Coffee Data Science

Sifting Green Coffee for Roasting

How does bean size affect taste?

5 min readMar 5, 2024

Green coffee beans are usually sorted at the farm so that the size of the bean is more consistent in a bag. Having the same size bean is also important for roasting, in theory, because all the beans should roast more evenly. So let’s test this out!

I borrowed some of the Kruve bean sifters from Hiver at Chromatic Coffee, and I went to work sifting coffee. It was not simple because beans got stuck. I used a massage gun to try to speed things up, but it was slow going.

The amount of beans less than 16/64" for Burundi was so small, I did not roast them separately.

All the sifted beans had similar moisture levels as green beans.

Roasting

I used Scott Rao’s roast profile that uses bean temperature as turning points for inlet temperature.

I used a Colombia coffee and a Burundi coffee. So I’ll show some curves and talk about taste.

Colombia

I was surprised at the curves having any difference, but a more consistent bean size should have some effect. The larger beans had a consistently higher temperature.

The Rate of Rise (RoR) has been a useful metric, and all four follow a similar trend.

The number of cracks is useful as well, and this metric seemed be more differentiating.

Burundi

The curves seemed much different. Larger beans went up in temperature faster.

The same was true for RoR.

The cracks followed a similar pattern where larger beans cracked sooner.

Roast Metrics

I looked at three main metrics:

Weight Percentage (Roasted/Green)
Moisture
Color

They all seemed pretty consistent.

Color seemed to show more of distinction than the other metrics, but I don’t know if it is significant because it is still within 5. Previous work shows the color can vary quite a bit depending on the bean orientation.

Tasting Equipment/Technique

Espresso Machine: Decent Espresso Machine, Thermal Pre-infusion

Coffee Grinder: Zerno

Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)

Pre-infusion: Long, ~25 seconds, 30 second ramp bloom, 0.5 ml/s flow during infusion

Filter Basket: 20 Wafo Spirit

Other Equipment: Acaia Pyxis Scale, DiFluid R2 TDS Meter

Metrics of Performance

I used two sets of 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.

Shot Data

I looked at taste, and it seemed for the Colombia coffee, the taste was better for sifted coffee.

Extraction yield varied, but there isn’t enough data just yet.

For the Burundi coffee, the taste came out a bit differently. The larger beans had slightly lower taste scores.

The EY story seemed more even than the Colombia EY story.

These tests were very interesting. I know there are competition coffees with a focus on sifting coffee beans before and after roasting for the most consistency. My tests are only initial experiments to get a better sense of impact. If sifting coffee was not as labor intensive, I would be more inclined to do it for all of roasts.

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