Baking dates to prehistoric times. At first it involved nothing more than the simple drying of grain seeds in the sun. Eventually the seeds came to be cooked in water, and the resulting gruel was baked on a hot stone, producing a kind of flat bread that was in many ways similar to the Mexican tortilla.

The process of leavening developed slowly. The Egyptians were perhaps the first to consciously use leavening in their baking and also were the first to use ovens. By the middle of the 3rd century BC, the Egyptians had developed baking methods that were similar to those in use today.

With the Industrial Revolution of the mid-19th century, the technology of baking begin to advance rapidly. The quality of ingredients improved, and automation began to replace the time-consuming manual process.

While some baked products are still unleavened [such as pie crusts, Mexican tortillas, and the similar chapatis from India], many recipes employ leavening, which is central to both their taste and their texture. 

Only wheat and rye flours have the qualities necessary for the expansion of an initial dough or batter, wheat being more satisfactory. Although various flours are used in baking, some amount of wheat flour must be added if any significant degree of leavening is desired. Protein in the flour, known as gluten, combines with water to produce an elastic and porous web capable of trapping gas bubbles released by the action of a leavening agent.

Sweet baking recipes as layer cakes, biscuits, cookies, and muffins make use of chemical reactions rather than fermentation for leaven, like in breads. These recipes generally employ a flour (all-purpose or cake) containing less gluten than that used (bread flour) in yeast-leavened goods. Baking soda is most commonly used, but it must be properly combined with counteracting acids in order to release a sufficient amount of carbon dioxide. Such a combination is provided in baking powder, whose formula also serves to regulate the timing of the gas's release.

Another important method of leavening batters is the mixing in of air bubbles from the outside atmosphere. This can be accomplished only by the inclusion of an ingredient [often egg whites] that can easily be beaten into a foam that can hold air bubbles. This method produces a particularly light and delicate product, like angel food cake.

The basic ingredients in any dough or batter are usually flour and a liquid (water, milk). Fat (fats, butter, oils, lard), sugar, eggs, salt, leavening agents and flavorings are also used, depending upon the recipe. 

In general, baking ingredients can be divided into two types, "tougheners / strengtheners" (flour, eggs) and "tenderizers / weakeners" (sugar, fat), sometimes overlapping. In order for a recipe to bake with all of the qualities we like, such as being tender, fluffy, moist, chewy, dense, etc, there needs to be a balance between the two. If one is increased, the other must be decreased, but there's more to it than that.

Recipes also vary by the amounts of each ingredient and the mixing techniques used to combine them. Professional baker's use Baker's Percentages to express their relationship to one another, where home bakers use recipes with ingredient amounts. Cooking temperatures and times also affect the final baked good. These relationships affect the color, flavor, texture, shape and volume.

Each ingredient in a recipe contributes to the final baked good.  In general:

Shortening tends to make dough more easily workable and the final product more tender, while also, in many cases, adding flavor. Egg whites, as mentioned, are often used to produce a light, airy texture, and yolks contribute to the color, flavor, and texture of baked products. Milk is used for flavoring, and sugars to sweeten and to aid fermentation.

Eggs are binders which help hold all the ingredients together. Eggs contribute liquid to a recipe and thus serve as a toughener, especially the egg white portion. But, too many egg whites, such as in a reduced-fat cake recipe make it dry. Including at least one whole egg helps to tenderize. Eggs can also act as leaveners especially when egg whites are beaten separately. The yolk functions to emulsify fat and liquids due to its lecithin content.

In a cake recipe, for example, butter and shortening are tenderizes because they help make it tender and moist. Sugar tenderizes (and of course makes it sweet) because it prevents the flour from forming gluten (gluten is formed when wheat flour is mixed with water or moisture). Sugar competes for water with the flour and wins, making less available. Buttermilk, an acidic ingredient, also tenderizes. 

Liquids bridge both categories as a toughener or a tenderizer. Water and milk enhance the development of gluten and/or gelatinization of starch in the flour or the setting of the structure (baking) and thus serve as a toughener. Milk also contains proteins which act as a structural enhancer. But, too much liquid will cause a baked good to collapse or the batter to become too thin, with the final baked good too heavy. The perfect balance of liquid offers both structural support and moistness which is perceived as tenderness.

The structural components of most baked products are egg whites and gluten from WHEAT flour.

A baked product may contain:

Eggs contribute to the structure of a baked product. They may serve to do this through their contribution of heat denatured proteins, steam for leavening or moisture for starch gelatinization. Egg yolk is also a rich source of emulsifying agents and, thus, facilitates the incorporation of air, inhibits starch gelatinization and contributes to flavor.

Wheat is the only grain with significant amounts of gluten-forming potential. It also contains starch which gelatinizes (absorbs water) and stabilizes the structure. Other grains like corn and oats, and therefore products like cornmeal and oatmeal, do not create gluten in a batter. They provide only flavor and bulk, and must be mixed with wheat flour for strength. 

