What is a TDS level? The importance of water quality in coffee


Anyone who has ever made their own cup of coffee will be aware that water makes up the majority of the cup. Most sources estimate that around 95%-99% of a cup of coffee is water, depending on the method you use to make it. The quality of the water can have a major impact on flavour.

The 2015 UK Barista champion, Maxwell Colonna-Dashwood, based his World Barista Competition (WBC) routine on the impact of water on coffee and in conjunction with Christopher Hendon, a PhD student at the University of Bath, he published an academic paper on the topic. If you want to really get to grips with the science of it all, I would definitely give it a read. While Colonna-Dashwood and Hendon’s work highlighted the complexity and importance of the issue, the coffee industry had been discussing the role that water had to play for quite some time.

The effects of low quality water on coffee are twofold; firstly, it can scale up machines, leading them to deteriorate faster. How does this happen? Well…coffee machines have boilers and over time, poor water quality can cause limescale to build up. In my old flat in Bethnal Green, I didn’t pay much attention to the quality of water until one day I glimpsed the inside of my kettle which looked like it was full of dandruff. Ever since dandruffgate, I have always considered water qality when brewing coffee. If you run a coffee shop, making sure the water running through your machines is filtered, if necessary, could save you a lot of money on future maintenance and repairs.

Low quality water also negatively impacts on the flavour of the coffee you brew. You may have heard of ‘hard’ and soft’ water. Some water companies offer to show you how hard or soft the water is in your house just by typing in your post code online. The terms ‘hard’ and ‘soft’ refer to the mineral content of the water. This mineral content is quantified by the amount of Total Dissolved Solids (TDS). Hard water has a high mineral content (roughly above 200 TDS) and soft water has a low mineral content (roughly below 100 TDS).

Coffee TDS

Why do the amount of minerals in your water affect the taste of your coffee? Well, the minerals can taste pretty nasty on their own and also, the amount of minerals can affect the rate of extraction of coffee into the water, thus affecting the final taste. If the water is too hard, the presence of lots of different minerals can adversely affect the flavour of your cup of coffee. If it is too soft,  the low mineral content can impede coffee extraction because you need a certain amount of minerals to help extract the flavours into your coffee. The general rule is that water needs to be ‘not too hard’ and ‘not too soft’, so as to maximise the rate of extraction.

As The Role of Dissolved Cations in Coffee Extraction (Colonna-Dashwood and Hendon’s scientific paper) went on to prove, however, it is what constitutes the level of TDS that matters more than the quantity. Hendon illustrates this point with regard to magnesium: ‘‘Hard water is generally considered to be bad for coffee, but we found it was the type of hardness that mattered – while high bicarbonate levels are bad, high magnesium ion levels increase the extraction of coffee into water and improve the taste.’

What does this information mean moving forward? If you use a filter, you can control the TDS level of your water. However, unless you monitor your water daily and have the ability to find out what minerals are in it, you cannot control nor always fully understand the mineral content. The SCAA recommend a TDS level of 150 parts per million (PPM) in their guide to water. However, as Colonna-Dashwood and Hendon show, one should be cautious about adhering to a one size fits all approach. I asked Mr. Colonna-Dashwood for some advice moving forward and he informed me to ‘…grab a simple KH (Carbonate Hardness) and GH (General Hardness) drop kit and use these as your primary, with TDS as a curious secondary…’ In short, learn about the water in question and find out how to get the best out of it. Being equipped with more knowledge on what is in your water will allow you to make the necessary changes to brew ratios and roast profiles etc., which will ultimately give you tastier coffee. As always, let your tastebuds guide you!

To sum up…water quality is of paramount importance to the coffee industry, not just because it wrecks a lot of the machinery involved in the making of espresso based drinks, but also the wrong kind of water will reduce and numb the brightness and complexity of the flavours.

Hendon & Colonna-Dashwood are publishing a book called ‘Water for Coffee’ in the second week of August. Check it out if you want to explore this topic further.


2 Responses

  1. Brian's Coffee Spot

    07/27/2015, 03:14 pm

    Hi Henry,

    Water is, as you say, very important and often overlooked. These days when travelling, I take grinder, beans, aeropress and scales with me. Now I’m wondering if I should start taking my own water as well!


  2. Henry

    10/11/2015, 07:51 am

    Hi Brian,

    Sorry for the late reply. Didn’t see you responded!

    I quite enjoy trying different waters around the country. Even when they’re not so great!


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