Effect of Acid and Sugar on the Rheology and Flow Properties of Corn Starch Paste

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10/08/08Brandy Ogg
Ms. Winston MPH, RD, CDEFCS 373
Lab # 2

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).

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 ¼ 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, ¼ 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

Cold- cloudy
Warm- Thick
Cold- thicker and clumpy
Warm- Cloudy
Cool- Yellowish
Warm- Thick with bubbles
Cold-Thick with no bubbles
Cold- Less translucency, cloudier.
Cold- Medium thick
Lemon Juice
Warm-slight cloudiness

Cool- Very Cloudy
Warm- yellowish and Thick

Cold-White &
Warm- Cloudy
Cool- white
Warm- Thick
Cool- Thicker
Juice 30ml
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