What is the reason a grain of salt alters the perceived bitterness of coffee by such a high degree?

Coffee served hot can be [quite bitter](http://drinks.seriouseats.com/2012/12/should-you-put-salt-in-your-coffee-reduce-bitterness.html) (presumably high pH). 


Several people I have known will add a single grain of [salt to the coffee](http://blog.khymos.org/2010/03/21/a-pinch-of-salt-for-your-coffee-sir/) - which [reduces the perceived bitterness](http://chemistry.about.com/b/2014/01/02/adding-salt-to-reduce-bitterness.htm). 


My question is: **What is the reason a grain of salt alters the perceived bitterness of coffee by such a high degree?** Can you explain the pH change that is occurring during this reaction?




Is simply a dissolution no reaction occur and as said Niculau the pH change, if occur, is absolutely negligible.




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Salt in general affect taste lowering the bitterness of the food enhancing the positive sensory attributes of foods. In fact the reason of this behavior is the sodium ion $Na^+$ (even litium ion works) and his interaction with the receptors in taste buds. So sodium acetate therethically works as well, Gery Beuchamp has studied many of this salt, quoting from [NCBI](http://www.ncbi.nlm.nih.gov/books/NBK50958/#ch3.r19):



> 
> various sodium-containing ingredients have been known to reduce the
>  bitterness of certain compounds found in foods, including quinine
>  hydrochloride, caffeine, magnesium sulfate, and potassium chloride
> 
> 
> 


[This is an article](http://www.ncbi.nlm.nih.gov/pubmed/8788095) about the suppression of bitterness by sodium. From here I'm afraid is more a question of physiology.


I read this the first time here:


This, Herve. Molecular gastronomy: exploring the science of flavor. Columbia University Press, 2006 p. 94


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