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Human Physiology - Food Tests

Grade 9IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Food tests are qualitative or semi-quantitative biological assays used to identify the presence of specific macromolecules: carbohydrates, proteins, and lipids.

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The Starch Test uses Iodine solution. A positive result is indicated by a color change from orange-brown to blue-black. This occurs due to the formation of a polyiodide complex within the amyloseamylose helix.

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The Reducing Sugars Test (e.g., glucose, maltose) uses Benedict's reagent (CuSO4CuSO_{4}). The mixture must be heated in a water bath at approximately 80∘C80^{\circ}C to 100∘C100^{\circ}C. The color changes from blue →\rightarrow green →\rightarrow yellow →\rightarrow orange →\rightarrow brick-red, depending on the concentration of the sugar.

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The Protein Test uses Biuret reagent, which consists of Sodium Hydroxide (NaOHNaOH) and Copper(II) Sulfate (CuSO4CuSO_{4}). A positive result is a color change from blue to violet or purple, caused by the reaction of Cu2+Cu^{2+} ions with peptide bonds.

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The Lipid Emulsion Test involves dissolving the sample in Ethanol (C2H5OHC_{2}H_{5}OH), then pouring the liquid into water. A positive result is the formation of a cloudy-white (milky) emulsion.

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Vitamin C Test: DCPIP (dichlorophenolindophenol) is used. The blue dye turns colorless in the presence of ascorbic acid (Vitamin C). The amount of Vitamin C is inversely proportional to the number of drops needed to decolorize the solution.

📐Formulae

Glucose+2Cu2++5OH−→ΔGluconic acid+Cu2O↓+3H2O\text{Glucose} + 2Cu^{2+} + 5OH^{-} \xrightarrow{\Delta} \text{Gluconic acid} + Cu_{2}O \downarrow + 3H_{2}O

C6H12O6 (Glucose structure)C_{6}H_{12}O_{6} \text{ (Glucose structure)}

Concentration of Nutrient∝Intensity of Color Change\text{Concentration of Nutrient} \propto \text{Intensity of Color Change}

💡Examples

Problem 1:

A student is given an unknown solution 'X'. Upon adding Benedict's solution and heating, the color changes to brick-red. When Iodine is added to another sample of 'X', the color remains orange-brown. What can be concluded about the composition of solution 'X'?

Solution:

The solution contains a high concentration of reducing sugars but contains no starch.

Explanation:

The brick-red color in the Benedict's test indicates a high concentration of reducing sugars (C6H12O6C_{6}H_{12}O_{6}). The lack of color change with Iodine (remaining orange-brown) confirms the absence of starch.

Problem 2:

Why is it necessary to use a water bath when performing the Benedict's test for reducing sugars?

Solution:

To ensure even heating and to prevent the degradation of the sugar molecules or the splashing of chemicals due to direct boiling.

Explanation:

Benedict's reaction is endothermic and requires an activation energy provided by heat. A water bath maintained at T≈90∘CT \approx 90^{\circ}C provides a controlled environment for the reduction of Cu2+Cu^{2+} to Cu+Cu^{+} ions.

Problem 3:

A sample of milk is tested with Biuret reagent. Describe the expected observation and the reason for it.

Solution:

The solution will turn from blue to violet.

Explanation:

Milk contains proteins (such as casein and whey). The Cu2+Cu^{2+} ions in the Biuret reagent react with the nitrogen atoms in the peptide bonds (−CONH−-CONH-) of the protein, forming a purple-colored coordination complex.