A couple of years ago I saw a picture of a beautiful rose-color soap which had been made with straight red cabbage juice.
Coloring soap with red cabbage juice has been on my to-do things since then and I eventually got around to trying it.
I did, however, quite a lot of research on the compounds of the juice and got to know that it owed its beautiful color to the great amount of polyphenols, and more specifically anthocyanins for its color. Having tried blueberry and red rose juice before, I thought that the color would not hold. Still, my research told me that depending on the last atom of the anthocyanin molecule, the color might hold even in a basic environment. I figured that since the blueberry juice contained fructose and therefore a lot of the blueberry anthocyanin molecules had a sugar “atom” attached to them (I am sorry for the dilettante explanation), it was the reason for the discoloration to beige in basic environment. Cabbage, being a vegetable, does not contain so much sugar, I thought, and therefore it might be a better coloring agent for cold process soap.
Several sources told me that anthocyanins are weak organic acids and have different colors that depend upon the numbers of removable protons that remain attached to the molecule.
To make a long story short, depending on the acidity of the environment, the correlation between acidity and color goes as follows.
This table basically means that the higher the pH, the more significant the change of color is when the cabbage juice or more accurately the anthocyanins in it are treated with NaOH.
Despite all scientific evidence found on the web, I hoped against hope that my soap would turn a beautiful rose color thanks to, of course, all the miraculously un-touched, un-damaged anthocyanin molecules from the red cabbage juice. I cut the cabbage, squeezed out the juice and got ready to use it.
The first step was using a few drops of it onto the NaOH flakes. The color turned from that supreme purple into dark blue-green and then, in less than a second, into yellow.
I figured that after I get a nice heavy trace, the pH would be somewhat lower so it would be a better timing to add the anthocyanin-loaded straight juice.
I discounted the juice-water mix and added the remaining juice after the batter got almost as thick as bread dough.
The finished soap is still pale yellow.
As a result of this experiment, I have decided to neither use red cabbage juice as a soap colorant nor for label appeal. The reason? The chemical reaction actually breaks the anthocyanin molecules into compounds which seem to have properties different from those of the anthocyanins and have not been researched well yet. As much as I would love to say that my red-cabbage-juice soap contains antioxidants I have no proof to show that any anthocyanins would be still present in my cold process soap.
And this is where the story ends. My advice: eat your red cabbage and get all the goodness through your digestive system. This way you will get a lot of fiber and vitamins in addition to a lot of whole-body polyphenols 🙂