Types of starch

Grain starches have some common characteristics:

• granules are medium sized
• contain significant amounts of lipids and protein
• have increased structureal stability and thus require higher temperatures to gelate
• have distinct "cereal" flavor
• contain a high proportion of moderately long amylose, so they thicken and congeal quickly

Examples:

• Wheat flour is only 75% starch, which makes it a less efficient thickener than cornstarch or potatoe starch. It adds a distinct wheat flavor to the sauce. Common rule of thumb is to add 1.5 times as much flour as starch.
• Cornstarch is practically pure starch. It is an efficient thickener, but it absorbs odors and develops flavors during processing.
• Rice starch have the smallest granule size and produce a fine texture. It is seldom available in Western markets.  

Tuber and root starches come from moist underground storage organs and have the following characteristics:

• larger granules that retain more water molecules
• cook faster
• release starch at lower temperatures
• contain less amylose, but the amylose chains are four times longer than cereal starches
• readily gelate
• do not require precooking to improve flavor

Examples:

• Potato starch has very large granules and very long amylose chains. Stringiness and initial graininess in the sauces are notable. However, the granules are fragile and fragment easily. It is unusual in that it contains a large number of phosphate groups that carry a weak electric charge and cause the chains to repel each other. This repulsion keeps starch chains evenly dispersed in a sauce and prevents them from congealing when cool.
• Tapioca is derived from the tropical plant casava (aka manioc). It does not develop any strong aromas and it is prized for its neutral flavor. It is mostly used in puddings.
• Arrowroot starch has smaller granules than potato or tapioca starches. Its gelation temperature is higher, more comparable to that of cornstarch.

There are also a number of modified starches available. Plant breeders have developed "waxy" varieties of corn that contain little or no amylose and nearly all amylopectin. Waxy starches make sauces and gels that resist congealing. Manufacturers have also used physical and chemical treatments to produce starches that readily absorb cold water or disperse in liquids without cooking. They have also added cross-linking chains, made them fat-soluble, and have given them other qualities that make them more effective emulsion stabilizers. Such starches are listed as "modified starch."

Starch

Starch is made up of thousands of glucose molecules linked up together. There are two structures of starch molecules: amylose and amylopectin. Amylose molecules are long and straight; amylopectin molecules are short and branched. Of the two, amylose is a more effective thickener than amylopectin as its long chains tangle with each other more readily and slow the motion of other molecules in the surrounding fluid.

When starch is mixed into cold water, its granules only absorb a limited amount of water and sink. Nothing happens. When starch is mixed into hot water, however, the granules absorb large amounts of water and swell up. As they do so, weak regions of the granules become disrupted; stronger regions lose their organized structure and become water-containing meshworks of long molecules. In other words, the granules become individual gels. A cloudy suspension of granules becomes more translucent as individual starch molecules become less packed and no longer deflect as many rays of light.

The temperature at which starch begins to behave in this manner is called the gelation range, usually around 120-140 degrees F/50-60 degrees C. 

Thickening of a sauce/dish with starch occurs as the granules become so saturated with water that they begin to leak amylose and amylopectin molecules into the surrounding liquid. The long amylose molecules form a fishnet of sorts that entraps pockets of water and blocks the movement of the swollen starch granules.

After reaching its thickest consistency, the starch-water mixture begins to thin out again. As more amylose leaks into the water, the starch granules break or otherwise become smaller. Heating close to boiling point, vigorous stirring, continued heating long after thickening, and addition of an acid accelerate the thinning process.

The cooling that follows allows amylose molecules to form stable bonds and water molecules settle in the pockets between the starch chains. As a result, the sauce/liquid gets thicker. If the temperature drops low enough, the starch particles begin to congeal. It is important to judge the consistency of a sauce/dish at serving temperatures, not at cooking temperatures. The best way to predict the final texture of a sauce is to pour a spoonful into a cool dish and sample it.

Staling

Though staling seems to involve the loss of moisture as the bread becomes dry, hard, and crumbly, it is more of an effect of starch retrogradation than net water loss. In 1852, Frenchman Jean Baptiste Boussingault conducted an experiment where bread was hermetically sealed to prevent moisture loss, yet it still became stale. Staling is now understood to be a manifestation of recrystallization and water migration out of the starch granules into the surrounding gluten network. The retrogradation of simple amylsoe molecules leads to the hardening of the bread. Branched amylopectins also retrograde, but do so more slowly. Certain emulsifying agents have been found to retard staling substantially. Manufacturers have added these to mass-produced bread dough for the last 50 years. True buttermilk and eggs have the same effects.

As long as the loaf is not too old, or has been refrigerated, staling can be reversed by reheating the bread above the gelation temperature of starch, 140 degrees F/60 degrees C. Toasting makes the interior of bread soft. A loaf of bread can be refreshed by heating in the oven.

