The Impact of Water in Coffee


I am currently in the process of writing Water For Coffee 2. Myself and Chris co-wrote the first version of this book back in 2015. In the almost ten years that have passed, much has changed, but there are also some key challenges that remain.

As we get together to write the follow up version, the conversation and recognition of waters impact in coffee has developed a lot since a guest roasters coffee in the Bath cafe didn’t taste very good. This began a journey of discovery to better understand how much of this was to do with the water.

I thought I would share a few of the key concepts that we are writing about in the book here in this newsletter as they swirl around my mind.

Mineral salts have become a big trend, whereby waters can be “built”, so to speak. Starting with deionised water and adding mineral salts like epsom salts to the water to create the mineral content. We definitely encouraged this approach in the first book as a way to actively get involved in exploring minerals in water. However, I also think this has created a false sense of control as well as the notion that this approach is optimal.

Last year I judged quite a few coffee competitions such as the Aeropress and also continued with my own tinkering and tasting. Every year I get the opportunity to taste a lot of coffee. Over time we each build up our taste experiences in our mind and start to notice some trends. Coffee is a particularly challenging subject with so many changing variables with every cup you drink. I started to notice I could quite predictably pick the coffees that had been made using mineral salt additions, particularly epsom salts, and not for positive reasons. There were negative sour notes, a lack of cleanliness, dry finishes and a reduction in the coffees origin characteristics. This appears to be a multi-fold phenomena.

The mineral salt choices themselves and the amount used both impact the brew heavily. The problem with using one or two mineral salts to make a water composition is that the resulting composition is abnormal and lacks the diversity found in typical waters whether filtered or not. Even utilising a wider array of salts has its challenges to match “natures” water. In the new book we explore this challenge and recommend some thresholds, whilst also recognising that source waters with a level of filtration will often provide a cleaner and balanced cup that displays origin characteristics well.

It is clear that the challenge to control water around the world and make the water we want at ease is still beyond our grasp. As well as this challenge it is as clear as ever that the feedback loops in coffee are as powerful as ever, whereby the different variables present in the coffees journey affect how we make decisions about other controllable variables. The clear impact being the concept of roasting coffee to taste good with a certain water. This isn’t just about roast though, it is also about growing and processing coffee as well as grinding and brewing.

Another aspect we only briefly touched on in the first book, but that has since become clearly vital to thinking about water for coffee as concentration of the coffee. The ratio of water to coffee used has a big impact on the actual amount of minerals interacting with the coffee. It may sound incredibly obvious but if you take a water with a given mineral content and brew a filter coffee, you use a lot of that water. Comparatively with espresso you use a lot less of that water relative to the amount of coffee. That means there are less minerals being used in the espresso brew. When we consider the buffering capacity of water we can understand why this change is so impactful and that it actually goes against what is desirable. Filter coffee is lower concentrate and highlighting acidity is typically a goal, whereas espresso is a high concentrate drink and managing acidity from being overwhelming is the goal. This is often why people find a higher amount of bicarbonate to be desirable in espresso but not when making filter coffee.  

At our roastery and two cafes we have differentiated our approach to water filtration for these two different brewing types. For the filter coffee we are using a reverse osmosis system that filters the water down to a low overall TDS and for espresso we are using an exchange cartridge with the bypass opened up moderately to allow for a higher overall TDS (180ppm) and bicarbonate content. I don't think you want to simply scale up your water hardness to match the ratio of filter coffee though. We are lucky I realise that our source water is hard with many minerals in a relatively desirable composition that we can play with to achieve some tasty results. These points and many others are expanded on and explored in the new book which we hope to have finished, printed and out in the world this summer.