Bacteria and yoghurtSpec 5.7
Yoghurt is made from milk using the bacterium _Lactobacillus_. The bacteria respire the sugar in milk, called lactose, and make lactic acid.
- The milk is heated to a high temperature (pasteurised) to kill unwanted micro-organisms. It is then cooled.
- Lactobacillus is added to the milk. The equipment is kept clean so that other bacteria do not get in.
- The mixture is incubated (kept warm), at about 40–45 °C, for several hours, so the bacteria grow and respire.
- The bacteria turn lactose into lactic acid. The pH falls, and the acid makes the milk proteins clump together, so the milk thickens and gets a sour taste. The low pH also stops many other micro-organisms growing, so the yoghurt keeps for longer.
- The yoghurt is cooled to slow down the bacteria, then packaged.
(opens full size in a new tab)Common mistake:
The acid is made from lactose, not from the milk fat or protein. The acid is what thickens the milk, by making its proteins clump.