Refrigerating bread can speed up the staling process by as much as 6 days! If bread is to be used in a couple of days, it is best to store at room temperature in a bread box or paper bag to reduce moisture loss. If bread is to be kept longer, it is best to wrap it in plastic or foil, and freeze it. Refrigerate bread only if it is to be toasted or reheated before consumption.

Bread tends to dry out before becoming spoiled. Still, bread that is kept at room temperature in a plastic bag allows moiture from the staling starch granules to collect on bread surfaces. The most commonly found toxic molds in spoiled bread are Aspergillus, Penicillium, Mucor, and Monilia sitophila.

Making bread

There are four main steps in making yeast bread.

Mixing

The first step is to mix the ingredients-flour, water, yeast, and salt-together. During mixing, starch granules absorb water and enzymes digest starch into sugars. The yeast feed on the sugars and produce carbon dioxide and alcohol. Glutenin proteins begin to react into gluten. Mixing can be done by hand, with an electric-mixer, or in a food processor. The later offer the advantage of limited oxygen exposure, which in excess can alter the pigments and flavor of the bread.

Kneading

The second step involves dough development. Through kneading, the dough is stretched, folded over, and compressed over and over. This manipulation strengthens the gluten network. It orients proteins to lay side by side which encourages them to form bonds. Overdevelopment of gluten causes bonds to break and turns the dough sticky and inelastic. Bewrae of overdeveloping the dough when kneading mechanically.

Kneading also aerates the dough. The more air pockets formed, the finer the texture of the final bread. Some breads call for minimal kneading. This results in fewer and larger air cells and their corresponding irregular and coarse texture. The gluten is weak and less developed, but it continues to develop through fermentation and can rise to an airy, tender crumb.

Fermentation (Rising)

The third step is fermentation, where the dough is set aside for yeast cells to produce carbon dioxide. As they do so and the carbon dioxide diffuses into air pockets, the dough rises. Yeast have the highest metabolic activity at 95 degrees F. This produces the greatest amount of carbon dioxide and metabolic by-products, some of which can be sour and unpleasant. For quick rising, it is suggested to keep fermentation at 80 degrees F for a couple of hours. Lower temperatures may extend the fermentation an hour or two, but generally produce more desirable flavors.

The end of fermentation is signaled by the dough's volume and the gluten matrix. Fully fermented dough is about twice its original size and has been stretched to its limit, so a finger impression remains when touched. Fermented doughs feel softer and are easier to handle than freshly kneaded dough.

Fermentation can be retarded by storing dough in the refrigerator. Yeast take 10 times as long to rise bread in cool temperatures. Retarding fermentation not only allows bakers to break up the work of making bread, but it has useful effects too. Long, slow fermentation allows greater flavor development by the yeast and bacteria in the dough. Cold dough handles easier without as much loss of leavening gas. The cycle of cooling and rewarming redistributes gases and promotes the development of a more open and irregular crumb structure.

Baking

The last step is baking. The kind of oven where a bread is baked has an important influence on the qualities of the finished loaf. Traditional bread ovens were made out of clay, stone, or brick. The baker preheated the oven by wood fire to temperatures up to 900 degrees F. The domed roof stored heat and radiated it down onto the loaves. The temperature declines during baking. The dough expands early on. The bread benefits from color and flavor development due to enhanced browning reactions.

Modern metal ovens are not ideal for bread making. The maximum cooking temperature is usually around 500 degrees F. Heat cannot be stored as well within their walls, so modern ovens maintain a heat source of gas or electrical elements. The necessary venting does not allow gas ovens to retain the loaves' steam. Some bakers use ceramic baking stones or ceramic oven inserts that mimic the traditional oven. The oven is preheated to its maximum temperature and provide more intensive even heat during baking.

Steam is important in the early baking stages because it increases the rate of heat transfer from the oven to the dough. As steam condenses onto the dough surface, it forms a thin film of water that temporarily prevents it from drying out into a crust. By doing so, it encourages the initial rapid expansion of the loaf. The hot water film eventually dries into an attractive glossy crust. Professional bakers often inject steam in the first minutes of baking. At home, one can spray water or throw ice cubes into the hot chamber to improve oven spring and crust gloss.

There are three stages of baking plus cooling.

Early baking: Oven Spring refers to the first 6-8 minutes of baking. Heat transfers first from the oven floor to the bottom of the dough, and to the top from the hot air and oven ceiling. Heat moves from the surface through the dough slowly through the gluten matrix; and rapidly through the gas network.  Alcohol and water in the dough vaporize. The gas cells expand, and the dough rises. The better leavened the dough, the faster it cooks.

