Roast Sensory Cues: A Detailed Analysis

We have worked very hard to build the most capable and most thoughtfully instrumented commercial coffee roasters ever built. We record data multiple times per second from 13 different sensors and readouts. We automatically record all of this in a virtual notebook. We graph most of it and calculate and display the rate of changes. That data can be used to automatically bring the system to a precise operating temperature and a known energy state and then execute precision automated production roasts worthy of specialty coffee.

Data absent sensory cues is meaningless

All of this is a stupendous leap forward for productivity, but if your goal is to produce better coffee, you’ll need to understand and interpret sensory cues like sight, sound, smell, and taste.

During the roast, operators primarily rely on sight, scent, sound and pattern recognition to identify, verify, and predict roast events.

Charge Temp and Charge Temp Theory

Generally speaking, coffee should be charged at the highest preheat temperature that doesn’t automatically produce heat defect. The goal is to make the drying phase as short as possible with the lowest application of burner heat to reduce the risk of heat defect.

Heat defects can be easily identified by the sight of blackened char spots on the top, bottom or ends of a seed. It can also be smelled in freshly ground coffee as an unpleasant sharply acrid scent. This imparts a similarly unpleasant taste.

Preheating the roaster to a higher charge temperature has three benefits. First, it’s simply more efficient. The sooner you get a batch out of the drum the sooner you can start the next batch. Second, and more importantly, it is important to drive the moisture out of the outer seed in a manner that conserves as much of the moisture in the inner seed as is possible. Conserving that moisture means you’ll have a shorter period of first crack and more precise control over post first crack flavor development. Lastly, many coffees respond better to an overall shortened roast time to preserve the flavor profile of the origin. If light roasted coffee is your jam, an aggressive hot and fast roast profile may be the way you get there.

If you are charging at a high temperature and you detect heat defect by either sight or scent, you should decrease your charge temperature by 10F.

The sight of End of Drying

At the beginning of the roast, as the coffee absorbs heat, seed color usually starts out as some version of dusty green. As the coffee heats it becomes a more vibrant green and then starts to yellow as the 10-12% moisture content is depleted in the outer seed.

In roaster speak, this change from green to yellow by sight is often marked as “end of drying.”

If the first crack event occurs around 390F on your roaster, end of drying color change is usually marked around 300F.

The challenge of using color changes as a development marker is that greens come in a wide variety of colors. Pulp and sugars on natural processed coffees will begin to caramelize and darken very quickly. Decaf coffees are often closer to a dark brown than green. Beyond that, comparing notes with other operators is tough because they can easily interpret color changes differently enough to make the sharing of roast data meaningless.

Although the initial few minutes of the roast do not produce any real chemical changes or produce or impart any appreciable change in taste, the period of time it takes to drive outer seed moisture off can greatly impact on development later in the roast.

The scent of Maillard: Green/Yellow Transition

In addition to or in lieu of calling the transition by sight, the operator can elect to call a green/yellow transition at the point of a scent change from drying hay or straw to something like the scent of baking biscuits or cookies. 

The initial drying period of the roast drives off free moisture and sets the stage up for Maillard development in the mid phase of the roast.

Maillard reactions only take place in the absence of water. Maillard reactions liberate soluble sugars from insoluble carbohydrates, deplete chlorogenic compounds and acids and create a cascade of higher level aromatics.

If the ‘end of drying” is marked at the sight of a color change at 300F and first crack is marked at 390F, Green/Yellow transition scent change is usually marked around 330F. 

The time between green/yellow transition and first crack is referred to as the “mid” roast or sometimes “Maillard’ phase.

See the roast go Exothermic?

Endothermic reactions absorb heat. Exothermic reactions produce heat. Coffee roasting is an endothermic process. Towards the end of the mid-phase as the roast approaches first crack, you might see the rate of rise increase suddenly, but it’s not exothermic.

Maillard reactions break apart long chain molecules inside the seed. Maillard reactions occur within a relatively narrow temperature range between 300F and 390F.

The actual chemistry is based on long chain molecules breaking apart with the application of heat. Big molecules break down into smaller ones. Stable smaller molecules of CO2 and H2O are carried away in the exhaust. Unstable molecules left behind in the seed combine in a cascade of reactions that create flavors.

The massive offgassing of CO2 and H2O significantly inhibits the seeds absorption of heat during mid-phase outer seed Maillard development. This slows the rate of the rise.

Relative to a first crack bean temperature of 390F, Maillard activity in the outer seed tapers off around 370F. The decrease in Maillard release of CO2 and H2O allows the seed to absorb heat much more quickly. 

For many years, roaster operators watching the climb in graphed ROR incorrectly identified this point as “exothermic”.

The change in the seeds ability to resist heat is a real problem for roast control. By the time it registers, it is too late to do anything about it.

Skilled operators anticipate this point and reduce latent heat energy in the roasting system with precisely timed airflow increases and fuel decreases to minimize unintentional deviations of the roast profile.

The sound of First Crack

The popping sound of first crack marks the end of the mid-phase 

First crack is the sound of the rupture of the endosperm deep inside the seed. If you look it up, a first crack temperature of 390F probably ruptures the endosperm at something like 400+ PSI.

