Adulteration Study: Revisiting Starch Iodine Reaction


Adulteration Study: Revisiting Starch Iodine Reaction

Dr. Anand S. Burange
Department of Chemistry, Wilson College, Chowpatty, Mumbai
anand.burange@wilsoncollege.edu.in ; asgburange@gmail.com
 
In this modern era, young generation is quite busy with their mobiles, laptops and other modern engineering feats for the recreation. They hardly get time to read books and to observe the world around. Lack of knowledge is one of the responsible factors for the degradation of moral values in a society we live. We consume lot of packed foods and sweets especially in India on occasion of various festivals. Do we really bother about the adulteration in variety of food products?
In our present work, our undergraduate students purchased variety sweet samples (Pedha: malai pedha, kesar pedha, etc) from various sweet shops in Mumbai (Area: Nalasopara). And the sweets were further evaluated for the presence of starch as one of the potential adulterants.
Typically, slurry of the sweet was first made in water and filtered under boiling condition. To the obtained filtrate, iodine solution was added to check the presence of starch. The presence of starch gives deep blue colouration with iodine solution. Surprisingly, all the samples were adulterated showed deep blue colouration with iodine samples. But during preparation of sweet some amount of starch is added as a binder. Starch is the most widely used adulterant in khoya which is used in pedhas. Few people use starch excessively to reduce the cost of the product. Therefore quantification of starch in the sample is necessary.

 Figure 1.  Wilson college research team  (From left Bhagyashri, Tushal, Ravish, Seema and Rubez)


Discovery of starch iodine reaction
Colin and Claubry discovered the starch-iodine reaction in 1814. The reaction is used in various qualitative and quantitative analysis in undergraduate and post-graduate practical.  

Absorption
Starch-iodine complex shows absorption at 620 nm. Various investigators tried to find out the nature of chromophore involve in this complex responsible for blue colour. 

Structural investigation
Starch is composed of unbranched linear amylose (α(1-4) linked glucoses) and branched amylopectin. Between them only linear amylose gives blue colour complex with iodine. Therefore one should call it precisely amylose-iodine complex rather simply starch iodine complex.


Figure 2. Structure of amylose


In a typical amylose-iodine complex, amylose acquires helical structure with wide central cavity. This cavity is occupied by iodine atoms. Helix of amylose-iodine referred as V-amylose (V: Verkleisterung) is probably left handed whereas inter-glucose hydrogen bonding is one of the reasons for its structural stability.  In the cavity iodine atoms are line up to form a polyiodide chin with average I-I separation of 3.1 A⁰.

Figure 3. Model of helical starch chain with iodine molecules in the center of the helix [Acquired from, J. Am. Chem. Soc., 1943, 65 (4), pp 554–558.]
 Reasons for deep blue colour of amylose-iodine complex
 Reasons for deep blue of the complex are enlisted below:
1. According to some researchers statistical disorder in I-I separation in polyiodide chain found to be one of the reasons along with some special (not well defined) type of I-I bonding.
2. Charge transfer interaction between iodine and hydroxyl oxygen and amylose glycosyl. 

How to deal with food adulteration?
Consumption of adulterated food and food products can cause various health hazards. As a social responsibility, we took up this project to find out current scenario in the market and results were shocking. Our next goal is to quantify the starch in the given sample. People who use starch excessively in food products, to such shopkeepers we can teach a good lesson by avoiding their food/food products and by creating awareness about such shops in our colony/area. 

 Bibliography
 1. Naturwissenschaften. 71, 31-36 (1984).
2. J. Am. Chem. Soc.  554–558 (1943)
 

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