How does the weather affect coffee roasting?

Karmen Yoong
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February 22, 2022
male coffee roaster holds handful of roasted coffee over large batch coffee roaster

Coffee roasting is a complex process, with every decision along the way influencing final flavour.

Roasters must monitor parameters, such as the airflow, applied heat, and roast time.

They must also consider external environmental factors such as the weather, as this can affect the entire roasting process. 

Therefore, roasters must consistently make adjustments to ensure every batch meets quality expectations, regardless of the season. 

To find out more about how weather conditions affect coffee roasting, I spoke with the owner of MABÓ Coffee, Bogdan Georgescu

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Lightly roasted coffee beans in a tangential roaster

What influences a roast?

To achieve the desired outcome of a roast, it is important to “dial in” roast profile by modifying charge temperature, airflow, and roasting time.

Charge temperature

Charge temperature refers to the initial temperature of the roasting drum before green coffee is added.

However, incorrect temperatures can affect the roast.

For example, low temperatures can limit the beans’ development or “stall” the roast. This may lead to a flat or baked bean and little sweetness. 

Then again, charging too hot may burn the outside of the beans, which could result in unpleasant flavours that may seem oily or smoky.

In order to determine an optimal charge temperature, roasters have to consider a range of factors such as batch size, the condition of green beans, and ambient temperature.

Airflow

Airflow is the movement of air through a roaster, and it largely influences how heat transfer occurs in a roast. Additionally, it is critical for the development of flavours. 

Similar to charge temperature, too much or too little airflow can affect the outcome of a roast.

For instance, Boot Coffee Campus suggests higher airflow can enhance the sweetness and brightness of a coffee. However, too much airflow may lead to sourness.

On the contrary, too little airflow can mask the coffee’s acidity and lead to smoky flavours. It may also “stall” the roast because of insufficient heat transfer.

Roast time

Coffee undergoes a few phases of roasting, including drying, caramelisation, and development. 

Chemical reactions that occur in each of these phases contribute to the coffee flavour profile. 

As an example, the browning phase, also known as the Maillard reaction, is essential in bringing out the best flavours. This phase has an impact on the coffee’s body, mouthfeel and flavour complexity.

Equally important is the development phase, which tends to affect sweetness and acidity. 

This phase contributes to the roast degree and the end colour of the beans.

Therefore, adjusting the total roast time is important for regulating the final cup profile.

close up image of freshly roasted coffee beans with suspended smoke or steam

How do weather conditions affect roasting parameters?

Factors such as ambient temperature, humidity levels, and pressure can have a considerable impact on the variables of a roast. As such, it’s important they are carefully monitored and controlled.

Of particular interest among roasters is the impact of ambient temperature on roast time, which is often longer when the weather is cold. This is because the roaster and the air drawn in take longer to warm up. 

This is especially true for those who roast outdoors, as cold wind will strip away heat energy from the roaster.

“It is true that atmospheric conditions affect roasting,” says Bogdan, who is the 2019 Romanian Coffee Roasting Champion. “This is because of its influence on green bean temperature.”

For example, beans that are too cold will roast slowly, potentially creating underdeveloped flavours. 

On the other hand, air humidity can also impact a roast in two different ways.

Notably, if the hot air inside the roasting chamber has a high moisture content, it transfers heat more efficiently than dry air. In that case, the roast may progress quickly. 

However, if the air is not heated, humid ambient air drawn into the roaster will compete with the beans for heat energy. Consequently, this leads to a slower roast. 

Similarly, in his book, The Coffee Roasters Companion, Scott Rao says humidity in the roasting air increases the efficiency of heat transfer, causing the bean to lose moisture quickly. 

Additionally, Bogdan says atmospheric conditions can generate lower or higher negative pressure inside the roasting chamber.

“Whether it is sunny, snowy, or raining, the negative pressure will be different,” says Bogdan, who was also a runner-up in the World Coffee Roasting Championship. “This pressure is related to how much air is flowing through the coffee.”

close up of dark roasted coffee beans,

Roasting in summer vs. winter

To ensure consistency in quality, roasters must fine-tune their roast profiles as the seasons change. 

“It is not possible to have the same profile for both summer and winter,” Bogdan says. Using a winter roasting profile in summer can lead to overdeveloped coffee and vice versa. 

Essentially, roasting approaches used between seasons vary depending on where a roaster is located.

During winter, Bogdan reveals he starts and ends his roast at a slightly higher temperature to compensate for the lower ambient temperature.

Furthermore, he usually maintains a fixed negative pressure for his roasts and uses a digital manometer to monitor and adjust the numbers accordingly. 

This is because the pressure inside the roaster changes with the weather – negative pressure is higher on warm days and lower in colder weather. 

Another key point is that during the colder months, Bogdan will take green beans out of storage a day before roasting. This allows the beans to warm to room temperature. 

Scott Rao says he applies less heat in his roast during the winter due to the effect of dry air on green bean moisture.

Apparently, he found his stored green beans lost moisture during the winter, and the cold, dry air made them roast too quickly. As a result, he now uses less heat in his winter roasts. 

Furthermore, Rao installed a humidifier in his roastery to maintain constant temperature and humidity levels all year, stabilising the green coffee’s moisture content.

Little's Christmas Spirit Flavour Infused Ground Coffee pouch, white package with golden stars sitting on wooden table top with blurred green, forest background

Fluctuations in environmental conditions are unavoidable. Yet, neglecting these changes can have a negative effect on your roasting outcome. 

To achieve consistent, perfectly roasted coffee year-round, roasters should ensure they understand all roasting factors at play. In addition, they should track the data and taste every batch produced. 

Essentially, a perfect roast will go to waste unless the coffee’s freshness is preserved. Investing in recyclable packaging or sustainable coffee bags made from rice or kraft paper can ensure consumers get the best tasting coffee every time. 

At MTPak Coffee, we are dedicated to helping specialty coffee roasters find a packaging solution to match the high-quality coffee they produce. Our range of sustainable coffee bags together with customisation options for design, material, pouch type and more will keep your coffee fresh for consumers. 

For more information on sustainable coffee bags, contact the team.

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How does the weather affect coffee roasting?

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