Specific minerals/ions
The impact of sulfate on coffee flavour
Sulfate
Admittedly, this is a little difficult to explain and is not well-researched compared to the metals like magnesium, sodium, etc.
However, from our first-hand experience, high sulfate levels tend to bring out the roast flavours of coffee.
Especially in milky drinks, sulfate levels >0.5 mM (50 mg/L) are advisable to give more impact (though this is subjective!).
The levels of sulfate in water tend to be a secondary concern, being the result of the magnesium/calcium balance you're trying to get and the source minerals.
The anions do play an important role in overall taste and mouthfeel, and in particular the sulfate:chloride ratio – which as luck would have it, is well-known and studied by beer brewers!
In fact, beer brewing can provide us a lot of insight about the anion effects on coffee, which is why we've created a section on it here.
Chloride
Chloride is going to be your major anion in water alongside sulfate.
The balance between the two can have important effects on overall flavour and mouthfeel, as is known by our fellow water hackers in the beer industry.
High chloride levels are often associated with salty tastes (usually because chloride comes alongside sodium/potassium).
The human tongue has a high capacity for chloride before finding it displeasing, so we wouldn't worry too much about chloride levels in the water used for coffee.
Better to focus on overall magnesium/calcium content, then consider the ratio of sulfate:chloride and its impacto on taste.
However, it's worth noting that high levels of chloride can be detrimental to espresso equipment – sulfate levels less so. So, if adjusting water for use in espresso machines, take more caution with what water you feed it.