Dairy Technology II – SS3 Livestock Farming Lesson Note (Second Term)

Dairy processing is the turning of fresh milk into products such as yoghurt, cheese and butter.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State three reasons for processing milk.
2. Describe the steps in making yoghurt.
3. Describe how cheese is made, including local methods used in Nigeria.
4. Describe how butter is made by churning.
5. State the safety and quality rules to follow when processing milk.

What is in milk?

Milk is made of water, fat, protein, lactose (milk sugar) and minerals, and each dairy product uses a different part.

• Fat: rises as cream and is used to make butter.
• Protein (casein): clumps into curds and is used to make cheese.
• Lactose: is changed to lactic acid by bacteria during yoghurt making.
• Water: forms most of the milk and becomes whey or buttermilk when solids are removed.

Why do we process milk?

We process milk because fresh milk spoils quickly, and processing keeps it longer and makes it more valuable.

• Preservation: processed products such as cheese and butter last longer than raw milk.
• Variety: one kind of milk gives products with different tastes and textures.
• Value addition: a cup of yoghurt often sells for more than a cup of raw milk.
• Less waste: extra milk that cannot be sold fresh is turned into products.

What is pasteurisation?

Pasteurisation is the heating of milk to a set temperature for a set time to kill harmful bacteria without spoiling the milk.

• Batch (holder) method: heat to about 63°C and hold for 30 minutes.
• High temperature short time (HTST) method: heat to about 72°C for 15 seconds.
• After heating: cool the milk quickly so that new bacteria do not grow.

How is yoghurt made?

Yoghurt is a thick, sour milk product made by fermenting milk with friendly bacteria that turn lactose into lactic acid.

Steps in making yoghurt

1. Heating (pasteurisation): heat the milk to about 85°C for 30 minutes to kill harmful bacteria.
2. Cooling: cool the milk to about 40 to 45°C. Hotter milk would kill the starter bacteria.
3. Inoculation: add a starter culture, such as a few spoons of plain yoghurt with live bacteria.
4. Incubation: keep the milk warm and still for about 6 to 8 hours until it turns thick and tangy.
5. Cooling and flavouring: chill the yoghurt in a fridge, then add sugar or fruit if wanted.

• Starter bacteria: the usual starter bacteria are Lactobacillus and Streptococcus.
• Why it thickens: the acid makes the milk protein set into a soft gel.

How is cheese made?

Cheese is a solid dairy food made by separating the curds (milk solids) from the whey (liquid).

Steps in making cheese

1. Coagulation: add a coagulant to make the milk protein clump together.
2. Separation: the milk splits into white curds and a yellowish liquid called whey.
3. Draining: pour the mixture into a clean cloth or basket so the whey drips away.
4. Pressing and salting: press the curds into shape and add salt for flavour and preservation.

What can be used to make milk clump?

• Rennet: an enzyme used in factory cheese making.
• Sodom apple leaf juice: used by many Nigerian farmers, especially the Fulani.
• Acids: vinegar or lemon juice can also make milk curdle.

What is wara?

• Wara: a soft local cheese made in Nigeria by curdling milk with Sodom apple leaf extract.
• Nono: a local fermented milk drink made by the Fulani.

How is butter made?

Butter is a solid food made by churning the fat (cream) of milk until it clumps together.

Steps in making butter

1. Cream collection: let fresh milk stand so the cream rises, then scoop it off.
2. Churning: shake or stir the cream hard so the fat globules stick together.
3. Separation: the fat forms a yellow solid (butter) and leaves a watery liquid called buttermilk.
4. Washing: wash the butter with cold, clean water to remove leftover buttermilk, which would make it go bad.
5. Salting and packing: add a little salt and pack the butter firmly.

How do yoghurt, cheese and butter compare?

Yoghurt is made by fermentation (bacteria), cheese by coagulation (curdling) and butter by churning (shaking).

What safety and quality rules apply in milk processing?

Milk processing must be done in very clean conditions, because milk is an ideal food for harmful bacteria.

• Fresh milk only: do not use milk that already smells sour to make yoghurt or cheese.
• Clean equipment: boil containers and spoons before they touch the milk.
• Clean hands: wash hands and keep hair and clothing clean.
• Cool storage: keep yoghurt, cheese and butter cool so they stay fresh.
• Safe water: use clean, safe water for washing butter and equipment.

Class discussion and practical activity

1. Local knowledge: Have you seen wara being made in your community? Which plant is used to make the milk clump?
2. Comparison: Why does yoghurt keep longer in the fridge than fresh milk?

The butter practical

1. Pour some fresh cream into a small, clean jar and close the lid tightly.
2. Let students take turns shaking the jar for 10 to 15 minutes.
3. Watch for the yellow butter to form and the buttermilk to separate.
4. Pour off the buttermilk, wash the butter in cold clean water and add a little salt.

Evaluation

1. State three reasons for processing milk.
2. List the five steps in making yoghurt. Why must the milk be cooled before the starter culture is added?
3. Name two things used to make milk clump in cheese making.
4. What is buttermilk, and why is butter washed with cold, clean water?
5. Why should you not use sour raw milk to make yoghurt or cheese?

Assignment

Draw a table showing the key component used, the key process and the liquid left over for yoghurt, cheese and butter.