Coffee Data Science

Measuring Moisture in Coffee Beans

To better understand moisture measurement

Robert McKeon Aloe


Recently, I have been working towards understanding moisture measurement in coffee roasting. I want to know how different changes in roast profile affect moisture. In this regard, I bought a Syncfo 4in1 to analyze moisture, and I have been working to make sure I understand how the meter works. How does it measure water? How does water absorb into coffee?

As a result, I looked at adding water to green beans and roasted beans to see how well the meter could measure the increase in moisture. This moisture measure addition is known because it is simply a weight change from water addition.

I looked at accuracy in terms of absolute and relative moisture change.

Green Coffee Beans

The first test involved adding 1%, 2%, and 3% water to three samples of the same beans. I let the water absorb for 24 hours, and then measured the moisture and compared it to what I would estimate the moisture being from the weight change.

I calculated this assuming the initial moisture measurement of the beans was correct. For these beans, it was around 10%.

While the change is off, there seems to be somewhat of a pattern, so it is possible the water was absorbed in some uneven way that the sensor can not accurately measure.

I looked at relative moisture measurements

This experiment made me think that adding moisture directly was problematic especially because when roasting, there seemed to be uneven spots as seen below.

So I tried humidity bags that were set to make an environment 69% humid. They were originally for cigars. The rate of change was too slow, so I switched to an 80F sous-vide where the beans are in a bowl above the water, but the moisture in the air can more slowly absorb. The result matched much closer in terms of added moisture vs measured.

When plotting as a scatter plot, these data plots gave me more confidence in the moisture meter, but I also felt like moisture in coffee must not be well understood in how and where it is absorbed.

Roasted Coffee Beans

I found through some experiments that adding moisture to roasted coffee is advantageous because it increases degassing. So I looked at measuring this addition of water for roasted coffees, and I found a similar issue as green coffee. This means water must not be absorbing as evenly as the meter is able to measure or that it assumes.

I looked at this metric over multiple roasts and I found the moisture measurement was off, but that doesn’t necessarily mean the meter was off. If the meter was designed to read moisture in a certain way, uneven moisture could be problematic.

Relative changes in moisture didn’t show much improvement.

I also put roasted coffee beans in a sous-vide like the green beans, and I looked at the moisture over time.

I also tried another batch of beans with similar results.

On a test batch, I went to a larger extreme over four days which had a strange cross over.

These results speak more towards me not understanding how water absorbed than that the moisture meter didn’t function properly. I think moisture is hard to measure, and it plays such a role in coffee roasting that I want to understand more.

The extra fun question is how does moisture leave coffee beans during the drying process to produce green coffee? This question should be well studied in the coffee industry, but maybe the data is not public.

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Further readings of mine:

My Second Book: Advanced Espresso

My First Book: Engineering Better Espresso

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Collection of Espresso Articles

A Collection of Work and School Stories



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.