Required practical

• Decomposition, or decay, is the breakdown of dead matter
• The rate of decay depends on:

  • Number of decomposing microorganisms
  • Temperature
  • Water and oxygen availability
• When plants wither and die, their pH reduces because bacteria in milk convert lactose sugar to lactic acid, reducing the pH
• Aim: To investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change
• Method:

  • Place 20 cm3 of fresh milk into three beakers
  • Decide on three temperatures to investigate (e.g. 5, 20, and 35°C)
  • Use universal indicator paper or solution to determine the pH of the milk in the three beakers
  • Cover each beaker and incubate at the appropriate temperature
  • Use universal indicator paper or solution to determine the pH after 24, 48, and 72 hours
• Results:

  • At 5°C, the pH reduced the least
  • Milk at 20°C and 35°C reduced to pH 4.8
  • Decay occurred quickest at 35°C
• Limitations in drawing conclusions:

  • No repeat readings were taken for concordant results
• To find out if skimmed milk decays less quickly than fresh milk:

  • Use similar quantities of fresh and skimmed milk at 35°C
  • Measure pH at the start, after 24 and 48 hours (not 72 hours)