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Demonstration of Microscale Projection Experiments - Chemistry en miniature

Test for Starch in Foods

Objective: Starch-Iodine Complex

Peter Keusch




German version



Chemicals:
LUGOL SOLUTION  (1.3 g of iodine and 2 g of potassium iodide, dissolved in 100 mL water

Foods: Noodles, waxy rice


Hazards and safety precautions:

Iodine is toxic - may be fatal if swallowed or inhaled. Harmful by inhalation and through skin absorption. Readily absorbed through skin. Very destructive of mucous membranes and upper respiratory tract, eyes and skin. Severe irritant.
Lugol Solution is harmful if swallowed or absorbed through the skin.

Safety glasses, nitrile gloves, effective ventilation. Lugol solution should be prepared in a fume hood!


Experimental procedure

The material that is to be analyzed - noodles and rice, respectively - is cut into small pieces. Water is added to a small amount of the material. The suspension is heated and afterwards cooled to room temperature. In order to obtain a low concentration the cooled and filtered samples are diluted with water.

Test tube 1   1.5 mL of diluted, aqueous "noodle sample"  
Test tube 1 11.5 mL of diluted, aqueous "rice sample"



Results:
  Color of solution  
Test tube 1 blue
Test tube 2 red-brown





Photo
Photo

Discussion:

          Amylose Helix
Starch consists of two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-80%). Waxy rice consists of 100% amylopectin and no amylose.

Amylose and amylopectin are both polymers containing 1000's of glucoses. Amylose molecules consist of single mostly-unbranched chains of glucose molecules. All of the monomer units are a-D-glucose, and all the alpha acetal links connect C-1 of one glucose to C-4 of the next glucose. As a result of the bond angles in the a-acetal linkage the chain of amylose is coiled in the shape of a left-handed helix. Amylopectin differs from amylose in being highly branched. The glucose momomers within the branches are linked by a-1,4 glycosidic bonds, but where the branch joins the main branch a-1,6 link forms. Approximately every 12th-20th D-glucose unit of one amylose chain is "cross-linked" to another amylose chain. Figure below gives a partial structure for amylose and amylopectin.


Structure of amylose and amylopectin


Lugol solution contains polyiodide ions. Iodine is dissolved in water in the presence of iodide producing tri-iodide- and pentaiodide-ions:
I 2  +  I -   ®   [ J 3 ] -      I 2  +  [ I 3 ] -  ®  [ I 5 ] -


When starch is mixed with Lugol solution, a blue colored starch-iodine complex is formed.
          Amylose - [ I 3 ] - Complex
          Tri-iodide ion slips into the
        amylose coil
·   Amylose reacts with Lugol solution to produce a deep blue color. [ I 3 ] - and [ I 5 ] - (primary substructures) are linearly arranged in the central groove of the amylose helices. The complexes may react repeatedly to bind iodine molecules along the length of the amylose chain forming structures such as [ I 9 ] - and [ I 15 ] -. The hydrophobic inner surface of helix amylose holds a spiral of water molecules. The intense blue color of the complex is produced due to donor-acceptor interaction between water and the electron deficient polyiodides.

When heat is applied, the amylose-iodine complex is destroyed. When the solution has cooled the 'blue' of the amylose/iodine combination appears.

·   Amylopectin reacts with Lugol reagent solution to form a reddish brown to purple solution. It seems that the amylopectin-iodine complex involves a nearly linear [ I 4 ] - stabilized in the hydrophobic cavity of a small "amylose-like" helix, which is responsible for the change in color.

·   The starch-iodine complex is very useful for indicating redox titrations that involve iodine because the color change is very sharp.



Sources for the pictures:
Microsoft PowerPoint - Carbohydrates_lecture 2006
Starch - Iodine

References:
  Demonstration Experiment on Video Starch - widely available in many Foods


General experimental instructions and index of experiments






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