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Most bakers are very familiar with traditional fat shorteners such as butter, margarine or vegetable shortening. Shorteners coat the flour proteins or water-proof them, contributing to tender baking recipe by reducing their contact with the moisture in the recipe and preventing gluten from forming. They also shorten the length of the gluten strands when the flour is stirred with that moisture (that's why they're called "shorteners"), preventing a tough baked good or tenderize. Fat coats the flour particles so the elastic formation slows down; it makes the gluten strands slippery so the gas bubbles can move easily; and it gives the final recipe a finer grain. Generally, when people refer to "moist" in a baked product, they are referring to the fat content and they way it lubricates the mouth when eating.
SARAH SAYS: When you add the fat in a recipe matters: in pastry making, the fat is rubbed into the flour. This essentially coats the gluten forming proteins, glutenin and gliadin, so they can't join together and form gluten. After the fat is worked in, then the liquids are added.
In traditional baking, where solid fats are creamed with crystalline sugar, tiny air cells are incorporated into the batter, so the baked good will have a fine, aerated texture. When a shortener is removed or reduced, it increases the chances that the end product will lack flavor and be tough and full of tunnels.
Different types of fat do different jobs in baking. A well-known baking fat, butter makes a very important flavor contribution, whereas margarine does not have as fine a texture and taste. When choosing a shortener, I always go for the butter, even in reduced-fat baking where the small amounts help to retain a great taste and aroma. If you have dietary restrictions that make it necessary for you to reduce saturated fats in your diet, you can substitute a butter-margarine blend. The recipe won't taste the same if you use margarine. Fat can be found in other baking ingredients, such as the egg yolk which serves as both a tenderizer and emulsifier due to its fat and lecithin content.
Oils do not act as a shortener because it is a liquid and won't cream with crystalline sugar in the same way that solid fat does. Oils tend to coat each particle of flour, which causes a lack of contact of moisture and helps prevent gluten development. It reduces dryness and enhances flavor. I use it sparingly in reduced-fat baking because it has the same number of calories and fat grams as butter, even though it has less saturated fat.
HOW FLAKY PASTRY WORKS
Pure fats, such as lard and shortening make for flakier baked goods than those that contain water such as butter, but butter imparts a better taste. Fats contribute to the tenderness (shortness) and especially flakiness of pastry. Pastry is often a trade-off between flavor and texture, much of which comes from the fat in the recipe. Some bakers use both butter and shortening to capture the best qualities of each, but I prefer to use all butter because of its better taste.
Fats contribute to the flakiness and tenderness of pastry by being layered in between sheets of thin dough. It can also be cut in or rubbed into the flour as pea-sized shapes before the final dough is made. The fat melts during baking, leaving air spaces. When placed in the oven, the flour starches set around the fat, leaving a layer or space when the fat melts which is reabsorbed back into the dough. The longer the fats take to melt in the oven, the more well defined the air cells. The melting point of shortening is higher than that of butter, and it stays solid longer. As a result, it forms better flaky pastry, but without the butter's wonderful flavor.
Cold butter or fats and the flakiness of the pastry are intricately connected. Because butter has such a low melting point, it must be well-chilled to ensure that it can withstand being rolled and handled without melting to produce flakiness. Butter that is too soft surrounds the flour particles rather than forming spaces, and the final texture of the pastry is flat and greasy.