Extraction Versus Time

The Merit Interval is an interval of time in which your extraction yield is optimal. As you are aware, and to your utmost subjectivity, there is some place during extraction where the after yield is either over or under extracted. The merit Interval would be the in-between area in which is neither over, nor under extracted. It is an area with a buffer, some play room, and is where every barista strives to cease extraction during - for producing the best tasting cup to their individual aspiration. It is also proportional to time and closely correlated with the rate of extraction. Here if a visual representation, where the area underneath the extraction verses time curve, from a to b, provides you with the Merit Interval. 

As you can see by the graph - a larger interval should produce a larger buffer. This means that you have a wider target to hit and will provide an interval that is easier to produce consistent, satisfactory extractions. The larger the area, the more forgiving the extraction should be. For example, it is no secret that pulling espresso shots is extremely sensitive. Compressing an extraction time around 25 seconds creates very small room for error - hence having a very tiny merit interval. However, looking at the near end of the spectrum - it becomes very difficult to mess up cold brewed coffee. With extraction times around 24 hours, you could cease extraction within a two hour interval and still have a perfectly adequate brew. 

I would like to take the aeropress into consideration. A typical aeropress extraction time may vary around 1.5 minutes having one of the shortest brewing times out of most methods. Because of this, and consequently having a small merit interval, both Jose and I have found it fairly difficult to produce consistent yields from. Comparing this to espresso, and although pulling a shot is very delicate, at the same time the elements are very much within your control. Granted your machine is dependable - and temporarily ignoring parameters - the extraction time is reasonable enough to consider consistent. The aeropress however is not quite so simplistic. While the infusion time is generally manageable - when adding it with pour time, rotation, temperature drop, agitation, plunge speed - it becomes a bit more to keep attention too. All of this while trying to end it on a dime can become quite the challenge. 

Looking at the curve below shows that the closer you are to zero on the time-axis, the higher the rate of extraction will be, and the less area the merit interval will have. Since there will always be coffee extracting, the extraction curve will be ever increasing, but at a slower and slower rate. In the graph it shows identical time intervals, in which the merit interval M2 is larger than M1. However, these time intervals should really decrease as they are shifted left (a1-b1 < b1-b2), but for illustrative purposes in the graph below, even if the time chuck was identical, the merit interval of M1 would still be less than M2. What this means is that by stopping extraction while the rate of extraction is very high and the total elapsed brew time is shorter, this will entail for a smaller target to hit. Again referring to espresso, because the rate of extraction is so entirely fast, you have very little time until being drastically bitter or astringent. With brewing coffee this is similar, but on a much larger scale. Things do not happen as fast, but at the same time the mechanical aspect can play a larger role because of how much more time there is for error. 

Looking at any traditional pourover brew method presents many volatile details. It can be a struggle to extract a uniform amount holistically because of things like high and dry grounds, erratic turbulences from varying pour speeds and heights, water temperature dropping in both the kettle and slurry, and even the inconsistency as water percolates through the slurry in a general sense. Despite these lovely battles, you could perhaps save yourself some sweat and coarsen up a bit, and extend your extraction time. I'm not trying to associate tastier coffee with longer extraction times, but conceivably produce tastier coffee more frequently and consistently instead. Extending extraction times is not always something that is even an option, for the quality of your grinder plays a large role in that as well. Having too many fines inside of your grind makes things trickier due to an even more dramatic over extraction, and may indeed lead to poor coffee. This is really an alliance towards pushing the best extraction out of a single brew, and then being able to repeat that same cup within a small deviation.