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 we all know, 98% of a cup of black coffee consists of water. Therefore, to brew a delicious cup of coffee, besides having certain requirements for bean quality, the choice of water is also extremely important.

Generally, people's requirements for water sources used in coffee brewing typically focus on its TDS (Total Dissolved Solids). For example, what range should TDS be in to be suitable for coffee brewing. However, it turns out that besides TDS, the number of times hot water is boiled has also become a requirement for some people. They believe that compared to coffee brewed with freshly boiled water for the first time, coffee brewed with repeatedly boiled or water boiled more than twice does not taste as good. Is repeatedly boiled water really not suitable for brewing coffee? True or false? To investigate the truth, FrontStreet Coffee specially conducted an experiment to see if repeatedly boiled water is truly unsuitable for brewing coffee!

Brewing Experiment

First, let's introduce the two heavyweight "guests" of this experiment. To make the experimental results more objective, besides tasting, we need precise numerical values for verification. And they are: a TDS testing pen that can measure water quality, and a concentration meter that can measure coffee concentration. Let's see if brewing with water boiled different numbers of times can affect the taste of coffee!

Because this experiment involves many trials, we need to minimize human impact on brewing as much as possible. Therefore, FrontStreet Coffee did not use pour-over brewing for this experiment, but instead used a smart dripper for coffee preparation. The smart dripper only requires pouring coffee grounds and hot water for steeping, and can extract a pot of coffee with suitable concentration and extraction rate without人为 creating turbulence with water flow, making it perfect for multiple experiments.

Meanwhile, to ensure practicality, FrontStreet Coffee brewed coffee immediately after boiling each pot of hot water, then let the remaining hot water cool to room temperature before measuring TDS (since TDS cannot measure high-temperature water) and reboiling for brewing! The bean used by FrontStreet Coffee this time is a medium-light roast Sidamo Guji 8.0! Parameters are as follows: 15g coffee grounds, grind level: 85% pass-through rate with a #20 sieve, Ek43 grind setting 9.5, fine sugar grind, coffee-to-water ratio: 1:15, water temperature: 94°C, steeping time: 4 minutes. First, we place the trapezoidal filter paper, then add coffee grounds and hot water in sequence.

Then take out a stirring spoon and stir for 10 circles to allow more thorough contact between water and grounds.

After waiting 4 minutes, we take out the container and place the smart dripper on it to filter the coffee.

After filtering, we measure concentration and taste the coffee respectively.

Then we let the coffee cool, keep a small portion for TDS testing. The remaining hot water is boiled again, then brewed, and repeated until the fourth brewing is completed.

FrontStreet Coffee used Nongfu Spring water. Measured with the TDS testing pen, the TDS values of water boiled 1-4 times are as follows:

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

As you can see, as the number of boils increases, the TDS value also continuously increases. In terms of concentration and extraction rate, the four cups of Guji were nearly identical, excluding differences in pouring and stirring intensity, with almost no distinction. The same was true for taste - all four cups of Guji had abundant aromas and prominent flavor presentations, with only subtle differences in mouthfeel, basically not much variation. Wait! Don't rush to conclusions yet. Although the results suggest that repeatedly boiling water doesn't significantly affect coffee taste, however! This is only because we boiled the water a relatively small number of times.

From the increase in TDS values, we can understand why people don't recommend reboiling water (this article only refers to coffee brewing)! The reason TDS values increase with the number of times water is boiled is that during the boiling process, some dissolved solid substances may form scale, or the water becomes concentrated due to evaporation, leading to an increase in TDS value. If we repeatedly boil a pot of water without adding fresh water, the water quality will indeed deteriorate to some extent, which in turn affects coffee brewing and the coffee's mouthfeel. However, such variables require boiling a pot of water many times without adding fresh water. If we only boil it a few times repeatedly, there's no need to be overly concerned. Unless you have very high requirements for coffee taste and want to drink an extremely refined cup of coffee. In that case, it's best to control the water amount to use it up in one go, which can avoid the negative effects of "thousand-boiled water"~

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Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473