Roast Approach: Roast Phases For Profile Development

Traditionally, roast education has centered around the idea of measuring three key parts of a roast; drying, Maillard, and first crack.  As a self taught roaster, I was always in awe of how my peers were able to make adjustments to their roast profiles to create the desired flavor outcomes in the cup.  As my knowledge and understanding of roasting grew over the years, I embraced a new methodology of roast profile development which focuses on manipulating key chemical reactions during the roasting process.  I use this phasic roasting methodology to create roast profiles and dial in the final cup.  

In the phasic roasting methodology, the roast is broken down into four distinct phases that align with key chemical reactions.  The phases are drying, Maillard, caramelization, and post crack development.  Key to this methodology is using seed temperature (from thermocouple reading) to establish consistent and objective "start" and "stop" points for each phase.  Time spent in each phase provides the roadmap for the roast profile construction. The temperatures that I provide with each phase are accurate to the thermocouple that I use on my roaster.  When I bring this methodology over to a different roaster, my first step is always to adjust the start/stop points of the phases to the new machine's thermocouple reading.  I do this by lining up my sensory observations at these key points from one machine to another, then adjusting my start/stop points accordingly.  For instance, going from the Proaster I normally use over to an Aillio bullet has a difference of -25 degrees F in the readings.  Once this is established, I am easily able to use this methodology on the new machine.

Roast Phases

Drying

The first phase of the roast is the drying phase.  The key chemical reaction during this time is evaporation.  Prior to entering the roaster, the processed green coffee is somewhere between 12-9% moisture.  In order for the Maillard phase to begin, the coffee must get down to between 6-7% moisture.  Once the coffee enters the roaster, the coffee begins absorbing heat and the moisture evaporates on the surface of the seed.  Internally, watermolecules try to preserve themselves by consolidating in the center of the seed.  This bit of moisture in the center will be critical to the audible "pop" experienced at first crack.  This phase is measured as the time spent in between the initial charge/drop of the coffee into the roaster and the temperature reaching 300 degrees. 

Sensorially, the end point is where the coffee seeds go from a neon green to a tan/yellow color.  

Maillard 

The second phase is Maillard, or more specifically, "early Maillard". Maillard is the non-enzymatic browning, whereby heat (rather than an enzyme) is the catalyst for the change.  These chemical reactions require a low moisture (6-7%) and typically begin happening around 300 degrees Fahrenheit.  Key to this phase of early Maillard, sugars and acids are broken down and recomposed into volatile and non-volatile reaction products.   As disaccharides are broken down into monosaccarides (fructose and glucose), then combined with amino acids, intermediate compounds such as furfurals and dicarbonyls are formed.  These intermediate compounds are critical precursors to chemical reactions which will happen later in the roast.  In the phasic roasting method, this phase is marked as time spent between 301 degrees Fahrenheit and 340 degrees.  Maillard will continue throughout the remainder of the roast, but during this phase, it is the predominant chemical reaction happening.  Sensorially, this is identified as the point where the coffee goes from neon green to tan/yellow through the time when the coffee begins to become a light mottled yellow-brown caramel color.  

Caramelization

The third phase of the roast is caramelization.  During this phase, there are numerous chemical reactions occurring.  Key chemical reactions in this phase include the breakdown of disaccharides, the formation of melanoidins, and the degredation of organic acids.  Maillard is continuing through this phase, adding to the development of volatile aromatics and the darkening color of the coffee seeds.  During caramelization, sucrose is breaking down into glucose and fructose.  This is key to the character of sweetness in the cup, as fructose hits the palate faster and has up to 80% more perceived sweetness than sucrose.  Breaking down the sucrose into fructose creates more perceived sweetness in the cup.  

The formation of melanoidins is the other key chemical reaction occurring during this phase.  As mentioned previously, during Maillard there are intermediate compounds formed as byproducts.  During this phase, the intermediate compounds link together and polymerize into high-molecular-weight, brown colored compounds called melanoidins.  These high-weight compounds are responsible for the body and mouthfeel of a coffee.  Melanoidins also have their own associated flavors of pastry and bread.

Caramelization begins to occur between 320-356 degrees, depending on the sugar.  For the purposes of consistency, the caramelization phase is marked as time spent between 340 degrees and first crack.  More time spent in this phase means more creation of reducing sugars like fructose and more creation of melanoidins.  The result is more sweetness and more body.  However, this comes at the cost of acidity.  As the roast continues, organic acids will begin to break down.  Citric acid will begin to breakdown first, followed by malic acid.  Spending too much time in this phase can result in coffee that is bready.  In this situation, both acids and sugars have broken down, leaving the prominent flavor of the melanoidins remaining in the cup.  We often refer to this coffee as "baked".  

Post Crack Development

The final stage in the roast is post crack development.  It is marked as the time between first crack and the end of the roast.  It is important to note that during this phase, all of the chemical reactions are happening.  Maillard is occurring, sugars are breaking down, acids are breaking down, polymerization of intermediate compounds is happening, and pyrolysis is occurring.  Of all of chemical reactions occurring during this phase, I believe that we most closely associate post crack development with flavors of pyrolysis.  As the roast progresses, pyrolysis continues the thermal breakdown of organic compounds leading to eventual carbonization.  Additionally, in the pyrolytic breakdown of chlorogenic acids, there is the formation of quinic and caffeic acids.  These contribute a sharp bitter acidity and astringency.  

The amount of time spent in the post crack development phase is really about controlling the amount of pyrolytic flavors in the cup.  More time during this phase will mean prevalence of bitter flavors.  It will also mean that acids (such as citric, malic, and lactic acids), sugars, and eventually melanoidins will all degrade.

Developing a roast profile

Once you have an understanding of the roast phases, we can take that knowledge and start using it to construct a roast profile with specific flavor outcomes.  Profile development becomes very simple.  To move quickly through a phase, thus a short time segment, more heat is required.  To move more slowly through a phase, less heat is required.  

Here is a model of the roast phases in a roast profile.  The short blocks denote a short time spent in this phase and high heat application.  The long blocks denote a long time spent in the phase, and thus a lower heat application.