Why recognize an under-extracted or over-extracted coffee
Recognizing the signs of under-extracted or over-extracted coffee is the first step to improving your technique, getting more balanced cups and bringing out coffee’s natural aroma, as also explained by the standards of the Specialty Coffee Association. Has it ever happened to you that you saw the coffee coming out drop by drop and taking forever? Or, on the contrary, seeing it pour out abundantly and fill the cup in just a few seconds? Well, in both cases we’re looking at an extraction error. In this article I’ll teach you how to recognize an under-extracted or over-extracted coffee, but above all we’ll learn how to understand the cause. A proper pre-infusion often helps avoid both errors.
The time factor for recognizing under-extracted or over-extracted coffee
The first parameter to consider in order to tell whether a coffee is under-extracted or over-extracted is time. Distinguishing between an under-extracted or over-extracted coffee requires paying attention to the rhythm of the extraction, because the flow immediately tells us whether we’re dealing with an under-extracted or over-extracted coffee. In the case of an under-extracted coffee, for example, the extraction time is short and the liquid leaves the filter very fast. There are cases in which this is obvious and so we can tell even with the naked eye whether we’re looking at under- or over-extraction. In other cases, though, when we’re very close to the correct parameters and need to make small adjustments, it’s not so easy to tell whether the extraction is right.
A short caveat is needed here: the rules we’ll consider in this article shouldn’t be taken as religious dogma; rather, they will serve, especially for less experienced users, as a baseline guide so that even a beginner can be able to tell if they’re getting a good extraction or not. With experience, everyone will follow their own protocol regarding extraction ratio, pressure, dose, temperature etc… but in the meantime, you have to start somewhere. So I want to share this information with you because in my case it has been decidedly useful, especially at the beginning of my journey.
We said the parameter to consider is time, right? So the first thing we need to do is gear up with something that measures time: a very simple watch or the timer on your phone will work just fine. To get a good espresso, ideally we should respect the 30-second rule, namely:
In 30 seconds you should get into the cup an amount of liquid that is double the dose of grounds you tamped into the filter
Let’s run a practical example assuming we use an 18/20 gram filter for a double shot (the single 7/9 gram filter isn’t ideal and we’ll see later why):
- I put 18 grams of coffee into the filter and tamp it
- I lock the portafilter into the machine
- I put a precision scale under the portafilter, place the cup on it and zero the tare
- I grab my watch/timer
- I start the extraction and the timer at the same time
- I weigh the coffee that comes out in 30 seconds
If we did everything correctly, the value we see on the scale should be close to 36 grams, namely double the 18 grams of grounds (so an extraction ratio of 2:1).
If we deviate significantly from these values, we probably have an under-extracted or over-extracted coffee. Let’s now analyze the first case, that of under-extracted coffee
Under-extracted coffee: what it is, why it happens and how to avoid it
A coffee is considered under-extracted when the water fails to dissolve and carry away a sufficient amount of soluble compounds from the ground coffee. In practice, this means that the extraction stopped too soon, leaving only a surface layer of the aromas and flavors contained in the bean inside the cup.
The main causes of under-extraction
Under-extraction can stem from several factors:
- Extraction time too short: especially in espresso, a flow that’s too fast (under 20 seconds) can compromise full extraction.
- Pressure too low: if the pressure is insufficient, the water flows too slowly or fails to properly penetrate the coffee bed, extracting only the most soluble acids. The correct extraction pressure should be between 9 and 10 bar.
- Grind too coarse: coffee grounds that are too large offer less contact surface with the water, reducing the amount of solubles extracted.
- Water temperature too low: below 88-90°C the extraction slows significantly.
- Insufficient coffee dose: too much water compared to the coffee available can lead to a weak, body-less brew.
- Uneven or insufficient tamp(in the case of espresso): if the coffee bed isn’t well pressed, the water passes through too quickly without extracting properly.
