Let’s talk about the Bitterness of Coffee
Coffee beans in their raw(green) state don’t contain bitterness. The bitterness is an add-on element that results from the roasting process.
So, the question is why we get bitterness.
The simple answer is carbonisation. Take sugar for example.
When we heat the sugar on a pan, it melts and then starts to turn into caramel.
Leave it on the heat a little longer and you’ll find your pan with some sticky black thing. That’s the end product of carbonisation and rather similar to what coffee can become if left in the roaster too long.
The Two Dimensions of Bitterness
So, I’ll be referring to a book by a coffee specialist and vice president of the SCAJ(Specialty Coffee Association of Japan), Taguchi Mamoru. It’s called 田口護のスペシャルティーコーヒー大全 which can be roughly translated as ” Mamoru Taguchi’s Specialty Coffee Encyclopaedia “.
He talks about the two dimensions of the bitterness of coffee. The good and the bad. He says that the roasting process brings about many chemical reactions within the coffee seeds.
There are two products which are detected as bitterness on our tongue.
- Chlorogenic lactone variety （クロロゲン酸ラクトン類）
This is, believe it or not, the so-called good bitterness as it is pleasant to our palate. I guess it’s similar to the pleasant bitterness of cacao in the dark chocolates we have.
- Vinyl Catechol Polymers （ビニルカテコール重合体）
This is the unpleasant bitterness, the bad bitterness that we’ve come to associate with when we taste something burnt due to carbonisation. Burnt toast, over-caramelised sugar or straight espresso(italian roast) for example.
The Two Factors Which Determine the Good and the Bad Bitterness of Coffee
As far as I know, and from what I’ve read….. there are two factors that determine which kind of bitterness you’ll be more likely to get from your coffee.
- Roast Degree
Yes, as far as the general rule applies the darker the roast, the more of the “bad” bitterness you’ll be likely to get. It also depends on the coffee. For example, one variety of coffee can produce a lot of the “good” bitterness around Full City Roast (just before the second crack) while another can produce it at a Vienna Light Roast ( mid second crack ).
But yes, generally you’ll get the “good” bitterness and then as you roast more the “bad” bitterness increases. So they have an inverse relation.
- Moisture Content in the raw state
Now, depending on the moisture content of the coffee beans at their raw state, you may get more or less of the “good” and “bad” bitterness.
Going back to the chemical reactions during the roasting process of coffee, the reactions that usually go on are Hydrolysis( the addition of water to break bonds) or Dehydration Synthesis( the taking away of water to break bonds).
For a roaster, the most important thing is to first “dry” the coffee beans before starting the roasting process.
I’m sure there are many reasons to do this, and most likely one reason to do this is to prevent Hydrolysis from occurring. Why? Hydrolysis is one of the reasons you could end up with Vinyl Catechol Polymers or the “bad” bitterness.
And so, it’s important to reduce moisture content so that Dehydration Synthesis occurs instead of Hydrolysis.
Including a Bitterness section in the cupping forms
Mamoru Taguchi touched upon this subject in his book and I totally agree.
The SCAA(Specialty Coffee Association of America) cupping forms as well as the COE (Cup Of Excellence) cupping forms do not contain any ” Bitterness ” section since it’s thought that one should only evaluate the coffee as you would find it in its raw state.
Bitterness is only a result of the roasting process. However, bitterness is the fundamental element that allows us to consume this amazing crop and it doesn’t do justice to ignore it.
As I introduced above, there is a pleasant side to the bitterness of coffee and if we can also introduce this into our evaluation, more emphasis may be put on producing coffee that not only tastes amazing at lighter roasts but also at darker roasts.