Question

If any food item gives a blue-black color with iodine then which nutrient is present in the food?

Answer 1

Hint: The compound which gives blue-black color when reacted with iodine is also present in potato and plant leaves. It is a polysaccharide.

Step by step answer:
The compound starch is the compound which gives blue-black color.
A solution of iodine and potassium iodide is made. The solution has an orange or brown color. The starch has a compound called amylase which is responsible for the deep blue colored complex. Without amylase, the complex will be of yellow coloration.
The reaction is written as follow:
${{I}_{2}}+{{I}^{-}}\to I_{3}^{-}+{{\text{C}}_{\text{12}}}{{\text{H}}_{\text{25}}}\text{N}{{\text{O}}_{\text{11}}}\to Blue-Black\text{ }Compound$
Iodine $\left( {{I}_{2}} \right)$reacts with iodide ions of potassium ions $\left( {{I}^{-}} \right)$to form a triiodide complex $\left( I_{3}^{-} \right)$. This complex then acts with the starch compound to form the blue-black compound. The iodide ions dissociate from the potassium ion molecule$\left( KI \right)$.
Additional Information: Starch is a compound having multiple units of glucose bonded using glycosidic bonds. It is a white powder. It has two major constituents. Those compounds are amylase and amylopectin. It is used by plants as a storage unit. The human body uses starch to form glucose which further converts into ATP or adenosine triphosphate. In the commercial industry, it is used to thicken the foods and also make sweeteners.

Note: The food items having starch are potato, sweet potatoes, bread, rice, corn, peas, beans, grains, and pasta. They are a very enriched source of carbohydrates for plants and human beings. They have a high concentration of calories as well.