Variable Parameters Affecting Roasting

The purpose of roasting is to bring out the best qualities of the coffee. However, to do this successfully, we need to manage some variables that can affect the roasting process and results.

Next, let’s take a look at how common variables such as time, temperature, and airflow affect roasting, and how these variable parameters should be controlled:


Variable 1 Lower Furnace Temperature

When we are ready to start roasting, we need to preheat the roaster first, and the drum temperature of the preheated roaster is the feeding temperature. This temperature should generate enough heat momentum to complete the roast. Too low a temperature can affect the flavor development of the coffee bean, making it taste flat and underdeveloped; too high and the beans run the risk of burning and pouring.

The temperature of the lower furnace does not exist whether it is correct or not. Both the size of the roasting batch and the ambient temperature in the roastery may affect this parameter, and different roasters and bean temperature probes will also result in different temperature parameters. So different bottom oven temperatures help us understand how our roasters perform at different temperatures.

Danilo Lodi, a certified judge of the World Barista Competition, believes that the density of coffee beans, storage time, and handling methods will affect our roaster’s ideal lower oven temperature setting.

He explained that fresh green beans require lower furnace temperatures, while older green beans require higher furnace temperatures. Bean density is also important, as denser beans can withstand higher furnace temperatures than less dense beans. Finally, natural processed coffee beans are processed at lower oven temperatures than washed processed coffee beans. Because the sugar content in the sun-treated coffee beans is high, the high temperature can easily make the coffee beans burn quickly.


Variable 2 Roasting Temperature

Once roasting begins, we monitor and adjust the temperature of the drums inside the roaster. If the roasting temperature is too high, and the roasting is done too quickly, the beans will not have enough time for the Maillard reaction to occur. This phase (starting at the end of the drying phase and ending when the first burst occurs) is where the aroma and flavor of the coffee develop. If the roasting temperature is too low, it may result in a flat coffee with an unpleasant mouthfeel.

To prevent this from happening, roasting expert Scott Rao recommends reducing the rate of rise (ROR). The ramp rate is the rate at which the temperature inside the roaster drum increases per minute. To make sure we lower this parameter throughout the roast, we need to pay attention to its state during and just before the first crack.

It should be noted that in the initial stage of roasting, that is, the stage after the furnace has not reached the temperature return point, the heating rate cannot be reduced, but the heating rate should be greatly increased to offset the coffee beans at room temperature entering the hot drum, cooling effect.

How the coffee is processed also affects our ideal roasting temperature. Some coffees (a few naturals and dehulled naturals) will reduce the tumbler temperature during the first crack. In some cases, it is best to ramp up the temperature just before the first crack to keep the curve going up until the end of the desired curve.

roast finished
roast finished

Variable 3 Airflow

Airflow plays a complex role in the roasting process. It increases the temperature inside the roaster drum as well as removes smoke, chaff, and particles that can affect the flavor of your coffee. Care should be taken to control this variable, as excessive airflow during roasting can deplete the moisture needed to develop the bean’s sweetness.

Airflow also affects the clean mouthfeel of coffee flavors. Airflow during roasting can make the coffee taste cleaner, but too much airflow can also cause astringency. A roaster with an air temperature probe will be able to determine when airflow should be increased and for how long.

Variable Parameters Affecting Roasting-airflow
Variable Parameters Affecting Roasting-airflow

Variable 4 Roasting Time

Coffee beans develop many important flavors and aromas during the Maillard reaction stage, which means the total length of time the coffee is roasted matters. Roasting for a short time will result in underdeveloped coffee beans without flavor layers while roasting for a long time will reduce the acidity and brightness of coffee beans. For example, if you want to accentuate the sweetness and caramel flavor of your coffee, you can roast it longer to give the sucrose in the beans more time to caramelize.

The longer the coffee spends in the Maillard reaction stage, the stronger the body and the less acidic it will become. Coffee beans roasted for espresso take more time to undergo the Maillard reaction to develop the acidity and body required for high-pressure extraction.

Maillard reaction stage, it only takes 20 to 30 seconds to change the mouth flavor of the coffee. So if we are roasting a coffee for the first time, we should sample and cup at this interval. This allows us to understand how the coffee flavor changes within each interval, which also allows us to choose the most suitable roasting plan for that batch of coffee to be roasted in the future.

Using software or spreadsheets to record and track this information will save us time and effort. It also helps us keep track of the temperature at each baking stage. We pay particular attention to the speed at which these changes occur, as this information can help us test different development and roasting times and prevent the development of bad flavors.

There are many complex variables involved in coffee roasting. As Scott Rao advises in The Coffee Roaster’s Companion, it’s important to look at all stages of roasting holistically, rather than limiting what’s happening in each stage of the roast.

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