Why do purple Pedialyte freezer pops turn green when frozen?

Question

I bought some Pedialyte freezer pops for my kids:

And I've noticed that after spending a long enough time in the freezer, the purple "grape" pops change color from purple to green:

After thawing (but before melting), the color changes back from green to purple. The dramatic color change happens to all the purple freezer pops, and only the purple freezer pops. They taste the same afterwards and are safe to eat.

I couldn't find any decent explanations for this effect searching Google. This Reddit thread is about the same effect happening to purple Powerade:

https://amp.reddit.com/r/mildlyinteresting/comments/4pi97r/grape_powerade_turns_green_when_it_freezes/

There are hundreds of comments, but they are mostly jokes. The only thing I learned was that it was not related to Anthocyanin because purple Powerade does not contain that.

These are the ingredients of the freezer pops:

Except for perhaps "artificial and natural flavors", the only difference between the different types of freezer pops are the coloring: "grape" has Red 40 and Blue 1. The others are different combinations of Blue 1, Red 40, and Yellow 6.

I initially had a naive hypothesis that maybe Red 40 turns yellow when freezing (since blue + yellow = green). But the Orange and Cherry pops also have this ingredient and don't change color.

Some answers on the Reddit thread suggested that the color change is due to the combination of electrolytes and blue coloring, which is plausible given that Pedialyte has a lot of electrolytes. But "Blue Raspberry" also has Blue 1 and doesn't change color after freezing.

So, my question is:

What specifically causes the purple freezer pops to change color, and why doesn't it happen to the other flavors of freezer pops?

Answer 1

I think this is an optical phenomenon and no chemical reaction is taking place. When you are looking at a solution, you are basically perceiving its absorption spectrum. When you have a solid, you are looking at the reflectance spectrum and the story is more complex, if you have a heterogeneous solid like this stuff.

If you originally had the solution and you froze at home, this is slow freezing and the size of ice crystals could be larger (phase separation). If you had a chance to look at frozen Coke/Pepsi, it does not look like "liquid" Coke or Pepsi. It is much lighter, light brown rather. Ice crystals are to be blamed for this color change. If you froze Pedialyte very fast probably, the ice crystals would be smaller and the color change would not be that drastic.

The second effect could be the solubility of this particular dye at freezing temperature. If you had an optical microscope you would probably see the segregration of particles. Since you are saying that other "colors" do not show this effect, most likely, this pigment is not very soluble at low temperatures.

Also note that certain anthocyanins (common flower/fruit colors) change color when frozen because at a low temperatures, a different conformation of the dye molecule exists, but this is rare. This is an extreme example of a dye changing its color drastically upon freezing. See for example, Unusual Color Change of Vinylpyranoanthocyanin−Phenolic Pigments, J. Agric. Food Chem. 2010, 58, 7, 4292–4297.Here