Identification of the Presence of Starch in Various Foods Using Iodine PDF

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Starch lab report
Identifying sample foods' starch level using an iodine solution...

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Identification of the Presence of Starch in Various Foods Using Iodine Name: Sarah Scop, Completed: November 2, 2021

Introduction: The iodine-starch test was first described by J.J. Colin and H.F. Gaultier de Claubry in 1814 and independently by F. Stromeye in 1815. The reagent used in the iodine test is a very diluted form of Lugol’s iodine. Lugol’s solution was first made in 1829 by the French physician Jean Lugol. It is an aqueous solution of elemental iodine and potassium iodide. Iodine on its own is insoluble in water. The addition of potassium iodide to the iodine results in a reversible reaction of an iodide ion with iodine to form a tri-iodide ion, which further reacts with an iodine molecule to form a penta-iodide ion, and so on. The iodine test is based on the principle that poly-iodide ions form colored absorption complexes with the helical chains of glucose residues of certain polysaccharides. The tri-iodide and penta-iodide ions formed are linear and slip inside the helix structures which then produce visual colors upon contact with these polysaccharide molecules. The iodine test serves to detect and distinguish certain polysaccharides (starch, dextrin, and glycogen) from other carbohydrates. A positive test is reflected by the development of color which is polysaccharide-dependent: 1. Amylose (blue-black) 2. Amylopectin (orange-yellow) 3. Dextrin (Red) 4. Glycogen (reddish-brown) Pure starch consists of two types of molecules: the linear and helical amylose and the branched amylopectin. Amylose forms a very dark blue-black color when exposed to iodine, which effectively masks the orange-yellow color formed by amylopectin. This is why starch appears to only produce a blue-black color when exposed to iodine.

Factors that Can Affect The Colored Complex Formed: 1. Length of the glucose chains: Longer chains have higher color intensity. 2. Temperature: The colored polysaccharide-iodine complex temporarily dissociates when heated, but is re-formed upon cooling. 3. Presence of water miscible organic compounds: Can disturb color formation 4. pH level: A very low pH can cause the hydrolysis of the polysaccharide and color therefore would not form.

Practical Applications of the Iodine Test: 1. It is widely used in photosynthesis experiments as an indirect test for photosynthesis. The formation and presence of starch in such plants is confirmed or ruled out using the iodine test. 2. There was a relatively recent study done on ways to apply the iodine-starch test for amputees who suspect they may be suffering from hyperhidrosis. In a starchiodine test, an iodine solution will be applied to the areas that one most experiences excessive sweating in. Once the iodine solution dries on their skin, a small amount of starch will be placed on the affected area. The chemical reaction causes the starch and iodine combination to turn a dark blue color where they are experiencing excessive sweating.

In this experiment, a test was done using iodine solution to determine the levels of starch in glucose, a potato, and an onion. There were both positive and negative controls present: starch and distilled water, respectively.

Materials and Methods: 5 test tubes were set up in the rack. Test tube 1 was filled with 2 mL of starch solution using a graduated cylinder; test tube 2 was filled with 2 mL of glucose solution using a graduated cylinder; test tube 3 was filled with 2 mL of distilled water using a graduated cylinder; test tube 4 was filled with 2 mL of potato solution made by crushing a piece of potato with the mortar and mixing it with distilled water using a pipette; and test tube 5 was filled with 2 mL of onion solution made by crushing a piece of onion with the mortar and mixing it with distilled water using a pipette. 1 drop of iodine was added using a dropper to each test tube. Each test tube was then stirred with individual rods.

Results: Test tubes 1 and 4 both exhibited high levels of amylose. Test tube 5 exhibited low levels of amylose, and test tubes 2 and 3 exhibited medium levels of amylopectin.

Figure 1: Table of results indicating which solutions contained starch based on the iodine-starch test.

Discussion: The results of this experiment show that both potatoes and onions contain starch, and that as expected, glucose does not contain starch because it is not a polysaccharide. Possible errors include contamination of test tubes by air-borne droplets of solution, minor variations in measurement, and variation in solution strength. Despite the possible errors, the results are consistent with experiments done by peers and with results that have been posted online.

Bibliography: 1. ThomasTKtungnung, “Iodine Test For Starch Practical Experiment”, YouTube (Majority of intro, direct quotation) 2. “Starch”,Wikipedia (types of molecules starch consists of, direct quotation) 3. Colby Hansen, Ben Wayment, Stephanie Klein, Bradeigh Godfrey, “Iodine-Starch test for assessment of hyperhidrosis in amputees, evaluation of different methods of application”, pubmed.gov, 2017 (practical example 2, paraphrased) 4. “Hyperhidrosis”, suttonplacedermatology.com (practical example 2, paraphrased)...