Page 46
STARCHES AND STARCH-THICKENED PRODUCTS
QUESTIONS- p. 88
1. Account for the differences in colour observed.
Starch is composed of two basic types of molecules, amylose and amylopectin. Amylose is an unbranched chain of glucose molecules which is coiled in the shape of a helix. If iodine is added to a solution containing amylose molecules, the iodine inserts itself into the helix making it rigid. This changes the color of the starch mixture to blue or purple depending on the length of the amylose molecule. Amylopectin is a branching molecule which does not form a helical coil. Thus the iodine is not able to bind to the starch molecule. Waxy cornstarch does not contain amylose so there is no reaction between the starch and the iodine and no color change occurs.
QUESTIONS- p. 88
1. Does the size of the starch granule influence the behavior of the starch during cooking? Explain.
Larger starch granules absorb water and swell first. The temperature or amount of heat energy required to cause swelling or gelatinization of larger starch granules is lower. For example, the gelatinization temperature for potatoes which have large granules is between 880 -920C.
QUESTIONS- p. 89
1. What happens when heat is applied to a starch suspension. Explain.
When heat is applied to starch granules suspended in a liquid, the starch granules absorb water and swell. Starch molecules have many hydroxy groups which react with the water molecules, attracting and holding them. The smaller amylose molecules diffuse out of the swollen starch granule and form a 3-D network which traps additional water. The loss of free water and restricted flow of the water due to the enormously swollen granules occupying more space, contribute to the increased viscosity of the dispersion. This process is irreversible.
2. What term do you associate with the change you described in (1)? Gelatinization.
3. What is the scientific explanation for lump formation?
When dry starch is mixed with warm or hot water, the exterior portion of the starch granule becomes gelatinized and sticky, and tends to cling together in clumps. The interior portion of the granule remains dry and does not gelatinize. The lumps tend to stay intact even under further heating.
4. Why does a separation agent help reduce or avoid the problem?
In order to obtain a smooth sauce free of lumps, each starch granule must be able to swell independently of every other granule. Separating agents allow individual starch granules to become well hydrated and keeps them uniformly dispersed throughout the medium thus reducing the chance of any lumping.
5. Would the addition of the water to the sugar/starch mixture in # 2 be more effective? Would how fast the water was added have any effect on the finished product? The temperature of the water?
Because sugar is used to separate the flour particles hot water may be added to the starch mixture, but to be sure of success, the water should be added slowly with stirring. Water temperature does present potential problems as it could result in some swelling of the starch granules and in turn lumping , especially if added without adequate stirring. Probably the best technique is to use cold water. However, if boiling water is to be used, it is best to slowly add the starch sugar mixture to the boiling water. This principle is applied in hot cereal cookery.
6. Besides the use of a separating agent, is there any other factor(s) that determine whether or not the resulting sauce will be lumpy? Explain?
Stirring is important to keep starch granules separated and in turn prevent lumping, so that a smooth sauce will result.
QUESTIONS -p. 91
1. At approximately what temperature does a paste of cornstarch reach maximum thickness? Wheat flour? Potato flour? Tapioca?
Cornstarch: Boiling 1000C
Wheat flour: Boiling 1000C
Potato flour: 700--800C
Tapioca: 850C
2. Why are starchthickened sauces, gravies and puddings usually cooked after the mixture has thickened?
Heating a starch thickened sauce after it has thickened ensures that all of the starch granules are fully hydrated. Each granule becomes very swollen and the sauce increases in viscosity and translucence.
3. For what kind of starch may additional cooking be undesirable?
Too much cooking of starch pastes that are high in amylopectin, such as tapioca or waxy cornstarch, may decrease their viscosity. These starch molecules gelatinize at lower temperatures.
QUESTIONS- p. 92
1. Compare the thickening power of flour and cornstarch. Give one reason for the difference.
Cornstarch has twice the thickening power of flour. Flour contains not only starch but other components such as proteins, fat, fiber, pigments, vitamins and minerals.
2. What factor(s) determine what starch or flour will be used an a thickening agent?
The type used as a thickening agent depends on the particular use of the product, whether it will be frozen or canned, and on the desired cooked characteristics. For example the final appearance may be cloudy or clear and the texture may be smooth or mucilaginous.
3. How does cooling or freezing affect cooked starch mixtures? Why?
On cooling, the starch granules recrystallize, forming a solid gel, a process known as retrogradation. As the mixture cools, there is a decrease in kinetic energy, which allows the amylose molecules to associate and form a 3D network. Over time, water is squeezed out of the gel as the amylose molecules pull together, and the gel network shrinks. This process is accelerated by freezing. When the starch gel thaws, water is lost as it unable to rebind to the fragile spongy mass.
