Livestock Products
Senior 2 · Agriculture
Livestock Products
Lesson 1 of 4
18 min
Let's begin
Milk collected at six in the morning on a warm day in Nyagatare can be sour by early afternoon. A goat slaughtered for a celebration will smell bad within a day if the meat is left in the open. Livestock products (milk, meat, eggs, hides and skins) are among the most perishable foods and materials a farm produces: they spoil quickly.
This is why Rwanda has built a network of milk collection centres where farmers' milk is tested and cooled, and why butchers use cold rooms. In this lesson you will learn what makes livestock products spoil, the principles behind every preservation method, and why preservation matters to farmers and to the country.
Microbes need water, warmth (most grow fastest between about 20 °C and 45 °C), food, and usually air and a non-acid environment. Every preservation method works by removing one or more of these conditions.
In warm milk, some bacteria can double in number about every 20 minutes. Suppose freshly drawn milk contains 1,000 bacteria per millilitre and is left at room temperature for 3 hours.
This is why milk must reach a cooler quickly. At about 4 °C most of these bacteria almost stop multiplying.
| Principle | How it works | Methods |
|---|---|---|
| Low temperature | Slows (cooling) or stops (freezing) microbes and enzymes | cooling to about 4 °C, refrigeration, freezing at −18 °C |
| High temperature | Kills microbes and destroys enzymes | boiling, pasteurisation, sterilisation, cooking, canning |
| Removing water | Microbes cannot grow without water | sun or air drying, smoking, milk powder |
| Salt or sugar | Draws water out of microbes by osmosis | salting and curing meat, salting hides |
| Acid | Most spoilage microbes cannot grow in acid | fermentation (fermented milk, yoghurt, cheese), pickling |
| Removing air | Stops microbes that need oxygen and stops rancidity | canning, vacuum packing, airtight sealing |
| Chemicals and smoke | Kill microbes or slow their growth | smoking, approved preservatives |
Methods are often combined: a smoked sausage is salted, partly dried and smoked, then kept cool.
Tip
Memory hook: microbes need "WWFA": Water, Warmth, Food and (usually) Air. Take one away and they stop. Every preservation method you meet removes at least one.
Exam technique: when asked how a method preserves food, name the condition it removes: "salting preserves meat because salt draws water out of bacteria by osmosis".
Distinguish between a livestock product and a by-product, with one example of each from cattle slaughter.
Solution: A product is the main useful item, for example beef. A by-product is a secondary item obtained along the way, for example blood, bones or horns.
Give three causes of spoilage of livestock products.
Solution: Any three of: microorganisms (bacteria, yeasts, moulds); the product's own enzymes; oxidation of fats (rancidity); pests such as flies; physical damage.
Bacteria double every 20 minutes. Starting from 1,000 per mL, how many are there after 2 hours?
Solution: 120 ÷ 20 = 6 doublings; 26 = 64; 1,000 × 64 = 64,000 per mL.
Explain how drying preserves meat.
Solution: Drying removes water. Bacteria and moulds need water to grow, so dried meat does not spoil as long as it is kept dry.
Give two ways in which milk collection centres help dairy farmers.
Solution: They cool milk quickly so it does not sour, which cuts losses; they test milk quality; they bulk milk from many farmers so it can be sold to processors, giving farmers a steady market and regular payment.
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.
