Kitchen Chemistry: Sponge Candy Recipe

I hadn’t had sponge candy in years and decided to dig through my old recipe books the other day to mix up a batch.  It’s a fun one to make and there is a lot of interesting chemistry going on here. Not everyone likes this stuff, but it’s a great kitchen chemistry experiment either way.

You will need: dark corn syrup, sugar, baking soda, and vinegar

1. Prepare a 9×13 baking dish by lining with foil and spraying with cooking spray.
2. Combine 1 cup sugar, 1 cup dark corn syrup, and 1 Tbs. vinegar in a sauce pan
3. Heat on stove to dissolve the sugar
4. Continue to heat and stir until you reach 300°F (use a candy thermometer to check)
5. Remove from heat and stir in 1 Tbs. baking soda, sifted. (This is the exciting part!)
6. Pour the mixture into your foil-lined baking dish.
7. Let cool and harden.
8. Break into pieces and enjoy.
9. It’s also good dipped in chocolate (what isn’t?), so you can melt some chocolate to dip the chunks into if you choose.

Here is what’s happening. There is actually quite a bit going on with this one. One part is what happens with the sugars and another part is what happens with the baking soda and vinegar.

We’ll start with the transformation of the sugars . This recipe has two sources of sugars.  The white table sugar gives us sucrose (with the formula, C₁₂H₂₂O₁₁) and the corn syrup gives us glucose (with the formula, C₆H₁₂O₆). The first thing that we do with these sugars is get them dissolved in the water. Not all of the sugar dissolved in the water until it was heated on the stove. This is related to a property called solubility, which determines how much of a solid can be dissolved in a liquid. For most substances, the solubility increases as the temperature increase, so when we heat the liquid we can dissolve more of the solid. As we heat the water, more of the sugars dissolve and eventually we have a sugar solution. As we continue to heat our sugar solution, we take the sugar through the different candy making stages. These stages are classified by the highest temperature that the sugar solution reaches: 235-240°F is called soft-ball, 245-250°F is called firm-ball, 250-265°F is called hard-ball, 270-290°F is called soft-crack, and 300-310°F is called hard-crack. (If you continue past 300°F, we enter caramelization territory, where the sugar molecules actually start to break apart and transform into other chemicals. Both glucose and sucrose start to caramelize around 320°F, so we’re not seeing that here.) The most important change that is occurring as we continue through these temperature ranges, though, is that the water is boiling off and the ratio of sugar to water is changing. This happens consistently, so we can aim for a specific temperature if we want a certain water content. The lower temperature stages have a higher water content than the higher temperature stages and the amount of water impacts the texture of the candy when it cools. As you can see in the examples below, less water content leads to harder candy textures.

In addition to the water content, the other big factor in the texture of the candy is how we let the sugar solution cool. We talked about crystallization in our crystal heart activity. Crystals, highly ordered structures, can form when a solid is dissolved in a liquid, like we have here. How we let this solution cool will impact whether crystals form and what type of crystals form if they do. If we let the solution cool slowly without disruption, we can get large crystals. If we let the solution cool slowly with agitation (stirring, for example), we can get small crystals. And, if we let the solution cool fast (pouring out in a tray, for example), we don’t see the formation of crystals. In this case, we are cooling our solution fast so we don’t have the formation of crystals. (Having two different types of sugar, glucose and sucrose, also keeps our solution from forming crystals.) So, by controlling the highest temperature of the sugar solution, we control how much water is left in the candy, which impacts the texture. And, by controlling how we cool the sugar solution, we impact the crystal formation, which also impacts the texture.

Okay, so what about that baking soda and vinegar?  This is another example of the same double replacement and decomposition reaction between sodium bicarbonate (baking soda) and acetic acid (vinegar) that we’ve posted about before. In this case, the heat of the sugar solution also helps drive the reaction. The product that is most interesting from this reaction is the carbon dioxide gas. We’ve seen how this fizzes as it bubbles through water in previous cases. It’s bubbling in the same way here, but it’s bubbling up through a sugar solution instead of water. This sugar solution is thicker than water, so it ends up trapping some of the bubbles and leaving us with this unique treat full of little pockets.

Hope you have a tasty experiment!