When roasting coffee in a traditional drum roaster, a flame applies heat to a rotating cylinder containing the beans.
The flame heats both the drum and the air going through the chamber, which not only “cooks” the raw coffee beans, but also produces various byproducts.
Among the prominent of these byproducts is smoke. Although not uncommon, too much can give the coffee unpleasant smokey flavours in the cup, as well as polluting the air and putting those within proximity at risk of smoke inhalation.
To combat this, many specialty coffee roasters take measures to reduce the amount of smoke produced, from installing afterburners to cutting down roast times.
To find out more about the impact of smoke and what roasters can do to reduce it, I spoke with head roaster at Cafes Belleville, Mihaela Iordache.
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Why does roasting coffee produce smoke?
Roasting coffee produces four main types of emissions: dust, chaff, odour, and smoke. Dust is generated during the handling of green beans, while chaff is formed when the silverskins of coffee beans fall off during a roast.
Smoke and odour, meanwhile, are combinations of organic constituents volatised at roasting temperatures and steam produced when the roast is quenched with water.
These fumes consist of several volatile organic compounds (VOCs), including diacetyl, 2,3-pentanedione, and other less harmful chemicals, along with carbon dioxide and carbon monoxide.
The concentration and exact VOCs released during roasting depend on factors such as roasting temperature, roast duration, and coffee bean variety. Dark roasts tend to produce more smoke than lighter roasts.
As well as imparting smokey flavours on the coffee, these fumes pose a risk to those within proximity of the roaster if inhaled. Carbon dioxide also contributes to global warming because it builds up in the atmosphere, which traps heat and causes the temperature of the planet to rise.
Mihaela Iordache is the head roaster at Cafes Belleville in Paris. She tells me that although managing the smoke produced during roasting is an essential job of any roaster, it’s not without its challenges.
“We roast on a gas-powered roaster, which produces large volumes of carbon monoxide and carbon dioxide,” she says. “Usually on bigger roasters, smoke and odours would be eliminated with an afterburner. But unfortunately these use a lot of energy, which means the pollution is trapped in a never-ending cycle.”
How does airflow affect a coffee’s characteristics?
Although a small amount of smoke can be expected during a roast, too much can adversely affect the distinct characteristics of the coffee.
A large part of this comes down to airflow – the movement of air around the roaster. Not only can airflow help regulate convection, it’s important for maintaining the volatile oils of the beans, ensuring they preserve all their best characteristics.
According to Mihaela, if airflow is not carefully controlled, it can impart unwanted flavours and aromas on the coffee.
“Poorly managed airflow can deliver a “smokey” character into the cup profile,” she tells me. “A lot of it is actually around properly calibrating the airflow depending on the day.
“Usually, roasted coffee with a lot of silverskin on it comes from insufficient airflow during roasting. So I would expect the coffee to taste like the smoke wasn’t properly managed.”
To maximise control over airflow, Mihaela explains that it’s paramount to keep on top of equipment cleaning and maintenance.
“Making sure you clean equipment and the roasting machine regularly is the biggest and most important time investment in the operation, especially the exit area where chaff and smoke are sent out,” she says.
How to eliminate or reduce smoke during roasting
In most towns and cities, there are rules and regulations around emissions that apply to roasteries.
Designed both to protect the local inhabitants and to reduce the environmental impact of a business’ activities, these regulations often carry hefty fines if they’re broken or actively ignored.
As such, specialty roasters must do everything they can to reduce the amount of smoke and odours they produce, not only to avoid breaking regulations but also for the welfare of their workers. Fortunately, there are a number of options available.
Afterburners
An afterburner (or thermal oxidiser) is an incinerator that burns off the majority of pollutants produced during a roast. It does this by trapping the smoke, and heating it at extremely high temperatures to generate a cleaner emission.
While not exclusive to coffee roasting, it is a popular choice among roasters looking to comply with local regulations and reduce the impact of their operations.
However, because afterburners require a significant amount of fuel to power, many question the logic of using one carbon-emitting machine to eliminate the carbon emissions of another.
They are also known for being noisy and expensive, both to buy and to install, putting them out of reach for most small-scale roasters.
Electrostatic filter
Electrostatic filters gives static charge to the dust particles in order to collect them. According to Willem Boot this method works reasonably well but unfortunately it takes a great amount of time to clean and maintain.
Vortex
The vortex is essentially a cyclone that uses the power of atomised water particles to grab odours, smoke, and organic compounds. While this system works well on small roasters (up to 15kg), a vortex for bigger roasters is yet to be developed.
Roast length
A long roast will tend to produce more smoke than a short roast. This is because moments before the second crack, the volume of smoke typically increases, becoming thicker and more pungent the longer the roast goes on.
Mihaela explains that to avoid this, roasters should consider light roasting their coffee.
“Our job as roasters is to identify the coffee’s potential and highlight it through roasting,” she says. “So with specialty coffee, you can roast a bit lighter, which will also help reduce the amount of smoke.”
For specialty roasters, it’s important to limit the amount of smoke emitted during a roast. Not only can it damage the health of those within proximity, you may be breaking rules and regulations around emissions.
Taking measures, such as installing an electrostatic filter or roasting lighter, can also help minimise your environmental impact, while protecting the wellbeing of you and your staff.
But don’t let your move to becoming more sustainable end there: by switching to recyclable or compostable coffee bags, you can further reduce your carbon footprint. At MTPak Coffee, we have a range of kraft paper, PLA, LDPE, and rice paper coffee bags that will both protect your coffee and preserve its freshness.
