Science

Water chemistry and brewing

Why water is not 'just water' for brewing, and how hardness and mineral content affect extraction.

Why water composition matters

Water makes up over 98% of a finished cup, and its mineral content affects how effectively it dissolves flavour compounds from coffee. Water that is too soft (very low mineral content) tends to under-extract; water with the right balance of minerals, particularly magnesium and calcium, generally extracts more effectively.

Hardness and alkalinity

Hardness refers to dissolved mineral content, while alkalinity refers to water's capacity to resist pH change. High alkalinity can mute acidity in the cup and make coffee taste flat, even if hardness is otherwise reasonable.

Practical guidance

Very hard tap water and heavily filtered or distilled water can both cause problems, for different reasons. Many home brewers get good, consistent results with a basic carbon filter; dedicated brewing water products or recipes exist for more precise control. A basic carbon filter jug also reduces chlorine taste and odour from tap water, which is a separate issue from hardness — even soft, low-chlorine tap water can still be poorly mineral-balanced for brewing, and vice versa.

Scale, corrosion and your machine

Hard water leaves mineral scale (limescale) inside kettles, moka pots and — most seriously — the internal boiler and pipework of coffee machines, where it gradually restricts flow, damages heating elements and can void a warranty. Very soft or aggressively low-mineral water can, less commonly, contribute to corrosion in some metal components instead. This is why manufacturers publish water-hardness requirements and descaling schedules for their equipment: follow the specific machine's manual rather than a general rule, since requirements vary by model and internal materials. See cleaning and descaling equipment for the general maintenance side of this.

Filtered, bottled, distilled and remineralised water

A basic carbon filter is a reasonable default for most tap water. Bottled water varies enormously in mineral content by brand and source — check the label's mineral content if pursuing more precise control, rather than assuming any bottled water is automatically better than tap. Distilled or reverse-osmosis water has had virtually all minerals removed; used on its own it tends to under-extract and taste flat (see why water composition matters, above), so it is typically remineralised with a small, measured mineral addition before brewing rather than used straight, if this level of control is wanted at all. None of this is required for a good cup — it is a refinement for readers troubleshooting a specific water-related taste problem, not a baseline requirement.

Sources

SCA brewing and espresso standards — Specialty Coffee Association (SCA)