Soaking and being stumped (updated with results)


Soaking and being stumped (updated with results)

To soak or not to soak ?.. that was today’s question.

Here you see two sets of curves. The lighter colour curves are the previous batch of this coffee with a ‘soak’ charge (I’ll explain). The darker ones are the current batch with no-soak. Bean Temp (BT) is Blue, Exhaust Temp (ET) Red. Now we all know BT isn't actually the 'bean temperature' don't we ? It's the air temp measured by a probe placed in the bean mass as they're being stirred and moved around. 

It’s the same coffee and batch size and I tried to match as close as I could the BT curve (bean temperature readings), drop temp and time. What I want to see is if the ‘soak’ makes a a difference in the cup.

I know, I said I was going to explain…

About 18 months ago I picked up on an excellent educational video series put together by Cafe Imports and Mill City Roasters with Joe Marocco and Dave Borton entitled “Roaster School”. A fantastic resource and I always looked forward to the next episode. Now and again Joe would talk about charging (emptying the green coffee into the roaster) the drum with low or no heat, then applying heat after a certain delay period. There was never any specific information as to why this was done but in a couple of episodes he eluded to the fact that it might be beneficial with softer (lower density) coffees. He didn’t refer to it as 'the soak’ but that’s essentially what he was talking about. I’d been struggling with roast appearance and consistency with low altitude Brazils so decided to give it a go. It worked and I adopted it as my go to approach for Brazil coffees. However I did not try it with high density coffees since I was getting results I was happy with using a non-soak charge.

Recently I met Scott Rao at an event held Clifton Coffee in Bristol. Scott’s blog is one of the goto places for speciality coffee folk and low and behold Scott had addressed the ‘soak charge' a few months back (which I initially missed). Plenty of folk commented on the blog with their experiences of using a soak charge, some good some not, some indifferent. Scott’s bottom line in his post was suck it and see.  No definitives about it being better or not and only one way to find out with you particulate roaster and your particular coffees. It brought the ‘soak’ issue back into my headspace. Then I happened to be talking (virtually) to a friend who is the head roaster for one of London's big-guns. We were both standing in front of our respective roasters having our virtual conversation.  He mentioned that they’d been soaking for a while and had great results. It had become their defacto approach. Then once more, this time I was watching one of the industry leaders (and godfather of specialty coffee) George Howell on an interview series where he drew the analogy of the soak charge with catching an egg. You don’t just stick your hand out and allow the egg to hit it and break, you match the speed of the egg with your hand and allow the energy to be absorbed ! Hah. Now it begins to make sense.

This brings me nicely back to my roast curves. Two curves, matched in everything (as closely as I could manage) except the initial charge phase (when the green beans are dropped into the drum). What do we expect ? If the likes of Scott Rao and Joe Marocco don’t know then I’m sure I don’t. Only one way to find out. Taste them side by side and see what’s what. Sweeter, cleaner, brighter, bolder, more balance, better finish ?? i.e. all the attributes we want from our coffees.

That’s tomorrow’s job. .. (and as if by magic tomorrow arrives with the usual activities in between).

In fact I've been misleading you (those that have made it this far) a little . I actually roasted two different coffees with the soak and my usual non-soak profile. One the El Salvador Finca El Miramar shown in the curve above, the other a naturally processed Ethiopian Kochere Yirgacheffe. Both are elements of my 'house' Espresso blend. El Miramar is a Bourbon varietal grown at 1400-1500m, whereas the Kochere is Heirloom varietals grown at 1900-2000. Different varietals, different processing and different altitudes.

I cupped these this morning and initially with the crust just broken I found it very difficult to distinguish any differences in the cups. As they cooled I started to notice small differences in the acidity, sweetness and finish. Not huge differences but noticeable. 

The soaked roast of the El Miramar had a softer acidity up front and in comparison the non-soaked version tasted a little harsh. The finish was also different. The non-soaked version had a slightly dry finish. We're not talking about unpleasant attributes at all just very small improvements with the soaked profile.

For the Kochere I struggled noticing differences. After 20 minutes of tasting I could easily have convinced myself that the soaked version had a better finish and was a little sweeter but to be honest I couldn't say that with any certainty.

I did measure the solubility of each roast with my VST Refractometer and there was no noticeable differences between the roasts. Not surprising really since we're only talking about changes during those initial few minutes with the overall roast time and temps being almost identical.

So, in conclusion. I'd have to agree with Scott when he said try it and see. It might show improvements. It might not. So many variables to take into account including the way the roaster applies the heat and maintains it's thermal inertia. ie convective versus conductive heating.

For me I've got a new standard profile for the El Miramar which includes a soak. That's a win so I'm happy. 

So what about Stumping ? Actually it's more like me being stumped. I've been asked to be on a panel of roasters for an event being held in Bristol as part of Cafe Imports 'UK Legendary Coffee Tour' supported by La Marzocco and Sprudge. My role is to sit on a panel with 3 other roasters and be asked questions by folk who presumably expect a sensible and knowledgeable answer. 

4 Comment

  • Carlos Avendaño on

    Great post! I have to agree on the soaking technique, however, is crucial to establish a protocol between batches to avoid energy lose or entering too high. Think about the inners of the coffee bean as the structure of a honeycomb, makes sense to achieve thermal stability between the machine´s mass-beans mass before putting in more energy. Either soaking or establishing low beginning temperatures (knowing how much energy your machine can transfer to avoid stalls).

  • Ben Toovey on

    Hey Dave, I tend to use the soak as a standard because I’ve yet to prove to myself that it causes problems, whereas going in too hot definitely can. I feel that its almost more important to use precise preheating and between batch protocols to mitigate heat buildup in the system, but then with a cooler machine soaking could cause problems e.g. not having enough energy in the first minutes of the roast, as apposed to having too much…

  • Steve Green on

    Thanks for the Roaster School video mention. We flattered to hear you’re watching and thrilled to hear you’re watching for something other than comedic value. We’ve made a lot of new friends and we hope to expand the series with shared knowledge, ideas, and techniques from coffee professionals all over the world.

    The soak technique is a control method that works best on high thermal mass roasting machines where the latent heat in the system in addition to constant heat from the burners makes it difficult to slow the roast enough to produce an optimal roast profile.

    On our roasters, it’s often beneficial to turn the air off during the soak to reduce the cooling effect of dilutive air.

    None of this automatically produces better coffee, but it often makes it much easier to achieve more precise control, repeatability, intentionality, and consistency.

  • Will LIttle on

    Great post. The soak charge definitely works for me. I’ve always seen it as a way of avoiding scorching and facing on single skin drums, which can be a definite problem for low density beans, but as I play around with it I also find that it gives me more control over ror later in the roast. Shouldn’t make any difference really, but I guess the dynamics in my roaster are linked to that in some way. Taste wise. Yeah, not scorched beans taste better! Also, stoked to see you on the panel for the CI tour!!

Leave a comment

Please note, comments must be approved before they are published