In food chemistry, the iodine test is a simple laboratory test used primarily to detect the presence of which type of nutrient in a sample?

Introduction / Context:
Simple chemical tests are often used in school laboratories and food science to detect different nutrients in food samples. One of the best known is the iodine test, which produces a characteristic colour change in the presence of certain carbohydrates. This question checks whether the learner can correctly identify which nutrient the iodine test is designed to detect.

Given Data / Assumptions:

• The test in question is the iodine test.

• The options list different nutrient types: carbohydrate, fat, protein, cholesterol and vitamin C.

• It is understood that this test involves adding iodine solution to a sample and observing a colour change.

• The learner is expected to have basic school level knowledge of food tests.

Concept / Approach:
The iodine test is used to detect the presence of starch, which is a complex carbohydrate. When iodine solution (usually iodine in potassium iodide) is added to a sample that contains starch, a blue black colour develops. If starch is absent, the solution remains yellowish brown. Other nutrients like fats, proteins and cholesterol require different tests, such as the Sudan III test or the Biuret test. Therefore, the iodine test is specific for starch type carbohydrates.

Step-by-Step Solution:
Step 1: Recall that the iodine test involves adding a few drops of iodine solution to a food sample. Step 2: Remember that a deep blue or blue black colour indicates a positive result for starch. Step 3: Recognise that starch is a form of carbohydrate, specifically a polysaccharide built from glucose units. Step 4: Examine the options and identify carbohydrate (starch) as the nutrient that matches this reaction. Step 5: Select carbohydrate (starch) as the correct answer.

Verification / Alternative check:
School laboratory manuals and basic biology textbooks describe a standard set of food tests: the iodine test for starch, Benedict test for reducing sugars, Biuret test for proteins and Sudan III or grease spot tests for fats. These manuals clearly associate iodine with starch and show the characteristic blue black colour as the identifying feature. Consulting any of these references confirms that the iodine test is for starch type carbohydrate.

Why Other Options Are Wrong:
Fats do not give a blue black colour with iodine; instead, they are often detected using Sudan dyes or by observing translucent grease spots. Proteins respond to the Biuret reagent by developing a violet colour and not to simple iodine solution. Cholesterol requires more specialised biochemical tests and is not detected in this simple manner. Vitamin C is commonly tested with reagents such as DCPIP (dichlorophenolindophenol), not with iodine in the school level context. Hence, these options do not match the iodine test.

Common Pitfalls:
Learners sometimes confuse tests for sugars and starch because both involve carbohydrates. They may think that iodine detects all carbohydrates instead of remembering that it is specifically sensitive to starch. Another pitfall is to mix up the colours of different tests. To avoid such errors, it helps to memorise a small table that pairs each test, the nutrient it detects and the colour change observed, for example: iodine for starch (blue black) and Benedict reagent for reducing sugars (brick red precipitate).

Final Answer:
The iodine test is primarily used to detect carbohydrate in the form of starch.