Action of Yeast on Carbohydrates
According to Zapsalis (1985), 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.
According to De Man (1999), Alcoholic fermentation is not performed by enzymes, but rather by yeasts. Yeasts are not bacteria but fungi. Yeasts are found in nature everywhere, particularly in the autumn when fruits ripen and their spores spread in large numbers in the air. These carry out a simple chemical reaction in sugar/water solutions according to the following formula:
CHO —-> CH3CH2 + CO2
According to McWilliams (1997), 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.
According to 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.
According to Zapsalis, the sample soaked in 50°Brix solution is chewier due to the water retained or held by sugar. The water retained in the mango acts as a plasticizer causing the fruit soaked in 50°Brix solution to become chewier.
According to McWilliams (1997), caramelization is the familiar browning of sugars through exposure to heat. The most common form of sugar—table sugar or sucrose—is a disaccharide, a combination of two monosaccharides: glucose and fructose. The two sugars can be easily separated using the enzyme invertase, which is essentially what bees do when they make honey from nectar. Fructose caramelizes more readily than glucose, so baked goods made from honey are generally a bit darker than those made with sucrose.
According to Lowe (1955), caramelization is the process of removal of water from a sugar (such as sucrose or glucose) followed by isomerization and polymerisation steps. As the process occurs, volatile chemicals are released producing the characteristic caramel flavor.
According to McWilliams (1997), in reality the caramelization process is a complex series of chemical reactions, which is still poorly understood. According to The 1,4 linkages between saccharide units of carbohydrates are covalent bonds; nevertheless, they are more susceptible to cleavage they are the covalent bonds within the ring structures. Consequently, chemical reactions under normal cooking conditions involve the 1,4 linkage between, rather than within, the basic saccharide units. The structure itself is threatened only under very severe conditions. Why sugars are subjected to very high temperatures, as occurs in production of such candies as peanut brittle, the energy provided sufficient to break some of the bonds holding the compound in its crystalline form. The result is considerable fragmentation of the monosaccharide and formation of a wide range of simple compounds from these fragments. This process is sometimes referred to as Caramelization. The compound formed includes organic acids, aldehydes, and ketones.
According to McWilliams (1997), when sugars are heated to such intense temperatures that they move (170C for sucrose), a series of chemical reactions will begin to take place, which ultimately can lead to a charred or burned product. However, 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 derivative undergoes change.
The highest rate of the color development is caused by fructose as caramelization of fructose starts at lower temperature. Baked goods made from honey or fructose syrup thus is generally a bit darker than those made with sugar.
According to Meyer (1968), the overall process of Caramelization involves a number of starches beginning with the inversion of sucrose (conversion to invert sugar) described above. After the ring structures in the components of the invert sugar are broken, some condensation of the compounds occurs, when creates some polymers ranging in size from trisaccharides to ologosaccharides (as many as 10 subunits polymerized). Severe chemical change 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.
According to Meyer (1968), 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.
Functional Properties of Polysaccharides
According to McWilliams (1997), Polysaccharides are relatively complex carbohydrates. They 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.
According to De Man (1999), Amylopectin is the branched chained glucose polymer containing both alpha – 1, 4- linear and alpha – 1, 6-branched linkages. At one time, we used to say that this branched bushy polymer with branches 20 to 30 glucose residues long, contributes primarily to the viscosity of a prepared food. This was contrasted to the gelling contribution of the alpha – 1, 4 linked linear polymer – amylose. Amylose is approximately one-fourth the size of amylopectin; however, the glucose residues are 400 to 4000 units long. The attributing of viscosity to amylopectin and selling to amylose may not be entirely valid.
Cereal Variety and Texture
According to McWilliams (1997), other waxy starches are essentially 100 percent amylopectin and are virtually lacking in amylose. The absence of amylose inhibits the formation of a gel structure when a waxy starch is used as a thickening agent; this characteristic makes waxy starches desirable in the preparation of fruit pies and other items in which a thickened, but ungelled consistency is desirable.
According to De Man (1999), The many different starches range in amylose, on the average, from 16 to 25% and maybe as high as 30%. You know there are some high amylose rice starch which may have as much as 80 to 90% amylose. These starches are unusual and are occasionally used in edible casings. At the other end of the scale, there are the waxy corn starches, waxy rice starches, and although not listed in the table, waxy sorghum, which contains 100% amylopectin and no amylose.
Rheological Properties of Starches
According to Zapsalis (1985), the gelatinization range refers to the temperature range over which all the granules are fully swollen. This range is different for different starches. However, one can often observe this gelatinization because it is usually evidence by increased translucency and increased viscosity. This is due to water being absorbed away from the liquid phase into the starch granule.
According to McWilliams (1997), the clarity and translucency is affected not only by the degree of gelatinization and swelling of the granule but by the association of amylopectin and amylose. Indicates that a high clarity and almost no whiteness occurs within a starch paste itself because of few swollen granular remnants and the amylose and amylopectin have a stable conformation due to inter- and intra chain repulsion with little association. Moderate clarity and high whiteness occurs because, even with few granular remnants, the amylose and amylopectin have collapsed conformation due to inter chain association of the polymers. Finally, he suggests that low clarity and low whiteness occurs when the granules are swollen remnants and the amylose and amylopectin starch molecules have either collapsed or associated.
With the temperature at 95 degrees Celsius, the viscosity of cornstarch began to rise. This is the pasting temperature and it varies with starch type and modification. This is a measure of the thickening power of a starch. For the tapioca starch the pasting temperature is 85 degrees Celsius. At the temperature of 60 degrees Celsius for both starches the viscosity dropped only slightly. This indicated that the starch pastes have good viscosity stability under heat. The viscosity of the cooked starch paste, after cooling at 60 degrees Celsius is a measure of the retrogradation or setback produced by cooling.
