April 20, 2020

4 Variables That Affect Roasting


Roasting is all about highlighting a coffee’s best qualities. However, to do this successfully, you need to manage the variables that can impact the roasting process and influence your results. 

Here’s how common variables like time, temperature, and airflow impact roasting, and how you can control them.

Lee este artículo en español 4 Variables Que Afectan el Tueste de Café

Credit: Matthew Hitcham

Variable #1: Charge Temperature

The temperature inside your roaster drum when its ready to start roasting is its charge temperature. This temperature should generate enough thermal momentum to complete the roast. Too low a temperature can impact the bean’s development, making it flat and underdeveloped. Too high a temperature and you risk scorching and tipping it.

There’s no single correct charge temperature. Your roast batch size and your roastery’s ambient temperature can influence this number, and different roasters and bean probes can also result in different numbers. Because of this, it helps to get to know how your roaster functions at different temperatures.

Danilo Lodi is a coffee ambassador for espresso machine manufacturer Dalla Corte and a World Barista Championship certified judge. He says that a bean’s density, age, and processing can influence your roaster’s ideal charge temperature. 

He explains that fresh beans will need a lower charge temperature, while older beans will need a higher one. Bean density is also important, as dense beans can handle a higher charge temperature than soft ones. Finally, naturally processed beans require a lower initial charge temperature than washed ones. This is because naturally processed beans have a higher concentration of sugar in them, making them burn quickly and easily.

You might also like A Guide to Bean Temperature & The Coffee Roasting Curve

Roaster roasting green beans

Credit: Matthew Hitcham

Variable #2: Roast Temperature

Once roasting begins, your roaster’s internal drum temperature should be monitored and adjusted. If the roast temperature is too high and the roast progresses too quickly, the beans won’t spend enough time in the Maillard zone. This zone (which starts at the end of the drying phase and ends at the beginning of first crack) develops a coffee’s aroma and flavour. If the roast temperature is too low, it can create a flat, unpleasant coffee. 

To prevent this from happening, roasting expert Scott Rao advocates for a declining Rate of Rise (ROR). ROR is the rate at which the temperature increases within the roaster’s drum every minute. To make sure this number reduces throughout the roast, you need to pay attention to it during first crack and just before first crack begins.

The only time a declining ROR shouldn’t be followed is in a roast’s initial stage – after it has charged but before its turning point. Here, the ROR should dramatically increase to counteract the cooling effect of room temperature beans entering the hot drum.

Danilo says that how the beans were processed will affect their ideal roasting temperature. “Some coffees (a handful of naturals and pulped naturals) will decrease the drum temperature during first crack. In some cases, it’s better to add heat close to this stage to keep the curve going up until the end of the desired profile.”

Credit: Matthew Hitcham

Variable #3: Airflow

Airflow plays a complex role in roasting coffee, increasing the heat inside the drum and removing the smoke, chaff, and particles that can influence the coffee’s flavour. This variable needs careful controlling, as too much airflow during roasting can leach beans of the moisture required for their sweetness.

Airflow can also impact how clean a coffee’s cup profile is. Danilo cautions that “Airflow during the roast can make a cleaner cup, but too much can also leave some astringency as well.” He mentions that roasters with an air temperature probe will be able to determine when airflow should be added, as well as for how long.

Credit: Matthew Hitcham

Variable #4: Roast Time

Many important flavours and aromas develop in the Maillard zone, which means that the total time that the coffee roasts is important. A short roast will create an underdeveloped, one-dimensional coffee, and an extended roast will reduce its acidity and brightness. For example, if you want to highlight a coffee’s sweet and caramel flavours, you’d roast it for longer to give the sucrose in the bean’s time to caramelise. 

The longer a coffee spends in the Maillard zone, the more body it will develop and the less acidic it will become. Beans destined for espresso will spend more time in this zone to develop the type of acidity and body required for high-pressure extraction.

Just 20 to 30 seconds in the Maillard zone can change a coffee’s flavour. For this reason, you should sample and cup a coffee at these intervals if you’re roasting it for the first time. This will show you how it changes every interval so that you can make informed decisions for future batches you’ll roast.

Using software or spreadsheets to log and track this information will save you time and effort, says Danilo. It will also help you track at what temperatures each roasting phase takes place. He recommends paying attention to how fast these changes occur, as this information can help you test different development and roasting times and prevent undesirable flavours from developing. 

Credit: Matthew Hitcham

Roasting coffee involves many complex variables. As Scott Rao suggests in ‘The Coffee Roaster’s Companion’, it’s important to view all roast stages as a whole, rather than focusing on what happens in each individual stage.

Danilo admits that while monitoring, recording, and comparing data can help you create a consistent roast profile, roasters should rely on their palates to help them understand the changes taking place in each phase. “[Y]ou need to evaluate all the coffees that you roast, every batch, record the data and compare. You’ll need to adapt the roasting profiles according to the changes that the coffee will show over time, and you can only do that by tasting [it]”.

Tasting and tracking your roasting efforts and its variables will be an ongoing process. However, every batch of coffee you treat this way will bring you closer to mastering your machine and improving your roasting skills.

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Feature photo credit: Barista Pilot

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