Surface Area and Time
February 08, 2015
In this post I’ll be covering how to increase or decrease the overall extraction of a brew from a theoretical standpoint.
(Angela Grieve, a Hustle subscriber asked for this via email. So Angela, this one’s kind of for you!)
There are a few variables that you can use when manipulating extraction. The two main contenders are Time and Surface Area. These two variables are intimately intertwined in every single type of brewing environment; you can’t change one without changing the other.
If you give water more time in contact time with the coffee, it will extract more of the flavours. If given enough time, extraction will continue until there’s nothing left to dissolve. So naturally we want to find a time where the extraction has reached a point that’s delicious.
Within the variable of time, there’s actually two different things happening that we need to separate and consider. The first relates to what the water extracts and when, which is purely based on contact time. The second relates to how easy it is for the water to do that, which is decided upon by a mixture of both time and surface area so I'll tackle that later on.
What the Water Extracts When
Every soluble component of coffee has a slightly different solubility (that sounds weird, but stick with me). Salts, sugars, acids, phenols, fats and lipids etc. all take different amounts of time to be dissolved by water. Some will be dissolved straight away, and others will need more time. We need to think about this when brewing coffee, because changing the contact time will also change which components of the coffee will be dissolved by the water. The first and most soluble parts of coffee are fruit acids and organic salts (light, bright, fruity flavours), closely followed by light aromatics created from the Maillard reactions and sugar browning during roasting (nuts, caramel, vanilla, chocolate, butter etc.) and lastly heavier organic matter (wood, ash, malt, tobacco etc.).
This makes it rather easy to err on the side of a shorter brew, as most coffee drinkers reading this article tend to steer away from ash and overtly bitter flavours. Unfortunately, this will also sacrifice sweetness in a lot of cases. Coffee, as always, is the master of making you compromise.
Increasing the surface area of the coffee makes it far easier for the water to dissolve its flavours. When you grind coffee, you’re increasing the surface area exponentially.
Think of a coffee bean as a cube. This cube is 1cm wide. It has 6 sides, each of which has an area of one square cm. So our little cube has 6cm2 of surface area.
Cut that cube in half and cut the resulting pieces in half until you have 8 little cubes. Each of them are now 0.5cm wide and have 1.5cm2 of surface area. This means there’s 12cm2 that the water can easily get at. Not much work and we’ve doubled our surface area!
Now lets put that cube through a grinder at a very coarse setting. This time, we get 64 little coffee cubes 0.25cm wide, each contributing 0.375cm2 with a total of 24cm2. Doubled again.
A typical espresso grind is going to break this 1cm cube into many hundreds of little pieces. As you can imagine, the surface area really starts to add up. This drastic increase in surface area allows the water to dissolve the majority of the coffee’s flavours pretty easily. No, I’m not going to draw 1024 cubes.
Inside these little imaginary cubes, there is still quite a large amount of coffee flavour that isn’t immediately accessible to the water. This is why coarser grinds take longer to extract than fine grinds; the water has to travel into the grind, dissolve the flavour, take that flavour back out, and deliver it into the brew. Compare this to a fine grind (where all water has to do is touch the coffee and it’s dissolved everything) and you see why surface area is so important for increasing extraction.
The most important thing to wrap your head around here is that grind size doesn’t really change what is being extracted. It only changes when the things are extracted. All of the flavour is right there, in the coffee bean, ready to be extracted. Grind size just puts more or less of that flavour in front of the water right away.
Here’s another way to think about it: When talking about grind size you could also say ‘how much coffee flavour am I going to hide inside the grinds, away from the water?’ or ‘how much should I delay the extraction of a portion of the total flavour?’. A finer grind will hide less flavour, and reduce that delay. A coarser grind will hide more flavour and increase the delay.
When you want to increase extraction, you need to grind finer. You need to increase the surface area of the coffee and ’show’ the water more of the flavour. Once adjusted, your extraction will increase because the water can ‘see’ more of the flavour and get to work dissolving it more quickly.
Remember: all of the flavour is waiting there in the grinds. Manipulating surface area just puts it in front of the water, or hides it.
How Easy is it for the Water
If you have lots of coarse grinds, it’s hard for the water to get inside them and dissolve the flavour. It has to work its way through the convoluted maze of structural cellulose, be in contact with the flavour for enough time to get what you want (see above) and then has to move those flavours back out again. Coarse grinds require a significantly longer contact time because it takes so long get these three steps done. If extraction takes 1 unit of time (1), getting into the majority of the flavour of the bean takes one unit of time (2), and getting that flavour back out takes one unit of time (3), then every time you hide flavour inside a large coffee ground, it’s going to take 3x longer to get it. Meanwhile, the external surface area continues to extract, most possibly farther than you’d like it to.
What does this mean for espresso?In espresso, you only have ~30 seconds to extract everything. This means that any coarse grinds are practically useless ‘fillers’ and will never be extracted completely or evenly. Espresso grind sizes are generally extremely small to give the water every chance to extract enough flavour in that short time period. You may have seen my 2013 World Barista Championship Routine. In this routine I helped to popularise (but not invent or discover) the use of a filter or ’shop’ grinder for espresso brewing. The grinder I used was Mahlkonig’s EK43. It’s crucial to use a grinder like this, or similar with flat burrs, that has a more even particle size for espresso brewing. It will help to ensure most (or all) of the flavour is accessible by the water within the short brewing time. Don’t worry, there will be much much more on this in future Hustles.
What does it mean for filter/brewed coffee?
In a filter coffee (eg. french press or pour over) we use much coarser grind settings. This is pretty much for one main reason. The brewing time of filter coffee is much longer than espresso because we don’t have the aid of a pump to push water through the coffee grinds and filter. Instead, we’re usually using gravity which is much slower. Because of this necessity for longer brewing times, we ‘hide’ more of the coffee’s flavours inside larger grinds to reduce the chances of over-extraction.
The most important thing to take from this: while you’re waiting for the larger grind's interiors to be extracted, the finest grinds are still extracting. The clock doesn’t stop for you. You have to decide whether that extra time will benefit the brew by giving you some more extraction (and yes, sweetness) from the larger grinds, or if it results in the smaller grinds being over extracted (and bitter/dry). Choose wisely!