Plants store glucose as the polysaccharide starch. Starch can be separated into two fractions–amylose and amylopectin. Natural starches are mixtures of amylose (10-20%) and amylopectin (80-90%). Rheological behavior of a starch involves studying its viscosity, elasticity and plasticity. Food processors measure starch viscosity as a quick assessment of product performance. The most common method for observing rheological starch behavior is studying viscosity changes during a programmed heating, cooking and cooling cycle.
A starch dispersion generally has high opacity and little clarity. The changes that occurred during gelatinization concurrent with the swelling of the granule increased in translucency. However, different starches will have different abilities. The clarity and translucency is affected not only by the degree of gelatinization and swelling of the granule but by the association of amylopectin and amylose. It has been said that a high clarity and almost no whiteness occurs within a starch paste itself because of few swollen granular remnants and the amylose and amylopectin have a stable conformation due to inter- and intra chain repulsion with little association. Moderate clarity and high whiteness occurs because, even with few granular remenants, the amylose and amylopectin have collapsed conformation due to interchain association of the polymers. It was suggested that low clarity and low whiteness occurs when the granules are swollen remnants and the amylose and amylopectin starch molecules have either collapsed or associated.
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.
The gelatinization range refers to the temperature range over which all the granules are fully swollen. This range is different for different starches. However, one can often observe this gelatinization because it is usually evident by the increased in translucency and increased in viscosity. This is due to water being absorbed away from the liquid phase into the starch granule. Starch granules are insoluble in cold water, but they will absorb water and swell slightly. High amylose starch requires high temperature for gelatinization.
Each type of starch has the unique endpoint temperature at which it will undergo optimum gelatinization. If starch is not completely gelatinized it certainly will not have optimum starch paste viscosity or gel strength. If over gelatinized, the dispersion may have decreased starch paste viscosity and gel strength because the swollen granule easily fragmented with stirring or actually imploded due to the extensive loss of amylose from the granule. If a typical starch paste is allowed to stand undisturbed, inter-molecular bonds begin to form, causing the formation of a semirigid structure or gel. This gel is a structure of amylose, molecules bonded to one another and, slightly, to the branches of amylopectin molecules within the swollen granule. This phenomenon is sometimes called retrogradation. The conditions of gelation are critical to the ultimate rigidity of the final product. The greater the amount of amylopectin, the more viscous the starch paste. Whereas, the greater the amount of amylose the firmer the gel the greater the gel strength.
A longer amylose molecule will, to a point, have greater gel strength due to its increased ability to associate through hydrogen bonding. This increased ability to associate increases the molecule’s tendency to retrograde. Smaller amylose molecules exhibit weaker association and, thus, are more resistant to retrogradation. Recent information indicates that amylopectin molecules with longer branches also are more susceptible to retrogradation. This is a particular concern to researchers trying to lengthen amylose molecules through cross-breeding.
Dextrinization may occur due to acid and enzymes. This means that the amylopectin would not be as “bushy” as previously and the amylose would be of a shorter chain length. Thus, there would be decreased starch paste viscosity and gel strength. Thus, since the granules do not swell and the amylose does not exude from the granule and there will be a decreased starch paste viscosity and decreased gel strength.
Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble. The triiodide ion slips into the coil of the starch causing an intense blue-black color.
The characteristic blue color of starch produced with iodine relates exclusively to the linear fraction. The polymer chain takes the form of a helix which may form inclusion compounds with a variety of materials such as iodine. The inclusion of iodine is due to an induced dipole effect and consequent resonance along the helix. Each turn of the helix is made up of 6 glucose units and encloses one molecule of iodine. The length of chain determines the nature of color produced.
Amylose has an affinity for iodine. It has been discovered that amylose complexes with iodine by forming a helical structure around the iodine in such an enclosure, the iodine exhibits a strong absorption of light (intense blue) The iodine molecule slips inside of the amylose coil. Cornstarch appeared blue under the microscope. It is classified as unbranched.
Amylopectin starches are characterized in being resistant to gelling and change in water holding properties. In contrast to amylose, amylopectin gives a reddish violet with iodine. Tapioca starch appeared reddish violet under the microscope. It is classified as branched.
According to Zapsalis (1985), 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.
According to Zapsalis (1985), 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).
Starch dispersion generally has high opacity and little clarity. The changes that occur during gelatinization concurrent with the swelling of the granule increased in translucency. However, it must be made clear that different starches will have different abilities. The clarity and translucency is affected not only by the degree of gelatinization and swelling of the granule but by the association of amylopectin and amylose. Indicates that a high clarity and almost no whiteness occurs within a starch paste itself because of few swollen granular remnants and the amylose and amylopectin have a stable conformation due to inter- and intra chain repulsion with little association. Moderate clarity and high whiteness occurs because, even with few granular remnants, the amylose and amylopectin have collapsed conformation due to inter chain association of the polymers. Finally, he suggests that low clarity and low whiteness occurs when the granules are swollen remnants and the amylose and amylopectin starch molecules have either collapsed or associated.
Microscopic Examination of Different Starches
According to Meyer (1968), 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.
According to Miller (1998), Starches have been widely used as thickeners for the food industry for many years. The primary sources of food starches are corn, wheat, potato and tapioca. Rice starches are the little known secret of the starch world, comprising a very small percentage of the total starch usage. Don’t be fooled by the fact that rice starches are not as common as the other starch sources. They have many unique attributes that make them some of the most interesting starches in the food industry.
Starch occurs in small particles known as granules. The granule size and shape are characteristic of each type of plant, so that an experienced person, by microscopic examination, can usually determine the source of the starch.