Sweet science

’Tis the season for peace and love and carols by the fire and decorated fir trees and all that sort of thing. ’Tis also the season for candy: candy canes, fudge, toffee, peanut brittle, bon-bons of all kinds.

A lot of it is store-bought, but a lot of it is made from scratch.

As Grandma will tell you, there’s an art to making candy. But you can tell Grandma there’s also a lot of science to it, and the science boils down (sorry) to one thing: the behavior of sugar molecules.

The ordinary white sugar we normally use is more properly known as sucrose. Sucrose molecules have 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. Most plants contain sucrose, but sugarcane and sugar beets have more than most, which is why most of our sugar comes from one of those two sources. Table sugar, under a microscope, appears to be made up of little cubes (though less perfect ones than salt). Those are sugar crystals, orderly arrangements of sucrose molecules.

Sucrose is actually made up of two simpler sugars, called fructose and glucose, stuck together. When you’re cooking with sugar and the recipe calls for something acidic, such as lemon juice or cream of tartar, the purpose is to break the sucrose down into fructose and glucose.

Sugar dissolves in water, but water can only hold so much. When as much sugar as possible is dissolved in a given volume of water, the solution is said to be saturated. The saturation point varies with temperature: the hotter the water, the more sugar you can dissolve in it.

So, when you make candy, you mix sugar (and various other ingredients) with water at a very high temperature, so high that even though a lot of the water boils away, the sugar remains dissolved.

Sugar solutions display different behaviors at different temperatures, something chefs have known since at least the 1700s. This led to the famous “thread-ball-crack” test, which allows cooks to determine the sugar concentration of a solution by dropping a small amount of it into cold water and observing what happens.

At about 70 percent concentration, the solution becomes threadlike; at 80 percent it forms a soft ball; at 90 percent it forms a hard ball, and from 98 to 99 percent, it forms a hard ball that cracks. It takes higher and higher temperatures to achieve each stage of solution.

Once you’ve got the sugar concentration where you want it, you let the mixture cool. As you do so, the water ends up holding more sugar in solution than would normally be possible at the lower temperature: it’s now “supersaturated.”

A supersaturated sugar solution is highly unstable: the sugar is just waiting for any excuse—just a bit of jostling, for example, or a speck of dust–to begin crystallizing again.

Whether you want that to happen or not depends on the kind of candy you’re making. In some types of candies, such a fudge, you actually want some crystallization, but you have to keep constantly stirring to break up the crystals as they form. This breaks them down into smaller crystals, which are what give fudge that creamy texture.

In some candies, however, such as lollipops, taffy and caramels, crystals aren’t wanted. To keep sucrose from crystallizing, you want to make sure there are plenty of fructose and glucose molecules mixed in. The sucrose molecules all want to lock together in a certain way, like Legos. The fructose and glucose molecules get in the way, so they can’t lock together.

There are a couple of ways to get these other sugars into the mix. The addition of acid, as I mentioned earlier, breaks sucrose into fructose and glucose: this is called inversion. Another way is to add a non-sucrose sugar, such as corn syrup, which is mainly glucose.

Fats can also interfere with sucrose’s efforts to crystallize,which is where butter comes in.

Of course, you can know all this and still be unable to make a decent fudge (I should know, I’ve tried).

Grandma may not know a molecule from a mixing board, but she sure knows how to put sugar through its paces.

Permanent link to this article: https://edwardwillett.com/2007/12/sweet-science-2/

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