Coffee roasting & Airflow control – “Creating flavour”

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My previous article described the unique feature of Giesen roasters which allows the operator to increase or reduce the roasting airflow pressure or Pa setting which is integrated into the control software of basically all Giesen roasting equipment. If you are more interested in the basics of coffee roasting, you can read that here.

By manipulating the Pa, which is basically the underpressure in the roasting system, the roast master can accelerate or slow down the Rate of Rise (RoR) and also highlight different qualities of the flavour profile of the coffee. In this article we’re putting 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 flavour notes black currants, liquorice, leather, pine, plums with a mild bittersweet aftertaste.

The Ethiopian Sidama displayed classical flavour 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 proofed to be very successful; the flavours came out as we had hoped. The Pa settings of 120 and 140 added increased perceived sweetness and less 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 flavour 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 flavour profile of the coffee.

Willem Boot

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