by Maxwell Colonna-Dashwood, Co-owner, Colonna and Small’s, UK Barista Champion 2012

This vital ingredient is the foundation of every cup of coffee you have ever tasted, apart from the bean itself of course.

It’s not just coffee that relies so dramatically on this everyday and seemingly straightforward substance. The worlds of craft beer and whisky are suitable comparisons, with breweries and distilleries proudly signifying the provenance of their water as being a vital part of their product.

A roaster, though, sells coffee, the water bit comes post sale. The water will be different and unique based on the locality of brewing, and this is on top of all of the other variables that define coffee brewing such as grinding, temperature and brew ratios. The reality is that the impact of water is rarely directly witnessed, with the other variables often being seen as the cause for dramatic flavour changes. You may be wondering right now, how big an impact can it really have?

I’m yet to present the same coffee brewed with different waters to drinkers and not have them exclaim “I can’t believe how different they are, they taste like different coffees’. These aren’t “coffee people” either, but customers who contested prior to the tasting that “you may be able to taste the difference but I doubt I can tell.”

It may make you question whether the coffee that you tried and weren’t particularly keen on, was a representative version of what the bean actually tastes like, or at the least what it is capable of tasting of like.

So, why the big difference, what is in the water?

Nearly all water that trickles out of a tap or sits in a bottle is not just water. As well as the H2O there are other bits and bobs in the water. Minerals mainly. These have a big impact not only on what we extract from the coffee but also how that flavour sits in the cup of coffee.

It’s fair to say that currently the way the coffee industry discusses water is through the use of a measurement called Total Dissolved Solids (TDS).

TDS has become the measurement which is relied upon to distinguish and inform us about how water will affect our coffee. It gives us a total of everything in the water. The problem though, is that TDS doesn’t tell us everything we need to know about the water; it doesn’t tell us about what those solids are. On top of this, TDS meters don’t measure some non-solids that have a huge impact on flavour.

In the water, we need the minerals calcium and magnesium to help pull out a lot of the desirable flavour in the coffee, but we also need the right amount of buffering ability in the water to balance the acids. This buffering ability can be noted as the bicarbonate content of the water. So for example an “empty” soft water with no minerals will lack flavour complexity and the lack of buffer will mean a more vinegary acidity.

A good test is to make the same coffee with both Evian and Tesco Ashbeck water. Evian has a good amount of calcium and magnesium to pull flavour out, but this is accompanied by a high bicarbonate content which flattens everything out and results in a heavy, bitter and chalky brew. The Ashbeck has little extraction power so is quite empty but has a low buffer so the acidity verges on sour. For bottled waters, Waitrose Essential yields pleasant results.

However the coffee shops in this guide will most likely have a trick up their sleeve. The industry filtration systems that have been developed primarily to stop scale build up in the striking and valuable espresso machines, also produce water compositions that are more often than not preferable for coffee brewing. Speciality coffee shops require all manner of specifics to be obsessed over and carefully executed. That cup of coffee that hits you and stops you in your step with intense, balanced and complex flavour will owe its brilliance to careful brewing, a knowledgeable brewer and superb equipment. However, it also owes a significant part of its beautiful character and flavour to the water it is brewed with.