Coffee Data Science
Measuring Groundtruth Extraction Yield for Robusta vs a Refractometer
Do extracts from a robusta cause different refraction
Previously, I explored the accuracy of a refractometer to measure extraction yield for arabica beans. There is an assumption that arabica and robusta have the same effect on refractive index, but I wanted to collect some data to help speak to it.
For a number of shots, I measured the Total Dissolved Solids (TDS) using a digital refractometer (Atago), and I calculated the Extraction Yield (EY).
EY = TDS * output/input.
Then I chopped up the spent espresso puck and dried it in a toaster oven set on 200F for 30 minutes. Previous tests of mine have shown that more than this amount of time is unnecessary to evaporate all liquid in the puck. I can then compare this weight to the original weight of the input puck.
In this way, I can see how accurate a refractor can measure the EY for robusta.
Equipment/Technique
Espresso Machine: Decent Espresso Machine
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped
Pre-infusion: Long, ~25 seconds
Infusion: Pressure Pulsing
Filter Basket: 20g VST
Other Equipment: Atago TDS Meter, Acaia Pyxis Scale
Metric of Performance
The main metric of performance is Extraction Yield via dried pucks and refractometers.
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).
EY from dried pucks is determined by using the weight of the dried spent puck and dividing it by the weight of the input.
Data
Previously, I found found there was an offset to the actual EY when measuring the EY with TDS. I have two sets of data, regular and adjusted. I adjusted the values based on the amount of moisture added during my humidified coffee technique.
On these graphs, if all the points were on the diagonal line, that means TDS measured EY is the same as the dry measured EY.
For Robusta, there is a strong trend showing a closer match between the two techniques for EY. All these values were adjusts by any moisture added to the roasted beans.
I broke this down by roast in case there was a roast dependency, but I didn’t notice something major. The caveat is that this experiment was over three roasts of the same green robusta coffee over shots spanning one month for each roast.
I wanted to quantify measurement error for both the scale and refractometer because that has an impact on how accurate we can calculate EY from either technique. The combined error is still oddly high at 0.111% which means because any error measurement below 0.111% is in the noise of the measurement tools.
A better scale would help, but I don’t have one that does micrograms.
This was a nice sanity check on measuring dissolved solutions to determine EY for robusta. It is easy to assume robusta behaves like arabica, but that would be a fundamental mistake.
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