The purpose of this lab is to determine the effects acid and sugar in various amounts have on a cornstarch paste. The importance of this experiment is to examine what causes starch to thicken or thin which is relevant to obtaining the desired viscosity for recipes that involve starch cookery without complication. Factors to be considered in the thickening power of cornstarch include the concentration of starch, extent of gelatinization thus temperature and duration of heat, and the addition of sugar or salt (1).
Background Gelatinization, a physical change, occurs when starch is heated in water. Due to the solubility of the amylose in the starch granule it migrates out causing hydrogen bonds to break between the amylose and amylopectin as water enters the granule. The amylopectin within the starch granule forms a bond to the hydrogen from the influx of water and swelling occurs. As a result, less free water is available and the mixture is thickened. With continued heating, past the point of gelatinization, viscosity is lost and the granules become compressed resulting in a paste (2).
Method and Materials Each method was prepared by a separate group and results were recorded for collective use. Method A served as the control which included one tablespoon (16 g) of cornstarch heated in an aluminum pot with the gradual addition of one cup of water (236 ml). The mixture was stirred prominently to prevent lumping while the boiling temperature was reached. The mixture was removed from the heat after boiling and allowed to cool at room temperature until 60 degrees C (140 degrees F) was measured using a thermometer.
A line spread test was performed to measure the spread of ? cup of the warm mixture. A metallic cylinder held the paste until it was released onto the line spread mat. After 30 seconds of flowing, four readings were recorded to average the spread. This value was used as a standard to compare all other starch paste mixtures with. The remaining paste was placed in the refrigerator to cool for 10 minutes. Then, ? cup of the cooled paste was measured using the line spread test again. The differences were recorded for each, warm and cool, paste mixtures.
Method B included the basic formula used in Method A with the addition of sugar in the amount of 25grams. In addition, Method C was carried out similarly to Method A with the exception of added sugar in the amount that was twice as much as the amount used in method B. One tablespoon (16 g) of cornstarch and 50 grams of sugar were placed into an aluminum pot to boil with the gradual addition of 236 ml of water. The mixture was stirred prominently to avoid lumping. Allowing the water to boil, a paste was created. The line spread test as described in Method A was completed for both warm and cool paste.
Method D included the preparation of a corn starch paste in Method A but with 16 g of cornstarch and less water than used in methods A, B, and C. Instead 206 ml of water and 30 ml of lemon juice were combined. In contrast, Method E used twice as much lemon juice as Method D, 60ml, and twice as much less water, 176 ml. Finally, method F used 25g sugar, the same amount of sugar used in method B except 30ml lemon juice and 206 ml of water were used which was the same for method D. Results and Discussion Treatment Warm Cold Clarity Description Sugar A. 0g 2. 25 8. 25 Warm- Cold- cloudy Warm- Thick Cold- thicker and clumpy B. 25g 7. 5 6. 25 Warm- Cloudy Cool- Yellowish Warm- Thick with bubbles Cold-Thick with no bubbles C. 50g 10. 5 9. 13 Warm-Cloudy Cold- Less translucency, cloudier. Warm-Thin Cold- Medium thick Lemon Juice D. 30ml 7. 0 6. 5 Warm-slight cloudiness Cool- Very Cloudy Warm- yellowish and Thick Cold-White & Thick E. 60ml 2. 0 1. 5 Warm- Cloudy Cool- white Warm- Thick Cool- Thicker Sugar F. Lemon Juice 30ml 19. 0 13. 6 Warm-Translucent Cold-Less translucency Warm- medium thick Cold-Thicker
The widest variance was between the rheology of the control, Method A, and method F. The flow of a cornstarch paste with acid and sugar results in a thin paste. The added sugar contributes to the translucency reported in the warm paste while the acid inhibits gelatinization by breaking down starch. In comparing method B to method C, method C had a much thinner paste with more flow. This is due to the increased sugar concentration. Sugar slows down gelatinization because it competes for water thus less water is available to enter the starch granules.
Therefore, the amount of swelling is significantly reduced. A higher temperature is needed to completely gelatinize the starch when sugar is added to the starch mixture. (2) However, a thinner and more translucent paste is the result. Furthermore, when more starch is present than liquid, the thicker the paste will be. In contrast, when more liquid than starch is used, the thinner the paste. Cereal starches such as cornstarch must usually reach a boiling temperature if they are to reach maximum viscosity (1). Notably, the ratio of liquid was consistent for each experiment.
In the case of method D and E, the only Independent variable that took place was how much lemon juice was added and how much water was removed from the liquid portion of each mixture. Due to the fact that the lemon juice is an acid, Method D, E, and F resulted in changes to the dependant variable, which was the viscosity. Increased thinning of the starch paste should have been the result in comparing the control to the mixtures in which acid was added. After partial gelation occurred from minimal cooling, the results were a much thicker paste due to hydrogen bonding between amylose molecules and the loss of energy.
The data recorded for rheology of the cooled starch supports this idea. The reason the paste would become increasingly thin with the addition of acid is due to hydrolysis of the starch granules by the acid (2). Since the lemon juice in methods D and E were probably incorporated after gelatinization occurred, the result was not a thin paste. This is the correct method for the addition of acid. Overall, in this experiment, some of the mixtures were poorly combined and heated without regard for when gelatinization would occur and some were carefully combined and heated.
Not all of the mixture were gelatinized beforehand which resulted in lumps. In Method C, for example, the water began to boil very quickly due to the small amount of liquid and some lumping occurred although it was to be avoided. Due to lab being completed by different people the results were not consistent. Conclusion Three physical changes including the viscosity, appearance, and strength of gelatinized starch mixtures were identified. Each change was a result of the concentration of added ingredients such as acid and sugar.