Peachy Keen

    Kids, did you ever think of freezing a whole peach or even one cut up into pieces?  Why not?  If you tried it, you'd find that all of the flavor was gone and that the perfectly peachy texture became mush upon thawing. So what can be done to save peaches beyond their growing season? 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    First, let's demonstrate why simply freezing a peeled and diced peach is a bad idea. Fill a plastic zip-top bag with water and seal shut. Have an adult partner find a pointy object such as a knitting needle. Imagine that the bag filled with water is a pulp cell inside the peach. Imagine that the needle is an ice crystal. Holding the bag over a sink, have your adult partner puncture the bag several times making holes for the water to leak out. This is what happens when ice crystals puncture pulp cells during the freezing process.

    What scientific trick can be used to prevent this? The substance that we call sugar is actually sucrose, a disaccharide, or double sugar. Sucrose is famous for being hygroscopic, meaning that it loves to grab hold of water at the molecular level. By adding some sucrose to diced peaches, some of the water is pulled out of the peaches and creates a syrup with the sucrose. The trick is that when the syrup freezes, the sucrose holds onto some of the water, and that prevents the ice crystals from getting so big that they poke holes in all the cells, which makes all of the moisture leak out when the peaches thaw.

    For one pound of diced peaches which you would like to freeze, add about ½ cup of sugar. Before doing that, however, be aware that neither sucrose nor freezing temperatures will stop peaches from turning brown. For that, more science is needed, in the form of an acid. Since browning is really an oxidative process, what's needed is an antioxidant. Ascorbic acid, which is good old vitamin C, works well for this. About 500 milligrams (mg) will do. Crush up a vitamin C tablet between two spoons, dissolve it in 3 tablespoons of water, and mix into the peaches before the sugar.

    Once the peaches, vitamin C, and sucrose are mixed, let the mixture sit for 15 minutes until a thick syrup forms. Transfer the peach mixture to zip-top freezer bags for long-term storage. Enjoy your kitchen chemistry all winter long! 


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    December 2009


    References:  Alton Brown of the "Good Eats" TV show on The Food Network: