Required Practical 10: Decay

Introduction

  • Aim: Investigate how temperature affects the rate of decay, using milk and enzyme lipase.

  • Decay is slow → we model it with lipase (breaks down fats into fatty acids, lowering pH).

  • Indicator: Cresol red (purple in alkaline, yellow in acidic).

Method

1. Setup

  • Label test tube Lipase → add 5 cm³ lipase solution.

  • Label test tube Milk → add:

    • 5 drops cresol red (indicator).

    • 5 cm³ milk.

    • 7 cm³ sodium carbonate solution (makes solution alkaline, purple colour).

2. Prepare experiment

  • Place thermometer in milk test tube.

  • Put both tubes (lipase + milk) into a beaker of water at chosen temperature (start at 20 °C).

  • Wait until both solutions reach same temperature as water bath.

3. Reaction

  • Add 1 cm³ lipase → milk tube, stir, start timer.

  • Lipase breaks down fats → releases fatty acids → solution becomes acidic.

  • Cresol red turns yellow → stop timer, record time.

4. Repeat

  • Use a range of temperatures.

  • Repeat with clean test tubes (to avoid contamination with lipase).

  • Share results with other groups → calculate mean.

Variables

  • Independent variable: Temperature of water bath.

  • Dependent variable: Time for milk to turn yellow.

  • Control variables: Volumes of lipase, milk, sodium carbonate, and cresol red.

Results & Graph

  • At low temps → reaction slow (enzymes work slowly).

  • At optimum temp → fastest reaction.

  • Above optimum → enzymes denature, reaction slows/stops.

✅ Key Points / Summary

  • Models decay using lipase breaking down milk fats.

  • Indicator shows pH change from alkaline (purple) → acidic (yellow).

  • Optimum = warm conditions; too hot = enzymes denature.

  • Mirrors real-life decomposition: microorganisms work fastest when warm, moist, oxygen present, but not too hot.

Step-by-Step Method (Exam Style)

  1. Label a test tube Lipase. Use a pipette to add 5 cm³ lipase solution.

  2. Label another test tube Milk. Add:

    • 5 drops cresol red indicator.

    • 5 cm³ milk.

    • 7 cm³ sodium carbonate solution (solution should be purple).

  3. Place a thermometer in the milk test tube.

  4. Place both test tubes into a water bath at the chosen temperature (e.g. 20 °C).

  5. Wait until both solutions reach the same temperature as the water bath.

  6. Use a clean pipette to transfer 1 cm³ lipase into the milk test tube.

  7. Stir the mixture and start a timer immediately.

  8. Watch for colour change from purple → yellow.

  9. Stop the timer when solution turns yellow and record the result.

  10. Repeat the experiment at a range of temperatures.

  11. Use clean test tubes for each repeat to avoid contamination with lipase.

  12. Share results with other groups and calculate a mean time for each temperature.