Mid-baking: The dough begins to transform into a sponge when the interior temperature of the dough reaches 155-180 degrees F/68-80 degrees C. At this range, the gluten proteins form strong cross-link bonds, the starch granules gelate, and the amylose molecules leak out. Gas pressure builds and ruptures the walls, turning the closed network of bubbles into an open network of pores similar to a sponge.

Late baking: Starch continues to gelate thoroughly. Continued cooking encourages surface browning reactions that improve color and flavor. Though limited to the crust, these reactions affecte the flavor of the whole loaf because their products diffuse downward. Bread is done when its crust has browned and the inner structure has set. Fully cooked bread feels light and hollow.

Cooling

The temperature varies inside a loaf immediately after being removed from the oven. During cooling, the differences even out.  Most moisture loss occurs at this stage as moisture in the interior diffuses outward. Small rolls dry out the most, while large loaves the least. As temperature declines, starch granules become firmer, which later allows even slicing. This firming continues over the next day and starts the process of staling.

Starch

The word starch comes from a German root that means "to stiffen or make rigid." While starch has been used in paper making and textiles to stiffen, it has also served the same purpose bread making. Gluten proteins make only 10% of flour by weight, whereas starch makes up 70%. Starch granules absorb water, swell, and set to form the rigid bulk. They account for more than half the volume of dough. Though starch provides gives structure to bread, it also tenderizes it by penetrating and breaking up the gluten network. As carbon dioxide is produced, the rigidity of starch-formed walls stops the expansion of the bubbles. Pressure forces the water vapor inside to pop the bubbles and allow carbon dioxide to escape. As it does, a spongy network is left behind. This accounts for the crumbly texture of bread.

Starch is particularly important in batters where gluten proteins are too dispersed in water and sugar to contribute to the solidity of the cake.

Dough, batter, and texture

Wheat flour has characteristic liveliness and cohesiveness that set it apart from other cereal doughs. These characteristics make it possible for light, delicate loaves, flaky pastries, and silken pastas.

There are three basic elements at work: water, the flour's gluten proteins, and its starch granules. When integrated, these elements create cohesive mass.

Mixture of flour and water is called a dough or batter depending on the relative portions of the ingredients. According to Harold McGee, "doughs contain more flour than water and are stiff enough to be manipulated by hand. All the water is bound to the gluten proteins and to the surfaces of the starch granules, which are embedded in the semisolid gluten-water matrix. Batters contain more water than flour and are loose enough to pour. Much of the water is free liquid, and both gluten proteins and starch granules are dispersed in it." Once cooked, the starch granules absorb water, swell, and create a permanent sponge-like structure made up of millions of tiny air pockets.The term crumb refers to this network. Crust is the outer surface.

The texture of breads and cakes is light and tender because the protein-starch mass is divided up by millions of tiny bubbles. Pastries are flaky and tender because the protein-starch mass is interrupted by hundreds of layers of fat.

Bread 101

What qualifies as bread? On the most basic level, bread is the result from cooking a mixture of milled grains and water. Here is a quick run down of the most common ingredients in bread:

• Flour - Wheat flour is most commonly used in raised bread because it contains two proteins, glutenin and gliadin, which form gluten when combined with water. As the baker kneads the dough, the gluten develops and becomes elastic. This elasticity allows the incorporation of carbon dioxide gas into the dough.
• Starch is a carbohydrate that makes up 70% of the flour by weight. Starch granules release sugars that the yeast feed on. Starch reinforces gluten and absorbs water during baking, helping the gluten contain the carbon dioxide.  
• Water is the most important liquid in bread. It dissolves and activates yeast and blends with the flour to create gluten.  
• Yeast is a live, single-cell fungus that begins feeding on the sugars in flour and releases the carbon dioxide that makes bread rise. Yeast also adds many of the flavors and aromas associated with bread.
• Baking powder and baking soda are chemical leavenings that participate in the reactions between acidic and alkaline compounds that produce the carbon dioxide necessary to inflate dough or batter. These chemical leavenings act much faster than yeast, and are best used in quick breads.
• Salt slows rising time, which allows the flavor of the dough to develop. Salt also adds structure to the dough by strengthening the gluten, which keeps the carbon dioxide bubbles from expanding too rapidly.
• Eggs add food value, color, and flavor. They also make the crumb fine and the crust tender. Eggs add richness and protein.
• Fat in the form of butter, margarine, shortening or oil add flavor and moisture to bread. Fat slows moisture loss and helps bread stay fresh longer.
• Other liquids such as milk, buttermilk, cream or juice may be added for flavor or to enhance texture. Only add warm liquids to dry ingredients. Too cool liquids slow or stop yeast action. Too hot liquids destroy the yeast and prevent bread from rising.
• Sweetners such as sugar, brown sugar, honey, molasses, jams, and dried fruits may be used to add flavor and color to the crust.