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
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.Tags: green coffee moisture rehydration