7.5 Sugars and fats crystallize

Many foods contain crystals. Butter, margarine, ice cream, sugar and chocolate each have their own kinds. Ice cream alone can hold fat crystals, ice crystals and sometimes crystals of lactose, the sugar in milk.

Crystals decide whether a food spreads, and whether it feels smooth or gritty in your mouth. Sometimes a cook wants them, and sometimes a cook works hard to stop them.

Sugar

Candy comes in two families:

• Crystalline candies keep crystals in the finished sweet. Fudge and fondant are examples.
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• Non-crystalline candies have none. Lollipops, taffy and caramels are examples. Their recipes are built to stop sugar crystals, because crystals would make them grainy.
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To keep table sugar (sucrose) from crystallizing, give it other sugars to get in its way. Fructose and glucose are the usual ones. An acid can also be added to "invert" the sugar, and fat added to some sweets has a similar effect.

When you boil sugar, you almost always want to avoid crystals. These encourage them:

• a dirty pot or dirty tools
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• sugar with bits in it, such as a scoop that went into the flour bin first
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• hard water, which is high in minerals
• stirring too much while the sugar boils
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To head crystals off, add an interferent while the sugar boils: an acid such as lemon juice, vinegar or tartaric acid, or glucose or corn syrup.

Ice cream

Ice cream is a mix of parts: ice and fat crystals, an emulsion, air cells and stabilizers such as locust bean gum. Together they give the ice cream its body. Lots of small ice crystals give it a smooth texture that melts and cools well. If those crystals grow bigger in storage, called recrystallization, the ice cream turns coarse and less pleasant. Sugar crystals in fondant and frosting, and fat crystals in butter, margarine and chocolate, raise the same worry.

Fats and chocolate

In fats such as butter, margarine, shortening and chocolate, crystals shape how the food looks, its texture, how it spreads and how it releases flavor. To control them, you control three things:

• how many crystals there are, and how big
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• how they are spread through the food
• their shape, called the polymorph

Chocolate shows this best. Tempering, also called pre-crystallization, coaxes the cocoa butter to set in one particular crystal pattern. That pattern keeps the chocolate shiny and gives it the right texture for a long time. It matters for moulding and decoration, and for how the chocolate feels when you eat it.

Badly tempered chocolate shows it in one of several ways:

• It never hardens at room temperature.
• It hardens but looks dull and patchy.
• It is spongy inside instead of crisp.
• It gets fat bloom: fat moves to the surface and leaves white streaks and patches.
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