Coffee Data Science
How Soluble are Coffee Solubles?
Improving the understanding of coffee extraction
A year ago, I had 10 shots of high extraction espresso from experimental coffee that I placed in the refrigerator in a cup. Over that time, all the water evaporated, and I was left with a very dense paste of coffee solubles. Of course I put that syrup into a basket and tried to brew them. However, I learned an interesting lesson that plays back to temperature which is that solubles need some heat to become dissolvable as well as time.
I ended up doing two tests where I pulled a shot with solubles on the bottom of a 7g single basket, and spent coffee was on top. I pulled the same shot with 5 minutes of thermal pre-infusion by locking in the portafilter into the machine for 5 minutes before pulling the shot. This allowed the solubles to have a temperature closer to the brew water.
The result was that channels quickly formed in the first shot because of differences of thickness of the solubles. For the second shot, the pressure pushed all of the solubles out of the filter until a small channel formed.
This experiment made me realize I had never tested how temperature affects the solubility of solubles themselves. So I designed a new experiment.
Sugar Tests
Mixing coffee solubles in spent coffee would be painful, but from the refractometry standpoint, we know they dissolve similar to sugar. So let’s start with sugar and see what we learn.
I mixed 5g of sugar in with 20g of spent coffee in a basket.
I mixed thoroughly, and I pulled flat 1 ml/s profile on the Decent. I then varied temperature.
One interesting note is that most pucks rose out of the basket afterwards, so some gases were either being generated or being added from the water source. Each shot had 5 minutes of thermal pre-infusion.
Total Dissolved Solids (TDS) is measured using a refractometer, and this number combined with the output weight of the shot and the input weight of the coffee is used to determine the percentage of coffee extracted into the cup, called Extraction Yield (EY).
I used 3 cups to pull salami shots, and then I varied the temperature. As expected, the hotter the temperature, the higher the extraction.
I also looked at the EY trend across brew ratios. Some of the EY numbers seemed off, but there was a good trend. 60C seemed too high.
Another way to view the cumulative EY across ratio.
Then we can simply this way of looking at EY by looking at just 3 temperature values.
Previously, there has been work to understand the temperature differential between the top and bottom of the puck, and I added some markings to shot a few interesting points for the temperature. This means the solubles, the sugars, are dissolving faster at the top than the bottom by temperature alone. So by doing thermal pre-infusion, a more even temperature also allows for the sugars throughout the puck to be more evenly dissolved.
Some times, I run a test just for fun, and often, this type of experimentation ends up providing some learning. The result of these tests help re-enforce the learnings from thermal pre-infusion.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
