This test is a qualitative test and doesn’t signify the concentration of starch. This test cannot be done under acidic conditions as the starch hydrolyses under such circumstances. Negative Test: A negative test is indicated by no change in color of iodine solution. Positive Test: A positive test is indicated by presence of a blue-black or purple color in the test tube. Note down the appearance of color seen in the test tubes. Heat the test tubes in the water bath until the color disappears. Normally, apples with a starch-iodine test score of 3 or 4 are good. Apples with scores of 7, 8, and 9 are considered overmature. Apples with scores of 4, 5, and 6 are considered mature. In general, apples with starch-iodine test scores of 1, 2, and 3 are considered immature. Observe the appearance of color in the test tubes. No single fixed score can be recommended to guide the whole industry. Add about 2-3 drops of Iodine solution to both test tubes and mix thoroughly. Similarly, take another 1 ml of distilled water in another tube. Take 1 ml of the test sample in a clean, dry test tube. Iodine Reagent: 0.5 ml iodine diluted in 5 ml distilled water and mixed with 10% potassium iodide to form Iodine solution (Lugol’s iodine). The Lugols iodine solution test indicates the presence of starch. Iodine test cannot be performed at very low PH due to the hydrolysis of the starch under these conditions. On the outside, the fruit ripening process is reflected in color, odor, and hardness. Also, the intensity of the color produced decreases with increasing temperature and with the presence of water-miscible organic solvents such as ethanol. The color obtained depends upon the length of the unbranched or linear chain available for complex formation. Monosaccharides on the other hand do not interact with the iodine, therefore no color is produced. Generally amylopectin, glycogen and cellulose do not form alpha-helices, they do not complex well with iodine, therefore, they do not show the blue-black color instead they show a purple or brown color. The amylopectic or branched portion of starch forms much shorter helices and iodine molecules are unable to assemble, leading the color to be of an orange/yellow hue. 
The amylose or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark-blue/black color. This reaction is as a result of the formation of polyiodide chains from the reaction of starch and iodine.
Triiodide anion instantly produces an intense blue-black color upon contact with starch. Starch contains alpha-amylose, helical saccharide polymer and amylopectin. This test depends upon the property of adsorption possessed by the large polysaccharide molecules. To test for the presence of starch in biological molecules.
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