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    Starch Background Information

    Definition

    Starch (amylum) is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.

    Basics

    Starch is a complicated carbohydrate. Pasta, potatoes, and other starchy foods are made out of carbohydrates. It is made of lots of sugars. Plants use it to keep energy.

    Starch in food: Some good food sources of starch are cereals, breads, potatoes, grains, peas, and beans. Starch is also used for thickening sauces in cooking. It is a white powder that is tasteless and odourless. It can also be used for thickening cold foods when they are pre-cooked.

    Laundry starch: There are also different kinds of starch like Laundry Starch which is a gives clothing a smooth and crisp feel. Sweat and dirt from a person’s wrist and neck would stick to the starch on the clothes, and not on the fibers of the clothes and would wash away along with the starch. Then, after each laundry load, the starch would be used again.

    Starch glues are used in wood, cotton, and bonding of paper. Animals and humans have amylase, so they can digest starch. Starch was used in Europe in the 16th and 17th centuries to stiffen the collars and ruffs of the fine linen which surrounded the neck. Starch also requires heat to thicken. The actual temperature depends on the type of starch.

    Topics of Interest

    Starch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. Starch is produced by all green plants as an energy store. It is the most important carbohydrate in the human diet and is contained in such staple foods as rice, wheat, maize (corn), potatoes and cassava.

    Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or alcohol. It consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75 to 80% amylopectin. Glycogen, the glucose store of animals, is a more branched version of amylopectin.

    Starch can be used as a thickening, stiffening or gluing agent when dissolved in warm water, giving wheatpaste.

    Industrial applications: Papermaking, adhesives and glues, clothing starch (laundry starch), textile chemicals printing industry, bioplastics, body powder, oil exploration, biofuel ethanol, hydrogen production.

    Name: The word "starch" is derived from Middle English sterchen, meaning to stiffen. "Amylum" is Latin for starch, from the Greek "amulon" which means "not ground at a mill". The root amyl is used in biochemistry for several compounds related to starch.

    History: Wheat starch paste was used by Egyptians to stiffen cloth and during weaving linen and possibly to glue papyrus. Romans used it also in cosmetic creams, to powder the hair and to thicken sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. In China, with the invention of paper, rice starch was used as a surface treatment of the paper.

    In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts and especially amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing season.

    Glucose is soluble in water, hydrophilic, binds much water and then takes up much space; glucose in the form of starch, on the other hand, is not soluble and can be stored much more compactly.

    Since starch is a reserve sugar for the plant, glucose molecules are bound in starch by the easily hydrolyzed alpha bonds. The same type of bond can also be seen in the animal reserve polysaccharide glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose and peptidoglycan, which are bound by beta-ties and are much more resistant to hydrolysis.

    Biosynthesis: Plants produce starch by first converting glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase. This step requires energy in the form of ATP. The enzyme starch synthase then adds the ADP-glucose via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP and creating amylose. Starch branching enzyme introduces 1,6-alpha glycosidic bonds between these chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes some of these branches. Several isoforms of these enzymes exist, leading to a highly complex synthesis process.

    Starch molecules arrange themselves in the plant in semi-crystalline granules. Each plant species has a unique starch granular size: rice starch is relatively small (about 2μm) while potato starches have larger granules (up to 100μm). Although in absolute mass only about one quarter of the starch granules in plants consist of amylose, there are about 150 times more amylose molecules than amylopectin molecules. Amylose is a much smaller molecule than amylopectin.

    Starch becomes soluble in water when heated. The granules swell and burst, the semi-crystalline structure is lost and the smaller amylose molecules start leaching out of the granule, forming a network that holds water and increasing the mixture's viscosity. This process is called starch gelatinization. During cooking the starch becomes a paste and increases further in viscosity. During cooling or prolonged storage of the paste, the semi-crystalline structure partially recovers and the starch paste thickens, expelling water. This is mainly caused by the retrogradation of the amylose. This process is responsible for the hardening of bread or staling, and for the water layer on top of a starch gel (syneresis).

    Hydrolysis: The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases

    Alpha-amylases are found in plants and in animals. Human saliva is rich in amylase, and the pancreas also secretes the enzyme. Individuals from populations with a high-starch diet tend to have more amylase genes than those with low-starch diets; chimpanzees have very few amylase genes. It is possible that turning to a high-starch diet was a significant event in human evolution.

    Beta-amylase cuts starch into maltose units. This process is important in the digestion of starch and is also used in brewing, where the amylase from the skin of the seed grains is responsible for converting starch to maltose (Malting, Mashing).

    Digestibility: Digestive enzymes have problems digesting crystalline structures. Raw starch will digest poorly in the duodenum and small intestine, while bacterial degradation will take place mainly in the colon. Resistant starch is starch that escapes digestion in the small intestine of healthy individuals. In order to increase the digestibility, starch is cooked. Hence, before humans started using fire, eating grains was not a very useful way to get energy.

    Dextrinization: If starch is subjected to dry heat, it breaks down to form pyrodextrins, in a process known as dextrinization. Pyrodextrins are brown in color. This process is partially responsible for the browning of toasted bread.

    Starch as food: Starch is the most important carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are rice, wheat, maize (corn), potatoes and cassava. Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta, porridge and tortilla.

    Depending on the local climate other starch sources are used for food, such as arrowroot, arracacha, buckwheat, barley, oat, millet, rye, banana, breadfruit, canna, colacasia, katakuri, kudzu, malanga, oca, polynesian arrowroot, sago, sorghum, sweet potato, taro, water chestnut and yams. Chestnuts and edible beans, such as favas, lentils, mung bean and peas, are also rich in starch.

    The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, sieving and drying. Today, the main commercial refined starches are cornstarch, tapioca, wheat and potato starch. To a lesser extent, sources include rice, sweet potato, sago and mung bean. Historically, Florida arrowroot was also commercialized. Still starch is extracted from more than 50 types of plants.

    A modified food starch is a starch that has been chemically modified to allow the starch to function properly under conditions frequently encountered during processing or storage, such as high heat, high shear, low pH, freeze/thaw and cooling.

    As an additive for food processing, food starches are typically used as thickeners and stabilizers in foods such as puddings, custards, soups, sauces, gravies, pie fillings, and salad dressings, and to make noodles and pastas.

    The Iodine test is used to test for the presence of Starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with starch producing a blue black color.

    Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents (removing the cell walls); they are used as food additives or flavourings, or as nutrients for bacterial culture media. They are often used to create savory flavors and umami taste sensations. Monosodium glutamate (MSG) is used for umami, but has no flavor. Yeast extract, like MSG, often contains free glutamic acid. Yeast extracts in liquid form can be dried to a light paste or a dry powder. Glutamic acid in yeast extracts are produced from an acid-base fermentation cycle, only found in some yeasts, typically ones bred for use in baking.

    Flour contains high proportion of starches, which are complex carbohydrates also known as polysaccharides. Leavening agents are used with some flours, especially those with significant gluten content, to produce lighter and softer baked products by embedding small gas bubbles.

    The dry heat of baking changes the form of starches in the food and causes its outer surfaces to brown, giving it an attractive appearance and taste, while partially sealing in the food's moisture. The browning is caused by caramelization of sugars and the Maillard reaction. Moisture is never really entirely "sealed in", however; over time, an item being baked will become dry. This is often an advantage, especially in situations where drying is the desired outcome, for example in drying herbs or in roasting certain types of vegetables. The most common baked item is bread. Variations in the ovens, ingredients and recipes used in the baking of bread result in the wide variety of breads produced around the world.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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