February 2, 2016

The S-Curve Roast Profile: Exploring Roasting Basics


Have you ever had a coffee that was roasted poorly? You know what I’m talking about – it has that bitter, smoky, and just plain unpleasant taste. It’s the kind of coffee that gives our beloved beans a bad name. And it’s the kind of roast that I’m going to teach you to avoid making, by helping you understand the S-curve roasting profile.

Spanish Version: La Curva S del Perfil de Tueste: Explorando las Bases del Tueste

I’ve been roasting for about two years now, and during that time I’ve learned some complex roast profiles. However, a large portion of my roasts are modified versions of one basic profile.

Now, let me give a quick disclaimer: roasting isn’t easy. Coffee is a complex product, and becoming an excellent roaster will require dedication, practice, and experimentation.

Dan Smith roasting

Roasting certainly isn’t easy – but once you’ve mastered it, you’re free to experiment!

Yet if you master this basic profile, you’ll be on the way to making some seriously great coffee. So, without further ado, my roast profile:

A Basic Roast Profile: The S-Curve

The S-curve is a very simplified, relatively easy-to-use profile for a medium roast coffee. More importantly, it’s a great starting point and, over time, you’ll be able to experiment with it.

s curve coffee roasting profile roaster

Please note: This profile is designed for a gas-operated classic drum roaster and has not been tested on other roasters (e.g. fluid bed, indirect heat, or recirculation).

This diagram shows the basic S-Curve roast profile. As you can see, temperature is on the Y-axis and time on the X-axis. I didn’t include any specific temperatures, since they’ll vary according to the probe (thermometer) location. You can add them to the graph once you’ve worked out where they fall for you and your machine, though.

Essential Information

Before you start roasting, you’ll want to know what temperature your roaster usually hits first and second crack at. This will give you an approximate window that you can work between for a medium-style roast.

Take an espresso roast, for example: I remove the beans around 1-2 minutes post-first crack, making sure that it’s at least 1℃ before second crack. This is actually a pretty large window (so don’t panic) and allows for a well-developed coffee, minus that oily taste that usually occurs in coffees roasted past second crack or the sour underdevelopment that can occur before first crack.

As for the timing, I recommend aiming for your first roast to be complete in 14-15 minutes, with first crack at around 12 minutes.

Dan Smith roasting

Checking the progress of the roast.

A Guide to the S-Curve Roast

Unfortunately, roasting isn’t as simple as throwing some beans in and waiting to see what happens. There are a few points where some action on your part will be required, usually in the form of adjusting the burners.

  1. Drop Temperature (DT)

This is the temperature your roaster should be before you load your beans and begin. During this period, you’ll usually keep your burners down low. The coffee should absorb heat with only a small amount of assistance. As a guide, you could use the second crack temperature as your drop temperature and then, depending on how your roast progresses, make any changes needed.

  1. Turning Point (TP)

Immediately after loading the beans the temperature will begin to plummet. You can see it on the graph, right? This will happen as the cold coffee absorbs heat from the roaster.

Shortly after loading, however, you will reach the TP. This moment is when the coffee has absorbed enough heat that the temperature stops falling and begins to rise again. (Note: if your roast runs too long, one way to decrease the roast time is to engage the burners before you reach TP or apply more initial heat to assist the turn.)

From this point, depending on roaster type, I would turn the burners up  to max so as to allow the heat to penetrate into the inner bean. As your roast progresses, try to control the burners to allow for a Rate of Rise of around 10℃ per minute.

  1. First Crack (FC)

Throughout the roast, the coffee beans are losing moisture, sugars are caramelising, and a multitude of other reactions are taking place. When the beans reach a certain temperature, moisture and gas get released along the seam of the bean and this results in a popping or cracking sound: first crack. (Don’t be alarmed if you hear three or four cracks rather than just one – this is very normal.)

At this point, the burners need to be turned down to very low or even turned off completely. The bean is going through a very rapid loss in moisture and weight; if the burners aren’t reduced, the ends of the bean can start to blacken (tipping) and the surface of the bean can start to scorch.

  1. Roast Development (RD)

The flavor of your coffee will develop throughout the entire roast, but within this small window your actions can have a large impact on it. What you do here will determine how much acidity and body your roast has.

acidity and body in coffee

Acidity vs Body: Their balance is down to how long you spend in the Roast Development stage

Generally speaking, the longer you spend in RD, the more body the coffee will have – but the more acidity will be sacrificed. Acidity can be associated with fruit, so if you want to bring out those fruity notes, roast for a shorter period of time. Alternatively, for a chocolatey flavour, roast a little longer. In my opinion, RD should range between 45 seconds and two minutes, depending on what you are trying to get out of your coffee.  

On a technical note, it’s important that the temperature remains stable during RD (take a look at the flat section of the graph). Aim for it to rise by no more than 6℃, but don’t let it drop either. You might want to apply a very small amount of heat towards the end of this stage to avoid it falling.

  1. End Heat (EH)

At the end of the RD time, you should increase the heat so as to bring it up to the temperature that the roast will finish at. Be careful, though: the beans are much more brittle due to the moisture loss, so too much heat will damage the coffee. When I tested out my roaster, I increased the heat to the point where the beans would tip just so that I knew how far I could push it before damaging the coffee.

  1. End Temp (ET)

This is when you check if your beans are done. You’ll see the surface of the beans starting to expand and becoming smooth. Be wary of finishing too early, though: my advice for beginners would be to start off darker than normal – think just before second crack. Then, as you start to grow more confident with the roasting process, you can finish it earlier.

Check the inside as well as the outside of your beans, too. Coffee isn’t like steak; you don’t want it to be medium-rare! Crack open a roasted bean under your thumb and look for a uniform colour throughout.

SEE ALSO: Coffee Video: Roasting UP CLOSE in 90 Seconds

Dan Smith roasting

Always watch the temperature closely to avoid any mistakes.

And there you go: your own home-roasted beans, soon to be ready for brewing.

This roast profile is a basic guide. Once you’ve grown confident with it, experiment. Try different temperature and heat profiles, and read plenty of coffee resources for inspiration (I strongly recommend The Coffee Roaster’s Companion). With roasting, it’s important to discover your own way of doing things. Be unique – you won’t regret it.

Happy roasting!

Edited by S. Parrish.

Perfect Daily Grind.