National BiologyProperties of micro-organisms and their use in

industrya: Yoghurt and Cheese

RecapThink about:The different types of cells we have come

across so farAny 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 yoghurtExplain 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

making1. Bacteria is added to the milk to thicken it2. Rennet (an enzyme) is added to clot the

protein in the milk3. The solid curds are separated from the liquid

whey 4. The curds are moulded into cheese blocks

Jimmy's farm: making cheese

Cheese MakingJimmy's farm: making cheeseExperimentCollect three test tubes/boiling tubesPour in 5mls of milkAdd rennet to each tubeLeave to stand for 20minsObserve resultsWrite report of experiment in jotter

Bacteria in yoghurtThe Nigrosin Stain TechniqueFollow 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 milkYoghurt forming bacteria are added to the milkThe 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 makingJimmy'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 Quiz1. What is used to make yoghurt?2. What do the bacteria do to the yoghurt?3. What is the lactose in the milk converted

into and what effect does this have on the yoghurt?

4. Why is rennet added to thickened milk during cheese making?

5. What are the solid and liquid parts of cheese making?