Roasting coffee is the management of heat in order to change the physical and chemical composition of coffee beans. Or as Candice Madison, Director of Roasting at green bean supplier Royal Coffee, puts it, “you are literally playing with fire.”
You are probably already aware that there are different types of heat at work during roasting, and that their behavior varies during the roast. Let’s review conductive and convective heating and how you can improve your roasts by managing them.
Lee este artículo en español Guía de Los Diferentes Tipos de Calor en Las Tostadoras de Café
The Main Heat Types
There are three ways in which heat can be transferred:
1. Conductive Heat in Coffee Roasting
Conductive heat transfer is straightforward: it is the transfer of heat between two objects that are directly touching. When you touch something that’s just come out of the oven and burn your hand, that’s conduction.
In coffee roasting, conduction occurs “if any type of heating element (say flames) touch the drum, when beans come into contact with the surface of the drum, paddles or faceplate, and even when the beans touch each other inside the roaster,” says Candice.
Conductive heat transfer plays a lesser role during coffee roasting than you might think. Candice points out that even in drum roasters, only about 30% of heat transfer comes from conductive heat transfer.
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2. Convective Heat in Coffee Roasting
Convective heat is the transfer of heat through a liquid or gas. When you open a hot oven and feel a rush of hot air against your face, that’s convective heat.
There are two types of convection: natural and forced. Natural convection describes the natural tendency of hot air to rise and cool air to fall. Forced convection is the forcing of air via a fan or pump to transfer heat. Almost every roaster uses a fan to some degree to pull air through their roaster. However, natural convection also takes place to a certain extent in drum roasters, where the air in the top of the barrel will tend to be warmer than the air near the bottom.
3. Radiation in Coffee Roasting
Radiation is the release of energy “via a process of electromagnetic radiation because of the vibrational and rotational movement of their molecules and atoms,” Candice explains. Probably the most well-known example of radiation is the heat of the sun traveling through the vacuum of space to heat our planet.
With current technology, radiative heat is difficult to measure and almost impossible to control in coffee roasting. Even roasters equipped with infrared burners are using infrared heat to heat a barrel (conduction) and air (convection), not directly applying radiative heat. Therefore, when it comes to radiation, it’s best just to acknowledge it’s there and leave it at that.
How Different Heat Types Affect The Roast Process
Your roasting style and equipment will determine how you want to approach conduction and convection. Every machine will use conduction and convection to different degrees and allow you different amounts of control over them. However, there are certain variables that you can always take into account:
For drum roasters, this is the amount of conductive energy you can store in your drum. You might want to consider this when working with more compact roasts to emphasize lighter tones. Candice tells me, “If I’m going to approach a coffee with a view to accentuating the floral and fruity notes, I’ll either apply a large amount of heat right from the start, from the turning point, and/or charge the drum at a higher temperature.”
At a certain point, the beans themselves become their own source of conductive heat. This means that larger batch sizes will carry more heat and be less responsive. To effect changes in the batch, you will need more energy.
This isn’t necessarily a bad thing: a slightly larger batch size may be easier to roast, while a smaller batch size can be overly responsive to every tiny adjustment. Ultimately, a larger batch size will require more energy at the beginning. Toward the end of a roast, it will also be a significant source of conductive energy.
Keep in mind that most roasters roast with no more than 75–80% of the manufacturer’s listed maximum batch size. As the beans expand during the roast, airflow can become restricted and cause problems. As Candice says, “cramming beans into the drum will simply mean more roast defects, not more great-tasting coffee!”
The amount of airflow in the drum isn’t directly correlated to the amount of convective heat energy. However, as the beans enter the Maillard reaction and then caramelize ahead of first crack, they release water and carbon dioxide. The water released has a cooling effect on the atmosphere inside the drum.
Read more in What Happens During Roasting? The Chemical Changes
That’s why, if you leave your roaster at a set heat and medium-to-low airflow, you will probably notice a slight dip in your rate of rise around 310°F/155°C and 360°F/182°C. At these points, opening your airflow a bit will actually increase your rate of rise. The increased airflow will vent moisture from the drum, keeping it hot.
Conversely, as you near first crack, you can lower your flame and close down the airflow to conserve convective heat. Depending on the characteristics of the coffee, however, you may need to open it up again at first crack.
