Hand Brew Theory: What Factors Cause Over-Extraction and Under-Extraction in Coffee?

When it comes to brewing coffee, everyone shares the same goal: making it taste good. Definitions of "good taste" vary, but as long as a cup is neither over-extracted nor badly under-extracted, we know it's at least a decent one. So for anyone wanting to improve, learning to identify extraction problems through real examples is essential.

Today, instead of discussing techniques, let's look at each stage of brewing and the telltale signs of both over- and under-extraction.

The Golden Cup Standard

Anyone who has studied coffee theory knows the mainstream research benchmark: coffee with an extraction rate between 18-22% falls within the reasonable range. Below 18% counts as under-extraction; above 22%, over-extraction. This is what many articles call the Golden Cup Standard.

Brewing coffee is ultimately an everyday activity, though. Not just hobbyists - even seasoned baristas don't pull out laboratory instruments to measure concentration and calculate extraction rates for every cup.

Through a series of comparative experiments, FrontStreet Coffee found that coffee squarely inside the Golden Cup range can still taste unpleasant, while some coffee above 22% remains delicious. It comes down to the flavor compounds in the beans and how differently they dissolve - with many details in play.

Factors Affecting Coffee Extraction

When brewing a pot of coffee, many factors shape extraction: the hardness of the beans, roast level, resting time, grind size, water temperature, coffee-to-water ratio, dripper geometry, pouring technique, and more. Each detail can alter the final cup to some degree. In FrontStreet Coffee's view, brewing should be a matter of adapting to circumstances rather than drowning in details. Rather than measuring and calculating every time, it's better to learn to judge over- or under-extraction by taste, combined with what you actually observe.

Under normal conditions, flavor compounds dissolve in a predictable order: the lighter, acidity-related substances first, then the sweet molecules, and finally the bitter components - the largest molecules, which dissolve most slowly.

Characteristics of Under-Extraction

When most particles are under-extracted, only a small share of the flavor compounds has dissolved. The early-stage acids dominate, while the sweetness and body never get their turn before extraction stops. The cup lacks aroma, with few distinct flavors on the palate. Light-roast, fruit-acid coffees show harsh raw astringency and sharp acidity, with a thin body and no aftertaste. Dark-roast coffees, missing their finest parts - body, caramel notes, pleasant finish - taste watery and shallow.

A salty taste can be another sign of under-extraction. Normally we rarely taste saltiness in coffee because acidity, sweetness, and bitterness mask it - but an under-extracted cup, lacking sweet masking compounds, lets the salt show. Still, saltiness is faint relative to other flavors, so the tongue often overlooks it.

Under-extraction has many causes: a grind that's too coarse, water that's too cool, a brew that's too short, or too little water.

FrontStreet Coffee once heard from a customer who bought our Colombia Sidra, followed our brewing recommendations, yet got a sharply acidic cup with no aftertaste. Investigation revealed her habit of cutting the brew short - removing the dripper before the liquid finished flowing. This broke the intended coffee-to-water ratio and, by skipping the tail end of the extraction, left the acidity sharp and unbalanced.

Characteristics of Over-Extraction

The other state - the one we run into more often at home - is over-extraction, where hot water pulls too much from the coffee: mostly the large-molecule compounds released late in the brew, showing up as bitterness, dryness, astringency, woody or straw-like harshness. These flavors usually trace back to too many fines prolonging the brew, or water that's too hot. For concrete fixes, see FrontStreet Coffee's earlier troubleshooting article.

Dealing with Over-Concentration

Some readers report a different problem: no obvious negative flavors, just a dull, indistinct cup with a wave of bitterness at the back of the throat when swallowing.

If this sounds familiar, your coffee is probably brewed too concentrated - an extraction rate that's high without quite reaching "over-extraction." The fix: add 10-20ml of bypass water to the pot and dilute the excess concentration down.

FrontStreet Coffee Recommendation

Dialing in extraction is far easier with forgiving, well-balanced beans. The FrontStreet natural Sidamo Huakui is a great training partner: traditional natural processing brings berry, floral, jackfruit, sweet melon, and honey notes, and its ripe tropical-fruit sweetness makes both under- and over-extraction easy to detect by taste.

If nutty, low-acid cups are more your style, try the FrontStreet Cerrado Red Bourbon from Brazil - cream peanut, toasted bread, caramel, and hazelnut aromas in a rounded, balanced body, with a gentle bitterness that stays smooth even when your technique isn't perfect.