Purple Cauliflower Indicator

    June, 2015:

    Kids, did you know that cauliflower also comes in orange and purple colors? And that you can make designs using purple cauliflower and lemon juice? Make a quick trip to the grocery store with an adult partner and find out how!

    First, a little science. The deep purple color of purple cauliflower comes from anthocyanins, the antioxidant that gives the purple color to the skins of grapes, plums, and eggplants. Anthocyanin is a harmless, water soluble pigment and so purple cauliflower is perfectly safe to eat.

    pH is a measure, used by chemists, of the strength of acids and bases. Anthocyanins can be used as pH indicators because their color changes with pH; they are pink in acidic solutions (pH < 7), purple in neutral solutions (pH ~ 7), greenish-yellow in basic solutions (pH > 7), and colorless in very basic solutions. You may have heard of red cabbage as pH indicators because, not surprisingly, they also contain anthocyanins.

    Have an adult partner steam some purple cauliflower in a steam basket on the stove, and collect the dark blue/purple water after it cools. Pour equal amounts of the cooled purple water into a few clear glasses. Add a teaspoon of cream of tartar (tartaric acid) to one glass and stir. Add a teaspoon of baking soda (sodium bicarbonate) to another glass and stir. Add some lemon juice (citric acid) to a glass and stir. What happens? Find other items around your home and test them using your cauliflower indicator to determine whether they are acids or bases.

    To some uncooked, raw purple cauliflower, add some drops of lemon juice. What happens? Here is where you can get creative and make some interesting designs in your vegetables.

    Heating the purple florets will also change their color from purple to gray or slate blue, especially if your water is hard or has an alkaline pH. You could add a bit of vinegar or cream of tartar to the water to minimize the color change.

    Many thanks to Antonya Sanders for the tip!

    Editor, Dr. Kathleen Carrado Gregar, Argonne National Laboratory