Coffee Data Science
Sourdough Green Coffee Fermentation
Getting weird with roasting
A few months ago, the World Barista Championship (WBC) saw many different coffees using a variety of processing. Quite of few of them used some kind of fermentation, and I was particularly intrigued by one method using Koji yeast (used to make soy sauce and miso soup). All of these methods were applied to the bean while the fruit was still on, and I got curious if I could apply anything like this to green beans.
So I grabbed my sourdough starter and hastily threw it into some green beans, and I was quite surprised by the result. Acidity and bitterness, be gone!
Yeast Processing
Typically, yeast can be used to process coffee cherries, and then the fruit is striped from the bean. After reading about people using yeast fermentation methods, I realized I probably would not get green beans this way. Acquiring coffee fruit is nearly impossible, so I would be unable to do what they do in my kitchen. I could find a place that sold it, but it could be challenging to compare yeast processed to some controlled sample, and usually, it is sold roasted and at a price.
I decided to do it on my own using what I knew from sourdough bread baking. I was impatient, so I used sourdough starter instead of ordering yeast.
Processing green beans with yeast isn’t new, but it is not widely known. Afineur has been doing it for some time with the criticism that they are buying lower grade beans and trying to increase the grade after buying from the farmer. They market their process as a way to reduce coffee acidity.
My aim was not to buy lower grade coffee, but to increase the grades of the coffees I was using.
Coffee Beans
Speaking of beans, let’s take a look at the coffee beans I was using. I bought them from Sweet Maria’s, and they have their own grading system similar to Q-grading. A top coffee will score around 92 for reference. Anything above a 90 is very good.
My aim was to get a variety of blend qualities so that I could see if any particular coffee saw more of an advantage. Here are the four roasts used for these experiments with their scores and origins. All of the blends were half bean 1 and half bean 2 except for 2/9/2022 which used 8% robusta.
Sourdough Processing
I used sourdough starter, mixed with some water, let it sit for a day, rinsed it, dried it in the oven, and roasted it.
Here is the key recipe:
- Add sourdough starter and warm water to coffee beans; mix.
- Cover and let rest for 24 hours
- Rinse thoroughly
- Dry
- Roast
The variables have changed from when I started. So I put them in a table. I recently got a multi-function Instant Pot, and that has been the game changer. It has allowed total control over the process, but it isn’t necessary.
The beans seem to germinate a bit in this mixture, which is interesting.
I usually dry the beans until they are the same weight as the originally were, but I know some material in them is consumed by the yeast because some of the roasts lost a lot more weight than the original during the process.
While roasting, I did find that it took a few minutes more to hit the first crack, and I had to modify when I ended the roast. Usually, I end a roast one minute past the first crack, but in this case, the beans weren’t developed enough until 2 minutes past the first crack.
Aside from roast times, the yeast processed coffees also lost more weight during roasting., However, the densities for all of the coffees was about the same.
Equipment/Technique
Espresso Machine: Kim Express & Decent Espresso
Coffee Grinder: Niche Zero
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped, Staccato, Inside-Out Staccato
Pre-infusion: Long, ~25 seconds
Infusion: Pressure Pulsing
Filter Basket: 20g VST
Other Equipment: Atago TDS Meter, Acaia Pyxis Scale, Kruve Sifter
Metrics of Performance
I use two metrics for evaluating the differences between techniques: Final Score and Coffee Extraction.
Final score is the average of a scorecard of 7 metrics (Sharp, Rich, Syrup, Sweet, Sour, Bitter, and Aftertaste). These scores were subjective, of course, but they were calibrated to my tastes and helped me improve my shots. There is some variation in the scores. My aim was to be consistent for each metric, but some times the granularity was difficult.
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).
Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.
Paired Shot Performance
The yeast processed shots were very noticeable from the beginning, and they turned into the best shots I have ever had up until that point. Their main strength was cutting out acidity and bitterness, and this allowed the sweetness to really come through.
Here are 34 shot pairs on a scatter graph to show how they compared. The TDS/EY/IR was pretty similar, so I didn’t find a good reason to split it out by roast. Not all roasts were affected the same. The 2/9/2022 roast (8% robusta) had mixed results. Oddly enough, the 2/4/2022 was not as good as the others, which was interesting because I used the highest scoring coffees for that roast.
In looking at timing, yeast shots ran fast at the start. They more quickly covered the filter (Time to Cover the Filter or TCF), but their total time was more than the control.
I broke down Sweet, Sour, and Bitter in my taste scale. The other metrics were not as directly affected by yeast processing.
We can look at the average taste scores to see how yeast processing affects taste. For me, I didn’t notice much of an affect on the syrup or mouthfeel component.
Finally, I looked at the general statistics using a two-tailed t-test for the N=34 shot pairs. The quantitative extraction methods didn’t see a statistically significant difference, but the Final Score (taste) metrics did.
If you home roast, try some yeast. If you are bothered by acidity, try some yeast. I’ve continued to roast with and without yeast processing the green beans, and I’m really interesting in exploring other yeast methods. I’m particularly excited about yeast processing used by farmers, and I’m glad I have a way to explore this at home. Hopefully, one day I can work with a farm.
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