RP10: Required practical activity 10: Decay and temperature
Not started yet — this one needs some love.
Aim: investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
Important: natural microbial decay in milk is too slow for a lesson, so this experiment uses a model — lipase is added directly to milk to simulate the enzyme activity of decaying microorganisms. In real decay, microorganisms secrete lipase, which breaks down lipids into fatty acids, lowering the pH. The model replicates this fall in pH.
Independent variable: temperature of the water bath (°C).
Dependent variable: time taken for the indicator to change colour (seconds), showing that the pH has dropped — fatty acids have been produced; rate = 1 ÷ time (s⁻¹). Alternatively, if using a pH probe: change in pH per minute.
Control variables:
same volume of milk
same volume of lipase solution
same volume of sodium carbonate solution (to set the starting pH)
same amount of indicator
freshly prepared lipase for each set of repeats
same starting colour (purple) of the milk mixture
Method (using Cresol red indicator)
Cresol red is purple at approximately pH 8.8 and turns yellow when pH drops below 7.2.
Set up a water bath at the first target temperature (e.g. 20 °C) in a beaker, using hot or cold water as needed. Check temperature with a thermometer.
Label two test tubes: one 'lipase' and one 'milk'.
Add 5 cm³ of lipase solution to the 'lipase' test tube.
Add 5 cm³ of fresh milk (not UHT) to the 'milk' test tube, then add 7 cm³ of sodium carbonate solution and 5 drops of Cresol red. The mixture should turn purple.
Place both test tubes in the water bath. Leave for 5 minutes until both reach the target temperature.
Transfer 1 cm³ of lipase into the 'milk' test tube using a pipette. Start the stop clock immediately. Stir continuously.
Record the time when the colour changes from purple to yellow — this is the endpoint (fatty acids have lowered the pH below 7.2).
Repeat steps 1–7 at different temperatures (e.g. 20, 30, 40, 50, 60 °C). Repeat each temperature three times and calculate a mean time.
Calculate rate = 1 ÷ mean time (s⁻¹). Plot a graph of rate (y-axis) against temperature (x-axis).
Alternative: replace Cresol red with a pH probe. Record pH every 30 seconds for a set time. Calculate rate as change in pH per minute.
Safety
Hot water baths — risk of burns and scalds from water above 50 °C; handle hot test tubes with a test tube holder; check temperature before placing hands near the water bath.
Sodium carbonate solution is an irritant at higher concentrations — wear safety goggles; wash any spills off skin immediately with water.
Enzyme allergy — some people are allergic to lipase; inform the teacher if you have a known enzyme allergy before starting.
Milk disposal — dispose of milk as instructed by the teacher; do not pour large volumes of acidified milk down the sink undiluted.
Common exam mistakes
The pH falls because microorganisms/lipase break lipids into fatty acids; do not say the milk simply decays by itself.
A lower pH means more acid has formed and more decay has occurred. Do not describe a pH decrease as "lower acidity".
On graphs, the higher-temperature line should usually start at the same initial pH and then fall faster, unless the temperature is high enough to denature enzymes.
Plot pH against time carefully and label axes fully; examiner reports repeatedly flag plotting errors, missing x-axis labels and incorrect tangent/gradient work.