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• relate the molecular and granule structure of various native starches to starch paste and gel characteristics.
• explain the process of sol and gel formation in starch-water dispersions.
• understand and predict how sugars, acids, lipids, and other ingredients affect the quality characeristics of starch-containing products.
• predict how processing, production, preparation, and storage of starch sols and gels affect the quality characteristics of the final product.
• summarize processing and modification of starches and relate this to dispersion characteristics.

Starch is found in almost every typical meal from the Northern Hemisphere. For a truly useful nutrient it is critical that there be some understanding of what starch is and how preparation processes will influence it.

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.

The structure of the granule of grain is crystalline with the starch molecules orienting in such a way as to form radially oriented crystals. This crystalline arrangement is what gives rise to the phenomenon of birefringence. When a beam of polarized light is directed through a starch granule, the granule is divided by dark lines into four wedge-shaped sections. This cross-hatching or cross is characteristic of spherocrystalline structures.

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.

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.

Starch in its processed, commercial form is composed of starch grains or granules with most of the moisture removed. It is insoluble in water. When put in cold water, the grains may absorb a small amount of the liquid. Up to 60 to 70C the swelling is reversible, the degree of reversibility being dependent upon the particular starch. With higher temperatures in irreversible swelling called gelatinization begins.

Composition of Starch

	Potato	Cassava	Wheat	Cornstarch
Moisture,%	19	13	13
Ash,%	0.4	0.2	0.2
Protein, %	0.06	0.1		0.4
Lipid,%	0.05	0.1	0.8
Phosphorus%	0.08	0.01	0.06
Amylose,%	21	17	28
Micrograph

1.Muhrbeck, P. and A.-C. Eliasson. 1987. Influence of pH and ionic strength on the viscoelastic properties of starch gels- a comparison of potato and cassava starches. Carbohydrate Polymers 7: 291-300.

Starch begins to gelatinize between 60 and 70C, the exact temperature dependent is the specific starch. For example, different starches exhibit different granular densities, which affect the ease with which these granules can absorb water. Since loss of birefringence occurs at the time of initial rapid gelatinization (swelling of the granule), loss of birefringence is a good indicator of the initial gelatinization temperature of a given starch. The largest granules, which are usually less compact, begin to swell first. Once optimum gelatinization of the grains has occurred, unnecessary agitation may fragment the swollen starch grains and cause thinning of the paste.

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.

',600,600)">	Raw starch that has not had moisture added does not undergo gelatinization. By definition, gelatinization is a phenomena which takes place in the presence of heat and moisture. The dry raw starch, if heated, would undergo dextrinization. This certainly would affect the starch paste viscosity and starch gel strength. The paste viscosity would be decreased and gel strength decreased. If a "limited amount" of moisture is added to the raw starch you may get partial gelatinization. This condition exists in baked products.
	Cornstarch at a 5% level in 95% water would have a slight change occur if heat is initiated. Water might be slightly ADSORBED onto the surface of the granule. Actually, in the research from which these images came, I found that I got a difference in paste viscosity and ultimate op as measured by viscosity if I allowed cornstarch to sit in water at room temperature. This led me to believe that there is some initiation adsorption upon the granule at room temperature (27C).
',600,600)">	If this 5% dispersion of cornstarch was heated to 40C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule might begin to be loosened slightly. In some types of starches water might even begin to be ABSORBED into the granule.
	If this 5% dispersion of cornstarch was heated to 50C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would begin to be loosened.. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, some of the amylose may begin to work itself off the granule surface, thus, opening the structure even more.
	If this 5% dispersion of cornstarch was heated to 60-65C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would loosen. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, some of the amylose would work itself off the granule surface, thus, opening the structure even more. This in turn would allow even more of the water to become ABSORBED and more amylose to work itself out into a colloidal dispersion outside of the granule. The long amylose polymer is a colloid in characteristics.
	This is intermediate between 60 and 70C. The precise changes are affected by rate of heating, condition of the starch and other factors.
	If this 5% dispersion of cornstarch was heated to 70-90C I would expect more water would be ADSORBED onto the surface of the granule, the hydrogen bonding between the starch polymers within the granule would loosen. This would allow the water to penetrate into the granule becoming ABSORBED by the granule. Additionally, the amylose would work itself off the granule surface, thus, opening the structure even more. This in turn would allow even more of the water to become ABSORBED and more amylose to work itself out into a colloidal dispersion outside of the granule. The long amylose polymer is a colloid in characteristics. At some point between 60-95C we would likely have gelatinization occur. This might be measured by loss of birefringence, increased viscosity, translucency, increased susceptibility to enzyme action, x-ray diffraction or some other chemical or physical means. At this point, the starch granule is swollen as much as possible. It is a starch sol until you remove it from the heat and begin to allow the amylose and some amylopectin to recrystallize, i.e. realign.
	In some instances, when heated to 90C the starch granule could reach optimum gelatinization and be a nice swollen granule sack. In other cases, this may allow the sack to "implode" and loose their contents as there is not enough structure and hydrogen bonding to hold the polymers together. It is interesting that overcooking, as with overstirring, will decrease the starch paste colloidal sol viscosity.

