How to rehydrate coffee

2 minute read

I’m so sick of the water.

Michael Phelps

Since I initially published my post about rehydrating coffee, many of you have tried it yourself, and others have inquired about the best way to go about it.

In general, I’ve found (as have others, including UK Barista Champ ’18 Josh Tarlo ahead of his 2019 routine) that you’ll get the intended effect of improving coffee through rehydration until about 16-17% moisture. It’s easy to overshoot: coffee is hygroscopic and pretty thirsty. Simply soaking green coffee in water will quickly lead to the coffee taking on more water than intended, swelling and having deleterious effects—including mold (particularly with longer than 12 hours of homogenization time).

So I recommend using weight to control your moisture input by calculating the percentage of solids versus water and deriving from that your final weight.

Tim Heinze of Yunnan Coffee Traders first taught me this method as a way to measure the moisture content of a fixed volume of drying parchment (since ~60% of cherry and ~45% of wet parchment is moisture) in the field more reliably and inexpensively than using a handheld moisture meter. Thus it came as no surprise to me when I saw him using the calculation, but in reverse, to control his rehydration of coffee.

Here’s how to do it:

First, take a moisture reading of your coffee. For the purposes of this demonstration, let’s say your coffee is 11.0% moisture. Begin by weighing however much coffee you’d like to rehydrate and calculate what percentage of that mass is water and what percentage is solids. From there, you can derive your total weight of water/solids.

Let’s say you’re working with 20kg of coffee at 11% moisture:

20 * 0.11 = 2.2 kg

Your lot is 2.2kg of water, and the remainder,

20 - 2.2 kg (water) = 17.8 kg

is solids.

If you’re shooting for 16% moisture (which is what I recommend), you’d simply derive how much water you need by dividing the solids by the percentage they will contribute to the final weight, which is, in this case, 0.84 or 84%:

17.8kg solids / (1 - 16%) = 21.19kg

That gives you the final weight of the lot, with moisture added. You can simply weigh out your starting weight of coffee (20kg) and add enough water to hit your final weight—21.19kg—which is an additional 1.19kg of water.

Close your container, shake or mix it to distribute the water uniformly throughout the seed pile, close the container, and allow it to rest at room temperature overnight or 8-12 hours, turning it a couple times throughout.

You should arrive at your final target 16% moisture, which you can verify using a moisture meter. If there is still standing water, you can allow it to sit for another few hours, but likely your moisture is too high. With 16% moisture, I had no issues in the cup with 24 hours of rest, but Tim notes that at 20% moisture the coffee that sat longer than 12 hours presented moldy. My recommendation: set 16% as your target upper limit, and shoot for 12-16 hours of homogenization rest (which happens to fit many production schedules well).

Once you reach your target moisture content, roast as soon as possible to prevent degradation of the coffee.


That's just, like, your opinion, man

  1. Hi Christopher,
    Referring to the reverse method–for measuring the moisture content of green coffee *without* a moisture meter (fourth paragraph). Could you please explain how?

    1. . . . Is it as simple as weighing your sample before and after roasting, and assuming that most of the difference is water loss (and that the roasted is 0% water)?

      Thanks again.

      1. Oh, to get your starting moisture content without a moisture meter you can either ask your importer (they’ll know) or use the (now withdrawn) ISO standard method — which is to place 100g of the green in a dehydrator at low temp (like 120°F for example) for 24-48 hours and weighing the result. If you have 8g of loss you can infer you lost 8g of moisture (8%).

        However, some moisture does remain in the cellular matrix and some of the loss appears to be due to material degradation:

        A better method would be to use a small dehumidifier, airflow and a lower temp of about 90°F/32°C and more time to make sure you aren’t damaging the coffee.

        1. Ah. Thanks, Christopher. A little beyond my means.

          In other news, happy to report a tiny but surprising success with rehydrating and roasting some old green coffee. Surprising because, although I’d struggled mightily and carefully to roast my control sample and the rehydrated sample as similarly as possible, my home-roasting equipment is very crude and short on data, and I’m very much in the dark. (A GeneCafe CBR 101) The first crack seemed to arrive a little later (I wasn’t confident I heard the beginning) with the rehydrated sample, and its final appearance was oddly mottled (patches of light and dark), which I guessed might have been because the seeds had not absorbed moisture uniformly. I kept total roast time constant for each, ensuring at least that the rehydrated batch did not have longer development, but I feared the rehydrated batch would be woefully underdeveloped and unpalatable.

          I have not cupped these blind yet, partly because I’d assumed the rehydrated batch would be a bust anyway. I’ll try and get to that. I made V-60s of each (as close as possible to Jonathan Gagné’s method). The control had a familiar papery underdeveloped flavor. I was sure the rehydrated would be the same, or likely worse. The very opposite was true, it turned out.

          The green coffee was a 2018 Kenya which I had frozen for a year then thawed in Novemebr 2019. I’ve been struggling to get anything decent with it since. It’s not bad, and I had resigned myself to erring on the side of under rather than over development. The window seems very narrow, and I by luck I hit the sweet spot a couple of times in maybe a dozen tries so far.

          As for the rehydration: I mostly followed your method, with a couple of slight tweaks. I used distilled water, and after blotting the seeds sealed them in a Fresh Saver bag and vacuumed the air out, thinking it couldn’t hurt and might ward off any bacterial growth. Then I stored them overnight, and roasted them about 20 hours after hydrating. When I opened the bag, I couldn’t smell anything off. Their appearance seemed about the same as the dried. When dry they had weighed 100g. Now they weighed 104.51g. So some water had been absorbed.

          As I said, to my surprise the result in the cup was very nice indeed, and I’m encouraged to try this some more. I have a second batch of Kenya, also from 2018 and frozen for a year. It has proven even more difficult to roast acceptably.

          At least on the strength of this it appears that the higher moisture level encouraged better development.

          So, one anecdote, one very poorly controlled experiment, one sample. But these days any hopeful coffee project, especially if it’s progress, is a small thrill. Thanks again for the inspiration!

          Best regards,

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