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Coffee extraction: under/over, TDS and the refractometer - the numbers behind flavour

Your coffee is sour or bitter, and you do not know why? The answer hides in one word: extraction. It is the process in which water draws the flavour compounds out of the coffee - and the whole flavour depends on how much it draws out. Too little (under-extraction) gives a sour, thin coffee, too much (over-extraction) a bitter, heavy one. The world of speciality learned to measure this with numbers: extraction as a percentage, TDS (the strength of the brew) and a special tool called a refractometer. This lets you turn guessing into precision and deliberately fix the flavour. Here is a guide to extraction: what it is, what under and over mean, how TDS is measured and why the Golden Cup standard says 18-22 percent.

What extraction is

Extraction is the process in which hot water draws the soluble flavour and aromatic compounds out of the ground coffee. Ground coffee is mostly an insoluble structure, but part of its mass - flavour compounds, acids, sugars, bitter substances - dissolves in the water during brewing. It is precisely these dissolved compounds that create the flavour of the cup. The key is that extraction happens sequentially, in a particular order: first the acids are drawn out, then the sugars, and finally the bitter compounds. This means the flavour of the coffee depends on how far the extraction goes - too short leaves only the acids, too long draws out the bitterness. Extraction is expressed as a percentage: how much of the bean’s mass dissolved in the water. Understanding that extraction is the drawing out of flavour in a particular order is the starting point for the rest. It is the foundation of understanding why coffee tastes as it does. We cover brewing methods more in coffee brewing methods.

Under-extraction: sour coffee

Under-extraction is the situation when the water has drawn too few compounds out of the coffee. Because extraction begins with the acids, an under-extracted coffee is dominated precisely by them: it tastes sour, sharp, thin and light, without sweetness and fullness. This is because the water did not manage to draw out the sugars and fuller compounds, which come later - it stopped at the stage of the acids. Under-extraction therefore gives a brew unpleasantly sour, weak and unfinished, as if it lacked the second half of its flavour. The causes are usually too coarse a grind, too short a brew time, too low a water temperature or too little coffee. If your coffee is sour and thin, it is probably under-extracted. Understanding that sourness means too little extraction is the key to fixing it - you have to draw out more flavour. It is the first of the two poles of bad flavour. We cover grinding more in grind size.

Over-extraction: bitter coffee

Over-extraction is the opposite problem: the water has drawn too many compounds out of the coffee. Because the bitter substances are drawn out last, an over-extracted coffee is dominated by bitterness: it tastes bitter, heavy, dry and astringent, sometimes downright unpleasant. This is because the water went too far, drawing out not only the pleasant acids and sugars but also the bitter, astringent compounds, which normally should stay in the grounds. Over-extraction therefore gives a brew bitter, heavy and overdone, as if too much had been squeezed out of the coffee. The causes are usually too fine a grind, too long a brew time, too high a water temperature or too little water relative to the coffee. If your coffee is bitter and heavy, it is probably over-extracted. Understanding that bitterness means too much extraction is the key to fixing it - you have to draw out less. It is the second pole of bad flavour, opposite to sour under-extraction.

TDS: the strength of the brew

Alongside extraction there is a second key number: TDS, that is total dissolved solids. It is something completely different from extraction. TDS describes the strength, that is the concentration of the brew: how strong or watery the coffee is, how many dissolved substances are in the finished liquid. It is expressed as a percentage. High TDS is a strong, dense, intense coffee; low TDS a weak, watery one. This is an important distinction: extraction tells you what part of the bean was drawn out (and whether the flavour is balanced), and TDS tells you how strong the brew is. You can have a well-extracted coffee that is too strong or too weak, depending on the ratio of coffee to water. TDS is regulated mainly by this ratio: more coffee to less water is a higher TDS. Understanding that TDS is strength, and extraction is balance, is the key to mastering coffee. They are two independent axes of flavour. We cover judging coffee more in home cupping.

