Understanding Water Quality for Coffee: TDS, Hardness, and How to Measure Them.

Water quality is a crucial but often overlooked factor that influences both the taste of your coffee and the health of your espresso machine. Beyond just purity and mineral content, two key concepts to understand are Total Dissolved Solids (TDS) and water hardness/softness. Here’s a breakdown of what they mean, how they affect your coffee, and how you can measure them—especially using a refractometer.

What is Total Dissolved Solids (TDS)?

TDS refers to the total concentration of dissolved substances in water, including minerals, salts, and other solids. It’s measured in parts per million (ppm) or milligrams per litter (mg/L), and it directly influences coffee extraction and flavour.

Why TDS matters: Water with very low TDS (like distilled water) lacks minerals and tends to over-extract coffee, resulting in bitter, dry flavours because it pulls too many compounds out of the grounds quickly. Conversely, water with very high TDS (hard water) already contains many dissolved solids, so it under-extracts coffee, leading to sour or flat tastes.
Ideal TDS range: The Specialty Coffee Association (SCA) recommends water with a TDS between 75-250 ppm, ideally around 100-150 ppm for balanced extraction and flavour.

Water Hardness and Softness Explained

Water hardness is a measure of the concentration of calcium and magnesium ions in the water:

Hard Water: Contains high levels of minerals (above 300 ppm). It can enhance body and mouthfeel but often leads to over-extraction, bitterness, and scale buildup inside your espresso machine. Hard water can also destabilise espresso crema.
Soft Water: Contains low mineral content (below 100 ppm). It tends to produce a lighter-bodied, more acidic coffee that can taste sour or underdeveloped because it under-extracts flavour compounds.
Balanced Water: Ideally, water should have moderate hardness (around 150 ppm minerals) to extract coffee flavours properly without causing machine issues or flavour imbalances.

How to Measure TDS Using a Refractometer

A refractometer is a handy tool that measures the concentration of dissolved solids in your brewed coffee, helping you understand extraction levels:

What it does: It measures how much of your coffee brew consists of dissolved solids (coffee compounds) versus water, giving you a TDS percentage. This helps you gauge extraction efficiency—whether your coffee is under-extracted (watery) or over-extracted (bitter)
How to use it: You place a small sample of brewed coffee on the refractometer’s prism, and it uses light refraction to calculate TDS. From there, you can adjust grind size, brew time, or dose to hit the ideal extraction range (usually 18-22% extraction).
Important note: Don’t rely solely on refractometer numbers—taste is the ultimate guide. Sometimes a brew tastes great even if the TDS is outside the “ideal” range.

Why Water Quality Matters for Your Espresso Machine

Hard water causes scale buildup inside boilers and pipes, reducing machine efficiency and lifespan.
Scale can block sensors and valves, causing inconsistent shots.
Soft water, while gentler on machines, may be too acidic and cause corrosion over time.
Regular testing and filtering help maintain machine health and consistent coffee quality.

Summary: Balancing Water for Great Coffee

Factor	Effect on Coffee Flavour	Effect on Espresso Machine
Low TDS / Soft Water	Sour, under-extracted, light body	Less scale, but possible corrosion
Moderate TDS (100-150 ppm)	Balanced, clear, flavorful	Optimal for machine health
High TDS / Hard Water	Bitter, over-extracted, flat flavour	Scale buildup, reduced efficiency

Final Tips for Coffee Lovers

Test your water’s TDS and hardness using home kits or professional services.
Consider using a water filter or blending tap water with purified water to reach ideal mineral levels.
Use a refractometer to fine-tune your brewing but trust your palate first.

By understanding and controlling water quality, you unlock the full potential of your coffee beans and protect your espresso machine for years of delicious coffee enjoyment.