Christmas cookie chemistry

A worker shows a famous Belgian speculoos, a traditional caramelised biscuit, at the Lotus factory in Lembeke in this picture taken on December 1, 2008.(REUTERS/Thierry Roge)

It's Christmas cookie time, and everyone's got a recipe or two they swear by, whether their loyalties lie with frosted sugar cookies or gently spiced gingerbread.

In a time of togetherness and seasonal cheer, though, cookies can be contentious: Is crispy better than chewy? Is a cakelike texture something to strive for, or avoid at all costs? How do you pick the perfect recipe when cookie tastes vary so wildly?

The chemistry of cookies can help answer that last question. These sweet treats require a delicate balance between sugar, flour and fat and the outcomes of varying an ingredient or two get confusing quickly. [11 Health Benefits of Christmas Dinner]

"Cookies are actually really complicated," said Jeff Potter, the author of "Cooking for Geeks: Real Science, Great Hacks and Good Food" (O'Reilly Media, 2010). "They're a whole microcosm. Every single thing in food science that happens pretty much happens in cookies."

The ingredients

To understand the complexity of a simple snowflake sugar cookie, it helps to start with the ingredients. There's flour, of course, the base of most baked holiday treats. When mixed with water, flour forms a protein called gluten, a long molecule that helps trap air bubbles in yeasty breads and pizza dough. Too much gluten in cookies makes them tough, but a little bit of gluten can give them structure and prevent them from spreading all over the cookie sheet.

Cake flour has fewer gluten-forming proteins than all-purpose flour or bread flour about 7 percent by volume compared with up to 15 percent in the highest-gluten bread flour, according to the King Arthur Flour company. All-purpose flour, with its middle-of-the-road protein content, is still perfectly acceptable for cookies, though some baking purists prefer to use cake flour.

But wait. Flour alone can't determine a cookie's ultimate texture. Water promotes the formation of gluten, Potter told Live Science. Sucrose (table sugar) and fat inhibit it.

Here's where the choice of fat and sugar start to matter. Butter, Potter said, is about 80 percent fat and 20 percent water. Shortening is all fat. Cookies made with shortening form less gluten, therefore becoming less cakelike.

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Christmas cookie chemistry