Tuesday, May 23, 2006

Physics Problem?

Recently I was baking cookies with a couple of friends and came upon this issue...

How can cookies start out square (out of the package) and end up round?

These are sugar cookies that come pre-made. They are little squares and you just put them on the baking sheet (left photo). After a while, when the cookies are in the oven, you can see that they still have a bit of "square" in them but are becoming "rounder". Finally, you see the cookies at the very end, when they are done cooking, and they are very round.

What gives? Can some reader of this blog enlighten me of the process that inexorably moves these cookies from a square to a round traditional cookie shape?

Admittedly, not a world shaking topic, but it is good to liven things up a bit from time to time. Posted by Picasa


Frank Borger said...

It all depends on geometry and size.

Pi R square
Cookies R round

Anonymous said...

Going from "solid" to sort of "liquid".
The liquid then "flows" evenly into a circle.

Michael Hiteshew said...

Anon is correct. Think of taking a square drop of water and placing it onto a flat, horizontal plate. Gravity pulls downward on all parts of the drop, flattening it. The drop continues flatttening until the pressure of water molecules sitting above other water molecules and pressing them downward and out of the way (we'd call this 'water pressure' in the everyday world) is balanced by the surface tension of the water droplet. The surface tension force acts equally in all directions. Were the droplet in free fall or zero gravity the surface tension would pull the droplet into a sphere. On a flat surface, with gravity pulling downward on the drop, the surface tension tends to pull the droplet into a flattened circle.

When the cookie dough heats up, it passes through a phase where it becomes a viscous liquid (like a syrup) before the ingredients combine chemically, dehydrate, then harden.

James R. Rummel said...

Being a bachelor with a big ol' belly and a sharp hunger for everything that is bad for you (hence the belly), I actually do a fair bit of cooking. In fact, I'm not half bad at it!

Sugar cookies are basically 1/5 sugar, 1/5 lard or shortening, and 3/5 flour. It is held together with a few whipped eggs acting as a mortar (so to speak). Flavorings such as vanilla and a pinch of salt are added to give it character. Baking powder is added so the mixture will rise when it is heated.

The square dough slumps in the hot oven as the lard melts, puddling out into that round shape you mentioned. The baking powder produces little bubbles through a chemical reaction, increasing the volume so the round cookies take up a greater area than the little squares of dough you started out with. The cookies firm up as the eggs cook.

So the short answer is that the lard melts when heated to produce the round shape. The cookies are soft and gooey when hot, but firm when they cool. Try popping them in the microwave and see if that softens them at a later date.

Chocolate chip cookies are essentially the same recipe, but brown sugar and chocolate chips are added as a flavoring.

Chocolate chip, sugar cookies, whatever you call them they are little matrixes of flour held together with baked eggs, and with a fair amount of fat mixed through it all.


Anonymous said...


Anonymous said...

The analogy to a free falling sphere would be interesting for some of those here in this group.
All across New England there are tall towers built a long time ago. When you ask why they were intially built, it was to drop molten metal, which would form during the fall into a sphere, for a musket bullet!

James R. Rummel said...

The analogy to a free falling sphere would be interesting for some of those here in this group.

What group?


Carl from Chicago said...

He is talking about the group in terms of interest generally on this blog. Dan posts some excellent articles about weapons, and lots of our readers are interested in those articles.