Two proteins found in wheat flour, glutenin and gliadin, form an elastic substance known as gluten when stirred with moisture. There are as many as 30 different types of protein in wheat, but only these two have gluten forming potential. When wheat flour is moistened and manipulated through stirring, beating and kneading and/or handling, these two proteins grab water and connect and cross-connect to form elastic strands of gluten. If a flour has a lot of these proteins, it grabs up water faster, making strong and springy gluten. 

The magical and elastic gluten network that forms serves many functions in a recipe. Like a net, gluten traps and holds air bubbles. They later expand from the gas from the leavening when a recipe is baked, causing the dough or batter to rise. During baking, the stretched flour proteins (gluten) becomes rigid as the moisture evaporates from the heat of the oven, and sets the baked goods' structure. The viscoelastic properties of gluten provide the perfect combination of elasticity and rigidity by expanding with the gas while still holding its shape. No other grain has been able to replace this function of wheat in baking.

Flour's strength is determined by its gluten content and mixing -- both work in concert together: if mixed too much, the cake texture toughens or too little, the cake falls. If the gluten is too strong for a recipe, it toughens and may not rise. If there is too little gluten, the recipe will collapse when taken from the oven or be mushy. Or, if you have the right amount of gluten and stir it too much, your recipe will be tough and dry. The recipe will direct you on which type of flour to use, which corresponds to a gluten protein %. That's why when you substitute one flour type for another, the recipe is always affected no matter how much or little you stir the batter or dough.

Every recipe is written with a specific flour in mind to give the best results: Breads rely heavily on gluten for structure, cakes to a lesser extent, and cookies almost not at all. Gluten also allows you to roll out pastry into thin sheets that don't fall apart. 

Recipes commonly use all-purpose flour, which has a moderate gluten or protein content. For a lower gluten content with a more tender outcome, I use whole wheat pastry flour or cake flour.  High-gluten flours, such as bread and regular whole wheat, as well as a moderate one, all-purpose, are typically used in yeast breads where a strong framework is desirable. But, in cakes, quick breads and pastries, a high protein flour makes a tough baked good.

WHEAT FLOUR & GLUTEN:  

When flour is milled, it is classified according to the ratio of its gluten forming proteins to starch. The protein content of a flour affects the strength of a dough. Depending on the type of wheat and where and when it was planted, the resulting flour can be high-gluten (milled from hard winter wheat), low-gluten (from soft spring wheat), or moderate (a combination of the two). All-purpose flour in the North has a high protein count; the one sold in the South is low-protein. Hard wheat, mainly grown in Midwestern U.S. has a high protein content. Baked goods made from high-gluten flours have a firm crumb; low-gluten flours give more tender results, and goods made from flours with a moderate gluten content fall somewhere in between. 

The percent protein in flour is a factor when baking (so is altitude): Gluten gives a framework to a baked good by swelling as they absorb water, some flour types absorbing faster than others. A higher-protein flour absorbs more moisture than a lower protein flour. Baker's have blamed the difference in absorption on humidity which only makes a minute difference. Instead, a flour's protein level directly affects the ratio of wet ingredients to dry. 

For example, a batter made with 2 cups of high-protein flour absorb 1 cup of water to form a soft, sticky dough. The same recipe made with 2 cups low-protein flour and 1 cup water make a thick soup. It takes 1/2-cup more low-protein flour to get the same consistency as the high-protein flour. 

When recipes are written, one type of flour in used and the person baking it uses another. That's because they probably live in different areas of the country or their flour brand is milled in different places.

The more that the flour and moisture are stirred or handled, the more the gluten strands strengthen and toughen. That's why many recipes say not to overmix them.  Fat, which is not present in reduced-fat baking in traditional amounts, plays an important role in coating the proteins in flour, minimizing their contact with moisture, and shortening the gluten's development. Without the fat lubricator, the gluten strands form more readily. That is why it is very important to never overmix a reduced-fat batter. It's not just how you handle the batter or dough to prevent gluten formation, many ingredients also do the job of interfering with its development. For example, butter and shortening coat the flour strands and prevent moisture from reaching them, while sugar acts as a tenderizer because it attracts water away from the proteins in the flour.

Most bakers are very familiar with traditional 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.

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.

Fruit purees, especially applesauce, are often used as fat substitutes. The pectin from the fruit forms a film around the tiny air bubbles in the batter, similar to what occurs when you cream solid shortenings with sugar, but not as effectively. My favorite fruit puree for baking is unsweetened applesauce. Not only is it readily available but it is inexpensive and versatile because it doesn't impart any strong flavor to the final result. Applesauce contains more pectin than other fruit purees, which helps to retain the moistness of baked goods. Even if a recipe is flavored with another fruit puree, I always add a little applesauce as well. You'll see recipes here that use pumpkin, banana, and prune purees, among others. 

