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