The various coffee associations depict ideal extraction boxes of a 4% span of extraction yield, all centred at 20%EY, encompassing 18-22%EY (although this standard actually goes back six decades). For a given brew method, how achievable is this across a range of coffees?
Several tests of 10 brews each, a different coffee for each brew (within my typical preference, e.g. various recent roasts from speciality roasters, not going from the lightest of the light to supermarket, old, dark roasted coffee sat on the shelf for months), shows that this range of 4% EY should be your absolute minimum target for your default grind setting & recipe. In fact, to achieve 95/100 brews in this range, 10 reference brews would typically need to fall within a 3%EY span & a standard deviation of ≤0.88%EY.
If you cannot keep the vast majority of brews within this range, assess your method, look at how you can reduce margin for error (careful weighing of dose/brew water/finished beverage & timing – repeating each step the same way, as much as possible, for each brew). Establishing whether or not you are achieving this is going to be very difficult without measurement.
With care, a standard deviation of ≤0.66%EY is certainly possible, suggesting the vast majority of brews, across different coffees, landing ±1.5%EY. This is without changing any aspect of the recipe, including grind setting.
Singling out a single coffee reduces variance further, a standard deviation of ≤0.5%EY is certainly achievable with care. For example, using the V60 recipe in the previous post (weighing brew water and killing the finished brew to the gram in the cup), 10 brews gave me a standard deviation of 0.34%EY, with 8/10 of the brews spanning 0.01%TDS (this is smaller variance than the precision specification of my VST Lab II).
I’m aware that there are differing views on the relationship between brew time and consistency of extraction, but these 10 brews all fell within a 15 second total span, with a standard deviation of 4.4 seconds (or, 3% of the flow time post bloom until no liquid standing above the bed). However, I wouldn’t discard, or write off a brew, based on time – always taste & assess, different coffees may take different times to reach the same EY.
Why is this relevant? Well, like many home brewers, I am regularly changing coffees. I want as little waste & disappointing cups as possible. I want to make as few & as small adjustments as possible, to maximise the strike rate of good cups. In short, I want to be able to spend less time chasing down variance from coffee to coffee & more time relaxing & enjoying good coffee. A little groundwork up front, evaluating your brew method & recipe, can pay off in the long run.