On Roasting Coffee: Probing the Merits of the “Convective Soak”

[Some of the research included in this piece was initially derived from my 2018 Roaster’s Championship Presentation]

Over the past couple of years, I have become quite enamored with probing the merits of technique known as the soak.

In a nutshell, the soak is a method of slowing down heat transfer from the roasting system to the coffee early in the roasting process. This is achieved by utilizing the thermal energy stored within the system to supply early momentum to the batch rather than with the added application of heat.

One central reason for desiring to achieve this slowed heat transfer is that, very simply, it seems to produce a sweeter cup.

But why? Why should coffee roasted using this technique produce a sweeter cup?

My hypothesis to explain revolves around the heating of sucrose.

Sucrose is the compound that is best known as table sugar, the same type of sugar that most people put into their coffee after roasting. Coffee in its unroasted state contains anywhere from 5-9% of this sweet compound as a percentage of its total weight.

By slowing down the initial conductive heat transfer from the drum/roasting system to your green coffee, you lessen the amount of immediate and aggressive surface heating that the seed and its cache of sucrose undergoes.

High speed heat transfer produces a blackened coloration (as proven by a hot pan home roast with little agitation)and a bittering impact on flavor; this is one reason why a high heat, short duration roast will often produce a darker surface color than a similar roast degree with a longer duration and incremental heat application.

This coloration is indicative of burnt sucrose (extreme pyrolysis) rather than the more desirable Maillard reaction (which requires input from amino acids) and leads to a sliding scale of potentially negative flavor inputs on your final product.

All this is to say that high amounts of heat, when applied rapidly, can make the sugars on the surface of the coffee seed burn.

This negative (my opinion) change happens to varying degrees depending primarily on how much early heat is applied.

For this reason, the implementation of a technique such as the soak slows the decomposition of the sugars, therefore minimizing the possibility of extreme pyrolysis on the bean surface.

This hypothesis gains validity with testing.

There are situations, however, such as those concerning small bean size (less than 15 screen), where even this technique can have its limitations. If the percentage of surface area to total bean mass is too high (testing in progress to ascertain the range in which these negative effects begin to take a noticeable stance) you are still left with a noticeable, albeit perhaps minimal, tinge of overly developed sugars. This also appears to be true for larger bean sizes, such as in Pacamara or Maracaturra.

For these cases, I have developed a technique that I am calling the “convective soak”. In this method, instead of using a low gas setting to temper the speed of heat transfer, one can use airflow.

I am aware that per conventional wisdom, early airflow can “excessively dry out” coffees early in the roast. However, we have tested this on multiple occasions and have found no measurable evidence to corroborate this long held belief.

It seems likely that this is due to the fact that at this early stage in development, the green coffee is still too dense to release anything but the minute amounts of surface moisture from the seed.  

The cases in which a difference in weight loss percentage occurred could be readily explained by the excess heat applied to sustain a pre-determined roast curve.

Furthermore, when utilizing this method one can still hit the coffee with a good amount of energy from the start, however, the air more evenly distributes this heat into the entirety of the system rather than directly into the coffee itself.

Thus, the beauty of this method is in the fact that this distribution of energy still imparts momentum to the batch, but it protects the seeds from extreme pyrolysis.

This is especially true if one only utilizes the airflow in the first few/couple minutes of the roast before removing it from the equation until again opening it up during mid-Maillard.

The resulting cups have had varied outcomes.

Small bean sizes utilizing the convective soak have resulted in nearly unanimous instances of very sweet cups with a high/mid acid and round/juicy body. 

Larger bean sizes have resulted in cups with a lowered perceived acid and an increase in sweetness and body (when compared to a “desirable” no-soak profile).

Middling bean sizes have been quite varied. Though again, it seems relatively conclusive that there is an increase in body and sweetness in comparison to no-soak scenarios.

More testing is undoubtedly necessary to hone this technique. However, enough has been done to prove that the convective soak holds merit in exploration and consideration.