Making Yoghurt - a delicious student activity

Each group will need:

»	2 measures of skim milk powder (each 40g)
»	500 ml of hot water
»	2 forks
»	2 mixing bowls
»	2 clean glass jars with lids or covers
»	Labels and marking pens
»	2 X 100ml measuring cylinder

What to do:

1.	Label the two bowls A and B. Put 40g of skim milk powder in each bowl.
2.	Gradually add half (250ml) of the hot water to one bowl, whisking with the fork.
3.	Using a clean second fork, repeat for the second bowl to use the last half of the water (250ml)
4.	Add the tablespoon of yoghurt to bowl A. The yoghurt is called the starter culture and usually contains the bacteria Steptococcus thermophilis and Lactobobacillus bulgaricus.
5.	Wash the forks and leave them to dry.
6.	Pour the contents of bowl A into a clean jar and label it "A".
7.	Pour the "B" bowl contents into the other clean jar. Label it "Control". This is the jar without the yoghurt.
8.	Cover and leave it in an incubator overnight. If you don’t have an incubator, place in a warm spot for four to five days.
9.	Record any changes. Write a summary of your investigations.

Question Time:

»	The Turkish word for yoghurt is ‘jugurt’. The word resembles the word junket. Try and find out what is the difference between junket and yoghurt.
»	When you made your yoghurt, why was it necessary to:
	a) have clean jar
	b) have a control
»	The breakdown of milk sugar (lactose) by the bacteria is shown by the equation: Lactose + Water = Lactic acid The lactic acid causes the casein molecules to coagulate in acidic conditions (pH of 4.6-4.7) From the above information, suggest why:
	a) yoghurt tastes sour
	b) the reaction will not occur if an antibiotic such as penicillin is added.
»	The traditional method for making yoghurt does not use homogenised milk. What does ‘homogenised’ mean?

Source: Janice Youl, Teacher Release to Industry 1997, Murray Dairy, Kyabram.