The solvent used to prepare coffee-based drinks and also their main ingredient.

Water makes up 90% of the content of an espresso, a figure that rises as high as 98% in the case of filter
extraction.
As such, water affects the extraction of every coffee, in terms of the quantity and quality of the substances extracted from the roasted bean. As a result, it also affects the taste and organoleptic properties of the extraction.
Choosing a water that is free from odours and chlorine (a substance that purifies water and rids it of bacteria, but that is also the worst enemy of coffee, because it significantly changes its organoleptic profile, making it more bitter) and with the right mineral content (mainly calcium and magnesium, which make up the total hardness of water) is therefore key to a good coffee extraction.
As such, making an informed choice on water involves assessing the TDS value (Total Dissolved
Solids), which in the case of bottled water can be considered, somewhat approximately, to be equal
to the total hardness (referred to on the label as total dissolved solids).

The Specialty Coffee Association has identified the following acceptable range of values:

• total hardness: 50 – 175 mg/l ( = ppm)
• temporary hardness (alkalinity): between 40 and 70 mg/l ( = ppm)
• pH: 6 – 8