11 May 2020
Our Roaster, Chris Bonney, takes us through the art of coffee roasting!
Part 1: Look At My Curve!
A fresh peach and a cooked peach taste pretty similar. A raw steak has a lot of flavours in common with a cooked steak. But a raw coffee bean tastes like, well, a broken tooth. And if you do somehow manage to chew a green bean, you'll taste almost nothing -- like a piece of plastic, maybe with hints of grass or straw. By cooking coffee, we actually CREATE the flavours and aromas that make coffee taste like coffee.
An enormously complex beverage, coffee has twice as many aroma/flavour chemicals as wine. And these chemicals are produced in cascading reactions during the roasting process. By adjusting roast conditions -- heat, time, airflow, etc. -- the roaster can control which chemicals and flavours are produced --from sparkling citrus, through rich caramel and cocoa to bitter charcoal and ash.
When I began roasting, the only way to learn was to find a roaster who was willing to share their secrets and then work as an apprentice (sleeping on a cold stone floor, gruel for every meal), slowly learning the craft. Two pivotal events in the coffee world changed this -- the publication of Scott Rao's Coffee Roaster's Companion, which laid out the basics of roasting in clear, black-and-white terms, and the invention of Cropster, a program for recording and comparing roasts. Rao's book introduced thousands to the hitherto secret world of coffee roasting, but also had the effect of promoting a single, right way to roast. At Toby's Estate our experience tells us that there are many ways to roast, and instead of forcing your beans to adapt to your roast style, you need to adapt your roast style to your beans.
So let's consider Cropster. You may have seen posts of roast curves, like so:
And they look super scientific and complicated, right? But really they're pretty simple. There are two temperature probes in a roaster -- one down low in the mass of roasting beans, called "bean temp." And one up high in the open area of the roaster, called "environment temp." If you compare this to your oven, environment temp is the temperature of the oven and bean temp is the temperature of your roast beef (or your celeriac if you’re vegan). This chart is just a graph of the output from these two probes over the course of a roast. The blue, v-shaped line is the bean temp; the red one is environment (See below).
The roast graph above (a Brazil) is nice and smooth after the initial dip (which comes from dropping the cold beans into the hot roaster) the temperature rises consistently until the end of the roast. Heat powers the chemical reactions we are trying to produce so we want to apply it evenly through the roast with no sharp dips or spikes
Each point on the curve is an adjustment of airflow or heat. (See below)
In the roast shown we are reducing the size of the flame as the beans get closer to the end of the roast. Note that the adjustments between the middle of the roast (defined at Toby’s by “Airflow 8”) and first crack are crucial in producing the finished cup.
The other two lines on our graph are RoR – “rate of rise.” If the first pair tells us how hot things are, the second pair tells us how fast things are heating up. Higher = Faster. Lower = Slower. The rate of rise is just a calculation based on temperature and time. It shows degrees per minute.
Roasting coffee is a lot like playing golf – you want to put all your energy into the initial drive so that the ball flies fast and then runs out of energy just as it drops onto the green. Similarly, you want your beans to heat quickly at the start of the roast and then slow down until they nearly stop heating at the back end of the roast. And that’s what we see in the graph above. The bean temp RoR (blue) and environment RoR (red) both start high (fast heating) and then get slower and slower. By the end of the roast the RoR is almost zero.
One last thing. The point marked “First crack” is a crucial measurement in roasting. It records the moment when the water in the coffee bean vaporises, making an audible cracking sound. First crack happens in every roast, and the time between first crack and the end of the roast is called “development time.”
So now you know how to read a roast graph. (The above coffee, Brasil Daterra Moonlight, was delicious, btw. It’s one of the May espresso specials so you can try it yourself.) But what, you ask, does a BAD roast look like? It looks like this:
In this graph the curves are bumpy, indicating uneven application of heat. Worse still, the graph flicks at the end, indicating that the roast slowed down and then sped up again. This is called “baking,” and produces a flat, harsh cup that lacks sweetness and structure. As a golf ball, this bean went fast, then slow, then fast again, and finally hit the green with too much energy and bounced off into the rough.
Part 2: What Does It Taste Like?
Coffee nerds love our numbers, and we can compare roast curves and total dissolved solids until the cows come home, but really, the only meaningful question is WHAT DOES IT TASTE LIKE? So let’s take a look at how the roast curve shapes the cup.
Kenya produces coffee of crystalline clarity with sparkling, champagne-like acidity and wonderful fruit notes. Below are two roast graphs for Kenyan coffees.
Both these curves are nice and smooth. But remember how we said the adjustments before first crack were crucial to the quality of the cup? The first roast has 2 gas adjustments before first crack, while the second roast has 3. If we taste these two coffees side by side, the 2-change Kenya is sparkling, clean and juicy, while the 3-change cup has a weird, intrusive savoury note like Vegemite on sticky date pudding. It’s not entirely wrong, but it most definitely isn’t right. We’ve roasted a bajillion Kenyans, and this rule is almost always true – more than 2 gas changes before first crack is fatal.
And you know natural coffees – intense and rich with winey notes and distinct berry flavours. Think Ethiopia Guji for example. Since flavour is the star, many roasters choose the obvious path – emphasise your star and don’t worry about anything else. So the cup tastes like: BERRY berry Berry BERRY. Other roasters consider naturals to be too wild and funky for polite society. They try to clean everything up, so the cup is more like: something something berry something something. At Toby’s, we believe that epic flavours deserve a great frame – the structure of the cup. It’s crucial that the acidity talks to the flavour, that the mouthfeel contributes to the finish, that every aspect is supported by measured transitions. In short, that the cup is a complete picture.
So this is how we roast (some of) our naturals – in this case a natural Myanmar:
There’s a lot going on here, but note that the starting temperature is much lower than the two Kenyas – 200° as compared with 220°. This is one of the keys to imparting structure without sacrificing flavour. This coffee, also one of our May specials, has a lovely raspberry acidity that transitions smoothly into a cup with blueberry and licorice notes and a silky body.
Roast parameters can affect much more about a coffee – the balance of flavour and aroma, the way the coffee ages, and on and on. But in the end the only thing that matters is producing the most delicious coffee possible. And that is what we aim to do every day at Toby’s Estate.