Required Practical 10 - Decay

Aim:

Investigating the effect of temperature on the rate of decay of fresh milk by measuring pH change.

Variables:

  • Independent Variable: Temperature

  • Dependent Variable: Decay of milk (pH of the milk)

  • Control Variables: Same volume of Sodium carbonate/lipase/milk/Cresol Red, Same indicator type (Cresol red), Same type of milk (full fat)

Method:

  • Add 5cm³ of lipase to a test tube.

  • Label a test tube to know it’s the lipase test tube (avoid confusion).

  • Label another test tube with “milk” then add 5 drops of Cresol red then 5cm³ of milk and 7cm³ of sodium carbonate to the same test tube.

  • Milk test tube should be purple as the solution contains sodium carbonate (an alkali) and cresol red is purple in the presence of an alkali

  • Place a thermometer into the milk solution test tube then add both test tubes into a beaker of water/ water bath at the first chosen temperature (e.g. 20oC).

  • Wait until solution is same temperature as the water in the beaker/bath.

  • Transfer 1cm³ of lipase using pipette to the milk solution then stir the solution, start a timer at the same time

  • The enzyme will start to break down the fat molecules in the milk into fatty acids which causes the solution to become more acidic, changing the colour of the solution due to the Cresol red (turns yellow).

  • Repeat experiment at different temperatures (same increment each time).