Starch is a form of carbohydrate that is a white, tasteless, odorless food substance found in potatoes, rice, corn, wheat, cassava, and many other vegetables. In grains, starch is contained in a husk of cellulose. Precisely speaking, starch is basically a complex carbohydrate called polysaccharide and is composed of several thousand glucose elements. All green plants produce starch and store it as energy. Starch contains amylase and amylopectin and is the most important resource for food for humans.
Uncontaminated starch is basically white in appearance, does not have any flavor and comes in an unscented powdered form. Pure starch is not soluble either in water or in alcohol. As discussed earlier, starch encloses two varieties of molecules - linear and helical amylose and the divided amylopectin. Based on the type of plant, starch usually encloses 20 per cent to 25 per cent amylose and about 75 per cent to 80 per cent amylopectin. It may be noted here that glycogen, the glucose stored by animals, is also a more branched variety of amylopectin.
When starch is dissolved in warm water it produces a wheat paste and this can be used for various purposes, including a thickening, stiffening or adhesive agent. The wheat starch was initially prepared and used by the ancient Egyptians to stiffen cloth as well as while weaving linen and also probably to paste papyrus. On the other hand, the Romans used the wheat paste in the manufacture of cosmetic ointments, to powder their hair as well as a thickener for sauces. In Persia and India, people used starch to prepare cuisines comparable to the gothumai wheat halva. With the invention of paper, the Chinese used rice starch to remedy the plane of the paper.
All green plants utilize the light energy to produce glucose from carbon dioxide by means of photosynthesis. Plants usually store this glucose or their energy as starch granules in plastids like chloroplasts and particularly amyloplasts. During the closing stages of their growing season, this starch usually mounts up in branches of trees close to the buds. In addition, the fruits, seeds, rhizomes and also the tubers stock up the starch for use to prepare for the subsequent growing season.
Generally, glucose is soluble in water, has a strong affinity for water, binds sufficient water and uses a lot of space. However, when glucose is in the form of starch, it is not soluble in water or alcohol and can be stored in a much more condensed manner.
As starch is considered to be preserved sugar or energy for the plants, glucose molecules bind to starch through easily hydrolyzed alpha bonds. A similar binding also occurs in the animal reserve polysaccharide glycogen. Such bonds are, however, in disparity with a lot of structural polysaccharides, such as cellulose, chitin and peptidoglycan that are attached by means of beta-bonds and are comparatively more resilient to hydrolysis or chemical decomposition by reaction with water.
Starch is the most essential carbohydrate included in the diet taken by the humans and it is found in numerous staple food consumed by us. The most prominent sources of starch consumed by people all over the world are contained in rice, wheat, maize, potatoes and cassava. On the other hand, foodstuffs that contain starch and are extensively consumed by people worldwide include bread, cereals, pasta, porridge, noodles, pancakes and tortilla. Depending on the local climatic and geographical conditions, other sources of starch used in food preparation include barley, oat, buckwheat, arracacha, arrowroot, rye, breadfruit, banana, colacasia, canna, katakuri, kudzu, oca, malanga, polynesian arrowroot, sorghum, sweet potato, sago, water chestnut, taro as well as yams. In addition, chestnuts and edible beans like lentils, mung bean, favas and peas also enclose high percentage of starch.
In fact, the starch industry hauls out and processes starch present in seeds, roots and tubers by means of several measures, such as sieving, drying and wet grinding. Presently, the most common processed starches available in the market include wheat, tapioca, cornstarch and potato starch. In a smaller degree, the sources for these processed starches include rice, sago, mung bean and sweet potato. Traditionally, people also commercially extracted starch from Florida arrowroot. As of now, starch is extracted from over 50 different kinds of plants all over the world.
A modified food starch is a type of starch that has been customized or treated with chemicals with a view to allow the starch to work appropriately under different conditions, such as extreme heat and/ or shearing that are very often come across during processing or situations that may occur during storage of the starch, such as cooling.
It may be mentioned here that pre-cooked starch is often used as a thickener by adding cold water to it. Such pre-cooked starch is known as pregelatinized starch. Alternately, starch needs to be heated before thickening or to 'gelatinize'. The precise temperature at which starch needs to be heated depends on the kind of starch being used.
In addition to the above uses, starch is also used as an excipient or a binder used in preparing medications to take the shape of tablets. There is also a type of starch known as resistant starch which is not digested in the small intestines of healthy people.
Starch has extensive industrial use as an adhesive or bonding agent. All over the world, apart from the food processing industry, the paper manufacturing industry is the largest user of starch and uses up millions of metric tons of the substance every year. For instance, the starch content in a usual sheet of copy paper may be as much as eight per cent. It may be noted that the paper industry consumes both chemically modified as well as unmodified forms of starch. Cationic starches having a positive charge binding to the starch polymer are used in the wet part of the paper manufacturing process that is known as 'wet-end'. Such substances obtained from starch connect with the anionic or negatively charged paper fibres or cellulose and inorganic fillers. Along with withholding (retention) and internal sizing agents, the cationic starches aid in providing the requisite strength to the final paper sheet known as the 'dry strength'.
During the dry end of the papermaking process the paper mesh is rewetted with a solution prepared with starch. This procedure is known as surface sizing. The starch industry actually depolymerize the starches used in this process either using chemicals or enzymes to produce oxidized starch. A range of mechanical processes known as size press are employed to apply the size-starch solutions to the paper web or mesh. The surface starches along with the surface sizing agent pass on added potency to the paper mesh and, in addition, afford water hold out or 'size' the paper for advanced printing features. In addition to providing additional strength and better printing properties to paper, starch is also used by the paper manufacturing industry for paper coating. For this, a special formulation comprising a blend of pigments, binders and thickeners (starch) are needed. When paper is coated, it becomes additionally smooth, hard, and white as well as has a surface shine and, therefore, augments its printing properties.
The use of sophisticated, value added starches in paper products is even more obvious when one takes into account the wide range of applications in that industry. Starches are used to provide greater strength to tissues and paper towels, and they allow a greater use of recycled paper in liner board and cardboard.
Apart from the food industry and the papermaking industry, worldwide the next most extensive use of starch is as corrugated board adhesives. Bonding agents prepared with starch are mainly founded in unmodified indigenous starches and a number of adhesives, such as caustic soda and borax. In this case, a portion of starch is gelatinized to prepare a catalytic suspension or slurry of uncooked starches and avoid sedimentation. This dense adhesive is known as Stein Hall adhesive and it is used on the tips of the fluting. This fluted paper is next compressed to paper known as a liner. Subsequently, it is dehydrated under extreme heat that results in the remaining uncooked starch adhesive to swell or gelatinize. Such a gelatinizing of the adhesive makes the glue a prompt and strong binding agent useful for the production of corrugated boards.
Besides these non-food and industrial uses of starch, it is also extensively used by the construction industry. In the construction industry, starch is used in the process to manufacture gypsum wall boards. Both chemically modified as well as unmodified starch forms are added to the stucco (an exterior finish for masonry or frame walls) mainly containing gypsum. In this formulation, top and bottom heavyweight sheets of paper are used and the procedure is let to heat and treat to take shape of the ultimate firm wall board. In addition, starches also function as glue for the treated gypsum rock with paper layering as well as offer firmness to the board.