Brewing Coffee Residue

An unfortunate experiments in grossness

Robert McKeon Aloe
6 min readAug 8, 2020

I take full responsibility for this mess. It is disgusting, gross, and well, I’m not sure why I did this experiment. However, in my defense, none of this coffee residue went through a cat’s butt, so there’s that.

I wanted to use the drip try from one of my espresso machines in a different one. I hadn’t looked at it for a few months. Turns out, it was dirty with the end drippings of 500 shots or more of espresso in the past year or two. The residue was the results of about 10g of coffee at the end of a shot of espresso that dripped to the bottom. The water would dry out, and over time, this monster formed.

It looked so solid and dense under a microscope:

Top row is from the top of the hardened residue, and the bottom row is from the bottom of the residue.

After pulling it off, I wondered how much was still soluble.

While cleaning the tray, the grossness was slow to dissolve.

Dissolving a Sample

I decided to take some, dissolve it in water, and test EY. I tried dissolving almost 1g of residue in about 25g of water at room temperature. The most I got was 24% EY, but there were strange things. Usually, if you leave a sample on the Atago, the TDS reading will increase as water evaporates. Instead, TDS continued to drop. I suspect suspended particles dropped or oil floated to the top.

I even tried filtering the sample for large particles but the aero press filter clogged.

So I used a paper towel. The TDS reading didn’t change.

I want to emphasize that at the very least this is not cat poop coffee.

Pulling a Shot

Then I ground it up with an old Hario. I had to grind again because it didn’t seem fine enough.

Then I prepared the puck with a paper filter at the bottom, and I used a 500g tamp.

Then I tried some pre-infusion, and that’s where everything got stuck. I did 15 seconds of pre-infusion, and then full pressure. The far right shows the result after one minute.

So I slowly depressurized the machine, and I carefully removed the portafilter without incident.

The puck was disgusting of course! I measured the TDS of the liquid, and it was 0.83 so only 2.8% EY. I was surprised because I expected more of the residue to be soluble. It was very oily and thick.

No, I didn’t taste it. I’d never drink a shot with such a low EY.

I pulled a cup of water through the machine got to make sure any of the grossness would not stay in there, and luckily the water came out clean.

The drip tray is still stained, but grossness free. I also took apart the grinder and washed it thoroughly. The smell was pretty grotesque. As I was grinding, I questioned why I was using minutes I wouldn’t get back on something bound to fail.

I didn’t set out to do this experiment; it just happened. And why you may vomit in your mouth, just remember, I didn’t drink it, and it didn’t come out of a cat’s butt hole.



Robert McKeon Aloe

I’m in love with my Wife, my Kids, Espresso, Data Science, tomatoes, cooking, engineering, talking, family, Paris, and Italy, not necessarily in that order.