Question

Given below from (i) to (iv) are some food items.
(i) Boiled mashed potato
(ii) Glucose solution
(iii) A slice of bread
(iv) Mustard oil

Which of the above will give blue-black colour when tested with iodine?
A) (i) and (ii)
B) (i) and (iii)
C) (ii) and (iii)
D) (iii) and (iv)

Answer 1

Hint:The presence of starch in the sample will cause it to change its colour to blue-black when a few drops of potassium iodide solution are put on the sample. The iodine test for starch cannot be conducted on very dark solids or liquids which do not cause colour change to be detected.

Complete answer:
Iodine testing for starch is used to assess the presence of starch in biological products. Starch is a storage molecule that can only be present in a plant. The presence of starch can be positive on plants and plant-based products. If an animal product tests positive for starch, it may be contaminated or combined with a plant product.

The reaction produced in the test is due to the formation of polyiodide chains from the interaction of starch and iodine. Amylose in starch forms helices where iodine molecules are assembled giving rise to a dark blue or black colour. When starch is broken down or hydrolyzed into smaller carbohydrate units, the colour blue-black is not formed. Therefore, this test can also show the completion of the hydrolysis if there is no change of colour.

Starch is a complex carbohydrate present in potatoes, whole grains and cereal grains, composed of multiple glucose chains. Therefore, boiled mashed potato and a slice of bread would give a blue-black colour when tested with iodine.

Thus, the correct answer is option C i.e., C. (ii) and (iii).

Note:All carbohydrates contain elements such as carbon, hydrogen and oxygen. Glucose and starch are two of the most common carbohydrates. A starch is a complex polysaccharide composed of many glucose units linked together by glycosidic bonds.