1. To identify starch in food.
2. To study the microscopic appearances of raw and heated starch. 3. To compare the viscosity of various gelatinized starch solutions. Introduction
Starch is one of the most abundant substances on the earth. It can be found in seeds, grains, and roots of many crops where it is synthesized in granular form. Starch granules are packages of starch molecules. The importance of starch is well-known, as is its central role in human diet. Many aspects of starch structure can be measured by chemical, physical, spectroscopic, or microscopic methods (Tina, 2000). Starch can be separated into two fractions - amylose and amylopectin. Natural starches are mixtures of amylose (10-20%) and amylopectin (80-90%). Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. The structure of amylose consists of long polymer chains of glucose units connected by an alpha acetal linkage. Amylose in starch is responsible for the formation of a deep blue colour in the presence of iodine (Frances and Ellie, 2008). Starch granules are more or less regular spheres which composed of a mass of radiating needle-shaped crystals when the granules are viewed under polarized light (Srilakshmi, 2003). Starch granules do not dissolve easily in cold water but they will form a temporarily suspension with the starch until the mixture is allowed to stand. When heated with water, the intermolecular hydrogen bonding is broken and grains absorb water and swell. The amylose will leach out from the starch granule as heating continues. Besides that, the viscosity increases until a peak thickness is achieved. This process is known as gelatinisation. The changes transform the temporarily suspension into a more permanent one (Srilakshmi, 2003). The thickness of the starch is called “viscosity”. The increase in the viscosity of heated starch solution is caused by the action of the enlarged starch granules bumping against each other, trapping the water and inhibiting its free flow. Most starches gelatinize when heated above 56°C to 75°C. Larger granules gelatinize at lower temperature while smaller granules gelatinize at higher temperature (Srilakshmi, 2003). Part 1: Identify starch in solution
Iodine solution (20mL), 10%(w/w) tapioca starch solution (20mL), 10% (w/w) corn starch solution (20mL), 10% (w/w) potato starch solution (20mL), 10% (w/w) rice starch solution (20mL), bread (1/4 slice), rice (a few granules), ripe & unripe banana (1/2 banana each), Pasteur pipette (x5), glass slides (x5), blade (x1), glass rod (x1). Procedures
One drop of tapioca starch solution was placed on a glass slide and it was smeared into a thin layer on the glass slide by using a glass rod.
A few drops of iodine were put onto the smeared tapioca starch solution and the observation was recorded.
The above steps were repeated by using corn starch solution, potato starch solution and rice starch solution instead of tapioca starch solution.
The ripe and unripe bananas were cut into thin slices and placed on a paper towel. A few drops of iodine were put onto the banana slices. The observation was recorded.
The bread and rice granules were put on a paper towel. A few drops of iodine solution were put onto the bread and rice granules and the observation was recorded.
Table 1: Colour changes of starch solution after added with iodine solution. Starch solution| Observation|
Tapioca starch solution| The colour of the starch solution changed from white to blue-black.| Corn starch solution| The colour of the starch solution changed from white to blue-black.| Potato starch solution| The colour of the starch solution changed from white to blue-black.| Rice starch solution| The colour of the starch solution changed from white to blue-black.|
Table 2: Colour changes of starch products after added with iodine solution. Starch products| Observation|...