Livestock Products
Senior 2 · Agriculture
Livestock Products
Lesson 2 of 4
22 min
Let's begin
Milk is the most valuable product of Rwanda's cattle, and the most perishable. A family that milks its Girinka cow twice a day must decide, every day, what to do with the milk: drink it, sell it at the collection centre, let it ferment into ikivuguto, or churn it into butter. Each choice is a way of preserving milk, and each keeps it for a different length of time.
In this lesson you will learn how milk is kept fresh on the farm and at the collection centre, how heat treatment makes it safe, and how it is turned into longer-lasting dairy products.
Cow's milk is mostly water (about 87%). The rest is milk fat (butterfat), proteins (mainly casein), milk sugar (lactose), minerals such as calcium, and vitamins. Lactic acid bacteria feed on the lactose and turn it into lactic acid: this is what makes milk go sour. When enough acid forms, the casein clots.
At a milk collection centre (MCC) the milk is tested, weighed, recorded for each farmer and cooled in a stainless-steel tank to about 4 °C. Two simple tests are common:
| Method | Temperature and time | Result |
|---|---|---|
| Boiling (at home) | Brought to the boil | Kills disease-causing bacteria; changes the taste a little. Boiled milk must still be kept cool and covered. |
| Low-temperature, long-time pasteurisation (batch) | 63 °C for 30 minutes | Destroys disease-causing bacteria and most spoilage bacteria; then cooled quickly. |
| High-temperature, short-time (HTST) pasteurisation | about 72 °C for 15 seconds | Same effect, faster; used in dairies. Pasteurised milk must be kept in a fridge and keeps for days to about two weeks. |
| Ultra-high temperature (UHT) | 135 °C for 1–2 seconds | Packed in sterile, sealed cartons, it keeps for months without refrigeration until it is opened. |
Pasteurisation is named after Louis Pasteur. It does not kill every bacterial spore, which is why pasteurised milk still needs refrigeration.
A cooperative has 100 litres of milk containing 4% fat. Butter is about 80% fat. What is the greatest mass of butter it could make? (Take 1 litre of milk ≈ 1 kg.)
In practice some fat stays in the buttermilk, so the real yield is a little less.
Tip
Memory hook: pasteurisation numbers go "63 for 30, 72 for 15": the lower the temperature, the longer the time. UHT is "135 for a blink" (1–2 seconds).
Exam technique: say what each process removes from the microbes: pasteurisation removes the live bacteria (heat), cooling removes warmth, ghee and milk powder remove water, fermentation adds acid.
Name the substance produced by bacteria that makes milk go sour, and the milk sugar it comes from.
Solution: Lactic acid, produced from lactose.
Why must milk be cooled to about 4 °C at the collection centre?
Solution: At about 4 °C bacteria multiply very slowly, so the milk stays fresh until it reaches the processor. Warm milk sours within hours.
A lactometer shows that a farmer's milk has a density of 1.01 g/mL. What does this suggest?
Solution: Normal milk is about 1.03 g/mL. A reading close to 1.00 (the density of water) suggests water has been added to the milk.
How much hard cheese can be made from 50 litres of milk, using 10 litres per kilogram?
Solution: 50 ÷ 10 = 5 kg of cheese.
Explain why ghee keeps much longer than fresh butter.
Solution: Butter still contains some water in which microbes can grow. Heating it to make ghee drives off almost all the water, so microbes cannot grow and it keeps for months.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
