Again, a word of warning to those reading; these words may be a bit dull to an average coffee drinker to which a “cup o’ joe is just a cuppa joe”. Then again, if you were only in it for the caffeine fix, you probably wouldn’t be reading this in the first place…. So I digress. Before continuing on, however, I will say that it may be helpful to read Coffee Science Part II via hartcoffee.com/coffee-science-pt2 which covers the temperature of water and its relation to extraction time (and perhaps re-read Part I, also available online). Coffee Science: Agitation and Contact Time will appear November 30th and Coffee Science; Brewing Methodology (different brewing techniques and ideals) will appear mid-January. Anyway, without more rambling, here is the overview of it all, Coffee Science: Extraction Overview
Water, Meet Coffee
Here you can see extraction occurring in a v60 pourover. The bloom has been completed (note the hill that the CO2 has formed in the middle of the brew bed while escaping) and the washing stage is beginning (note the browning on the sides of the filter). You can see positive outgassing occurring where the water is entering the bed (in the form of a foam or “spit”)
Your coffee is fresh and your grind is set with your window of variance as small as is capable (good job sharp grinder burrs!); your purified (not distilled) water is at a balmy 203®F and you’ve pre-wet your filter and warmed your vessel (be it a v60 or a French press or a drip coffee SS carafe).
You’re ready now to introduce your water to your precious coffee.
The roasted coffee seed is formed by a cellulose matrix, which in plain English means that under intense magnification we can see that it looks much like honeycomb or Swiss cheese. Here, trapped within the holes or chambers of the coffee are both the aromatic gasses of the particular bean as well as a large amount of CO2.
While much of this gas is jailbroken during grinding, some chambers remain unbroken or merely fractured. When water is added, these remaining vestiges of matter will then begin to do what is called “outgassing”, or simply begin an outpouring of gas that continues throughout the brewing process.
The initial, most intense part of outgassing occurs within the first 15 to 45 seconds of contact (depending on the freshness of your coffee), and is known as the “bloom”.
The bloom is an essential component of brewing fresh coffee because, when done properly, it prepares your grounds for nutrient extraction. Another, more visually apparent reason for the bloom, is that the outpouring of CO2 gas is so great, that each particle has the ability to hold itself far enough apart from other particles that incoming water will not be able to gain access to the ground coffee’s cache of nutrients (basically, each particle will have a gaseous force field to protect itself from water).
After this initial outpouring of gas, however, the coffee becomes susceptible to nutrient extraction.
This extraction occurs in two steps, the first of which, I refer to as the “washing” stage.
When the water comes into contact with coffee (after the bloom), the solubles on the surface of the ground coffee particles bind to the H2O molecules and wash away into the catching vessel below.
This stage occurs rather quickly and simply and is responsible for about half of the nutrient extraction required for a delicious cup of coffee.
The remaining brew time is concentrated on the swelling of the ground coffee due to large amounts of moisture (your water) and the extraction of the sugars and nutrients held within the inner parts of the fractured coffee bean. This occurrence is called the “expansion and extraction” stage.
This stage is where you separate a good cup of coffee from a bad cup of coffee, taking for granted obviously, that your coffee is wonderful from the start.
When done properly, your water will penetrate the now porous particles and extract the soluble nutrients (mostly comprising of sugars and sulfur containing compounds) and stop at the moment when the larger, less soluble compounds begin to break down.
When your brewtime runs too long and these larger compounds begin flowing into your cup, you are now over extracting your coffee.
Over extracting coffee has a pronounced bitter flavor that is, well it’s unpleasant.
Perhaps a question has arisen? Why can’t you simply stop your brew after the washing stage and choose not to test the troubling waters of the expansion stage and risk over extracting?
Unfortunately, while over extraction produces bitter elements to your cup, under extraction will result in a grassy, flat, astringent, and bodiless cup of coffee; which is quite as unappealing as an overtly bitter one.
In order to get the most out of your coffee the washing and expansion stages must both be complete.
Here is the dance we dance.
The variables must all come together, in unison, to create the perfect extraction. If one element is off, the picture becomes skewed and in order for the desired extraction to occur, another variable must react to the change to create a counter action to achieve the same result.
There are many more factors that can agitate or contribute to this process, as outlined in other “coffee science” articles. These are merely the occurrences that transpire in a well-brewed cuppa.