Signals to recognize an under-extracted coffee
Here are some clear indicators of under-extraction:
- A taste tending toward sour or tart
- An excessive amount of coffee in the cup
- Weak or watery body
- Short and not very intense aftertaste
- Pale and scarce crema
- Extraction too fast: we’ll see the coffee form a continuous, watery stream that quickly fills the cup
Over-extracted coffee: what it is, why it happens and how to fix it
Over-extraction happens when water extracts too many soluble compounds from the grounds. Unlike under-extraction, where the coffee is bland and sour, here you get a result that is bitter, astringent, woody or burnt. An over-extracted espresso can ruin even the best blend: the water, by staying in contact with the coffee too long, extracts undesirable substances that hide the more delicate aromas.
The main causes of over-extraction
Over-extraction is often the result of one or more poorly calibrated parameters:
- Extraction time too long: an espresso that exceeds 30-35 seconds is at risk: the slow flow favors the extraction of bitter compounds.
- Excessive pressure: pressure above 10 bar can create resistance and favor unwanted over-extraction.
- Grind too fine: grounds that are too fine slow the water’s passage, lengthening contact time and increasing extraction.
- Water temperature too high: above 96°C, the water tends to “cook” the coffee, increasing bitterness.
- Excessive coffee dose: too much coffee in the filter can slow the flow and over-extract.
Signals to recognize an over-extracted coffee
Here are the main signs to look for:
- Extraction too slow: we’ll see the coffee coming out drop by drop
- Scarce amount of coffee in the cup
- Marked, persistent and unpleasant bitter taste
- A dry or astringent sensation in the mouth
- Body often flat or dry
- Unpleasant aftertaste, like tobacco, wood or burnt rubber
- Dark crema, with large bubbles and a fragile consistency
How to fix an under-extracted or over-extracted coffee
Correcting an under-extracted coffee may take several attempts. Clearly, the more we’re able to measure the parameters we’ve used, the easier it’ll be to spot the error. For this you’ll need a few tools:
- Precision scale. It’s essential if we want to avoid wasting kilos of coffee for no reason. On the various shopping sites you’ll find some particularly suited to this purpose. They’re generally compact and not very thick, so they can be placed on the machine’s drip tray while still leaving room for the cup
- Thermometer: unless your machine has a temperature control system (PID), the only way to get an idea of the extraction temperature is to measure the temperature in the cup. This will obviously be different from the extraction temperature, and we’ll see later what influences these values. For now, it’s enough to know that the ideal beverage temperature measured inside the cup should be between 60° and 70°. You can find these too on the various shopping sites by looking among those designed to measure hot drinks (tea, coffee, milk, etc.)
- Timer or any other tool for measuring time
Having a pressure gauge to monitor extraction pressure is important. If your machine doesn’t have one, there are kits that can be installed aftermarket. I know this might sound fiddly, but if we do everything correctly and then extract the coffee at 15 bar of pressure because the machine is set that way by default, we won’t be able to achieve the desired result.
Obviously an under-extracted or over-extracted coffee can depend on a mix of factors. That’s why it’s important to measure them. As a general rule, if your coffee comes out too quickly, with little body and little crema, you know the signs point to under-extraction. So first of all, make sure you put an adequate amount of coffee in the filter (each filter has its own spec, which should be written on the filter itself), set the grind a little finer, try tamping a bit harder, and then measure how much coffee comes out in the canonical 25/30 seconds. You’ll see that after a few tries you’ll get closer to the ideal parameters. In the case of over-extraction, obviously, we’ll do the opposite.
Conclusion: how to handle under-extracted or over-extracted coffee
A good coffee lives in the right balance between under- and over-extraction. The key is to listen to the cup: if it’s too sour, it’s probably under-extracted; if it’s too bitter, there’s been over-extraction. The perfect coffee lies in between: aromatic, full-bodied, sweet and persistent. Over-extraction is a subtle error that can go unnoticed if you don’t know the extraction parameters well. But once you understand the signs and causes, correcting it becomes simple. Remember: in the world of coffee, control is everything, and every detail counts on the way to the perfect cup.
Every mistake is an opportunity to get to know your coffee better! If you want to keep discovering the secrets of the perfect extraction, subscribe to our newsletter:
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