Fresh beans? Check. Grinder? Check. Scales? Check. Aeropress, French Press, piston or pump? Got it. You’re all set to be a domestic barista, producing tasty coffee in minutes, any time you choose. I don’t want to rain on your parade, but… Have you tasted London’s tap water recently? Not so good. As a result, even the most illustrious and expensive coffee beans will be affected by it if used to make your brew.I recently went to a cupping by James Hoffmann, owner of Square Mile and 2007 World Barista Champion, where he illustrated this point with great clarity.

A range of interesting coffees were cupped in the usual way, we slurped and made informed comments. At the end we shared our thoughts on the different beans, then James focused on the last two bowls. We were unanimously enthusiastic about one – a delightful blend with notes of caramel, toffee and a little burst of citrus. The other? “Dull”, “mucky”, “flat” and “lifeless” were some of the words used to describe it. Then James revealed that he’d used exactly the same beans, brewed in exactly the same way. Only for one he used bottled water, for the other, tap. The difference was astonishing.It’s not just a matter of taste, London tap water does filthy things to your equipment. It’s hard as nails and will scale up expensive, state-of-the-art machines in the time it takes to say “Reverse Osmosis filtration system”. When water is the main constituent of a cup of coffee, it’s little wonder that coffee shops take the subject of filtration very seriously indeed. As James says, “A Reverse Osmosis filtration system (RO) is essential. I now consider them essential in a commercial environment, and if you’re opening or operating a coffee bar in London without one then I strongly suggest getting one installed as soon as possible.”An RO washes away all the minerals from your mains water supply, leaving very soft water behind. However, pure RO, or very soft water, does not make nice coffee: it’s acidic, harsh and too bright, almost extracting too much of the coffee’s character, just as having too many minerals, or a high “TDS” (total dissolved solids), doesn’t allow enough space for the coffee’s flavour to shine through, resulting in a flat and muddy-tasting brew.

The genius of ROs aimed at the coffee industry is that a small amount of tap water is blended back into the stripped-down water to a desired level. (An ideal range is 80-120 TDS, where there’s a balance between the optimal taste level of a coffee and a low enough minerality to prevent your espresso machine from clogging up.)My explanation is very basic, but essentially an RO changes the make-up of your water in a way that no water jug filter can. The latter can remove bad flavour and odour but does not significantly reduce TDS levels.An RO system is not really a realistic option for the home, however. You need to be a real connoisseur to have one installed as they require proper plumbing and are therefore not cheap. The other issue is environmental: as you’re effectively washing your water, depending on the system, you could be flushing away almost three times the amount of water that you use. So what are the other options?An excellent solution in terms of flavour is bottled water. Many relatively inexpensive, own-brand bottled waters produce a really good cup of coffee with mineral and hardness levels perfectly aligned.

The main downside is the negative impact on the environment, and I admit I find buying bottled water ethically unsound. However, when you’re spending all that money on wonderful beans, sourced responsibly, roasted to perfection and brewed using equipment designed to enhance them, water becomes an issue, and nearly everyone I know that’s serious about coffee swears by bottled water as a solution. Perhaps shops could sell branded bottles that could be refilled for a minimal charge?In a recent Twitter debate about bottled water (thanks @joecoelho7,@LeeGazeprophets and @cominsoon coffee), Tesco’s Ashbeck (£1.10 for5 litres) unanimously won first place, with Volvic (from 65p for 1.5l) a close second, for consistency, price and availability. Brewers Cup champion James Bailey, who currently runs quality control for Workshop Coffee, prefers the slightly pricier Duchy Originals (81p for 750ml at all good supermarkets) and Voss (about £2 for 800ml from specialist suppliers) for exceptional flavour and clarity. The rule of thumb is to check the “dry residue value” at 180% – this mean’s you’re looking for a TDS level of 80-120 and a fairly neutral pH level of 7 or so.James Hoffman thinks cafés, coffee shops and roasters could offer their RO water to customers. I certainly use RO water at work and take some home with me to use there. It’s a nice idea and could be a kind of aftercare service, the way some shops grind beans for customers who don’t own a grinder.If you’ve ever spoken to your barista about coffee, you’ll notice how they come alive and feel genuinely passionate about it.

I think I can safely say that no one is happy about negatively affecting the taste of their beans with sub-standard water.Gwilym Davies, 2009 World Barista Champion and owner of Prufrock, tells me how uncomfortable he feels that, after careful husbandry and roasting, the beans he sells are misrepresented by bad water, especially when results are so dramatically different with good water. The solution he is currently developing is to offer customers his own RO water at a small charge. This is exciting news for the future of home coffee brewing, and where Prufrock leads, hopefully more will follow. In the meantime, you might politely ask your coffee supplier if they would be willing to fill your bottle with RO water: be prepared for dramatically better results when you get home.Chloë Callow is a food and coffee blogger, known as The Faerietale Foody. She also works for Bespoke Water System (@BespokeWater)