The Chemistry of Coffee Extraction: What Actually Happens in Your Brew

When water meets coffee, extraction doesn’t happen all at once: it progresses in stages.

1. Early Extraction: Acids & Bright Flavours
   In the first few seconds, the most soluble compounds dissolve first — mainly organic acids such as citric, malic, and chlorogenic acids. These give coffee its brightness and tang. If your coffee tastes overly sharp or sour, it’s often under-extracted, meaning this early stage dominated the cup without enough sweetness to balance it.

2. Mid Extraction: Sweetness & Balance
   Next, sugars and desirable aromatics dissolve. This is the “sweet spot” where coffee becomes round, balanced, and complex. Caramelised sugars formed during roasting bring body and sweetness. Most well-brewed coffees aim to highlight this middle phase.

3. Late Extraction: Bitters & Dryness
   Towards the end, heavier compounds such as tannins and bitter alkaloids extract. If the brew runs too long, these compounds overwhelm the cup, resulting in dryness or bitterness — a sign of over-extraction.

Professionals measure extraction using two metrics:

A typical ideal extraction yield is 18–22%. Below 18% is under-extracted; above 22% becomes over-extracted. You don’t need lab tools to aim for this – a balanced cup usually tells you you’re close.

Several variables influence how quickly and how much flavour dissolves:

• Grind Size: Finer grinds extract more quickly (bigger surface area). Too fine = bitterness; too coarse = sourness.

• Water Temperature: Hotter water extracts faster. The ideal range is usually 92–96°C for most methods.

• Brew Time: Longer contact = more extraction. Each brew method has its own ideal time (e.g. espresso ~25–30 sec, filter ~2.5–4 mins).

• Coffee-to-Water Ratio: Changing the ratio alters strength and extraction. A standard starting point for filter is 1:16 (1g coffee to 16g water).

  
  

How easily flavour compounds dissolve depends on the roast:

• Medium roasts are less soluble because the beans retain more density and structure. They usually benefit from a finer grind or a slightly longer brew time to bring out full flavour. 
• Medium-dark roasts sit in the middle, with increased porosity from extended roasting. They extract more readily, producing a fuller body and deeper flavour, often well suited to espresso, moka pot or richer filter brews.
• Dark roasts are the most soluble as the longer roasting creates a more porous cellular structure. They extract very quickly and can become bitter if over-extracted, so a coarser grind, lower temperature, or shorter brew time often works best.

Good extraction is about harmony — acidity, sweetness, and bitterness all playing their part. When brewing, think of yourself as managing a chemical process: adjusting time, temperature, and grind to guide extraction into that balanced middle zone.