These terms - strength and extraction - frequently give rise to confusion. In this post, we'll define each, and explain how they relate to each other in the context of brewing coffee.
Strength is not primarily about flavour. It's about how concentrated something is. Strictly speaking, no coffee is any stronger than any other coffee, regardless of what that bag in your local supermarket says.
Consider a latte and a cappuccino. Both drinks have the same amount of coffee in them - but one has less milk than the other and therefore that beverage is stronger.
All the drinks that we serve in a coffee shop contain the same amount of coffee (espresso) - but we give different names to them depending on how much milk is added. A splash? Macchiato. More? Piccolo. Even more? Cappuccino. More? Latte. You get the idea.
We all have our strength preferences which is why we have a menu of different drinks to suit us all.
Strength, in this context, is therefore a measure of how much of your beverage is coffee, and how much of it is milk (or water in the case of an americano).
We can describe strength in the context of the coffee itself. Consider a V60, Aeropress, or any other brewing device. Here, people tend to prefer beverages that are made up of around 1.15-1.45% coffee and around 98-99% water. Brews with more coffee than this tend to taste strong for most people.
The exception is espresso, which contains around 10% coffee and 90% water - too strong for a lot of people, so often diluted with either water or milk.
If we want our coffee to be stronger, we need to use more coffee, or less water (same thing) when brewing.
Extraction is all about flavour. Extraction is a term that describes how much of the coffee that we're using to brew our drink ends up in the beverage.
When we grind roasted coffee, around 25-30% of the resulting powder is potentially soluble in water. The rest is the organic matter that's left over when we've finished brewing. That's the puck in the espresso machine handle or the sludge in the bottom of the cafetiere after plunging.
Ideally, we want something like 18-22% of the powder to end up being contained within the brewing water. That's because, when we brew coffee, different compounds extract at different rates. Broadly speaking, we get acids first, then sweetness, then bitterness.
By extracting the right amount of these compounds, we get a brew which has a good balance of all three elements.
'Under' extract, and the acids will be too dominant because we haven't extracted the sweetness and bitterness to balance those acids. 'Over' extract and there will be too much bitterness, which will dominate the brew. This is why 'under-extracted' coffee tastes sour (acidic), and 'over-extracted' coffee tastes bitter.
How strength and extraction relate to each other.
To make delicious coffee, we need a good extraction (for the right balance of flavours) at our given strength preference.
Now that you understand both elements, it's easy to see how your coffee can end up either weak and over-extracted, strong and under-extracted, weak and under-extracted or strong and over-extracted!
We can increase extraction when brewing coffee in several ways, including by increasing the contact time between the coffee and the water or grinding the coffee finer, or by brewing with hotter water for example.
However, we can only make the coffee stronger by using more coffee. As an aside, you'll want to brew coffee with around 16-18 parts water for every 1 part coffee, for resulting strength that's in the right ballpark. This is commonly referred to as a brew ratio and expressed as 1:16 - 1:18.
Seems simple so far, why the confusion?
Let's go back to that V60 again.
As we pour water over those coffee grounds, we are extracting flavour (compounds) from them. It's true to say that if we keep pouring more and more water, we'll extract more and more flavour from the grounds. The problem though, is that we risk making the beverage weak in the process (i.e. too diluted with water) and we risk extracting too much flavour. This is how we get our weak, over-extracted brew.
The solution? We have two problems. Weak coffee and too much flavour.
If we want less flavour, we need to use less water, which is the same as using more coffee - we will increase the strength at the same time as reducing the flavour.
Conversely, the most effective way to increase the extraction (flavour) is to use lesscoffee. Less coffee = more flavour. More coffee = less flavour. That's what people struggle with.
Changing the brew ratio is the most effective way of extracting more or less flavour, but it also has an impact on the strength of our beverage. If we don't approach the whole process systematically, we can solve one problem and create another.
Is that it?
Well, no. This is one deep rabbit hole, but having a basic understanding should help you to figure out how to get off to a solid start. Like with everything, you can go much deeper if you want to, but there's no real need.
In general, try and get your strength dialled in first. This is done by using the right brew ratio for you (typically 1:16-1:18 for brewed coffee, and 1:2-1:3 for espresso). Incidentally, strength is perceived mainly as 'mouthfeel'. Weaker brews will have a more tea-like mouthfeel, whereas espresso is much thicker, with a more oily texture on your tongue.
Once you're happy with the strength, use other options to get more or less flavour depending on how the coffee tastes. If it's too acidic, extract more. If it's too bitter, extract less.
Extract more by grinding finer or brewing longer. Extract less by grinding coarser or brewing for less time.
I hope that helps. As always, I'd be happy to hear your thoughts and try to answer any questions over on our Discord server. In the meantime, happy brewing!