I read on the Roaste Facebook page about adding salt to make coffee less bitter. I told that to my dad who was intrigued. He "found" his cup of coffee from this morning in the microwave (he forgot it was in there) and it had been in there since the morning. He decided to try the salt in it to see if it did anything to the flavor.
We were drinking Kana Cuban Roasters Havana Blend. Normally this is a bold coffee but very smooth.
After sitting in the microwave the flavor was different. It was bitter. Flat. When rewarmed it even had that old coffee smell. Something not to drink unless truly desperate.
My dad slowly added salt and kept sipping until it reached a "drinkable" flavor. (He added a total of half of an eighth of a teaspoon.) To his surprise the flavor did mellow out. While it was still nothing to drink unless truly desperate the bitterness did fade.
Of course we had to take it to the next level. Fresh coffee but store brand. We noticed the same thing - it slightly mellowed out a harsh taste. Gave it a surprise. We now have a new "secret" ingredient when serving coffee. C8 H10 N4 O2 (caffeine) meet NaCL (salt).