Sugar serves a number of roles. All sugar is an important and versatile food ingredient in baking recipes, other than merely providing sweetness and flavor:

Besides its pleasant sweetness, sugar performs a host of less-obvious and important functions in cooking, baking, candy-making and the like.

Flavor Enhancement—Sugar "potentates," blends and balances flavor components, much like a seasoning. For example, a pinch of sugar added to corn, carrots and peas produces a better-tasting product. In most tomato based products, such as barbecue, spaghetti, and chili sauces, sugar softens the acidity of the tomatoes and blends the flavors.

Solubility—Sugar is readily soluble in water. The ability to produce solutions of varying degrees of sweetness is important in many food applications, particularly beverages and confectionery. Sugar’s capacity to produce a supersaturated solution and then crystallize when cooled is the basis for rock candies. The wonderful variety of confectionery draws from the candy maker’s ability to vary sugar concentration, along with temperature and agitation, to produce different crystal sizes and textures.

Boiling Point Rise, Freezing Point Depression—In solution, sugar has the effect of lowering the freezing point and raising the boiling point of that solution. These are important properties in preparing frozen desserts and candy, respectively. In ice cream, for example, sugar’s ability to depress the freezing point slows the freezing process, promoting a smooth, creamy consistency. In shortening-based cakes, sugar raises, delays and controls the temperature at which the batter goes from fluid to solid, which allows the leavening agent to produce the maximum amount of carbon dioxide. The gas is held inside the air cells of the structure, resulting in a fine, uniformly- grained cake with a soft, smooth crumb texture.

Hydrolysis (inversion)—In food processing, hydrolysis decreases the tendency of sugar to crystallize from thick syrups or jellies.

Caramelization (thermal decomposition)—When sugar is heated to a sufficiently high temperature, it decomposes or "caramelizes." Its color changes first to yellow, then to brown, and it develops a distinctive and appealing flavor and aroma. The melted substance is known as caramel. The brown color of toasted bread is the result of caramelization.

Browning (Maillard reactions)—Color is also produced in cooking when sugars and proteins interact in complex ways. This is known as the browning (Maillard) reaction, important in candy making, baking and other processes.

Yeast Fermentation—Sugar is consumed by yeast cells in a thoroughly natural process called "fermentation." Carbon dioxide gas is released, and alcohol is produced, reactions vital to bread rising and baking and alcoholic beverage production.

Bodying/Bulking Agent—Sugar imparts satisfying texture, body, mouthfeel and bulk to many processed foods, such as ice cream, baked goods, icings, beverages and candy.

Texture Modification—Granulated white sugar and brown sugar are integral to the creaming process that incorporates air into batters. For example, as sugar is creamed with shortening in baked goods, the irregularities of the of the sugar crystals help create air pockets that contribute to a uniformly fine crumb structure. In gingersnaps and sugar cookies, the desirable surface cracking pattern is imparted when sugar crystallizes by rapid loss of moisture from the surface during baking.

Preservative—By binding water, sugar acts as a very effective, natural preservative. For example, the high sugar levels in jams, jellies and sauces make them more immune to the microorganism development common in thinner, high-moisture products like commercial applesauce. Sugar is the preferred sweetener in cereal coatings because of its ability to crystallize into a frosty surface forming a hard, continuous glaze. This protects the product from air and moisture, extending its shelf life.

Dispersant—In dry beverage, dessert and bakery mixes, sugar prevents lumping and clumping when the mix is hydrated.

Whipping Aid—In foam-type cakes, such as angel and sponge, sugar enables the creation of a light foam that serves as the basic structure of the cake.

Humectant—When the sucrose molecule is "inverted", by the application of heat, acids or enzyme, the resulting fructose (especially) and dextrose contribute a moistening property, desirable in such foods as icings, fudge, cakes, marshmallows, soft cookies, and so forth.

Microwave Properties—Sugar has unique dielectric properties that enable it to produce desired surface browning and crisping. Sugar can shield lower food layers from heating, as in microwavable ice cream toppings. Sugar can function as a control agent to minimize uneven heating. (from sugars.com)

Honey, molasses, maple and corn syrup are liquid sweeteners, and while they do provide sweetness, they do not cream well, just as liquid vegetable oils can't substitute for solid shorteners.  

Sugar plays many important roles in baking recipes:   

All liquids fall into one of three categories having to do with what is called pH. That is to say that liquids are either neutral, like water, acid like citrus fruits and vinegar, or alkali (sometimes called "basic") like ammonia, lye (which is in soaps), or soda.

