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Food tests

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Lesson notes

Introduction to Food Tests

  • Food tests are used to identify the presence of specific nutrients in food samples.
  • Different chemical reagents produce characteristic colour changes when they react with particular nutrients.
  • The four main nutrients tested are **protein**, **sugar**, **starch**, and **lipids**.

Testing for Protein (Biuret Test)

  • Use **Biuret solution** (blue in colour).
  • Add a few drops of Biuret solution to a food sample in a test tube.
  • If **protein** is present, the solution turns **purple**.
  • If no protein is present, the solution remains **blue**.
  • Foods high in protein include meat, fish, eggs, cheese, milk, and nuts.

Testing for Sugar (Benedict's Test)

  • Use **Benedict's solution** (blue).
  • Add Benedict's solution to the food sample and heat in a **water bath** for a few minutes.
  • If **sugar** is present, the solution changes colour: **green → yellow → orange → red** (depending on concentration).
  • No colour change (remains blue) indicates no sugar.
  • Foods high in sugar include fizzy drinks, fruit juices, sweets, and cakes.

Testing for Starch (Iodine Test)

  • Use **iodine solution** (yellow/brown).
  • Add a few drops of iodine solution to the food sample.
  • If **starch** is present, the solution turns **blue/black**.
  • No colour change (remains yellow/brown) indicates no starch.
  • Foods high in starch include bread, pasta, potatoes, and rice.

Testing for Lipids (Ethanol Emulsion Test)

  • Use **ethanol** (alcohol).
  • Add ethanol to the food sample and shake, then pour into water.
  • If **lipids** are present, the mixture turns **cloudy/milky white**.
  • No cloudiness indicates no lipids.
  • Foods high in lipids include oil, cheese, butter, and some meats.

Summary of Colour Changes

  • **Protein** → Biuret: blue to **purple**.
  • **Sugar** → Benedict's: blue to **green/yellow/orange/red** (after heating).
  • **Starch** → Iodine: yellow/brown to **blue/black**.
  • **Lipids** → Ethanol: clear to **cloudy white**.

Benedict's test for reducing sugars: colour gradient from blue (negative) through green, yellow, orange to red (high concentration).

Benedict's test — reducing sugarcolour on heating — concentration increases left → rightBluenoneGreentraceYellowlowOrangemediumBrick-redhighincreasing reducing-sugar concentration

Iodine test for starch: yellow/brown solution turns blue/black in presence of starch.

Iodine test — starchadd iodine solutionOrange-brownnegativeBlue-blackpositive+ food

Biuret test for protein: blue solution turns purple when protein is present.

Biuret test — proteinadd biuret reagentBluenegativePurple / lilacpositive+ food

Ethanol emulsion test for lipids: clear solution turns cloudy white when lipids are present.

Emulsion test — lipidadd ethanol, then waterClearnegativeCloudy whitepositive+ food

Slides

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Practice questions

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  1. 1.Which food contains a lot of lipids?

    Easy
    • ABread
    • BApple
    • CButter
    • DRice

  2. 2.Which food contains a lot of protein?

    Easy
    • APasta
    • BEggs
    • COrange juice
    • DPotato

  3. 3.Which food contains a lot of sugars?

    Easy
    • ABread
    • BCheese
    • CFruit juice
    • DButter

  4. 4.Which food contains a lot of carbohydrates?

    Easy
    • AMeat
    • BFish
    • CBread
    • DEggs

  5. 5.An investigation is carried out to determine if a food sample contains sugars. Which test should be used and what colour change indicates sugar is present?

    Medium
    Benedict's test — reducing sugarcolour on heating — concentration increases left → rightBluenoneGreentraceYellowlowOrangemediumBrick-redhighincreasing reducing-sugar concentration
    • ABiuret test; blue to purple
    • BBenedict's test; blue to green/yellow/orange/red
    • CIodine test; yellow/brown to blue/black
    • DEthanol test; colourless to cloudy white

  6. 6.An investigation is carried out to determine if a food sample contains lipids. Which test should be used and what colour change indicates lipids are present?

    Medium
    Emulsion test — lipidadd ethanol, then waterClearnegativeCloudy whitepositive+ food
    • ABiuret test; blue to purple
    • BBenedict's test; blue to red
    • CIodine test; yellow/brown to blue/black
    • DEthanol test; colourless to cloudy white

  7. 7.An investigation is carried out to determine if a food sample contains protein. Which test should be used and what colour change indicates protein is present?

    Medium
    Biuret test — proteinadd biuret reagentBluenegativePurple / lilacpositive+ food
    • ABiuret test; blue to purple
    • BBenedict's test; blue to red
    • CIodine test; yellow/brown to blue/black
    • DEthanol test; colourless to cloudy white

  8. 8.Iodine solution is dropped onto a slice of bread. What will be observed and why?

    Medium
    • AThe iodine turns blue/black because bread contains starch.
    • BThe iodine turns purple because bread contains protein.
    • CThe iodine turns red because bread contains sugar.
    • DThe iodine remains yellow/brown because bread contains lipids.

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