During a roast, temperature and, specifically, a roaster’s control over it, is often one of the most important contributing factors to a coffee’s quality.
As soon as the beans come into contact with heat inside the roasting drum, they begin the journey from their raw form to a product packed full of flavour that’s ready to be ground, brewed, and consumed.
However, if the initial temperature of the drum is too high or too low, it can have a considerable effect on the entire roast. Known as charge temperature, it’s crucial roasters learn how to control if they want to avoid “baking” or “scorching” the beans.
What is charge temperature?
In coffee roasting, charge temperature refers to the temperature reading of the drum at the moment the beans are added (or “dropped”).
Measured using a temperature probe, it can have a considerable impact on the entire progression of a roast, as well as on the outcome of the coffee itself.
For example, if the charge temperature is too low, it can slow down the speed of the roast, leading to hollow or flat coffee, known as “baking”.
If, on the other hand, the charge temperature is too high, it can burn the outside of the beans, while leaving the inside uncooked, a practice known as “scorching”.
“Charge temperature has a huge impact as you can make a lot of mistakes in the beginning of the roast,” Bogdan says. “Most common is creating a roast defect called ‘tipping’ or ‘scorching’ with a highly negative impact on flavour taste.
“Another problem that can occur is loss of the moisture very fast, taking us to a massive rebound after first crack.”
The ability to understand and control charge temperature is very important and will improve consistency and overall quality of the roasting phase.
It is usually indicated on the roaster display screen or by the roasting software connected to it. On a roast curve, it creates a “tick” shape as the temperature plummets when the probe comes into contact with the room temperature beans before rising in equilibrium.
What influences charge temperature?
During a roast, there are many variables to consider, including the origin of the coffee, the batch capacity of the roaster, the density of the beans, and the preferred roast profile.
When it comes to choosing a charge temperature, it’s important to consider all of these variables and make adjustments accordingly.
However, some variables carry more weight than others. Bogdan explains that, for him, there are a few key variables that roasters should pay particular attention to when deciding on charge temperature.
“Charge temperature is a decision based on several aspects,” Bogdan explains. “One is the roaster itself. If you use a Diedrich with IR burners, you’ll need to charge differently than if you use a Probat with an atmospheric burner or a Giesen with a direct flame.
“It also comes down to the batch number. Basically, if you’re at the beginning of the roasting session you need to charge higher than if you’re after, let’s say, the fifteenth batch.”
He says that green bean humidity and the way in which the coffee is processed will also influence charge temperature. For example, a washed Kenyan will be approached differently to a natural Brazilian, which typically burns more easily.
Finally, the brewing method needs to be factored in as this determines the length of the roast. A filter roast, for example, is typically shorter than an espresso roast, which requires an extended development time.
“Charging temperature is a combination of all these variables, as well as the roaster’s experience of how their equipment is operating.”
Why consistency is key
Once you’ve found the perfect charge temperature for your coffee, the next challenge is to maintain it.
In roasting, ensuring one batch doesn’t differ wildly from the next is essential. Your customers want to know that each time they buy your coffee, it will be the same as the last time they bought it.
If not, it can affect your brand’s reputation and push potential customers to try competitors’ coffee instead.
Bogdan explains that achieving a consistent charge temperature over the course of several hours often involves using your roaster’s gas settings to balance things out.
“Keeping a constant charge temperature throughout the day depends on your roasting style,” he says.
“You need to charge differently as the roaster continues to accumulate heat during the day. But also, by keeping the charge constant and using the gas settings, you can achieve better consistency between each roast.”
It’s also crucial to consider the batch size. If the amount of coffee you roast changes, you’ll need to factor this in to produce the same results.
The bigger the amount of coffee loaded into a hot roasting machine the higher is the temperature drop generated.
So choosing a higher charge temperature when roasting bigger volumes of coffee is important to avoid extremely long turning points of temperature reading and overall long total roast time.
According to Bogdan, a failure to make adjustments can lead, above all, to inconsistent flavours in the cup.
“In my experience you mostly see the impact of charge temperature on flavours when you make mistakes, like scorching or tipping will add some roastiness to the taste.
“On the other-hand if you charge too low, you might not have enough heat to finish the roast correctly so the coffee would be underdeveloped, but it’s incorrect to put the vegetal taste on the charging.
“Another example of manipulating the taste of the coffee when you charge lower or higher is related to the maillard zone, you will pass through maillard less or more time, impacting the body and the aromatics of the final taste”.
For roasters, it’s important to understand how changes in charge temperature can influence their roast. A good understanding can help improve consistency and ensure each batch delivers its full potential.
Once you’ve mastered charge temperature, the next step is to preserve the freshness of the roasted coffee. For this, you’ll need high-quality bags with good barrier properties.
At MTPak Coffee, we can help you design the perfect packaging for your roasted coffee. We have a range of sustainable materials, including kraft paper, PLA, and LDPE, as well as recyclable degassing valves and resealable zippers.