Investigate food nutrients in various food samples

When writing a food test report, use a table with four columns:

| Food sample tested | Procedure | Observation | Inference |

• Food sample tested: The food being tested (e.g., onion, potato, egg white)
• Procedure: The steps followed during the test
• Observation: The changes observed (e.g., color change,沉淀 formation)
• Inference: The conclusion about which nutrient is present or absent

Carbohydrates are classified into three types: monosaccharides (simple sugars), disaccharides (double sugars), and polysaccharides (complex sugars).

Test for Monosaccharides and Reducing Sugars (Benedict's Test)

Principle: Reducing sugars (like glucose) can reduce copper ions in Benedict's solution from blue (Cu²⁺) to orange or brick-red (Cu⁺).

Materials: Benedict's solution, food sample solution, test tube, test tube holder, heat source, water

Procedure:

1. Put 2 ml of the food sample solution into a clean, dry test tube.
2. Add 2 ml of Benedict's solution.
3. Gently heat the mixture for 2–3 minutes.
4. Observe the color changes.

Observations and Inferences:

Worked Example: Testing onion for reducing sugars

When 2 ml of onion solution was mixed with 2 ml of Benedict's solution and heated, the color changed from blue to orange. This indicates that onion contains reducing sugars (monosaccharides).

Test for Non-Reducing Sugars (Sucrose)

Principle: Sucrose is a non-reducing sugar that must first be hydrolyzed (broken down) into glucose and fructose using hydrochloric acid (HCl) before testing.

Procedure:

1. Put 2 ml of food sample solution into a test tube.
2. Add 1 ml of dilute hydrochloric acid (HCl).
3. Heat gently for 2–3 minutes, then cool.
4. Add sodium hydroxide (NaOH) or sodium bicarbonate to neutralize the acid.
5. Add 2 ml of Benedict's solution and heat again.
6. Observe color changes.

Observation: If the color changes to orange or brick-red, non-reducing sugar is present.

Test for Starch (Iodine Test)

Principle: Starch reacts with iodine solution to produce a blue-black color.

Materials: Iodine solution, food sample solution, test tube, dropper

Procedure:

1. Put 2 ml of food sample solution into a test tube.
2. Add 2–3 drops of iodine solution.
3. Shake thoroughly and observe the color.

Observation and Inference:

Worked Example: Testing Irish potato for starch

When iodine solution was added to potato solution, a blue-black color appeared. This confirms that Irish potato contains starch.

Principle: Proteins react with sodium hydroxide (NaOH) and copper sulphate (CuSO₄) to produce a violet or purple color.

Materials: Egg white solution, 1M sodium hydroxide solution, 1% copper sulphate solution, test tube, dropper

Procedure:

1. Put 3 ml of water and 2 ml of egg white solution into a test tube and shake.
2. Add 2 ml of sodium hydroxide solution.
3. Shake the mixture.
4. Add copper sulphate solution drop-wise while shaking.
5. Observe the color.

Observation and Inference:

Lipids include fats (solid) and oils (liquid). Several tests can confirm their presence.

Test 1: Grease Spot Test

Procedure:

1. Rub a food sample (e.g., groundnut seeds) on a piece of white paper.
2. Hold the paper up to light.
3. Compare the rubbed area with the unrubbed area.

Observation and Inference:

Test 2: Sudan III Test

Procedure:

1. Put 3 ml of water and 2 ml of cooking oil into a test tube.
2. Add 2 drops of Sudan III reagent.
3. Shake well and leave for 3–5 minutes to settle.

Observation and Inference:

Test 3: Emulsion Test

Procedure:

1. Dissolve the food sample in ethanol (or ether).
2. Add water to the solution.
3. Shake well.

Observation and Inference:

• Never taste any food or chemical in the laboratory.
• Handle all chemicals with care.
• Use a test tube holder when heating; never hold test tubes with your hands.
• Heat test tubes at the top of the solution, not at the bottom (to prevent explosions from rapidly heated gas bubbles).
• Keep flammable substances away from heat sources.
• Ensure proper ventilation when heating.

In Tanzania, knowing how to test for food nutrients helps you make informed decisions at the market or in your household. For example, when buying groundnut oil for cooking, you can perform a simple grease spot test using a white cloth or paper to verify that the oil is pure and not mixed with cheaper substances. This practical skill ensures you get value for your money, especially when purchasing food items in local markets such as Kariakoo in Dar es Salaam or Mwanza's Central Market.