Sucrose is by far the most common sugar available in the market place. Regardless of whether it has been refined from sugarcane or from sugar beets, sucrose is a comparatively inexpensive and widely acceptable sweetener for use in the home. During the production of table sugars, juice is squeezed from the sugarcane the sugar is carried with the liquid or, in the case of sugar beets; strips of the beets are soaked to extract sugar. The next step is clarification to eliminate extraneous substance prior to crystallization of the end product, sugar (McWilliams, 1997).
Monosaccharides are the simplest form of carbohydrates. They consist of one sugar and are usually colorless, water–soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose, galactose, and ribose. Monosaccharides are the building blocks of disaccharides like sucrose (common sugar) and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports a hydroxyl group (except for the first and last) is chiral, giving rise to a number of isomeric forms all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but they have different chemical and physical properties (De Man, 1999).
A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides. ‘Disaccharide’ is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide) (Meyer, 1968).
It is formed when two sugars are joined together and a molecule of water is removed. For example, milk sugar (lactose) is made from glucose and galactose whereas cane sugar (sucrose) is made from glucose and fructose. The two monosaccharides are bonded via a dehydration reaction (also called a condensation reaction) that leads to the loss of a molecule of water. The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties. Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting (De Man, 1999).
Polysaccharides are polymers made up of many monosaccharides joined together by glycosidic linkages. They are therefore very large, often branched, molecules. They tend to be amorphous, insoluble in water, and have no sweet taste (De Man, 1999).
Polysaccharides are also called glycan the form in which most natural carbohydrates occur. Polysaccharides may have a molecular structure that is either branched or linear. Linear compounds such as cellulose often pack together to form a rigid structure; branched forms generally are soluble in water and make pastes (Meyer, 1968).
When all the constituent monosaccharides are of the same type they are termed homopolysaccharides; when more than one type of monosaccharide is present they are termed heteropolysaccharides (Miller, 1998)
Action of Yeast in Carbohydrates
The simple sugar, glucose, obtained during these transformations is used by the yeast to generate carbon dioxide and alcohol. During the fermentation process, most of the starches used are the ones damaged during the milling process. Because the particles are damaged, they can easily absorb water during the dough making process. This water contact triggers the enzymatic activity. A non-damaged particle of starch will only retain water at its periphery and not inside the particle itself (Lowe, 1955).
Yeast exhibits a variable preference for different sugars. It readily assimilates four sugars, namely, sucrose (after hydrolysis to glucose and fructose by yeast invertase or sucrase), glucose, fructose, and maltose (after hydrolysis to glucose by yeast maltase). In yeasted doughs, an increase in maltose occurs during the first stages of fermentation, until the initial supply of glucose and fructose is exhausted, after which the maltose content gradually declines (De Man, 1999).
Since the second stage of fermentation involves the conversion of maltose into ethanol and carbon dioxide, the behavior of this sugar in the fermentation process is of some significance. This is especially the case since different yeast strains have been shown to vary in their maltase activity. The rate of maltose fermentation by yeast also has been shown to be influenced by pH to a much greater degree than is true of glucose fermentation (De Man, 1999).
The main simple sugars, glucose and fructose, represent about 0.5% of the flour. Yeast can directly assimilate them by penetration of the cell membrane. Simple sugars are transformed into alcohol and carbon dioxide by zymase, an enzyme naturally present in yeast cells. Because of this easy absorption, these sugars are the first ones used in the fermentation process. Their consumption takes place during the first 30 minutes or so at the beginning of the fermentation process (Zapsalis, 1985).
A humectant is a substance used primarily in foods and cosmetic products to help retain moisture. These substances are called hygroscopic, which means that they are able to absorb ambient water. Some humectant additives are beneficial when consumed or used (McWilliams, 1997).
Sugars, to varying degrees, are able to attract and hold water. The capability, known as hydroscopicity, can be useful in maintaining the freshness of some baked products, but can be a source of potential problems in texture when the relative humidity is high. An elevation temperature also increases the absorption of moisture from the atmosphere (McWilliams, 1997).
Caramelization is one of the most important types of browning processes in foods, together with Maillard reactions and enzymatic browning. Carmelization leads desirable colour and flavour in bakery’s goods, coffee, beverages, beer and peanuts. Undesirable effects of caramelization are for example burned sugar smell and blackening (De Man, 1999).
