryness and astringency in the cup.
But what charge temperature is best? Let’s take an in-depth look at this complex issue.
Coffee and roast profile line graph. Credit: Roastworks Coffee Co
WHAT CHARGE TEMPERATURE IS BEST?
Carlos emphasises that charge temperature is a relative parameter. The exact reading will depend on the size of the drum, the size and location of the temperature probes, the air temperature and local climate, and more.
For example, Joe Morocco of City Mill Roasters, Minneapolis, says that his charge temperature will generally lie between 375ºF/191ºC and 425ºF/218ºC. However, for Luisa Quintero of Libertario Coffee Roasters, Colombia, her standard charge temperature on her San Franciscan 3kg roaster is 320ºF/160ºC to 374ºF/190ºC. She tells me that she needs to begin with less heat because her roastery is based in Bogota.
Even the location of the roaster inside the room can affect charge temperature. Brian Kendall, a home roaster using a Behmor 1600 Plus, tells me, “The best way I have found to keep the charge temp constant is to roast indoors away from drafts. If the air outside of the roaster is constantly changing, it has a major impact on both charge temp and the time the beans take to reach first crack.”
To work out the best charge temperature, you need to get to know your roaster (and roastery!). However, while we can’t give you an absolute temperature, we can help guide you into recognising the best range relative to other factors.
How Different COFFEE TYPEs Impact Results
You should adjust your charge temperature to suit the beans you’re roasting. The density, moisture content, processing method, coffee variety, and batch size should all be considered.
When it comes to batch size, Brian tells me that he always roasts in one-pound batches on his Behmor home roaster. “I only roast in manual mode, P5, because I find that the flavour profiles develop more… [but] I limit my ability to play with the charge because of how I order green beans. I guess I could experiment more if I played with 1/2lb batches. That would expose more when playing with charge temps.”
Density – which can be affected by altitude and variety – is also important. With denser beans, Luisa explains that she wants a slower development. For this reason, she’ll roast a smaller amount of beans; this will help the heat to transfer quicker. The beans will also benefit from a higher charge temperature and shorter roast time.
As for processing, Luisa would use a charge temperature of around 365ºF/185ºC for a washed coffee. However, with a natural process, she explains that she’d opt for a lower temperature to avoid the beans burning. A natural coffee’s concentrated sugars could burn and create an ashy flavour.
Carlos reminds me that it’s also important to know your beans’ moisture content. The more moisture there is, the longer the coffee needs for drying. This means using a lower charge temp. However, you also need to be careful that the coffee doesn’t bake. When the moisture content is too high, getting the right drying time becomes a difficult balancing act.
Different stages of roast development. Credit: Strandvejsristeriet
CHARGE TEMPERATURE & ROAST TYPE
Another thing you need to consider, of course, is your roast type. Luisa tells me that when she wants an espresso roast – which is typically darker – she’ll use a higher charge temp. On her roaster, this will be at around 356ºF/180ºC. She’ll also use more air to control the roast.
However, when she wants a filter profile – which is normally roasted lighter to maximise complexity and acidity – she’ll use a lower charge temperature of 320ºF/160ºC–347ºF/175ºC.
Using a data-driven approach to determine blend percentages and roast profiles. Credit: Horsham Coffee
When roasting coffee, every variable matters. Carlos and Luisa both recommend that you get to know your machine and your location. And then, once you really understand how it performs in different contexts, you can focus on bringing the best out of your coffee based on its origin, variety, processing method, and more.
Maybe it will take you a little while to master this – but that’s okay. Take the time to experiment. Roasting is about making controlled decisions, and trial and error is a totally valid way to discover the impact of different variables on your overall roast.
And when you start to understand the relationship between your roaster, charge temperature, and different coffees, you’ll be able to better control your roasts – from accentuating sweetness or acidity to doing justice to a high-density Ethiopian Yirgacheffe.