Extraction: The Heart of Brewing

Brewing coffee is the act of extracting soluble flavour compounds from roasted, ground coffee using water. It sounds deceptively simple — just add hot water to ground beans — but this process involves over 800 volatile aromatic compounds and hundreds of chemical reactions happening simultaneously. Roughly 30 percent of a roasted coffee bean is soluble in water, yet the ideal extraction window is far narrower — typically 18 to 22 percent, according to research by the Specialty Coffee Association. Extract too little, and the cup tastes sour, thin, and grassy, dominated by the fast-dissolving acids. Extract too much, and bitter, astringent, ashy flavours take over as heavier compounds flood the brew. The goal of good brewing is to land in the sweet spot where sweetness, acidity, and body harmonise.

The Four Variables

Every brewing method manipulates the same four variables, and understanding them is the key to consistently good coffee. The dose is how much coffee you use relative to water — a typical ratio is 1:15 to 1:17 (grams of coffee to grams of water), though Scott Rao and other brewing experts recommend starting at 1:16 and adjusting to taste. Grind size controls how quickly water can penetrate the coffee particles: finer grinds expose more surface area and extract faster; coarser grinds slower. Water temperature affects which compounds dissolve and at what rate — most methods work best between 90°C and 96°C, with lighter roasts generally benefiting from higher temperatures. And contact time is how long the water and coffee interact, ranging from 25 seconds for espresso to 4 minutes or more for immersion methods like French press.

Water makes up over 98 percent of a brewed cup of coffee. Using filtered water with a mineral content of 75 to 150 parts per million (TDS) can dramatically improve flavour — too soft and the cup tastes flat and lifeless, too hard and it becomes chalky and muted. The Specialty Coffee Association’s water standard recommends a target of 150 mg/L TDS with no chlorine.

Immersion vs Percolation

Brewing methods fall into two fundamental categories, each producing a distinctly different cup experience. Immersion methods — like French press, AeroPress, and cupping — steep coffee grounds in water for a set time, then separate them. The water saturates around the grounds, and extraction gradually slows as the liquid approaches equilibrium with the coffee bed. This produces a forgiving, full-bodied cup with a heavier mouthfeel and rounder flavours. Percolation methods — like pour-over and espresso — pass fresh, unsaturated water continuously through a bed of coffee. Because the water is always hungry for solubles, percolation extracts more efficiently and produces cleaner, brighter, more transparent cups — but demands greater precision in technique and timing.

Measuring Quality

Advanced brewers use two metrics to evaluate extraction with scientific precision. Total Dissolved Solids (TDS), measured with a refractometer, tells you how concentrated the brew is — essentially, how much “stuff” from the coffee ended up in the water. Extraction yield, calculated from TDS and brew ratio, tells you what percentage of the coffee’s solubles ended up in the cup. The Specialty Coffee Association defines a “gold cup” standard as 1.15 to 1.35 percent TDS at 18 to 22 percent extraction. But as Matt Perger of Barista Hustle reminds us, numbers are guides, not gospel — the best cup is ultimately the one that tastes right to you. The real art of brewing lies in developing your palate to know when you have found it.

Further Reading

• Craft Coffee: A Manual by Jessica Easto — an accessible, thorough guide to brewing science and method for home enthusiasts
• The World Atlas of Coffee by James Hoffmann — includes a comprehensive section on brewing principles and equipment
• SCA Brewing Standards — the industry’s gold cup protocols and water quality specifications
• Barista Hustle — advanced brewing science, extraction theory, and technique guides