Decay
Science Practicals: GCSE, topic 1.1.11
The Rate of Decay of Biological Material
Biological material in the form of waste products or dead organisms decays as a result of the activity of decomposers. Decomposers are organisms such as worms (which eat the decaying matter) or microorganisms (which use enzymes to break down the organic matter). The rate of the decay process depends on:
Temperature
- High temperature is usually best.
Moisture availability
- High moisture availability is usually best.
Oxygen availability
- High oxygen availability is usually best.
Practical - Temperature and Milk Decay
Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
Outline
- The effect of temperature on the rate of decay can be observed using milk.
- As milk takes a long time to decay, lipase and sodium carbonate are added to speed up the decay.
- As milk decays, its pH reduces. This can be observed with an indicator called Cresol red.
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Method
- Use a water bath to heat all of the components to a given temperature.
- Mix the milk, sodium carbonate and Cresol red. The resulting solution should be purple.
- Add lipase to the solution and start timing.
- When the solution turns yellow, stop timing.
- Repeat using different temperatures.
Independent and dependent variable
- The independent variable is the temperature.
- The dependent variable is the time taken for the solution to turn yellow.
Test yourself on Decay
What are the ideal conditions for decay?
Which of the following conditions does NOT affect the rate of decay?
What is the independent variable in an experiment investigating the effect of temperature on the rate of decay of fresh milk by measuring pH change?
Which substances can be added to speed up the decay of milk?
What is the name of the indicator used to detect the changing pH of milk?