Properties of micro-organisms and their use in industry
a: Yoghurt and Cheese
Recap
 Think about:
 The different types of cells we have come across so far
 Any processes that you are aware of that involve the
use of micro-organisms in day-to-day life
What are we learning today?
We are learning to:
 State what type of micro-organism is used in the
manufacture of cheese and yoghurt
 Explain the processes involved in the manufacture of
cheese and yoghurt
Success criteria:
 Explain lactic acid fermentation by bacteria
What type of micro-organism is
used to make cheese and yoghurt?
 Bacteria are used to make cheese and yoghurt
Cheese Making
• Cheese is also made from milk
• There are several steps involved in cheese making
Bacteria is added to the milk to thicken it
2. Rennet (an enzyme) is added to clot the protein in
the milk
3. The solid curds are separated from the liquid whey
4. The curds are moulded into cheese blocks
1.
Jimmy's farm: making cheese
Cheese Making
 Jimmy's farm: making cheese
 Experiment
 Collect three test tubes/boiling tubes
 Pour in 5mls of milk
 Add rennet to each tube
 Leave to stand for 20mins
 Observe results
 Write report of experiment in jotter
Bacteria in yoghurt
 The Nigrosin Stain Technique
 Follow your instructions carefully
You may
see rods
or cocci or
both on
your slide
How is yoghurt made?
 Yoghurt is made by the souring of milk
 Yoghurt forming bacteria are added to the milk
 The bacteria carry out lactic acid fermentation
lactose sugar
lactic acid
 The lactic acid thickens the milk and gives it the sour
flavour
 The pH of the yoghurt drops and acts as a natural
preservative
Yoghurt making
 Jimmy's Farm: making yoghurt
 You will now make yoghurt in the lab using bio
yoghurt as a starter culture (contains bacteria already)
The yoghurt making process
• The milk should be warm to provide good growing
conditions for the bacteria
• Once the starter culture has been added to the milk
the mixture is kept warm for at least 7 hours to allow
the bacteria time to multiply and act on the lactose
• After the milk has been converted to yoghurt it is put
in the fridge to prevent the bacteria continuing to sour
the milk which would make it inedible
Cheese and yoghurt Quiz
1.
2.
3.
4.
5.
What is used to make yoghurt?
What do the bacteria do to the yoghurt?
What is the lactose in the milk converted into and
what effect does this have on the yoghurt?
Why is rennet added to thickened milk during
cheese making?
What are the solid and liquid parts of cheese
making?