(Updated 2019 July the 31st )
Whatever method of coffee extraction we use, coffee needs a solvent.
Water is one of the best solvents that we have in nature especially for its chemical properties: it’s super attractive with hydrophilic substances, substances that have molecules that can absorb water.
The better way to extract the molecules inside of a coffee’s beans is to make as more contact surface as possible to let the water get closer and extract as many flavours as we need. Once we have our ground coffee we can go on to the extraction. Now we have to ask ourselves: which is the proper dose of coffee that we are going to use?
If we are going to used arabica beans instead of robusta beans we will need more grams of coffee. Arabica beans grow at high altitudes and are, therefore, thicker and less soluble than robusta beans; it is usually preferred to be extracted at higher temperature instead of robusta beans. We can set a range between 88-94 grades centigrade which is the standard range of the water temperature in the majority of the coffee extraction systems.
After setting the proper ratio between grams and water temperature, we can proceed with the time. If our recipe will be flat and/or sour we will in the case of under-extraction, if it will be astringent and/or bitter we will in the case of over-extraction.
We can use a tool called refractometer whenever we want to know if we are extracting the proper amount of solids in the cup. For brewing recipes, the recommended range is between 1,20% and 1,45% whereas espresso recipes are between 8% and 12%. However, as regards the coffee extraction, we are in the range between 18-22 % for brewing recipes and 18-21% for espresso recipes. Below 18% we are going to under extract and above 22% we are going to over-extract (23% in the case of brewing recipes).
The refractometer is a great assessment tool but we don’t have to dwell only on these criteria. The most important evaluation relates to our palate. There will be cases in which the solids will not be within the established range (determined by the Coffee Brewing Institute around 1950), but it will not mean that we will extract improperly.
ABOUT THE YIELD
The Yield is the weight of the liquid that we obtain in the cup.
The Yield is the starting point for every barista that wants to improve itself and serve the best for its customers.
When we talk about the yield, we don’t focus only on the coffee extraction time: the time is a consequence of what recipe we want to obtain.
In simple terms, more or less yield means more or less liquid in the cup. More yield means as well less body, fewer flavours and more bitter; less yield means more body and more flavours, but as well more sourness. The quicker we learn about this concept the more we can try to find a proper balance in the cup.
A standard yield is usually twice the amount of the dose we grind. So, for example, if we grind 20 grams of coffee, the yield will be of 40 grams. When we will be satisfied with the yield, we can memorize it on the machine so that we can replicate it.
A short explanation: the caffeine will never increase if we increase the yield: more than 70% of the caffeine is extracted in the firsts few seconds.
Too often we overlook or we take for granted that we are the ones that prejudice the final result. If we don’t pay attention to every single action the result can only be affected.
We have to make sure that we are grinding with the proper granulometry, putting the proper dose of coffee in the filter holder, tamping correctly with the tamper and close the filter holder without hitting it on the group, avoiding any channelling effect that would lead to an under extraction in the cup.
So now that we have learnt about the yield, we can have fun extracting a balanced shot of coffee for every origin we are going to use.
We can define an espresso as “balanced” when sweetness, sourness, and body are perfectly mixed.
The acids and the fats are the first components that are being soluted in the cup while the sugars are next to be extracted, followed by the bitterness components. The body then will be given to the brew ratio that we will use.
The brew ratio is the ratio between the amount of ground coffee and the quantity of water that we will use.
The water will, therefore, affect the flavours: less water means we will not extract all the flavours we wanted, while more water means that we will extract undesirable flavours like bitter ones. If for espresso the standard ratio is 1:2, the brewing method is about 1: 1,16 which means one gram of coffee ever 16 grams of water.
COFFEE EXTRACTION TIME
As I said before, the coffee extraction time is just a consequence of what we want to achieve and it’s also related to some variables such as:
- the grind size of the coffee dose
- the roast profile of the bean
- the amount of the coffee dose
- the yield
All these variables are related when it comes to coffee extraction time.
When baristas make their firsts steps into coffee, they tend to rely on how much it takes (in terms of seconds) to extract a shot of coffee rather than looking at what just made that happen.
The faster way to see the extraction time being changed is by making coarser or finer the grinding size. By making coarser we are increasing the surface area and making easier for the water to pass through the coffee bed. In this case, there’s not enough resistance and the water is not going to extract some of the flavour compounds we are looking at to balance our cup.
We will end in the cup with a sharp cup, lacking sweetness and complexity. It’s what we call an under-extracted coffee shot. On the other hand, by making finer we are minimizing the surface area and making harder for the water to pass through the coffee bed.
In this case, there’s too much resistance and the water is going to extract all of the flavour compounds including the unpleasant ones. We will end in the cup with an astringent, bitter cup, lacking acidity and sweetness. It’s what we call, an over-extracted coffee shot.
But it’s not only because of that
For example, a medium-to-light roast profile calls for a higher dose of dry coffee instead of a darker one. This also has an impact on the ratio between the coffee and the water in case of espresso drink and the coffee and the milk in case of milky drink.
So, with the same grind size, it’s pretty obvious that increasing the coffee dose leads to more seconds of extraction while decreasing the dose leads to fewer seconds of extraction. And about concerning the yield, is even more clear than a more yield requires more time while a less yield needs less time to end the extraction.
I am an Italian coffee lover that pushed for the love of this “amazing drug” decided to come to London to study about coffee and its different extraction procedures and tastes.