The refractometer: measuring flavour

To measure TDS, and indirectly extraction, a special tool is used: a coffee refractometer. It is a small device that works on an optical principle. The refractometer passes light through a drop of coffee and measures how much that light bends (refracts). The more dissolved substances in the coffee, the more the light bends - and from this measurement the device calculates the TDS. It is the preferred tool for measuring finished coffee, giving an objective number instead of a subjective impression. Knowing the TDS and the ratio of coffee to water, you can calculate the extraction as a percentage. This turns brewing from intuition into science: instead of guessing why the coffee tastes bad, you measure and you know. The refractometer is a favourite tool of baristas and roasteries aiming for repeatability. Understanding that flavour can be measured by a number opens the door to precise brewing. It is technology in the service of flavour. We cover measurement in roasting more in the roast curve.

A table: extraction and flavour

Let us gather the relation of extraction and flavour in one place:

The table shows the heart of it: extraction below 18 percent gives a sour coffee, above 22 percent a bitter one, and between them lies the sweet, balanced ideal. It is a simple map to good flavour.

The Golden Cup standard

These numbers arrange themselves into the famous standard called the Golden Cup, developed by the Specialty Coffee Association. It sets the ideal range of two parameters. The extraction should fall within 18-22 percent: below 18 percent the coffee is under-extracted and sour, above 22 percent over-extracted and bitter, and in this window balanced and sweet. The TDS for filter coffee should be about 1.15-1.35 percent (sometimes up to 1.45), which gives a pleasant strength of brew. These two ranges together create the so-called brewing control chart - a rectangle of ideal coffee on the axes of strength and extraction. The Golden Cup is a practical target: if you hit this range, the coffee should taste good and balanced. It is not a rigid law but a proven point of reference. Understanding the Golden Cup standard gives a concrete target for brewing. It is a compass for anyone who wants repeatably good coffee.

How to fix the flavour

The most practical value of this knowledge is the ability to fix the flavour of coffee. If the coffee is sour and thin (under-extracted), you have to draw out more: grind finer, brew longer, use hotter water or more coffee. If the coffee is bitter and heavy (over-extracted), you have to draw out less: grind coarser, brew shorter, use cooler water or less coffee. If the coffee is well balanced but too strong or too weak, adjust the ratio of coffee to water (this changes the TDS, not the extraction). These are simple, logical tools that let you deliberately improve any coffee. Instead of frustration and guessing you have a concrete plan of action based on understanding extraction. With a refractometer you can do this precisely, but even without it the mere knowledge of under and over is enough to fix most problems. Understanding these relations is the key to good coffee at home. It is a skill that really changes the everyday cup. We cover grind size more in grinding coffee.

How to sense it in the cup

Extraction is easy to sense once you know what to look for. An under-extracted coffee is sour, sharp, thin and light, with an empty, unfinished flavour - as if it lacked sweetness and body. An over-extracted coffee is bitter, heavy, dry and astringent, with an overdone, unpleasant aftertaste. A well-extracted coffee (in the Golden Cup window) is balanced and sweet, with a pleasant acidity, full body and a clean, satisfying flavour. If you sense sourness, draw out more; if bitterness, draw out less. You do not need a refractometer - the flavour itself will suggest the direction. It is worth experimenting, changing one parameter at a time (grind, time, temperature), to feel how it affects the extraction. Over time you will learn to hit the sweet, balanced middle by flavour alone. It is one of the most practical skills for a home barista.

The essentials in brief

Let us gather it up. Extraction is the drawing out of flavour from coffee by water, happening sequentially: first the acids, then the sugars, finally the bitterness. Under-extraction (below 18 percent) gives a sour, thin coffee, over-extraction (above 22 percent) a bitter, heavy one, and the ideal lies between them. TDS is a separate number - the strength, that is the concentration of the brew, regulated by the ratio of coffee to water. It is measured with a refractometer, which examines the bending of light in the coffee and converts it to TDS, from which the extraction is calculated. The Golden Cup standard sets the ideal: extraction 18-22 percent, TDS about 1.15-1.35 percent. Knowing these relations, you can fix the flavour: a sour coffee needs more extraction, a bitter one less. Now you know what numbers hide behind the flavour of coffee and how to steer them.

Note every coffee in GustoNote - including the extraction you sense (sour, bitter, balanced). Over time you will start to hit the sweet, balanced middle.