Liquid in a recipe may be milk, water, fruit juices, potato water and even eggs. The amount of liquid determines whether a "dough" or "batter" is produced. Liquids also serve to hydrate the flour, for gluten formation, and to hydrate the starch, for gelatinizing, which results in formation of the basic structure of a baked product. Liquids also dissolve the sugar and salt, making possible the leavening action of baking powder, soda and acid, or growth of yeast. Liquids contribute moistness to the texture and improve the mouthfeel of baked products. When water vaporizes in a batter or dough, the steam expands the air cells, increasing the final volume of the product.

Milk contains fats and proteins in a solution (water) and contributes valuable nutrients to baked goods. It helps browning to occur and adds flavor. When making yeast dough, milk should be scalded and cooled before adding to other ingredients. This is done to improve the quality of the dough and the volume of the bread.

Juice may be used as the liquid in a recipe, but do not substitute milk with juice and vice versa. Because fruit juices are acidic, they are probably best used in baked products that have baking soda as an ingredient.

The three basic leavening gases commonly found in baking recipes are air from whipped eggs, or beating, stirring, creaming and kneading; water vapor or steam from liquids; carbon dioxide from chemical leaveners, baking soda and baking powder; and yeast, both packaged and from a starter (sourdough or sponge). In many baked items, all three of these agents participate in the leavening process. 

A leavening agent provides a source of gas to the recipe called carbon dioxide. When moistened, fermented and/or heated, it expands the millions of air bubbles previously created in a batter or dough from mixing, creaming, beating, folding, whipping and kneading trapped in the structural framework by the gluten strands. If the batter is over mixed or not baked promptly, the gas will escape and the final recipe will have poor texture and low volume.

During mixing, some air is always incorporated. Although it is usually not the major leaven, it plays an important role. Beaten eggs aerate recipes due to their ability to foam and by contributing water for steam, such as with sponge or angel food cakes. A foam is created by incorporating air into a mixture through "beating". Whole eggs, egg whites or egg yolks can each be beaten into a foam, with whites having the potential of producing the most. Air is also incorporated into cakes when fat and sugar are beaten together. 

Steam is produced when water, in the recipe, is heated to 212 degrees F by baking. Most batter recipes are to some degree leavened by steam. To get maximum steam production in a system, a 1:1 ratio of liquid to flour is needed, which recipes already have. As the amount of water relative to flour decreases, less leavening from steam occurs. In steam-leavened products, the changes that occur in the volume occur at the end of the baking cycle. Popovers are a good example of the rapid volume expansion which leavens a product late in the baking period.

Chemical leaveners include baking soda (bicarbonate of soda) which produces carbon dioxide gas when moistened and/or heated. The pH (the amount of acid or base) of the baked product is affected by the leavener. They are alkaline, and when they comes in contact with an acidic ingredient like applesauce, buttermilk, honey, brown sugar, molasses and lemon juice, the alkali/acid combination creates carbon dioxide. In some recipes, depending on the quantity of acidic ingredients included, a combination of baking soda and baking powder is used for better flavor and texture.

Baking powder, another chemical leavener, does not need an acidic ingredient to release its leavening power. Double-acting baking powder begins releasing carbon dioxide as soon as it is moistened, and again when heated in the oven. If there is not enough acid, color and flavor changes may appear. The color of a more alkaline gingerbread is darker due to the effects of pH on the pigment; there is also a less pronounced molasses flavor. An extremely alkaline product may even taste bitter or soapy.

Some baking powders include sodium aluminum sulfate, but there are aluminum-free baking powders that work just as well, and I prefer them. Look for a brand like Rumford's at natural food stores or many supermarkets.

Yeast, used in bread baking, is either packaged or created through a sourdough or sponge starter. Yeast, a single-celled live organism, feeds off of the flour's starches and sugars, moistened by liquid, usually water, fermenting it to carbon dioxide and ethanol (alcohol). Carbon dioxide is the primary leavening gas that makes yeast breads rise. The alcohol evaporates during baking, leaving behind flavor.

The trapped gas which raised the dough escapes from the bubbles as the recipe cools.

Not every recipe includes a thickener, although flour certainly has thickening attributes. But many fruit fillings include cornstarch to thicken the juices. I occasionally use tapioca as a thickener, as well.  Eggs are used as thickeners and are used in such recipes as custards, puddings and sauces.  

Flavorings enhance a baked good's aroma and taste. Salt, sugar or an acidic ingredient, such as buttermilk, cocoa powder or lemon juice are the three most important ones used to give interest to a recipe (a wide variety of flavorings and other ingredients add greatly to a recipe, too).

Butter also plays an important flavoring role. The butter in traditional recipes contributes to and carries flavors throughout the batter. Even more important, butter has flavor of its own that, when it interacts with sugar, is responsible for the caramelized baked taste we associate with baked goods. In reduced-fat baking, the flavorings must be increased to compensate for the reduction in butter.

Some material: c Sarah Phillips, The Healthy Oven Baking Book, Doubleday, 1999