When sugars are heated to such intense temperatures they tend to participate in series of chemical reactions which will eventually take place and can ultimately lead to a charred or burned product. However, such caramelization of sugar creates pleasing color and flavor changes, the color ranging from a pale golden brown to a gradually deeper brown before burning actually occurs. Similarly, the flavor begins assume new and distinctive overtones as the mixture of sugar derivatives undergoes changes. The overall process of caramelization involves a number of stages beginning with the inversion of sucrose. After the ring structures in the components of the involve sugar are broken, some condensation of the compounds occurs, which creates some polymers ranging in size from trisaccharides to oligosaccharides. Severe chemical changes at the very high temperatures involved also lead to dehydration reaction and the formation of organic acids and some cyclic compounds, as well as many other substances. Caramelization can be halted abruptly by very rapid cooling of extremely hot sugar mixture. This is done in home food preparation adding boiling water, which is much cooler than the caramelizing sugar. Of course, the addition of cool water also will halt the caramelization process; however, this practice is not recommended because of the extreme splattering and thus potential for burning one’s skin that result when the two liquids come into contact and equalize their extreme difference in energy (Miller, 1998).
An extremely important browning reaction in the preparation of foods is the Maillard reaction. This reaction, like the series involved in caramelization, is classified as nonenzymatic browning. Actually, the Maillard reaction is a series of reactions involving the condensation of a sugar and an amine. During the course of this series, the product is transformed from an essentially colorless substance to a golden color and onto a somewhat reddish brown and then a dark brown. This range of colors can be followed during the baking of a plain or white cake as the crust color develops. Similarly, the reactions can be traced by watching the color development in sweetened condensed milk when it is heated in a water bath (McWilliams, 1997).
Starch is a polysaccharide (meaning “many sugars”) made up of glucose units linked together to form long chains. The number of glucose molecules joined in a single starch molecule varies from five hundred to several hundred thousand, depending on the type of starch. Starch is the storage form of energy for plants, just as glycogen is the storage form of energy for animals. The plant directs the starch molecules to the amyloplasts, where they are deposited to form granules. Thus, both in plants and in the extracted concentrate, starch exists as granules varying in diameter from 2 to 130 microns. The size and shape of the granule is characteristic of the plant from which it came and serves as a way of identifying the source of a particular starch (De Man, 1999)
Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of Amylose and Amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units (Meyer, 1968).
There are two types of starch molecules amylose and amylopectin. Amylose averages 20 to 30 percent of the total amount of starch in most native starches. There are some starches, such as waxy cornstarch, which contain only amylopectin. Others may only contain amylose. Glucose residues united by a 1,4 linkage form the linear chain molecule of amylose. Amylose is the linear fraction and amylopectin is the branched fraction (De Man, 1999).
Starches are insoluble in water. They can be digested by hydrolysis, catalyzed by enzymes called amylases, which can break the alpha-linkages. Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet (McWilliams, 1997).
Cornstarch typically ranges between 24 and 28 percent amylose. Amylopectin the other fraction, amylopectin, is also a polymer of -glucose. However, the presence of 1,6- -glucosidic linkages in addition to 1,4- -glucosidic linkages results in quite a different spatial arrangement. In contrast to the linearity of amylose, amylopectin molecules are dendritic as a result of the shift in direction of the -glucose chain at each 1,6- -glucosidic linkage. In essence, amylopectin molecules are linear for a span of about 10 to perhaps 25 or more glucose units, at which point a 1,6- -glucosidic linkage occurs, causing the molecule to branch. Within a single amylopectin molecule, these branches are found very frequently. Thus, an amylopectin molecule is described spatially as nonlinear, actually rather bushy and dense. Typically, amylopectin is far more abundant in starches than is amylose. In root and tuber starches, amylopectin exceeds amylose content by approximately four times; amylopectin ordinarily constitutes about 80 percent of the starch. Cereal starches are composed of around 75 percent amylopectin. However, genetic variations containing starches composed of virtually only amylopectin (e.g., waxy maize) have been developed and are of commercial significance. The relative proportions of amylopectin and amylose in starches are of considerable importance because of the different behaviors of these two starch fractions in cooked starch products (McWilliams, 1997).
Cereal starches are found in the endosperm of grains. Cornstarch is the most common of the cereal starches used in this country. Its granules as about comparable in size to those of tapioca, but their tendency to polygonal rather than round distinguishes the starch granules of corn from those of tapioca. In contrast, rice starch granules are the smallest of the starches, usually between only 3 and 8 microns in diameter. Like cornstarch granules, rice granules also polygonal. Wheat starch has two basic forms of granules: small spheres about 10 microns in diameter and large disks about 35 micron wide, significantly larger than the other cereal starch granules (McWilliams, 1997).
