Today, drum roasters are the most popular machine for professional coffee roasters around the world. However, in the past few decades, some manufacturers have started looking away from drum roasters in favour of an alternative model. 

The fluid bed roaster has technically been around since the 1970s, but has become more popular in recent years. It heats the beans in a fundamentally different way, leading to some key differences in the roast.

To learn more, I spoke with a roaster and two manufacturers. Read on to find out what they told me. 

You might also like our guide to airflow in roasting and how you can control it.

A history & guide to fluid bed roasters

The technology for fluid bed roasters has been around since the early 1970s. In the 1960s, chemical engineer Michael Sivetz realised after working in a polyurethane plant that he could adapt a process used for drying magnesium pellets to roast coffee, thus inventing fluid bed roasting.

For years, fluid bed roasters were consequently referred to as “Sivetz roasters”. And while they attracted a small following in the 1970s, they didn’t really become popular until much later.

The difference between drum and fluid bed roasting is simple. In a drum roaster, the metal drum is heated, which then transfers heat to the beans. However, with a fluid bed roaster, the air is heated first and then blown through the roaster bed, eliminating the need for a heated drum.

Tim Monson is the Head Roaster & CEO at The Monsoon Roastery in Springfield, Massachusetts. He says: “Fluid bed roasting uses convection heat, rather than conduction or induction. 

“In a typical drum roaster, whether it is electric or gas, you have a rotating drum with heat coming from underneath it; sometimes it’s direct, sometimes indirect,” he continues. “In a fluid bed roaster, you’re creating your heat and your airflow from the bottom. 

“This keeps the beans in a constant rotation, so that they’re not actually roasting on a surface; they’re being roasted from the air and the pressure moving in the chamber.” 

As the hot air flows through the roaster, the force lifts the beans into the air, causing them to float. This effectively means the coffee beans are resting on a “bed” of heated air, which is where fluid bed roasting gets its name from.

Ken Lathrop is the owner of Coffee Crafters, based in Post Falls, Idaho. He notes that after the beans are roasted on this bed of heated air, they still need to be cooled externally, even though it might look like this can take place inside the machine.

“We tried to do that in the beginning, and it didn’t work,” he explains. “If you do a good job of insulating your heat chamber, it holds the energy in, so when you shut it off, it’s still roasting.  

“What we do is we release the beans into a separate chamber and then flash cool them. A ten-pound load will cool in about 90 seconds,” Ken tells me. “We direct a lot of air into the bean cooler to suck the heat out of the beans as quickly as possible.”

Gökhan Mithat Karakundakoglu is an Export Manager and SCA Authorised Trainer at Toper, a Turkish roaster manufacturer. He says that Toper has been producing roasters since 1954, but has only recently started producing fluid bed roasters.

“There has been a growth in demand from customers who wanted to start a more boutique style of coffee company,” Gökhan says. “They [often] feel that the appearance of the machine is more stylish than a classic drum roaster.

“Our fluid bed roasters only run on electrical power, which is useful for roasters who don’t have a direct gas supply.” 

What are the key differences in flavour when roasting on fluid bed and drum roasters?

For starters, Tim says that certain origins and processing methods can really shine on a fluid bed roaster.

“I find that a lot of naturally processed coffees tend to really pop with the direct convection,” he explains. “They seem a little bit cleaner, and it helps propel some of the sweetness.”

However, Gökhan notes that testing Toper’s fluid bed models led to both positive and negative feedback. “We have found that some fluid bed roasting results end up a little dry compared to a drum roaster,“ he tells me. 

With drum roasters, roastmasters monitor the amount of air entering and leaving the roaster to maintain a certain moisture level for specialty coffee. If coffee is roasted beyond a certain moisture level, becoming “dry”, it will lose complexity and its more subtle flavours.

However, because fluid bed roasters only use air to heat the coffee, rather than residual heat from the drum, moisture loss is more of a concern.

If too much moisture is lost early in the roast, the beans can end up being flat tasting once roasted.

However, Gökhan also notes: “Because you are only using hot air to roast the beans, [it is easier] to prevent tipping (also known as scorching), which can happen easily in a drum roaster with certain beans.”