4. What is the name of the process that accounts for the changes described in (3)?
Retrogradation, or gel formation, accounts for the formation of the gel network. Syneresis is the term used to describe the weepiness that occurs when water is squeezed out of the gel.
QUESTIONS -p. 93
1. Explain how large amounts of sugar affect the gelatinization of starch and the characteristics of the sol/gel.
Sugar tenderizes a starch gel and reduces paste viscosity because it retards gelatinization. Due to the hydroscopic nature of sugar, the temperature required for gelatinization is raised thus delaying the process. Sugar can attract and hold water molecules leaving less water available to hydrate the starch granules. The starch molecules do not swell as much as they would in the absence of sugar and too much sugar can cause a mixture to not thicken sufficiently. The addition of sugar also contributes to the translucence of a starch gel.
2. Explain how acid affects the gelatinization of Starch. How does the time of addition affect results? How would you expect the rate of cooking to influence the thickness obtained in the sauce? What recommendations would you make for working with cooked acidstarch mixtures?
The combination of acid and heat causes a hydrolytic reaction that breaks down molecules of starch into smaller molecules. These shorter molecules are able to move more freely in the thickening paste, resulting in a somewhat thinner paste. To avoid thinning of the paste, acid should be added after gelatinization has been completed. However, if acid has to be added before gelatinization has occurred while the starch paste is being heated, a rapid heating rate will minimize the length of time for acid hydrolysis to take place. The end result will be a somewhat thicker starch paste.
QUESTIONS -p. 94 (G)
1. What are the reasons for the differences you noted between the sauces?
When flour is heated until brown, the starch granules undergo a process known as dextrinization. Dextrins are fragments of starch molecules composed of chains of glucose molecules. When they are dissolved in water, they have a sweet taste, and contribute to the color and pleasant flavor of brown gravies. As the starch undergoes dextrinization, it loses its thickening power. Extensive dextrinization, as occurs when flour is browned with dry heat, may cut the thickening power by as much as one half.
QUESTIONS- p. 94 (H)
1. Explain the effect the enzyme preparation had on the cooked starch mixture.
Diastase is a starch splitting enzyme which catalyzes the hydrolysis of starch molecules. It is composed of both alpha and betaamylases. Betaamylase attacks damaged starch molecules, splitting off maltose molecules by hydrolizing alternating 1,4 glycosidic linkages. Alphaamylases attack the large starch molecule randomly in the chain, producing large dextrins. As it continues to act, the large dextrins are further split into maltose molecules and 6-unit dextrins. Because the starch molecules are broken down, the starch paste loses its thickness and becomes fluid. Diastase is sometimes used commercially to thin liquids and improve clarity.
QUESTIONS- p. 99
1. How does the starch used in the instant pudding mix differ from that in the homemade?
Homemade puddings rely on regular cornstarch to thicken the mixture Instant puddings contain a modified starch that has been pregelatinizated. These starches are precooked to gelatinization then dried with hot rollers. This process breaks down the large swollen starch granules into smaller particles. These smaller particles are easily hydrated and increase the viscosity of the mixture without the application of heat.
2. Why are cornstarch puddings cooked after the mixture has thickened?
To improve flavor and to assure "elation of all the starch granules and in turn optimum final viscosity.
3. What causes a "skins to form on the surface of cornstarch pudding: How can skin formation be limited?
Skin formation is due to loss of water from the starch and protein molecules near the surface of the mixture. To reduce this problem, cover the hot pudding tightly with plastic wrap or greased paper.
4. If you wish to prepare a moulded cornstarch pudding or Blanch Mange, what must you do? Why? (see p.103)
To produce a firmer gel that will retain its shape when unmolded, use an extra 5 mL of cornstarch.
5. What is the difference in formulation between a regular cornstarch pudding and a Cream Pudding? (see p.103). Why does the cream pudding have less starch?
A cream pudding has added egg, less cornstarch and more vanilla. Less starch is needed because the egg also acts as a thickener. As egg proteins coagulate under heat, the viscosity of the mixture is increased.
6. What ingredients are used to prepare a homemade pudding mix? (see p.104)
Skim milk powder, sugar, cornstarch, salt.
7. What advantages has mix B or mix A? Disadvantages?
Mixture A - probably has a more traditional flavor
-
has less fat in the formula
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would be a better one to store
Mixture B - has less sugar in the formula
-
would be cheaper
Disadvantages of B - the addition of milk powder might
give an off flavor, depending on the freshness
of the milk
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The pudding might be more likely to deteriorate during storage.