Coffee Roasting and Pressure Profiling
By Willem Boot
What is Pressure Profiling?
Many drum roasting machines share similar concepts in their design and function. Giesen machines come with unique features that aren’t offered by any other manufacturer. In general, drum roasters feature a revolving drum and an impeller that continually sucks air through the machine and through the drum itself. The air intake is usually situated near the burners of the machine and preheated roasting air is mixed inside the roasting chamber with the coffee beans and evacuated by the roasting impeller. The design of most drum roasters allows the roasting impeller to continually evacuate more air than the roasting machine is taking in. In simple terms, the roaster is always a bit out of breath. With the force of the roaster impeller, the machine collects chaff in the roasting cyclone and roasting exhaust (mostly smoke) is blown out through the exhaust stack.
Some roasting machines utilize the concept of variable airspeed. How does this impact the heat transfer? More air at higher temperatures can accelerate the heat transfer and increase the Rate of Rise (RoR). More air at lower heat input levels, will slow down the heat transfer and decrease the RoR. The impact on flavor works as follows: more airflow can accentuate the sweetness and brightness of the coffee; too much airflow can create sourness. Too little airflow will mute the acidity and potentially lead to leather-like or even smoky flavor notes. So, in theory, air speed offers positive benefits but nevertheless few roasting companies have embraced airspeed as a roasting parameter.
Giesen has taken airflow control to the next level by introducing the highly innovative concept of pressure profiling. How does it work? The control panel of basically all Giesen machines features a setting for “Pa” (which stands for Pascal), which is an expression of the underpressure in the coffee roasting system. By increasing the Pa setting, the machine will increase the rotation speed of the roasting fan to maintain the higher pressure target. During the roast, the operator can increase or reduce the Pa setting confirm the requirements of the profile. If this is done while recording the roasting profile using the Giesen roasting control software, then all the modulations can be repeated automatically with the next batch of the same profile. With the integration of Cropster, this software will also record all modifications made with the Giesen user interface, which allows for the maximum possible consistency of roasting profiles.
In my opinion, these features make Giesen roasters truly unique!
Impacting Flavor Through Pressure Profiling
At Boot Coffee Campus we put the Pa feature to the test by roasting 2 different coffee types, a Guatemalan Antigua washed process and an Ethiopian Sidama natural process.
Before the test we performed a benchmark cupping of both coffees to identify the characteristics of these beans using a standard cupping roast, followed by a tasting session of both coffees. The Guatemalan Antigua revealed some age, due to the fact that the coffee was processed more than 9 months before the test. Overall flavor notes: black currants, licorice, leather, pine, plums with a mild bittersweet aftertaste.
The Ethiopian Sidama displayed classical flavor notes reminiscent of this type of Ethiopian coffee: strawberry sweetness, vanilla, red grapefruit and a lingering floral aftertaste.
Next we performed 5 separate roasting trials of each coffee type. Each profile was designed to repeat a similar roasting curve and roasting time, within a window of 10 to 11.5 minutes. By varying the Pa settings from 80, 120, 140 180 to 220 we were able to put this feature to the test by itself. All other variables were unchanged.
With the Guatemalan coffee, the experiment proved to be very successful; the flavors came out as we had hoped. The Pa settings of 120 and 140 increased perceived sweetness and decreased bitterness. The lowest possible setting, 80 Pa, muted the acidity and increased the perceived bitterness. The highest setting, 220 Pa, produced less sweetness and the acidity was a bit harsh, producing an astringent aftertaste.
The Ethiopian Sidama trials produced inconclusive results! One panel members preferred the lowest possible setting of the Pa and another panel members liked the highest possible setting. The third taster preferred the 140 Pa setting. All tasters agreed that the flavor profiles were significantly different but there was no consensus about preferences.
Summarizing, I can conclude that experiments like these are extremely helpful in understanding the complex dynamics of roasting profile parameters and how these influence the chemistry and flavor profile of the coffee.
Willem Boot is founder of Boot Coffee Campus, a coffee training and tasting laboratory in San Rafael, CA, USA – www.bootcoffee.com
Can you please post, what defines your “standard cupping roast”?
Hi Michael By “standard cupping roast” we are referring to a roast that meets the protocols defined by the Specialty Coffee Association and adopted by the Coffee Quality Institute’s Q Grade program.
– Coffee is roasted within 24 hours of cupping and allowed to rest at least 8 hours
– Coffee is roasted between 8 and 12 minutes (we operate in a narrower range in our lab – generally 9 minutes +- :15
– Roast color is Agtron Gourmet 63 +- 1 for the ground sample.
I think that using the term “pressure” is misleading.
What is really happening is simply changing the airflow.
To accurately utilize the term pressure, the drum would need to be a semi-sealed environment, with much higher pressures measured in W/C inches or PSI.
The Giesen coffee roasters in our lab are unique in that they have an air pressure sensor in the return air exhaust which measures pressure in Pascals (1 Pa = 0.000145038 PSI) of pressure. On the roaster your actually set the Pa of desired pressure on the control panel (as indicated in the first photo of this post), and the software manipulates the fan speed to maintain that air pressure. Yes the airflow is changing, but (as I understand the system) as long as the speed of the incoming air exceeds the rate which it escapes, you’re building pressure in the roasting environment.It makes these machines truly unique.
Willem or Marcus,
At what point did you apply the increase in PA? Or did you keep it constant during the whole roast. Also, how much would drum speed play into this? What size Giewen did you roast on?
We used a consistent air pressure throughout in these experiments. Drum speed would be another factor to analyze separately. We are roasting on a W1A, W1M and W6A in our lab. For these experiments we used a W1A.
currently I am using a constant airflow around 70% for all stage of roasting and playing with flame through gas adjustment .
but it doesn’t work for all coffee origins .
I am wondering the pressure reading here should be a negative one indicating the differential pressure between in drum and the atmospheric one. This differential pressure is indeed a measure of the speed of the air flow.
Does not stack height and number of bends on the flue have an effect on Pa,? and would it not be useful to talk about airflow in terms of %, as some one Giesen’s 100Pa will not have the same % airflow as on another machine.