When I wrote The Coffee Roaster’s Companion, I was on the fence about The Soak. These days, I’m on the side of the soak, most of the time. I’m overdue to write about it.
What is the soak?
Broadly, from what I’ve seen, the soak is a roasting method in which one charges a batch with low-to-no gas and increases the gas to a high setting sometime in the first two minutes after charging. Just to avoid the inevitable question, no, the soak does not involve adding water to coffee beans :).
The soak is not necessary to produce stellar coffee. But I have found the soak makes good development and clean flavors easier to produce in most situations, with most machines.
Why does the soak work?
This is my hypothesis— it’s speculative, and I have no proof that this is the reason the soak works, so please take it in stride:
In a typical 10–12 minute roast, coffee does not release moisture for at least the first minute, and probably a little longer. This is because a portion of the coffee beans’ mass has to reach the boiling point before releasing its moisture as steam. Even at the boiling point, it’s likely steam won’t be released immediately upon its formation, as the cellulose bean structure surrounding the steam may be too tough, and not porous enough, to allow the steam to escape. So much for “the drying phase."
Moisture released by beans during roasting forms an “evaporative front” that acts like a force field, slowing the transfer of heat into the beans. That's why beans with higher moisture content require more gas during roasting: the heat has to overcome a greater amount of (cooler) moisture to penetrate the beans.
Given that there is no evaporative front for the first, let’s say, 1:30 of a typical roast, one doesn’t need to apply as much gas during that phase to heat the beans at a reasonable rate. Charging with a lower gas setting also has the (presumably) beneficial side effect of lowering the peak drum-metal temperature in a classic-drum roaster; that should produce softer flavors in the cup. Setting a machine's thermal energy slightly higher before charging a soaked batch facilitates development. Therefore, the soak may help softness and development; that should yield better coffee and provide a greater margin for error.
But in CRC you advised charging with sufficient energy, and the soak calls for less gas at charge.
Good point. When I wrote CRC, I knew that an inadequately-warmed-up machine was more likely to produce underdeveloped coffee. Therefore, a sufficiently hot charge, and pre-charge actions, were important for good development. What I didn’t know then was how much of the “sufficient energy” should come from the machine’s thermal energy and how much from the gas setting at charge.
Soaking has taught me that the initial thermal energy of a roaster is critical for development, but the initial gas setting is not. Please don’t take this concept too far: I do not recommend charging too hot or soaking with the gas completely off. My apologies for not being wiser and clearer when I wrote the book; I’m always learning and will try to comment publicly when I’ve realized I have made a mistake. Unless it’s a really, really embarrassing mistake. :)
Should I always soak?
Probably not. Some machines aren’t as well-suited to soaking as others. As well, some coffees seem better suited to the soak than others. I simply recommend you try the soak, but keep an open mind about its benefits and its limitations.
I’d like to hear about your experiences with the soak. Please don’t ask me how or when to soak— that’s a little deeper than I’d like to delve in this blog, and also, a proper answer may require more information about your particular machine and roasting style than would be reasonable to discuss here. Thanks for reading.