So, depending on where you’re at in the roast, airflow affects convective heat differently. On top of that, you also need a minimum amount of airflow to pull smoke and chaff out of the barrel.
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For roasters with variable drum speed settings, an increase in drum speed means lifting the beans into the hot air accumulated at the top of the drum, increasing their exposure to convective heat. As with charge temperature and batch size, going too high or too low can cause problems.
Candice tells me, “If your drum speed is too high, you risk roasting inconsistently, and, as cellulose matter is eroded through pyrolysis, actually damaging and smashing the beans before they’ve even left the roaster. If the drum speed is too slow, you will by nature be using more conductive heat, as more of the bean mass will stay in contact longer with the walls of the drum. This leads to beans that will potentially suffer from scorching or facing.”
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Different Heat Types in a Drum Roaster
Although we’ve looked at how heat transfer affects the roast process, it’s important to remember that every machine is different. Let’s look at the parts of a typical drum roaster and how different heat types could come into play.
Several factors influence heat retention in the drum. Most modern roaster manufacturers construct it from machined steel. It may be solid or perforated. In the latter case, far less conductive heat will affect the roast. The thickness of the drum also plays a big role in heat retention.
The more heat your roaster’s drum can store, the more lag there will be when you make an adjustment to your flame setting. “Your roaster may be made to be very responsive (poor heat retention – not a bad thing!) or very stable (good heat retention),” Candice says. “It may take a little time to get used to one over the other, but you can make coffee taste great either way. You may simply prefer one [type].”
This is your primary means for manipulating convective heat currents. Some larger roasters are designed with a damper on the exhaust air pipe that isn’t really meant to be adjusted during the roast. The idea is to set it at the appropriate setting and use the flame height to effect changes in the roast.
On some smaller roasters, the fan can actually be strong enough to cool the barrel. This can stall your roast (another moment of convective energy transfer, but not a good one).
Whatever your approach is to airflow in roasting, it’s worth emphasizing that regular maintenance and cleaning is critical for maintaining proper airflow (and avoiding roaster fires).
Sample roaster at Matraz Café in Guadalajara, Mexico. Credit: Ana Valencia
Don’t forget that your cooling tray is another point of convective heat transfer, this time with the flow of heat from the beans to the cooler air. The sooner the coffee is cool to the touch, the better.
Measuring and monitoring heat is key to achieving a dialed-in and consistent roast. Candice recommends that you “ensure that you have and maintain clean, adequately sized thermocouples/probes. Too thick, and the data will be insufficient or not precise; too thin, and it may be too sensitive to background noise.
“Are your probes grounded or ungrounded? Find out if you don’t know. Grounded probes act a little differently from ungrounded probes, and issues may be easier to diagnose if you have all of this information before things go awry.”
If you only have one probe, it should be positioned so that it’s completely in the bean mass. If you have two probes, you’ll probably want the second to be positioned near the exhaust pipe at the head of your roaster. A measure of the exhaust air temperature will give you an insight into the amount of convective heat energy in the drum.
There are other places you could place additional probes, if you know how to interpret the data you see from them. However, there is no probe placement that will give you solely conductive or convective heat. For this reason, you need to understand what the probe is really measuring and what that tells you about your roast.
Learning How to Use Convection & Conduction in Roasting
Although there are only two controllable heat types in roasting, the interplay, management, and measurement of them can be complex. If you’re a beginner roaster, you can set yourself up for success by learning about it.
Make sure you know your equipment. As we’ve seen, heat retention and transfer are affected by all parts of the roaster’s design. When you’re familiar with how your roaster works, you can make better calculations about how to run it. Also, keep your equipment clean and well maintained – this will affect your heat transfer.
Second, recognize that you don’t have to manipulate every single variable. Controlling fewer variables (by not changing the batch size or charge temperature, etc.) can make it clearer how heat is naturally accumulating and flowing through the roast.
On the other hand, making constant airflow and burner adjustments makes it difficult to understand what is occurring in the roast. You end up trying to effect changes that you don’t completely understand. Stick to one set of parameters. Then, once you feel you’ve gone as far as you can with them, you can make other adjustments.
As you start to get a more intuitive understanding of heat types, you will gain better control of your roasts and greater insight into how they are developing.
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