Coffee Brewing vs. Water Hardness

This week, we talked to a supplier of water filters and she gave us some interesting feedback about local water conditions. She said that Klang Valley water tends to have a purity of ~70 ppm Total Dissolved Solids (TDS). Further more she said that this is with a "hardness," of "15-20," whereupon I guessed she was referring to Total Hardness (TH). She then said that this is regarded as too soft for coffee brewing, by certain customers.

I'm trying to read up on this, and have obtained some view-points, though not yet a conclusive view.

Google search term: "coffee water too soft"

This search yielded some interesting results.

(i)

Jim Schulman's Insanely Long Water FAQ. It tends to go missing, so you may have to search for it by name in the future. A choice quote:

Taste testing by various authorities over the past twenty years has established a rough agreement that neutral pH water with 90 mg/l hardness is optimal for coffee taste.

Note that the unit "mg/l," ought to be equivalent to "ppm by mass." This article has the best documented data of all the sources linked in this article. It is referred to by some of the resources below.

(ii)

The third result quotes an SCAA source, saying

As a point of reference, SCAA’s specifies* a target for TH of 150PPM with a tolerable range of 75 to 225 PPM.

and took the view, that overly-soft water (i.e. water which is too pure) makes for under-extracted coffee.

I commented my reaction:

It's interesting that you have observed water softness being positively correlated with under-extraction. (I've seen folks espousing this view, as well as its inverse.)

I'm trying to make sense of this in my head. Did you observe specifically an increase in acidity/sourness when the softness of the water increased? If so, could this simply mean that the ionic compounds responsible for acidity/sourness in fact were extracted at a *higher* rate than usual, due to the softness of the water? If so, then "under-extraction," might not be the term to use... rather, perhaps we could say "over-extraction of acidic substances?"

Just a thought.

(iii)

This article states,

Water with 20ppm (parts per million) to 120ppm of calcium is good. Calcium is essential for good extraction.

but does not say why, or based on what evidence.

(iv)

This article states,

For plain coffee, a minimum mineral content of 150-200 parts per million is essential to a good extraction. Water softer than this will result in weak and flavorless coffee. For espresso, some mineral content helps the flavor, but lots of mineral content will not help more,

but again, we're lacking in documented evidence.

(v)

This chap talks a bit about how water dissolves coffee in general, and his views on water for espresso are unquantified.

(vi)

This conversation (which makes references to this earlier conversation) takes the view that any under- or over-extraction due to water hardness or softness can be compensated for by jiggling the other parameters of brewing coffee.