What it shows
Milk sours because bacteria grow and make lactic acid, and their speed depends on temperature and whether the milk was boiled. Smell, pH strips and curdling on heating show how quickly each sample spoils.
The science
Fresh milk has a pH near 6.7. Bacteria that survive or fall in from the air ferment lactose into lactic acid, which lowers the pH and finally curdles the proteins. Warm conditions speed it up, cold slows it, and boiling kills most bacteria though some spores survive, so boiled milk lasts longer but not forever.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 1 litre of fresh packet milk
- Six identical clean glasses or jars with lids
- A fridge shelf
- A shaded shelf and a sunny window
- Turmeric or pH paper strips
- A thermometer
- A notebook
Safety
For this project: An adult must boil the milk. Do not taste any sample. Dispose of soured milk down the drain with water and wash hands after sniffing from a distance only.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Divide the milk into six clean jars of equal volume and ask an adult to boil two of the portions before cooling.
Place one raw and one boiled jar in the fridge, two in a shaded shelf and two in the sun.
Record the temperature of each place with the thermometer.
At the same time every four hours or every day, smell each jar from a distance and note any change.
Dip a strip in each jar and record its pH colour.
Ask an adult to heat a spoon of each sample and see if it curdles.
Continue for three days and draw a graph of the pH over time.
What you should see
Raw milk in the sun sours first, usually within a day, while boiled milk in the fridge stays fresh longest. The strips show a falling pH as each sample sours.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does milk sour?
Bacteria change milk sugar into lactic acid.
2Why does the fridge help?
Cold slows the growth of bacteria.
3Why boil milk?
Boiling kills most bacteria so it lasts longer.
4What happens to the pH as milk sours?
It falls, because acid is building up.
Take it further
Test how an added pinch of baking soda delays souring and find out why it is not a good idea to use it.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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