Coffee roasters are strange and interesting machines. Elegant arrangements of gears, bearings, fans, and motors, roasting systems are the final gatekeepers of coffee flavor; their properties and mechanisms have a massive effect on the type of coffee they produce. Here at Centri, I roast all our coffees on a restored 1995 Probat L12, and today we’re going to take a deep dive into the way that our roaster functions. I’ll also discuss how the roaster operates, how that operation affects flavor, and why I like roasting on the L12 so much. It’s my hope that this discussion will yield some interesting insights into the roasting process, as well as how our roaster produces the deliciousness in your cup.
The Roaster
Founded in 1868, Probat is a roaster manufacturer headquartered in Emmerich am Rhein, Germany, with subsidiaries and factories all over the world. In the context of the modern coffee industry, Probat is renowned for their UG roasters, a series of machines built from 1949 to 1958. These machines defined the features for which modern Probat roasters are known; the integration of cast iron components into a sturdy, long-lasting build, consistent airflow, and incredibly powerful direct flame burners. UG series roasters are especially coveted because of their extensive use of cast iron, which makes them extremely thermally stable, but all modern Probat roasters hold to the same design principles; high build quality, consistent airflow, and powerful burners, all of which combine to produce roasted coffee of stellar quality.
Our 95’ L12 is no exception to this rule. Boasting cast-iron in the, faceplate, backplate, and body of the roaster, as well as a rolled steel drum, our machine is extremely durable. Dillon, our founder, also completely rebuilt this machine, so while the bones of the roaster are original, much of the hardware has been replaced, which enhances its already stellar build quality. The roaster can handle up to 12 kilos (around 26 pounds) of coffee per batch, but we prefer the taste results of 9 kilo, or 20 pound batches. The roaster only has one fan, which can be toggled between roasting and cooling applications, with no further airflow modulation possible. This means that airflow through the roaster itself is held at a constant level during roasting, a staple feature of Probats and major point of contrast with modern roaster designs, which often feature completely controllable airflow. The heating setup of the roaster features three rows of gas-powered burners which contact the drum directly from below. Roasters with this setup are called direct flame roasters, and are the most common type of coffee roaster on the market. The burners themselves are extremely powerful, boasting a total output of around 96,000 BTUs/hr, which is 8,000 BTUs/kg/hr at a full 12kg batch size, or around 10,600 BTUs/kg/hr at our slightly reduced batch size . By comparison, the other roaster in our facility, a Diedrich IR-20, has a 150,000 BTU/hr rating, or around 7,500 BTUs/kg/hr for our full 20 kilo batches, which is 3,100 BTUs/kg/hr less than a standard batch on the Probat, or 500 BTUs/kg/hr less on a full batch. These numbers make clear that the L12 is a very powerful roaster with the ability to push a lot of heat into green coffee. Overall, the L12 is sturdily built of cast iron and steel, mechanically simple, and very powerful, with no airflow modulation. Now, onto what that means for the flavor of the coffee it roasts.
The Coffee
One of the reasons that vintage Probat roasters like our L12 have been so coveted by third wave roasteries is that they excel at producing well developed coffees at a wide variety of roast levels. The reason for this lies in their construction. Cast iron components to the build make the roaster extremely thermally stable. As a low-density metal (relative to steel), cast iron takes a long time to heat up, but retains heat very well once it is hot. Heating up our L12 usually takes around 45 minutes (average warm up time for a roaster is around 30 minutes), but once it’s hot, I can walk away for an hour and come back to a machine still ready to roast. Like most Probat roasters, our machine’s drum is made of steel, in this case rolled steel. Steel is denser than cast iron, which for heat transfer purposes means it is more sensitive to temperature change; it heats up and cools down faster. This sensitivity is an important quality because in the beginning of a roasting cycle, heat is primarily conveyed to beans by means of conduction from the drum; that is, the green coffee gets warmer because heat is being transferred from the drum directly to the beans themselves. A high-density drum thus makes for a roasting system in which conductive heat transfer is especially pronounced. When combined with a powerful burner setup like that of our L12, this in turn means that the roaster operator has a great deal of influence over the temperature of the beans through modulation of the gas, particularly at the beginning of a batch; the roasting system is very responsive and can be modulated to create a wide array of profiling options. The lack of airflow control is often seen as a negative by modern roasters, but in my opinion, it plays into the strengths of the machine; the number of variables one has to track are kept to a minimum, so the variable you can control, in this case the roaster’s gas/burner setting, has a large amount of influence over the roasting process. This laser focus on single-variable manipulation rewards skilled operation by reducing distractions, airflow being one potential distraction.
The analogy I often make is that using modern roasters of different manufacture is like driving a car with an automatic transmission, while the L12 is like driving a car with a stick shift. Much like an automatic car, modern roasters are typically easier to use, with simple functions and lots of controls over those functions; the coffee equivalents of cruise control, automatic braking, and backup cameras. The automatic offers much in the way of function and asks nothing in return, as its corners are rounded off. Our L12 is like a stick shift in that it offers little in the way of frills or features, but centers around the precise operation of a few simple variables; instead of the clutch, throttle, and brake, it’s gas setting. The machine is simple to use, but difficult to operate effectively; it rewards precise operation, which can only be learned through intimate knowledge of the machine and lots of practice.
To track all this back to taste and flavor, our L12 and Probats in general are unique because they can produce coffees with flavor profiles in a very wide range. The simplicity and modularity of their controls and construction allow for effective production of light and dark roasts, all of which taste extremely clean due to the precisely calibrated airflow. Coffee from the Probat just tastes good, and this is not a result of operational skill; the machine itself is precisely tuned to produce properly developed, well rounded, and altogether tasty coffee.
Why I Like the Probat
It’s probably easy to surmise from the preceding paragraph that I enjoy roasting on our L12. The reason for this is simple; it’s a powerful tool which allows me to roast a wide variety of green coffees effectively. Having to only control for a few variables makes deciding on a roast profile a simple process, and once a profile is defined, the roaster’s thermal stability makes repeating it a breeze. There are challenges associated with the Probat, to be sure; working with such a sensitive instrument takes time to learn, but once that initial hurdle is overcome, its ease of use is unparalleled. I love our L12, and I’m extremely happy with the quality of coffee it produces. We’re lucky to have such a powerful tool at our disposal.
In conclusion, today we’ve discussed the technical details of our roaster, how that affects the taste of the coffee it produces, and why that makes it machine beloved not only by me, but our whole team. I hope this has been a helpful deep dive into our roasting technique and philosophy, as seen through the lens of our equipment. Stay tuned to this blog for all things coffee related. Until next time.
Article By : Bret Colman, Director of Coffee / Head Roaster, Centri Coffee