I got a little debate started via the comments on this answer. The poster suggests the use of salt to make a sour kiwifruit-sauce taste sweeter in the same way you would use salt to make something taste less bitter.
I was interested to see if this would really work, so I did a simple experiment. I'll repeat the details from the comment,
Salt will only make fruit taste sweeter if it is already sweet. Here's an experiment I tried with two glasses of dilute lime juice. I added enough sugar so that the mixture was just a little too sour. I added a very small amount of salt to one glass, stirred until disolved and tasted. The glass containing salt was noticeably more sour. [...]
And the poster's reply to this,
Kiwifruit typically has more sugar content than grapefruit, which is typically 'made sweeter' with a touch of salt. It is certainly much sweeter than lime juice; kiwifruit averages over 8 grams of sugar where the same amount of lime juice (as in your example) averages just over 1.5 grams.
My assumption has been that salt acts as a flavour enhancer and so will accentuate whatever taste is predominant (unless the taste is bitter). My little experiment bears me out, but one experiment is hardly conclusive as any number of things can go wrong. In any case, I'm willing to believe that things are more complex than I have assumed.
Does salt help sour fruit taste sweeter?