Coffee Data Science

Thermal Pulsing for Coffee Roasting: High or Low Start

More charge explorations

Robert McKeon Aloe
4 min readJul 2, 2024

Previously, I looked at charge temperature, which is the starting temperature of the roaster before you add the beans. I wanted to explore the beginning of the roast profile to see how that impacts taste.

The Roest profile waits for the turning point to commence which is the point when the bean temperature starts going up. I used a default setting, so instead, I set the setting for the low of the first curve for the Low Start, and then I started with the peak of the first pulse for the High Start:

  1. Thermal Pulsing (TP) Baseline
  2. Thermal Pulsing (TP) Low Start
  3. Thermal Pulsing (TP) High Start

Roasting Curves

All the curves had similar forms with a little offset.

The Rate of Rise (RoR) showed a strong peak for High Start with a smaller dip at the beginning of the Low Start.

All the crack curves had a similar trend.

Roast Results

The High Start ended up not having as much loss from roasting.

The moisture levels were pretty similar.

Water activity was particularly low for TP Low Start.

For roast color, the TP High Start came out a bit lighter than the other two roasts.

But the roasts didn’t have much difference in density.

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).

Shot Data

TP High Start underperformed the other two in taste, but TP Low Start seemed pretty even to the Baseline.

EY came out similar for all the shots, which is somewhat expected.

These experiments didn’t yield an improvement, but they did give insight into how the temperature at the beginning of the profile affects the end taste quality. More testing should be done with more focus on lower start temperatures.

If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.

Further readings of mine:

My Second Book: Advanced Espresso

My First Book: Engineering Better Espresso

My Links

Collection of Espresso Articles

A Collection of Work and School Stories

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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.