Tim also says that while roasters might assume roast curves and profiles are fundamentally different on drum and fluid bed roasters, they are actually deceptively similar. 

“Fluid bed roast curves are within similar parameters for classic roast curves,” he notes. “However, it depends on the bean; the convection means factors like how high it was grown and the processing [play a part].

“Because you’re throwing heat directly onto the bean, you need to be a little bit more gentle with your rate of rise to avoid scorching.”

Ken adds that fluid bed roasters can be used for both light and dark roasts, but notes that they’re excellent for more developed profiles. “One of our customers would tell you is that he likes dark roasts better out of a fluid bed roaster because you can take it to a darker roast profile and cool it more quickly. 

“At some point when you’re roasting, you’ll hit that breakpoint where you start drying the oils and sugar down in the beans with a drum roaster. Fluid roasters are really good at doing a dark roast.”

Energy consumption: Fluid bed vs drum

Understandably, energy efficiency is a key priority in today’s coffee sector. Consumers are increasingly looking for more responsible and sustainable products. In addition, technology with a lower carbon footprint and less energy use will be cheaper for a roaster to maintain. So, are fluid bed roasters more energy efficient?

The answer is that it depends on the machine. Ken says that at Coffee Crafters, energy efficiency is a key focus for the roasters they manufacture. However, historically, this has not been the case. 

“The original Michael Sivetz design required a huge 350,000 BTUs of heat and a massive amount of air,” he says. “However, we’ve done extensive research in the last seven years. We’ve started raising the air pressure in the heating chamber, allowing us to [roast] larger loads with a lower amount of air. 

“Our new 25lb roasters are probably going to end up needing less than 100,000 BTUs of heat,” he adds. “We’re trying to achieve about 3,500 BTUs per pound of green coffee, which would actually be more efficient than a gas roaster.”

Tim also notes that much like drum roasters, fluid bed roasters can be fully electric, and says that he has the option to choose where his electricity comes from.

“I source my electricity entirely from renewable energy sources,” he tells me. “This means that we’re not burning fossil fuels and that the energy we’re getting comes from a combination of wind and solar.”

As well as helping reduce the company’s carbon footprint, this may also make it more attractive for consumers who exercise greater environmental awareness in their buying habits.

What are the advantages?

For many, the advantages of a fluid bed roaster over a conventional drum roaster come down to two main factors: cost and ease of use. These are particularly important for roasters at the beginning of their journey.

Tim tells me that he started his journey at Monsoon on multiple small-batch home machines. He says that fluid bed roasters were more affordable, and helped him to scale up and roast larger batches while still maintaining quality.

“I actually went with the fluid bed roaster as a little bit more of an economic point,” Tim explains. “I found myself having to relearn a little bit of what I was doing, but it was actually closer to what I started roasting with, the popcorn popper right at the beginning.”

Ken adds that the ease of use is a key point for most Coffee Crafters customers. “One of the advantages is in the simplicity,” Ken explains. “Some 75% of our customers have little to no roasting experience when they start their business.”

Given that fluid bed and drum roasters also offer the same markers for development throughout the roast – and that traditional curves and profiles are at least somewhat similar – it can also be easy for more experienced roasters to make the switch.

Ken also adds that the simplicity isn’t just in the roasting, explaining that it’s “simplicity in operation and simplicity in maintenance”. Fluid bed roasters are often designed in a much more accessible way, making them easier to repair if something does go wrong.

“Our roasters only have one moving part, which is the loft motor that locks the beans,” Ken says. “Everything else is solid-state electric, meaning [the parts are] really durable, easy to work on, and easy to install. We designed them to be installed in home roasting environments, and certified them for food safety and coffee shops, but people can install these in their garage or their basement.”

While fluid bed roasters are by no means everyone’s first choice and perhaps less viable in a commercial setting where volume is the main priority, they certainly have their place in coffee roasting and are becoming more popular. 

There’s no doubt that drum roasters will remain mainstream, but fluid bed roasters can be an excellent and often more economically viable option for roasters who want something more affordable and compact. However, whether or not they will become more popular in the years to come remains to be seen.

Enjoyed this? Then try our article on defining your roast profile.

Photo credits: Coffee Crafters, The Monsoon Roastery

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