Identify the Starch in Food and Study the Microscope Appearances of Raw and Heated Starch

Name: Yap Hui Nie                     Student ID: 28903315

Experiment 2: Starch

Objective(s):

  The aims of the experiment are to identify the starch in food and study the microscope appearances of raw and heated starch.

Introduction:

  Starch is the most abundant carbohydrate reserve in plants and is found roots and seeds of plant. It is also the most important sources as it provides human energy. The granules have ordered structures which are semi-crystalline and birefringent. Starch granules in storage tissues can vary in shape, size and composition. The shape and size of the granules depends on the source, but in each tissue, there is a range of sizes and shapes. The diameter of the starch granule changes during the development of the reserve tissue (Cornejo-Ramírez et al., 2018).

  The starch granules consist almost entirely of two major polysaccharides, namely amylose and amylopectin. Amylose is a linear component of starch with 1,4 α glucose linkage and amylopectin made up by 1,4 α glucose linkage and 1-6 α glucose linkage thus it is extensively branched. Large starch grains with amylose and amylopectin commonly found in potato tubers, fruits and seeds of cereal (Bertoft, 2017).

  Raw starch is insoluble in water and it forms suspension when reacts with water. Starch granules exhibit a birefringence pattern that looks much like a Maltese cross under polarized light. When the starch is heated, it starts to absorb water and swell. Gelatinization starts when amylose leach out and the solution turns thicker and opaque. Different starch granules have different gelatinization temperature (Carolina and Almeida, 2015).

  The iodine test can be used to test for the presence of starch in food. Starch turns into an intense dark blue colour upon addition of aqueous solutions of the triiodide anion, due to the formation of an intermolecular charge-transfer complex. In the absence of starch, the brown colour of the aqueous solution remains.  

Materials:

Part 1:

Identify starch in food: iodine solution, 10% of tapioca starch solution (100mL), 10% of corn solution (100mL), 10% of potato solution (100mL), 10% of rice starch solution (100mL), bread (x1/4 slice), rice (a few granules), ripe and unripe banana (x1/2 banana each), Pasteur pipette (x5), glass slides (x5), blade (x1), glass rod (x1).

Part 2:

Microscope examination of raw and heated starch: iodine solution, 10% of tapioca starch solution (100mL), 10% of corn starch solution (100mL), 10% of potato solution (100mL), 10% of rice starch solution (100mL), light microscope (x1), magnetic stirring hot plate (x1), thermometer (x1), 500mL beaker (x4), Pasteur pipette(x4), cover slip (x1 box), glass slide, glass rod (x5).

Methods:

Part 1:

1. 100mL of water is poured into beaker that contains the respective starch sample. The solution is stirred well using glass rod until the starch is completely dissolved.

2. One drop of starch solution is put on a glass slide and the droplet is smeared into a thin layer on the glass slide using a glass rod.

3. A few drops of iodine are put onto the smeared starch solution and the observation is recorded.

4. The banana is cut into thin slices and the banana slices are placed on a paper towel. A few drops of iodine are put onto the banana slices and the observation is recorded.

5. The bread and rice granule are placed on a paper towel. A few drops of iodine solution are put onto the bread and rice and the observation is recorded.

Part2:

1. Starch solution that was stained with iodine (prepare in part 1) is examine under a light microscope.

2. The shape and size of the starch granules are sketched.

3. 100mL of tapioca starch solution is heated up to 85°c. A few drops pf the starch solution is put on a slide and a thin layer of the starch solution is smeared using a glass rod. The starch granules are examined under a light microscope.

4. Step 3 &4 for corn, potato and rice starch is repeated with the final heating temperature of 95°c, 70°c and 98°c, respectively.  

5. The shape of size of the starch granules are sketched. The raw and heated starch are compared.

6. The heated gel are cooled. The appearance of the gel whether the gel is translucent or opaque.  

Results:

Part 1:

Part2: