Investigating Decay

• Background information

  The decay of foods like milk is influenced by temperature and the action of enzymes such as lipase, which breaks down fats into fatty acids and glycerol. In this practical, you will investigate how temperature affects the rate of milk decay by monitoring pH changes due to lipase activity. By adjusting the temperature, you will measure the time it takes for the pH of milk to change, reflecting the rate of decay.
• independent variable

  temperature of milk
• dependant variable

  time taken for pH of milk to change
• control variables

  • type of milk
  • volume of milk
• Who is conducting the decay required practical?

  1. Chip
• What is the independent variable in the decay practical?

  The temperature
• What range of temperatures will be tested in the experiment?

  15, 30, 45, 60, and 75 degrees Celsius
• What is used to measure the rate of decay?

  An indicator that provides a color change
• What volume of milk is required for the experiment?

  5 milliliters
• What is the purpose of adding sodium carbonate to the milk?

  To ensure the milk solution is alkaline
• What is the concentration of the sodium carbonate solution used?

  0.5 moles
• What color does phenolphthalein produce in an alkaline solution?

  Pinky purple
• At what pH does phenolphthalein become decolorized?

  At pH 8.2 and below
• What happens to the milk as it decays?

  It becomes more acidic
• What do lipids in the milk break down into during decay?

  Glycerol and fatty acids
• What is the dependent variable in this experiment?

  The time it takes for decolorization
• What enzyme is used in the decay practical?

  Lipase
• Why is lipase used instead of microbes in this experiment?

  For speed in observing decay
• What temperature should the water bath be set to for the first experiment?

  60 degrees Celsius
• What should be done to ensure the test tube contents are at the correct temperature?

  Measure the temperature with a thermometer
• What is the relationship between microbial growth and decay rate?

  More microbes lead to faster decay
• What happens to the pink color during the experiment?

  It fades as lipids are broken down
• How much lipase is added to the milk?

  1 milliliter
• What should be done after collecting results at different temperatures?

  Repeat the experiment for mean averages
• What should be plotted on the graph after calculating mean averages?

  Temperature on the x-axis and time on the y-axis
• What is the optimum temperature for lipase activity?

  Around 37 degrees Celsius
• What are the steps to set up the decay practical?

  1. Measure 5 mL of full-fat milk.
  2. Add 7 mL of 0.5 moles sodium carbonate.
  3. Add 5 drops of phenolphthalein.
  4. Measure temperature and equilibrate to desired temperature.
  5. Add 1 mL of lipase and start timing.
  6. Record the time taken for decolorization.
• What factors control the decay rate in this experiment?

  • Temperature (independent variable)
  • Enzyme concentration (lipase)
  • Mixing consistency (control variable)
  • pH level of the solution
• step one

  measure out 5 cm cubes of lipase solution and add it to a test tube. Label this tube with L for lipase
• step 2

  • measure out 5 cm cubes of milk and add it to a different test tube
  • add 5 drops of phenolphthalein indicator to this
• step 3

  • measure out 7 cm cubed of sodium carbonate solution and add it to the tube containing milk and phenolphthalein
  • this makes the solution in the tube alkaline so it should turn pink
• step 4

  • put both tubes in a water bath set to 30 degrees celsius and leave them to reach the temperature of the water bath
  • use a thermometer into milk tube to measure this
• step 5

  • once tubes have reached 30 degrees celsius use a calibrated dropping pipette to put 1 cm cubed of lipase solution into milk tube and start a stopwatch straight away
  • stir contents of the tube with a glass rod
  • the enzyme will start to decompose the milk
• step 6

  • as soon as solution loses its pink colour stop the stopwatch and record how long the colour change took in a table
• step 7

  • repeat the experiment at a range of different temperatures
  • make sure to carry out experiment three times at each temperature then calculate the mean time taken for colour to change at each temperature
• step 8

  • use results to calculate the rate of decay using formula
  • rate =1000/time