Background

Recently, my wife spilt some Pomegranate juice on a white top. The instant we added some soap to wash the stain out, the stain turned a dark blue.

I turned to Google to find out what was going on - but all that immediately came up was this post asking why their cheesecake had failed to pick up the pomegranate colour

(The above link has some interesting comments about two points - emulsification, and pH. I'll focus on the latter)

Theory

The following explains roughly what is going on here:

The colour of Pomegranate Juice appears to be caused significantly by anthocyanins - which are a group of secondary metabolites found in some plants (Anothocyanins are given the E number E163 in the EU).

The interesting thing about Anothocyanins are that they can be used as pH indicators - they change colour depending on the pH of the surrounding liquid.

Perhaps the best known example of this effect is red cabbage - which is commonly used in lower school science experiments as an example of pH indicators (or at least it was when I was at school)

Experiment

I thought I'd test my theory that the colour change was pH based.

First, I set out three Petri dishes (Yes - I have petri dishes in my kitchen - and yes, they're only used for food!)

Secondly, I prepared an acid an an alkali - using some handy Vinegar (Acid) and Bicarbonate of Soda (Alkali).

With solutions made, I layed them out in the petri dishes (Note the control dish - containing just water):

Finally, I used a pipette (again - food quality) to distribute a few drops of pomegranate juice evenly between the dishes:

Results

The results are fairly clear - the colour of the pomegranate juice clearly changes considerably with the pH of the solution.

Interestingly, the alkali solution can be seen reacting with the added juice. Many Anthocyanins degrade in higher pH solutions like this - and it appears that the pomegranate colour readily degrades in alkali solution

Take-aways

In a quick one-liner: If you want to use Pomegranate (or any anthocyanin-containing plants - including blueberries, cranberries, raspberries and cherries) then make sure the pH of your mixture is low (acidic) by adding some citric acid (E number E330 - lemon juice) or other acidity regulator before adding the pomegranate juice (so it does not degrade).

As a bonus - if you have an alkali base mixture for your food, and you want to colour it with a similar red-purple colour, then it might be worth using Beetroot or Swiss Chard - which contain a different family of Pigments (Betalains) - which (from what I can find) do not appear to vary with pH.