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Background

During the weekend, my mother made a plum cake and put slices of plum on top. She gave me a piece to try and it was sweet and good for ~1.5 seconds before my face puckered from how sour it was. I can usually take sour thing (I like to eat lemons/limes with salt and keep dipping into a bottle of citric acid), but I was surprised by how a cake could be so sour, even with a small slice of plum.

She took the slice of plum off of another piece and gave it to one of her neighbors who also puckered from how sour it was.

“Problem” / Mystery

The next morning, she had another piece and was surprised to find that it was not sour whatsoever. She gave me a piece later in the day to confirm that it was not just her tongue that was broken and sure enough, it was not sour in the slightest. All traces of sourness had completely gone, literally, overnight. In fact, she made two of them and they both gave the same results (really sour, then suddenly not).

Question

I know that in the culinary field, some kinds of food molecules can break down under certain circumstances, but I have never heard of anything that could explain this.

Now we are both baffled by this mystery. What the heck could cause a homemade cake that was shockingly sour to lose every last bit of sourness in less than a day? Nothing had changed (she did not cook or freeze it); she left it on the counter, covered in a mesh overnight.

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Have you considered it may have been an effect of something else you had been eating on the first occasion? –  SAJ14SAJ Jul 9 '13 at 2:01
    
I observed something similar years ago with sour-coated candy (sweet gummy confections coated with a citric acid powder). Intensely sour at first, but noticeably less after a few days in storage and merely tangy after a few weeks. I have no good explanation for this though. –  Shog9 Jul 9 '13 at 4:05
    
@SAJ14SAJ, no, it was definitely the plum. Besides, at least three different people confirmed how tart it was, including her neighbor. –  Synetech Jul 9 '13 at 6:17
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1 Answer 1

It may have been the act of leaving the plums on top of the cake, covered overnight at room-temperature. This might have concentrated the natural ethylene produced by the plum enough to cause it to ripen by breaking down the starches and turning it into sugars.

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Even if this were true, how would it affect the amount of acid? –  SAJ14SAJ Jul 11 '13 at 2:05
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@SAJ14SAJ sugar wont affect the amount of acid but it will reduce how sour it tastes. –  Jay Jul 11 '13 at 4:09
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Interesting theory. I never heard of sliced fruit ripening, but I guess this could happen. What makes me doubt it is that even on the tree, under best conditions, one night of ripening won't turn a very sour plum into a sweet one. –  rumtscho Jul 11 '13 at 8:51
    
My mom's Pflaumkuchen (plum cake), similarly sounding to the OP's, had the same results quite often. If slightly underripe plums are used to top the cake, they may retain some tartness immediately after baking but would mellow to just a nice sweet flavor by the 2nd day. I don't know the cause, I just experienced it almost every time. –  Kristina Lopez Jul 11 '13 at 17:25
    
Interesting theory. The next time she makes it, I’ll suggest putting some slices of both ripe and unripe plums on different sides to test it. –  Synetech Aug 11 '13 at 17:54
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