Good espresso should not be bitter, burnt or ashy. However, I seem to run across said espresso this week. After pulling numerous shots, drip, pour over and even a try at cupping, I can say without a shadow of doubt this coffee was a dud. It must have had a low quality Brazilian coffee in it that could not handle the darker roast that it was given.
However, it allowed me to use a few trouble shooting techniques when you are not getting the right taste foryour espresso shots. So to be specific this is what I did when trying to comprehend why this particular coffee was bitter.
Temperature. this is probably the most straight forward one. Burnt espresso can be made by water that is too hot. I am still getting use to my new espresso machine, so when I had the first taste I thought I must've pulled it too hot. And with the espresso machine that I have (a heat exchanger) you pull the water from the group until it stops hissing off and then can count from there to get your rough temp of brew 205 degree minus x seconds. In this case I started at about 200, then down to 198, and 195. I can't say there was a much of a difference from 200 to 198, however going down to 195 I got the most awful taste of burnt and sour.
Grind, the other big factor is grind. If the grind is too fine, you will start brewing up bitter elements of the coffee. Since in the above I was merely focusing in on temp this was the next guess. Now I only have double baskets at my disposal, so I went with the ridgelesss Espressoparts basket that I have because it can hold the most coffee and requires the largest grind. I can fit 17 grams in my basket before it's too big of a dose. I was at the 16 gram range pulling a short shot, 1-1.5 ox. This time I let it rip at 200 degree and for a normal 2 oz shot in about 24 seconds. No luck still bitter/burnt just not nearly as much.
Cleanliness, well the other big factor in bitter espresso is a dirty machine. The espresso grounds can get caked on and then over extract in every shot you make. Now I do clean my machine pretty well after every time I am done using it, but it's a different machine from the Gaggia Classic I was using so who knows perhaps I was sloppy. I took a look at it and it seemed fine. Then I pulled a shot of the Gold Medal coffee I got from Mr Espresso, rich balanced chocolate goodness. Yeah it was not cleanliness in this case.
After this I was frustrated. It had been such a long time since I had a burnt coffee that I though well perhaps it's just me. No triple basket, or accurate temperature management that I could bet my life on. So I did a pour over in the V60 and did a cupping of it.
In both cases I got the smell of ash off the top and wretched black water underneath.
Overall it was a very frustrating experience, however it did provide for a nice evening of getting to try and figure it out even if it wasn't really what I wanted to find out.