|MadSci Network: Chemistry|
First, let me explain what fat does in baking. When flour and water are mixed together (like in a cake or cookie) it forms gluten which is a very elastic, stringy material. This gives the baked item structure so it doesn't collapse and end up flat. Now, for this to happen the flour and water have to mix together and be manipulated - sometimes we stir to get this part done, other times we knead, like for flour. If all the gluten that could be developed was developed we would have a baked item that is very tough to chew and flat. So this is where fat comes in. We almost always mix the fat with the flour before we add any liquid (even if the liquid is not water, liquids are more water than anything else, so it works the same way). The fat coats the pieces of flour so that the water cannot get to it. This means that not all the gluten will be developed that could be because some of the flour won't get mixed with the water. So that makes the final product, cake, cookie, etc, more tender and not tough. This is called the shortening ability of a fat because the fat helps shorten the strings of gluten that will be developed. This is where the name shortening came from - have you ever heard the old song "Mama's little baby loves shortening, shortening; Mama's little baby loves shortening bread" - this is bread that has a lot of fat in it.
Another thing fat does is help trap air bubbles. As you mix fat and sugar together - the first step in many recipes - you mix some air bubbles in as the fat surrounds air. This is important later when the item is baking because these air bubbles are part of what makes the item rise. This means that your baked item will be puffed up and not flat. Notice sometime when you make a cake - the finished cake is much taller than the heighth of the batter in the pan, before you bake it. That is partly due to the fat.
So, what are the differences among different fats?
Well butter is mostly fat but has milk solids in it. So, butter doesn't work as well as the same amount of some other fats because there is less fat to do the job. But, it works well enough for most things. It does a pretty good job of shortening - that is covering the flour to decrease the amount of gluten that will be formed. And it is very good at trapping air bubbles.
Margarine/Crisco - these are very similar so I combined them - they are very good at shortening and very good at trapping air bubbles.
Oil - very good at shortening - since it is liquid it can really cover well but not good at all about trapping air bubbles
Now, there is one other baked item that I really haven't talked about - pie crust. This is a special product to deal with. The two things you want a pie crust to be (besides good tasting) are tender and flaky. Fat is important in each of these. But, what makes pie crust more tender makes it less flaky and vice versa. The same thing that makes a cake tender - covering the flour particles with fat so that the water can't get to it and form gluten - will make pie crust tender. But pies become flaky when you leave sort of big pieces clumped together. These big clumps will be rolled flat when you roll out the pie crust, and end up between the gluten that has been developed. Then, when the pie crust bakes, the fat melts, so you have spaces between the gluten that is filled with melted fat. This makes the crust flaky - it will come apart in thin layers where the fat was.
So the more you stir (and then knead) pie crust, the more you spread the fat around, cover more flour, leading to a tender crust. But the less you stir the crust, the more big clumps of fat you have, the flakier the crust will be. So, it is kind of a guessing game - how much to stir to get a tender and flaky crust.
Crisco/Margarine are great for making crusts because they spread very well and clump together well. Butter is almost as good. Oil is not good to make pie crust with because it spreads great but doesn't clump together at all so doesn't make a flaky crust.
Some fun experiments are to make different baked goods with different kinds of fats or with a different amount of stirring to see what affect it would have on the final product.
Thanks for an interesting question. You can find out more about this subject and other food - chemistry topics by looking in Food Science books. The two below are one I used in college when I was taking my food science classes and teaching them so they would be a little technical for you but there might be some others that are more for your age. Good luck!
Food Science, 2nd edition by Helen Charley
Introductory Foods, 10th edition by Marion Bennion
Ohio State University
Try the links in the MadSci Library for more information on Chemistry.