What it shows
Curd forms when lactic acid bacteria grow in warm milk and turn lactose into lactic acid, which sets the milk proteins. You test milk at different temperatures to find the range in which the bacteria work best.
The science
Lactobacillus bacteria in the starter use milk sugar and make lactic acid. As the milk becomes more acidic its casein proteins clump together to form a soft gel, which we call curd. The bacteria grow best around 35 to 42 C. Cold slows them, and above about 50 C they are killed.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 1 litre of boiled milk, cooled in steps
- 4 spoons of fresh curd as starter
- Six small steel or glass bowls
- A kitchen thermometer
- A stopwatch or clock
- pH paper or turmeric strips, optional
- A warm box or a cloth to cover
Safety
For this project: An adult must boil the milk and pour it into bowls. Check the temperature before use and do not taste curd that smells bad or tastes bitter.
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.
Ask an adult to boil the milk and then pour portions into six bowls.
Let the bowls cool until they reach 20, 30, 35, 40, 45 and 55 C and label each with its temperature.
Add one spoon of starter curd to every bowl and stir gently with a clean spoon.
Cover each bowl and keep them all in the same room, or wrap bowls with the same cloth.
Check every two hours and note the time at which each bowl sets enough to hold a spoon upright.
Test pH of the finished curd with a strip if you have one.
Compare the texture and sourness at the end of 24 hours.
What you should see
The bowls at 35 to 42 C set first and are firm, the 20 C bowl is slow and the 55 C bowl may not set. A bar chart of setting time against temperature shows the best range.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What turns milk into curd?
Lactic acid bacteria that make lactic acid from milk sugar.
2Why does the 55 C bowl set poorly?
The heat kills or weakens the bacteria.
3Why is a starter needed?
It carries the bacteria that start the souring.
4Why does curd set slowly in winter?
The cold makes bacteria grow slowly.
Take it further
Compare setting time for different amounts of starter, from half a spoon to four spoons.
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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