If you couldn’t perform the experiment, here is a video:
Explanation:
After pureeing, the drink quickly turns into a pudding-like mass that no longer tastes good. The cause must lie in a reaction between components of the blueberries and the milk.
Blueberries contain a lot of pectin, which can swell significantly (bind water). You might know pectin from making jam. Pectin is added to the fruit, e.g. in the form of gelling sugar, and the mixture is heated so that the jam sets. Pectin belongs to carbohydrates, i.e., macromolecules made up of sugars. When pectin encounters protein, e.g. milk protein, pectin-protein complexes are formed. Especially in the acidic pH range, the electrostatic attraction of oppositely charged proteins and pectins is promoted. (We found this explanation from the pectin manufacturer Herbstreith-Fox, which in turn refers to scientific literature, including this publication:
Schmitt, C. and S.L. Turgeon, Protein/polysaccharide
complexes and coacervates in food systems. Advances
in Colloid and Interface Science, 2011. 167(1): p. 63-70.
However, whether this is really the explanation for the experimental result can be tested with the following experiments:
- Puree cooked blueberries with milk to make a shake
- Apple-milkshake
Hypothesis for Experiment 1: Since pectins are used for making jam, they should not be impaired by the heat of cooking, at most activated (unlike proteins, which denature with heat). If you then cool them and puree them with cold milk, the same thing should happen as with uncooked blueberries.
Result of Experiment 1: A blueberry-milkshake made with cooked blueberries works very well, tastes delicious, and does NOT gel even after standing for a long time!
This refutes the hypothesis.
Hypothesis for Experiment 2: Since apples, like blueberries, contain a lot of pectin, an apple-milkshake should also gel.
Result of Experiment 2: The apple-milkshake works and does not gel. It only separates after standing for a long time, but tastes good. You could refine it with cinnamon. Note: The apple variety used was “Pink Lady”.
This also refutes the second hypothesis. A simple complex formation between pectins and milk proteins does not seem to fully explain the phenomenon. We are therefore grateful for any insights from the community!
Another assumption is that the tannins in blueberries (polyphenols) react with milk protein. Tannins are chemical compounds that form poorly soluble compounds with proteins and other molecules. They can alter the structure of proteins, for example, by precipitating or cross-linking them. In this respect, this could also be an explanation that could be pursued. However, polyphenols are also said not to be heat-sensitive, so experiment 1 cannot be explained with this theory either.
The phenomenon is also discussed in detail on Chefkoch. There, pectins are mainly suspected, possibly also in conjunction with a blender that chops more finely than crushing blueberries with a fork, which perhaps releases fewer pectins. And the tannins in blueberries are also considered a possible cause.
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