There are a couple of ways of making your home brewed coffee taste better. Choosing the appropriate bean and the grinding method is one. The second one, which might be a little more tricky to figure out, is the water you use for your brew.
Water temperature and water composition are the two main factors that need to be checked to maximise the extraction of your coffee bean.
What temperature is best for coffee brewing?
A popular idea is that if coffee is brewed with boiling water (which means water at 100 Celsius), it will taste burnt. But, this is not necessarily true. Generally the hotter the water, the more energy it provides for chemical reactions such as the one happening when water absorbs coffee - coffee making. So yes, temperature does effect but to go as far as saying that boiling water will burn the coffee is simply not true.
The actual maximum temperature water can reach by the time it is poured in your french press or pour-over is about 95 degrees Celsius (203 degrees Fahrenheit). This temperature is usually ideal for most coffee types available on the market - like medium and light roasts. However if you are using a darker, more developed roast then using the maximum temperature might more easily express very bitter, ashy brunt flavour notes. Trying temperatures closer to 85 degrees Celsius for brewing such coffee will give you a smoother, less bitter flavour notes. Try further reducing the temperatures if you are still experiencing unpleasant flavour notes with your roast.
As a general rule of thumb, the hotter your water, the more coffee will be extracted from the bean. This can sometimes be a good thing, and sometimes such as in the case of darker roasts become a bad thing. Water temp for coffee (filtered coffee) can be summarised below:
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Light roast - 95 degree Celsius
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Medium roast - 95 degree Celsius
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Dark roast - 85 degree Celsius
All that said and done, the best water temperature for coffee is subjective since flavour and taste is itself subjective. Taste and preferences and unique to everyone and your best temperature for filtered coffee can only be found through experimentation with different roasts and different water temperatures.
Espresso water temperature
Water temperature is a factor when brewing coffee in general - but when it comes to espresso, water temperature can really make the difference between a really good tasting espresso and a mediocre harsh tasting one. The general guidance for espresso water temperatures can be found below. You can adjust the temperature of your brewing water directly from the setting of your espresso machine. If no instructions on how to change the water temperature are displayed on your machine, odds are that your model is not equipped with the functionality.
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85 to 90 celcius for darker more developed espresso roasts
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88 to 92 Celsius for medium style espresso roasts
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90 to 95 Celsius for lighter espresso roasts
These are of course ballpark figures and many different factors have to be considered when making an espresso. James Hoffmann has a great video on his YouTube channel, explaining the differences between brew temperatures for espresso.
What water composition is best for brewing coffee?
There are 3 main factors that affect your overall coffee quality: Coffee bean and roast type, grinding level and water. Arguably since coffee is mostly made of water (98%), water quality is the most important of them. As simple as it might seem, water is actually a very complex substance but for the sake of simplicity only the most important aspects will be covered in this article. The Speciality Coffee Association (SCA) coffee standards are what is commonly referred to as the gold standard when it comes to water composition for an ideal brew. Their suggested composition is described below.
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Clean, tasting good and transparent
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pH - 7
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TDS - 150 mg/L or ppm
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Total Alkalinity (CaCO3) 40 mg/L or ppm
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Calcium & Magnesium hardness
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Magnesium 17-85 mg/L or ppm
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Calcium 17-85 68 mg/L or ppm
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Sodium 10 mg/L
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Total Chlorine 0 mg/L
The first criteria is pretty straightforward, if the water looks blurry or muddy, don’t drink it and you should probably get it checked out. Now for the pH - when in proximity to 7 (which is halfway between being acidic and being alkaline) the water is considered ideal for coffee. It is typically challenging to alter the pH of your water without influencing other factors like the mineral composition, which makes pH hard to test for good coffee taste. However, if you have a pH below 6 or above 8, you should definitely try changing your water source for your brewing water.
The water hardness must be about 150 mg/L according to the SCA to achieve a smooth and soft brew. Any higher than that would result in a hard water which would result in a negative impact on the flavour of your coffee. The magnesium and calcium hardness is essential to maximise coffee bean extraction and enhance the flavour of your coffee. However if this amount is too high (i.e higher than 100mg/L) the water hardness levels are automatically increase as a result and lower brew quality will be achieved.
There are essentially 3 types of water out there commonly used for brewing coffee. Each of them have their own set of advantages and disadvantages.
Tap water composition
Tap water is rarely considered ideal for brewing coffee. In the UK, tap water is usually pretty hard with an official TDS value anywhere between 100 to 300 ppm according to Thames Water. The real measured TDS is generally higher than that due to the aging water infrastructure and piping across the UK. The levels of calcium are also very high with quantities reported to be about 100 to 365 mg/L. Just like with Calcium, the Chlorine levels are high with values about 1 mg/L but depending on its source, can be as low as 0.18. Similarly, sodium levels are too high with values of 38 mg/L found in UK tap water. Finally Alkalinity tends to be around 200 mg/L according to Thames Water. This is once again too high for optimal coffee brewing water.
Filtered water composition
Filtered water, through one of the very popular water filter jugs like Brita is excellent to get rid of chlorine and other large impurities in water like limescale. They also slightly reduce the TDS and sodium levels through their ceramic ion exchange beads present in the filters. However this reduction is not significant enough to have any real benefit on your coffee brew quality. This also means that these filters do not have the capacity of reducing Calcium, Sodium and Alkalinity levels from the tap water - making filtered water not an ideal water for brewing coffee.
Purified water composition
Purified water can be obtained by two ways: a reverse osmosis system or a distillation unit. These systems essentially transform tap water into pure water, which significantly reduces TDS, Alkalinity, Sodium, Calcium and Chlorine levels. The purity of the water is actually its downfall - since purified water is actually corrosive for coffee machines, which tends to reduce pH levels and result in an acidic tasting brew. Minerals like calcium & magnesium must be added back to the water in order to achieve a composition close to the one recommended by the SCA.
Conclusion
Now, following this guidelines will not guarantee a perfect cup of coffee, but it will certainly increase your chances of achieving it. When it comes to mineralisation of your water, there are many products out there that aim to make your brewing water as close as possible to the SCA's recommendation. However they tend to be messy to use and require you to mix and match mineral concentrate and purified water. Skuma on the other hand, is a system that purifies your water using reverse osmosis and then ads a specific amount of minerals to recreate the SCA's recommendation.
