Pour-Over Coffee: Should You Use Soft or Hard Water? What's the Ideal TDS for Brewing Coffee? Why Does Boiling Water Affect TDS?

As everyone knows, 98% of a cup of black coffee is water. So to brew a delicious cup, besides having standards for bean quality, your choice of water matters enormously.

People usually judge brewing water by its TDS (Total Dissolved Solids) - asking, for example, what TDS range suits coffee. But some have added another criterion: how many times the water has been boiled. Coffee brewed with repeatedly boiled water, they say, tastes worse than coffee brewed with water boiled fresh for the first time. Is repeatedly boiled water really unsuitable for coffee? True or false? To find out, FrontStreet Coffee ran an experiment.

Brewing Experiment

First, meet the two heavyweight "guests" of this experiment. For objective results we needed more than tasting - we needed numbers. Enter a TDS testing pen for water quality and a concentration meter for the coffee itself. Would water boiled different numbers of times actually change the taste?

Since the experiment involved many trials, we needed to minimize human influence on brewing. So instead of pour-over, FrontStreet Coffee used a smart dripper: just add grounds and hot water, steep, and it extracts a pot with consistent concentration and extraction rate, with no turbulence from manual pouring - ideal for repeated trials.

To keep things practical, FrontStreet Coffee brewed immediately after each pot boiled, then let the remaining water cool to room temperature to measure TDS (the pen cannot read high-temperature water), and reboiled it for the next brew. The bean was a medium-light roast Sidamo Guji 8.0, with these parameters: 15 g of coffee, grind at 85% pass-through on a #20 sieve (EK43 setting 9.5, fine-sugar size), a 1:15 ratio, 94°C water, and 4 minutes of steeping. We placed in the trapezoidal filter paper, then added grounds and hot water in sequence.

Then a stirring spoon - 10 circles to ensure thorough contact between water and grounds.

After 4 minutes, we set the smart dripper on a server to filter the coffee.

After filtering, we measured concentration and tasted each cup.

Then we let the coffee cool, keeping a small portion for TDS testing. The remaining hot water was boiled again, brewed with, and so on - until the fourth brew was complete.

FrontStreet Coffee used Nongfu Spring water. Measured with the TDS pen, the TDS of water boiled 1-4 times was:

First boil: 34
Second boil: 38
Third boil: 43
Fourth boil: 47

As the number of boils rose, TDS climbed steadily. In concentration and extraction rate, the four cups of Guji were nearly identical - differences in pouring and stirring ruled out - with almost no distinction. Taste told the same story: all four cups had abundant aromas and prominent flavors, with only subtle mouthfeel differences. But wait - do not conclude just yet. These results show that a few reboils do not significantly affect the cup, but that is only because we boiled the water relatively few times.

The rising TDS explains why people discourage reboiling (in coffee brewing, at least). Each boil can form scale from dissolved solids or concentrate the water through evaporation, pushing TDS upward. If you repeatedly boil the same pot without adding fresh water, water quality genuinely deteriorates over time, which eventually affects the brew and its mouthfeel. But reaching that point takes many reboils with no fresh water. Boiling a few times? Nothing to worry about. Unless you demand an extremely refined cup - in which case, measure out just enough water to use in one go, and avoid "thousand-boiled water" altogether.

FrontStreet Coffee Recommendation

For beans that reward careful water work, the FrontStreet Hua Kui 8.0 from Guji, Hambella - the very bean used in this experiment - is a superb choice: a medium-light natural with white florals, bright citrus and tropical fruit, honeyed sweetness, and a thick body. Because it is a light-roasted, hard-density bean, FrontStreet suggests a grind with 78-85% pass-through on a #20 sieve, water at 91-94°C, and a 1:15 ratio.

On the V60, the classic pairing for such bright, floral beans, FrontStreet's Guodingding Cooperative and Red Cherry Project lots from Yirgacheffe deliver the same delicate fruit-and-floral clarity - 15 g of coffee, three pours, and an extraction of roughly two minutes will show them at their best.

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