Coffee Extraction Fundamentals – Nuts & Bolts
My goal with this piece is to help you understand the fundamentals of coffee brewing well enough that you can take a new coffee, or brewing method, and “Find” its full potential using this guide. Lets examine the main variables that influence the extraction of coffee at a microscopic level and this will guide our decisions in the brewing process.
A roasted coffee seed only contains of approx 28 – 30% solubles. The other 70% of the bean is made of insoluble fibers and carbohydrates that create the beans structure. Only 20 – 22% of the solubles are desirable, so we have to understand what’s happening and when, to avoid getting the ‘bad’ 8 – 10% soluble in our brew.
- Age of Coffee / Time off Roast
- Roast Level / Bean Density
- Grind Size
- Water Temp
- Water Quality
- Dose / Brew Ratio
- Contact Time
- Filter Media
How can we account for this many factors??
Age of Coffee & Roast Level
Let’s start with the structure of the coffee bean. Simply, its a honeycomb of closed cells that lock up the soluble, and gases that are created during the roasting process. The age of the coffee then affects the changes that happen inside those cells. This in turn affects the interaction with the water. This is a hard one to factor in because roasting methods and green quality / age can really affect this one. Generally though the lighter the roast the less the effect age has. A lighter roast equals a higher density, meaning that the heat from roasting has had less effect on the bean, leaving its cellular structure more intact.
So we grind to make these cells, and their tasty soluble, accessible to the brew water. The size of the grind determines how quickly the extraction happens. Here is where a quality grinder starts to come into focus. How uniform the particle size distribution is impacts the cup. Generally a more uniform particle size offers more flavor clarity and acidity, while more fines can add sweetness and body to the cup.
Water temp. There’s probably been too much focus placed on water temp. Rather, when it’s viewed as just another factor, it can be varied within the dynamics of the other variables. It’s very likely that water quality with the correct mineral content and/or TDS is more important and one of the least considered variables in the brewing process!
Brewing ratio is one variable that most coffee professionals watch closely. Mostly because it’s easy to measure and control with a gram scale. It’s worth noting that when one, or more, of the other variables can’t be controlled that changing ‘the dose’ can offset those unchangeable factors.
Contact time is the total time of water / coffee contact, from pre-infusion thru the draw-down. This is probably affected as much by water temp / TDS and bean density as by the grind size (This becomes more apparent with high quality grinding equipment.) It’s helpful to think of contact time as the ‘result of’ grind size, water temp/quality & brew ratio. I believe too much emphasis is placed on contact time and more consideration should be given to the grind size / brew ratio relationship.
Filter media is very important as it’s the medium between the insolubles and all the great tasting things we want in the cup. It also has a big part to play in the acidity / sweetness balance. Although filters are important, a majority of the filtering is done by the coffee bed itself. Agitation can really affect this and the paper’s ability to flow properly. Don’t be afraid to try different filters, or at the very least purchase filters from reputable manufactures.
Now let’s talk a bit more about these 20 – 22% desirables. These come in many different shapes and sizes. But for simplicity let’s reduce them down to four main categories: Sugars, Acids, Melanoids & Lipids.
- Sugars, fruit acids and caffeine are the easiest to dissolve and are responsible for light and fruity flavor notes.
- Melanoidins that are responsible for the browning color of coffee, both in bean and liquid form.
- Lipids are the natural fats and oils (Brewing methods that use metal filters like French Press and espresso allow lipids to pass through into the cup, producing the mouthfeel those methods are known for. The pores in paper filters are so small that they prevent most lipids from passing through.)
Thru testing with a TDS meter we find that 80 – 90% of these desirables are extracted into the brew within the first minute. (Whew that’s a relief!) But it’s important to realize that for a satisfying cup, we still need some of those compounds that add fullness and body, the fats and lipids. We are also lucky in that the 8 – 10% bitter compounds are the slowest / hardest to extract and spoil our efforts so far.
So now how to balance this dynamic process? Reducing the unknowns helps. Achieving perfection every time likely won’t happen, but this adds satisfaction to the times we experience that ‘Oh so good’ cup. I believe with time and thinking about what’s happening at the cellular level we start to connect the dots and make sense out of the mystery of the brew…
This is another very good article that does a good job of explaining the Nuts & Bolts of Brewing Coffee https://handground.com/grind/an-intuitive-guide-to-coffee-solubles-extraction-and-tds