Odd Chemical Reaction In My Smoothie

[GraphicNerdity]

I made a smoothie with strawberries, blueberries, pineapple, a small amount of pineapple juice, a banana, a little ice, and a spoonful of sugar. I poured it into a cup, and left what didn't fit, in the blender. After about half an hour, I came back to find the smoothie completely and totally coagulated, into a soft solid. I'm curious as to what caused this to happen. I'm assuming it has something to do with the acid in the pineapple, because I blended up the coagulated smoothie and tried to drink it again and could no longer detect the acid taste of the pineapple as strongly as when it was fresh, but I don't know much about chemistry as I'm still in High school. Can anyone shed some light on the situation?

[exchemist]

I made a smoothie with strawberries, blueberries, pineapple, a small amount of pineapple juice, a banana, a little ice, and a spoonful of sugar. I poured it into a cup, and left what didn't fit, in the blender. After about half an hour, I came back to find the smoothie completely and totally coagulated, into a soft solid. I'm curious as to what caused this to happen. I'm assuming it has something to do with the acid in the pineapple, because I blended up the coagulated smoothie and tried to drink it again and could no longer detect the acid taste of the pineapple as strongly as when it was fresh, but I don't know much about chemistry as I'm still in High school. Can anyone shed some light on the situation?

I'm not sure, but my first guess would be you made a thixotropic mixture. https://en.wikipedia.org/wiki/Thixotropy

 

You made a slurry of these fruit which, when left to stand became more solid and when you remixed it became fluid once more. This behaviour can arise from the way the liquids are distributed. When intimately mixed in very small droplets, it flows, but left for a period the droplets merge and some of the more solid particles that were previously in suspension ("lubricated" by the film of liquid in between them) touch each other and stick, forming a gel with the liquid trapped in the gaps between. When you re-blend it, you break this structure up again, "lubrication" is restored, and it flows once more.

 

You comment about the acid taste disappearing is interesting. It could be a real neutralisation reaction, or it could be a masking by the sweetness of the other ingredients. Why not bring home a few pieces of pH paper from school and repeat the experiment, measuring the pH of pineapple alone, then the mixture immediately after blending and then after standing?

 

All the ingredients are in principle acidic, even the bananas (pH 5 approx), but there could be an ingredient in the mixture that acts as a "buffer"  and reduces the initial acidity. 

Edited February 22, 2018 by exchemist