Some interpret first crack as the first audible pop of the inner seed, but initial pops are usually outlier events from cracked or broken seeds. It is better practice to mark the onset of first crack by 3-5 pops in rapid succession marking instead the point of a “rolling first crack.”

The steam release from the endosperm both briefly slows the roast and allows a very brief, very energetic and very important period of Maillard development in the inner seed.

That same release of steam also briefly inhibits the absorption of heat. On the graph, this is labeled "crash."

If the operator is in control of the roast enough to avoid the "flick" without stalling the ROR, the roast will typically quickly return to the original ROR trend line in about 30 seconds without much effect on flavor.

If the roast stalls out and additional heat is required to achieve a light or medium finish temp, some flavors will often present as depleted or muted and/or earlier than normal "roast flavor" notes will present in the cup.

Can you see the math? Development time ratios.

If the first three roast stages are defined as:

“End of Drying” or EOD 

In the shortest time and charging at the highest temperature possible without heat defect

The second roast postulates a turning point of 230F at 1:30 with applied heat sufficient to achieve an average ROR of 20F/min for 5 minutes to 330F G/Y.  EOD is at 6:30 minutes.

Driving the Mid-phase 60F from 330F G/Y to 390F first crack in 4:45 minutes will require an average ROR of 12.6F/min for a first crack at 11:15 minutes.

Post development from first crack to a finish temp of 405F at an average ROR of 8.2F/min finishes the roast at 13:05 minutes.

Chemistry is most reactive at heat

When development time ratios work, it’s because the temperature the seed reaches has a greater effect on flavor development than the time it takes to reach the temperature.

The reason we manipulate the time of the roast is to more advantageously match inner and outer seed development.

Post first crack Maillard development deep in the inner seed has a huge effect on the flavor of your coffee and as little as 15 seconds difference from roast to roast is easily detectable in the cup.

Moreover, all throughout your roast, the rate of chemical change and the rate of flavor development or flavor depletion increases 2x with every 18F or 10C increase in temperature.

Guiding the roast to a targeted flavor profile isn’t like catching a speeding bullet. It’s like catching an accelerating bullet. Time matters a lot in the final seconds of the roast.

Finish Temp and Flavor profiles

Finish temp is navigated by pattern recognition of sight and scent in the tryer as a gauge of development and flavor profiles are developed by an iteration of variables like finish temp, development time and time of first crack.

Experienced roasters will recognize the scents of sweet smelling furfuryl and pre-smoke roast notes. They will note the relative darkness of the roast and the swelling and smoothing surface of the seeds. These cues will clue the operator to the speed of development and the conclusion of the roast.

Remember how chemistry is most reactive at heat? This makes finish temp about 70% of what your customers taste because the outside of the seed heats first. 

At worst, a poorly chosen finish temp will burn up your coffee or be raw and insoluble in the cup. 

“Dialing in” a finish temp means developing a roast profile and then repeating that roast to 3 or more finish temps with 2F degrees difference maintaining the same development time. Ideally, these roasts should be cupped blind against each other to identify the finish temp closest to your ideal presentation of that coffee.

Post and Mid Phase Development 

Development time dictates the relative roast level inside the seed. Your post first crack development time is probably 20-25% of what your customers taste.

Poorly controlled or chosen development times can waste 25% of the potential flavor you paid for. Neither case will build your brand. Get it right and you’ll be more in control of your product and probably make more money.

Similar to identifying the optimal finish temp, skilled operators iterate on their post first crack development by plus or minus 15 seconds to the same finish temp. These roasts are then blind cupped against each other to determine the optimal development time to the previously determined finish temp.

If the coffee still seems in want of improvement, operators now further manipulate the period of mid phase development by changing the time of first crack 20-30 seconds while maintaining the same development times and finish temp.

For any given commercially viable specialty coffee roast profile, professional roasters will routinely correctly identify a 2 degrees F difference in finish temp and/or a15 second difference in post first crack development 95% of the time.

Second Crack

Second crack is the ‘Rice Krispies” popping sound coffee makes as the cell walls of the seed swell with liberated CO2 and H2O and burst. If first crack starts around 390F, second crack usually starts around 420F.

Second crack is very hot and very aggressive. Sugars inside the seed caramelize and burn. Oil deep inside the seed is pushed to the surface and burns off. That oil creates the strongly scented roaster exhaust your neighbors will complain about. 

Modern specialty coffee roasteries seldom roast beyond second crack. Many use the sound of second crack as a signal to discharge their darkest blends, but most of us have figured out how to roast longer, slower roasts that deliver the solubility espresso and home brewers require.

Control tastes good

Roasting is cooking and by necessity a sensory driven process. 

Better coffee out of the cooling tray starts with a superior green and tight control of the roast to intentional results.

Craft in coffee is the intersection of the ability and understanding of the operator, the quality of the operator’s tools, and the systematic and intentional way the tools are used to surprise and delight our customers with our coffee.

Good machinery combined with good greens and good technique is a great way to attract and retain good customers.