Starch, a glucose polymer of very large dimensions, actually comprises two fractions: amylose and amylopectin. The simpler of these is amylose, which is very large molecule consisting of considerably more than 200 glucose units linked by 1,4-glucosidic linkages. Amylose molecules are somewhat linear in their spatial configuration, enabling them to hydrogen bond to each other under certain conditions. Amylose is slightly soluble, but does not have a sweet taste. In the structure of amylose presented, note that like the disaccharides, the glucose unit’s link by elimination of a molecule of water. As can be seen, the structure of amylose and dextrin are basically the same. The difference is in n; that is amylose has far more glucose units than does dextrin (Zapsalis, 1985).
Amylose molecules contribute to gel formation. This is because the linear chains can orient parallel to each other, moving close enough together to bond. Probably due to the ease with which they can slip past each other in the cooked paste, they do not contribute significantly to viscosity. The branched amylopectin molecules give viscosity to the cooked paste. This is partially due to the role it serves in maintaining the swollen granule. Their side chains and bulky shape keep them from orienting closely enough to bond together, so they do not usually contribute to gel formation. Different plants have different relative amounts of amylose and amylopectin. These different proportions of the two types of starch within the starch grains of the plant give each starch its characteristic properties in cooking and gel formation (McWilliams, 1997).
Amylopectin, the other starch fraction, is more complicated structurally than is amylose, but it also comprises only glucose units. There are two types of linkages in amylopectin: 1,4- -glucosidic and 1,6- -glucosidic. There are for more 1,4 linkages than there are 1,6 linkages. The usual configuration contains between 24 and 30 glucose units linked together consecutively between 1 and 4, at which point a single occurs. The 1,6 linkage results in disruption of the linear extension of the molecule wherever it occurs, the result being a branching of the molecule at this linkage. Other glucose units continue to be linked to the unit involved with the 1,6 linkage because the first and fourth carbons are still available for bonding to the other units. In nature, this new segment will again have between 24 and 30 glucose units linked by 1,4 linkages before another 1,6 linkage causes additional branching of the molecule. (Zapsalis, 1985).
Amylopectin molecules are extremely large and may have a molecular weight of a million or more. The branching of amylopectin results in a molecule with little solubility; like amylose, it also does not contribute sweetness to food flavors (McWilliams, 1997).
Starch is the reserve carbohydrate of plants and occurs in granules in the cell in plastids, separated from the cytoplasm. It occurs in granules with the size range and appearance characteristics for each species. These granules can be seen using light microscopy and by X-ray. And its rheological properties or texture components are considered because it affects quality of a product.
Starch is composed of two different polymers, amylose and amylopectin. Amylose is a linear fraction with glucose units joined by a-1 → 4 glucosidic bonds. As stated by Fennema (1996), it is evident that amylose in solution assumes the form of a long, flexible coil that readily bends in a wormlike manner. With iodine, the polymer chain takes the form of a helix which may form inclusion compounds with a variety of materials. A blue color will characterize the presence of linear change upon addition of iodine. However, amylopectin is a combination of a-1 →6 glucose linkages, resulting to a branched chain. Testing this with iodine solution will yield a red color. The length of the chain determines the nature of the color produced (DeMan, 1976).
These granules are absolutely insoluble in cold water and upon heating it will go through a process of gelatinization, a process where starch begins to swell. The swelling temperature is influenced by several factors such as pH, pretreatment, heating rate, and presence of solutes. According to DeMan, at this point optical birefringence disappears indicating a loss of crystallinity. The thermal energy permits some water to pass through the molecular network on the granule surface as the heat is applied. With continued heating, the energy level becomes high enough to disturb hydrogen bonding in the crystalline areas, thus the granule and the mixture becomes viscous and translucent (Campbell et al, 1979).
Starch which contains other ingredients interfere overview since it has individual effects. Acid addition on a heated starch will due to reduction of paste viscosity, as well as its gel strength. Gelatinization is then accelerated, as hydrolysis of some molecules on the granule surface occurs. Meanwhile, monoglycerides decrease the degree of gelatinization, the effect tending to increase with fatty acid chain length and with monoglyceride concentration (Campbell et al, 1979).
Microscopic Examination of Starches
Starch granules are observed using light microscopy or x-ray. According to Fennema (1996), the microscopic appearances of starch granules from different plant species are unique that identification is possible by this method alone. Unique characteristics are the size, shape, and uniformity of the granules. Concentric ring appears on some starch granules, since it has an arrangement of linear and branched molecule mixture, in potato starch for instance.
Granules from different sources vary as to size, ranging from 3- 100 µm, rice as the smallest, and potato as the largest among starches. Amylose constitutes 20-30% of the total starch in the nonwaxy cereal starches and in potato starch, whereas waxy starches has no amylose. High degree of branching and unorganized orientation causes portions of starch granules to become vague. Yet, in other portion of the granule, crystallinity exists as a result of a high degree of linear chain orientation. With this, a distinct outline of birefringence is observed under a polarizing microscope (Campbell et al, 1979)l.