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 is critical to the ultimate rigidity of the final product. Gelation of starch occurs due to its 3-D structure.

A 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. Craig et al. [1989] 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 remenants, the amylose and amylopectin have collapsed conformation due to interchain association of the polymers. Finally, he suggests that low clarity and low whiteness occurs when the grnules are swollen remnants and the amylose and amylopectin starch molecules have either collapsed or associated.

CHANGES IN GELATINIZATION OF STARCH

hydration and swelling to several times original size
loss of birefringence
increase in clarity
marked, rapid increase in consistency and attainment of peak
"dissolution" of linear molecules and diffusion from ruptured granules.
with heat removal, retrogradation of mixture to a paste-like mass of gel.

All these changes may occur with a typical "native" starch. We know they occur with a 5% cornstarch mixture. The problems and the "unusual" occurs primarily when we deal with modified starches.

If gelatinization has occurred correctly and IF the starch is the correct type and concentration, upon removal from heat a three dimensional structure may form. This is a messy structure made up of cross-linking of amylose as an intra- and inter-molecular bonding within and exterior to the swollen starch granule.

Factors Affecting Starch Paste Viscosity and Starch Gel Strength

STRESS OR OTHER FACTOR	Stirring Amount and Type This is a gelatinized cornstarch dispersion that, likely due to stirring, the granules broke apart.
Types and Amount of Starch	Certain the type of starch will influence the characteristics of the starch paste viscosity and gel strength. Generally speaking, with "native starches" the greater the amount of amylopectin the more viscous the starch paste, whereas, the greater the amount of amylose the firmer the gel (greater the gel strength).
heating rate	It is of interest that, generally, the faster you heat a starch-water dispersion the thicker it will be at the identical endpoint temperature. This is of importance to those interested in household versus foodservice dispersions.
endpoint temperature	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.
cooling and storage conditions	Research has well established that the cooling conditions will impact the strength of the gel. Generally, if cooled too fast, the amylose will not have time to form the vital micelles necessary for the three dimensional structure. If cooled too slowly, the amylose fractions will have a chance to align too much and become too close together and the liquid portion will not be trapped in the micelles. In both instances there will be weeping and syneresis.
dextrinization: heat	Heat dextrinization frequently occurs during the household production of gravies with meat drippings. Essentially, the heat will hydrolyze the starch polymers and the short chains mean that the granule will not remain intact. This means that the starch paste will be more runny with the dextrinization.
ingredients added acid (dextrinization) Sugar affects the gelatinization of starch, whether a starch pudding or a high-moisture cake. In either instance, both research and qualitative observations have shown that sugar will delay or inhibit gelatinization of starch. The starch pudding may be less viscose or have a less firm gel. The cake may collapse as the structural contribution of starch is delayed or inhibited. The type of sugar also influences the degree of gelatinization. As seen in the graph the types of sugar impact the degree, temperature and/or speed of gelatinization. Bean, M.M. and W.T. Yamazaki. 1978. Used with permission, Institute of Food Technology The reason for sugars delay upon the gelatinization of starch is still not clear. Generally, researchers indicate that it is likely due to the competition for water. It becomes less clear as to how this competition actually influences the water structure. Does it actually form the "hydrate" structure, does it affect its plasticity or mobility, or does it actually prevent the adsorption of water. This is the easy logical answer; however, there has been a number of researchers that indicate that sugar actually interacts with the amorphous areas of the starch granules. This interaction also leads to increased starch gelatinization temperature. Johnson and other researchers observed not only was their a linear relationship between sugar concentration and onset of starch gelatinization. Bean and Yamazaki observed similar results. That is, as sugar concentration increased the onset of gelatinization temperature increased. Additionally, Johnson et al observed the order of effectiveness at delaying the onset of gelatinization, it was found that fructose was most effective followed by glucose, maltose, and sucrose. fat: triglycerides, surfactants	Dextrinization may occur due to acid and enzymes. In the making of lemon pudding or pie, if the lemon juice is added early in the gelatinization process, dextrinization of the starch will occur. 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. Fat, composed of triglycerides and surfactants, will serve to "waterproof" the starch granules so that water will not penetrate as readily during the gelatinization process. 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. The addition of sugar to a typical starch dispersion will serve to decrease starch paste viscosity and gel strength because the sugar will compete for water and it won't be available for gelatinization.

