Starch, amylase, and iodine test

1) Starch (amylose + amylopectin):

"Starch is generally insoluble in water at room temperature. Because of this, starch in nature is stored in cells as small granules which can be seen under a microscope. Starch granules are quite resistant to penetration by both water and hydrolytic enzymes due to the formation of hydrogen bonds within the same molecule and with other neighboring molecules.

However, these inter- and intra-HYDROGEN BONDS can become weak as the temperature of the suspension is raised. When an aqueous suspension of starch is heated, the hydrogen bonds break. "

(Source: http://www.eng.umd.edu/~nsw/ench485/lab5.htm)

Hence for boiled starch --> H bonds broken --> exposed amylose and amylopectin structure --> allow amylase to hydrolyse starch. 


In contrast, for unboiled starch --> amylase unable to hydrolyse starch as the starch granules are intact. 

2) Amylase:

Acts on α-1,4 glycosidic bonds (present in amylose and amylopectin). 

However, amylase is unable to break down amylopectin completely due to the α-1,6 glycosidic bonds present at the branch points. 

(Further reading: http://books.google.com.sg/books?id=xIj3YPKSpJkC&pg=PA453&lpg=PA453&dq=amylopectin+red+brown&source=bl&ots=uokPwYlqHY&sig=v_zrQ-kF7hCYj7So3lKARSbhBCo&hl=en&sa=X&ei=NM2dT6LCH8TJrAe8kcBm&ved=0CCcQ6AEwAQ#v=onepage&q=amylopectin%20red%20brown&f=false)

3) Iodine test 

The amylose (unbranched/linear portion of starch) forms helices, which allow iodine molecules to assemble, forming a dark blue/black color (refer to diagram below). 

The amylopectin (branched portion of starch) forms much shorter helices due to the branching present, and iodine molecules are unable to assemble, leading the color to be of red brown or red violet. 

As starch is broken down or hydrolyzed into smaller carbohydrate units, the blue-black color is not produced. Therefore, this test can determine completion of hydrolysis when a color change does not occur.

(Further reading: http://www.webexhibits.org/causesofcolor/6AC.html)

***Please refer to the SPA answer scheme (when it becomes available at IVLE) to see how the answers should be phrased for Q18,19 and 22.