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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?

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@belisarius Tried the recipe with one 300g grapefruit, 12.7g sugar (roughly 1 tbsp), and 800mg salt on one grapefruit half (about a quarter of what is specified in the recipe for 4 grapefruit halves). My wife acted as a blind taste-tester. We both preferred the grapefruit half without salt and also found it much sweeter. This supports neither the theory that salt can help sour fruit taste sweeter, nor my theory that it will make a predominantly sweet fruit taste sweeter (since the half with sugar and no salt was plenty sweet). Will repeat tomorrow at breakfast :) –  Chris Steinbach Jan 5 '13 at 20:34
    
Thanks for sharing your experiment! (I never tested it as I like the sourness of grapefruit) –  belisarius Jan 5 '13 at 20:39
    
@belisarius There was definitely a contrast between the sweeter upper layer and the relatively sour interior so you don't lose the sour notes entirely. –  Chris Steinbach Jan 5 '13 at 20:50

2 Answers 2

It seems at the least plausible, on two fronts.

  1. If I'm reading http://ajpgi.physiology.org/content/291/6/G1005.full correctly, saltiness and sourness can cancel each other out to some extent.

  2. Salt can increase perceived sweetness: T1r3 taste cells have sodium-glucose co-transporters which may provide the explanation.

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Goodness, that wasn't light reading. The first paper, states '[...] salt is often added to acidic foods and beverages to improve their palatability. We generally perceive a decrease in the saltiness of food when mixed with acid.'. You might want to infer that sourness is cancelled to reveal an underlying sweetness. But this is not a necessary inference and the results of my tests indicate that this is not the case. –  Chris Steinbach Jan 7 '13 at 23:50
    
Regarding the second paper, I don't know whether the sweet tasting amino acids studied in cited paper are sufficient in fruit to make any noticeable difference. It seems that dates have the highest concentration of alanine, glycine, and serine (about 0.3% in weight) but I've never had a sour date, so I'm not sure that testing with them would help. For the fruits that I have tested, sweetness is repressed by salt and so even though there are trace amounts of these amino acids, there appears to be a net-negative affect on sweetness. –  Chris Steinbach Jan 8 '13 at 7:42
    
@ChrisSteinbach, the first is focussed more on the effect of sourness in inhibiting saltiness, but it does refer in passing to saltiness inhibiting sourness a couple of times (e.g. "Sour taste aversion is sufficiently potent so that the ingestion of acids is only tolerable when sourness is masked with sweet- or salty-tasting substances"). –  Peter Taylor Jan 8 '13 at 13:41
    
With respect to the second paper, I wonder whether PNAS is serving you a different paper - the one I see doesn't mention amino acids at all, but suggests that "the enhancement of sweet taste by sodium salts (37–39) and the sweet taste of low concentrations of Na+ (40, 41) may be mediated by sodium-dependent glucose uptake into T1r3 taste cells via SGLT1". –  Peter Taylor Jan 8 '13 at 13:43
    
I followed the citations that attest to "the enhancement of sweet taste by sodium salts (37-39)". Citations nr. 37 and 38 were for cats and dogs respectively, so I skipped them. Citation 38, "Enhancing effects of NaCl and Na phosphate on human gustatory responses to amino acids" is the paper I referred to in my comment. –  Chris Steinbach Jan 8 '13 at 14:01

It does help the taste and make it more complex. I don't agree that salt makes fruit taste more sweeter. However, it is known that salt water reduces acidity so the sour fruit juices doesn't leave a weird feeling in your mouth.

My experience with this is learning from my parents how to eat pineapples by dipping them into salt water.

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How does salt reduce acidity? –  SAJ14SAJ Jan 5 '13 at 0:47
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Salt water changes the pH of an acidic solution to be more neutral. –  O_O Jan 5 '13 at 0:53
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That is saying the same thing as reducing acidity. The question is how? I could not find any citations when googling, and the disassociation of the salt into ions will be close to neutral in total ph. –  SAJ14SAJ Jan 5 '13 at 0:54
    
My mom would always rub a freshly cut pineapple with copious amounts of salt and then rinse off the excess. I also sometimes sprinkle a little salt onto green apple slices. Can't say if it's sweeter, but definitely less acidic. Don't know why ... –  Megasaur Jan 5 '13 at 8:35
    
@O_O I'd like to try the pineapple dipped in salt-water as an experiment. About what salt to water ratio would you use? –  Chris Steinbach Jan 5 '13 at 21:06

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