Johnson, J.M., E.A. Davis, and J. Gordon. 1990. Interactions of starch and sugar water measured by electron spin resonance and differential scanning calorimetry. Cereal Chemistry 67(3): 286-291.

Bean, M.M. and W.T. Yamazaki. 1978. Wheat starch gelatinization in sugar soutions. I. Scurose: Microscopy and viscosity effects. Cereal Chemistry 55(6): 936-944.

Just a note, one can determine the viscosity by a simple linespread test. This is the dumping of a portion of hot starch sol onto a diagramm of concentric circles. The spread determines the viscosity.

The first step would appear to be to understand the roles and functions of starch. Quality characteristics of the starch itself depends upon which role or function. These may be listed as:

Modified Starches may be prepared as follows:
-"pear" and powdered starches
-pregelatinized starches
-acid-modified starches (thin cooking)
-enzyme-thinned starches
-white dextrins (dry heat)
-yellow dextrins
-oxidized
-starch ethers and esters
-cationic (+) starch ethers
-cross-bonded starches
-dialdehyde starches
-linear starch fractions

	Many different parts of the plant may contain the amyloplasts or leucoplasts in the cytoplasm. This storage mechanism in the cytoplasm is present as organized plastids. In reviewing starch as food sources, it has been observed that the plastid and associated granules may take up much of the cell in seeds, tubers and root vegetables. It is of interest to know that the storage of starch in the leucoplasts is part of the photosynthetic cycle. A number of years ago, a forester was attempting to evaluate the affect of the light cycle and photosynthesis. The starch in pine needles was able to plot the day night photosynthetic cycle.
Modified from Lineback, D.R. 1984. The starch granule organization and properties. Baker's Digest 58: 16, 18.	The starch granule is held within the plastid. The granule itself has an organized structure which can be seen in the diagram to the left. The amylose and/or amylospectin is laid down from the interior outward. There has been some interesting research that indicates that not only does the starch have a structural radiation out from the center of the granule, but there is other organizational patterns. There are different types of patterns to the x-ray diffraction. They are called either A, B or C pannterns This arrangement contributes to different crystallinities. X-ray diffraction patterns indicate that cereals give an A pattern; legumes, roots, and tuber starches give a B patterns; andC patterns occur as a between the two. It is likely that the A and B patterns are due to the crystallized amylopectin.
representation from T.J. Schoch. Brewer's Digest 37(2): 43.	There appears to be concentric layers of starch from the center. This historical representation indicates the light and dense packing. These may be due to the light-dark [daytime-night time] time periods. Work done at Oregon State University looked at these concentric circles using a scanning electron microscope to view the photosynthesis of pine starch. From this, they could plot the active light and dark periods in the forest, relating to the availability of light during the day.
	Actually, the organized pattern is due to the arrangement of amylose, amylopectin and the lipid complex. The arrangement of these components will differ for different starches and within starch granules. This arrangement forms different gelatinization [swelling] characteristics and ultimate food quality characteristics.

GLOSSARY

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Updated: Wednesday, July 22, 2009.