Coffee Data Science

Gaggia Classic: Tricks and Critiques

Some data on a classic espresso machine

4 min readSep 12, 2023

I borrowed a Gaggia Classic from my brother-in-law, and I pulled some shots to better understand the machine. I generally have thought the machine is a good starter machine for espresso, but then I dove deeper into the grouphead.

The Grouphead

First, let’s talk about the grouphead. I think it is not designed well because the water comes in from one side into the water diffuser. Once the basket is pressurized, the location of the water input doesn’t matter as much, but depending on the flow rate, this causes uneven water input while filling the basket which causes channeling.

When the flow rate is slow, water will come out of the left side faster than the right. When the flow rate is fast, the opposite is true.

I measured this by measuring the water from each side of just the water diffuser (without the shower screen). I ran a few tests, and every test, even the hacks were uneven. The 5 minutes of warm-up helped, but I wasn’t sure if the machine continued to heat, would that change? Is part of the temperature surfing problem related to water flow?

I talk about these tests in this video that shows quite a bit of testing.

Potential Hacks

I looked at two hacks: pulsing and reducing flow using the steam valve. I have no idea how these tricks will affect the lifetime of the machine, but they were effective in pulling better espresso.

Here is a video comparing a few tricks using the steam wand to reduce tank pressure and the on/off switch to do a blooming profile. I also collected some data on shot performance.

Additionally, I modified the inside of the grouphead using aluminum foil as I have tested with on the Decent. This shower screen fix improved shot performance and helped reduce this water diffuser issue. It seems I forgot to take a picture of the modification too, but it is similar to the Decent ones. I would do this modification now with silicon.

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).

Data

This data is a small sample of data. It is not intended to be decisive, but a feasibility test of performance. The taste differences were very noticeable.

There were also big differences in TDS and EY that are hard to ignore.

Overall, the Gaggia Classic is a great espresso machine to start with. If they could use a better diffuser and improve water input, the machine would be on another level because of the simplicity of profiling between the buttons and the steam wand.

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