Introduction to Milk, Cheese, and Yoghurt
Welcome! In this chapter, we are going to explore how milk—a highly nutritious but very delicate food—is processed to make it safe to drink and last longer. We will also look at how "friendly" bacteria and enzymes are used to transform milk into delicious yoghurt and cheese. Since milk spoils very quickly due to its high water and nutrient content, understanding these preservation methods is a key part of Food and Nutrition.
Think of milk processing like a "security check" for your food: it removes the "bad guys" (harmful bacteria) so the food stays fresh and safe for you to enjoy!
1. Milk Processing Methods
Because milk is a perfect environment for bacteria to grow, it needs special heat treatments to make it safe for us to consume. Here are the key methods you need to know for your exam:
Pasteurisation
This is the most common method for fresh milk. The milk is heated to a specific temperature (usually around \(72^{\circ}C\)) for a very short time (about \(15\) seconds) and then cooled rapidly.
The Goal: To kill harmful bacteria (pathogens) without changing the flavour or nutritional value of the milk too much.
Storage: It must still be kept in the refrigerator.
Sterilisation
In this process, milk is sealed in bottles and heated to a much higher temperature (around \(113^{\circ}C\) to \(130^{\circ}C\)) for a longer time.
The Goal: To kill all bacteria and their spores.
The Result: It lasts much longer than pasteurised milk, but the heat changes the flavour and slightly destroys some vitamins (like Vitamin B and C).
Ultra-Heat Treatment (UHT) / Long-Life Milk
Milk is heated to a very high temperature (at least \(135^{\circ}C\)) for only \(1\) to \(2\) seconds.
The Goal: To create a product that is sterile but tastes more like fresh milk than "sterilised" milk does.
Storage: Unopened UHT milk can be stored in a cupboard for months. However, once you open it, you must keep it in the fridge!
Evaporated Milk
To make evaporated milk, about \(50\%\) to \(60\%\) of the water is removed from the milk through evaporation. It is then sealed in a can and sterilised.
The Result: It is thick, creamy, and has a slightly "cooked" or caramelised taste.
Condensed Milk
This is similar to evaporated milk because water is removed, but there is one big difference: lots of sugar is added.
The Goal: The high concentration of sugar acts as a preservative by making it hard for bacteria to grow. It is very sweet and thick.
Dried Milk (Milk Powder)
Almost all the water is removed from the milk, leaving behind a fine powder.
The Goal: Without water, bacteria cannot grow.
Storage: It has a very long shelf life and is lightweight, making it easy to transport.
Quick Review: Pasteurisation makes milk safe for a short time; Sterilisation and UHT make it safe for a long time. Evaporated and Condensed milk are concentrated forms, while Dried milk removes the water entirely.
2. The Manufacture of Yoghurt
Making yoghurt is a brilliant example of using "good" bacteria to preserve food. Don't worry if the idea of "bacteria" sounds scary—these are helpful ones!
The Process
1. Preparation: Milk is pasteurised and then cooled to a warm temperature.
2. Inoculation: Specific "starter" bacteria (such as Lactobacillus) are added to the warm milk.
3. Fermentation: The milk is kept warm. The bacteria eat the lactose (milk sugar) and turn it into lactic acid.
4. Thickening: The lactic acid causes the milk protein to thicken or "set," giving yoghurt its creamy texture and tangy flavour.
Key Takeaway: Yoghurt is made through the action of bacteria converting sugar into acid, which preserves the milk and changes its texture.
3. The Manufacture of Cheese
Cheese making is a slightly more complex process that uses both bacteria and enzymes to preserve the nutrients in milk.
The Role of Bacteria and Enzymes
1. Acidification: Just like in yoghurt, "starter" bacteria are added to warm milk to turn lactose into lactic acid.
2. Coagulation: An enzyme called rennin (often found in rennet) is added. This enzyme acts as a "biological scissor" that cuts the milk proteins, causing them to clot.
3. Curds and Whey: The milk separates into solid clumps called curds and a watery liquid called whey.
4. Processing: The liquid whey is drained away. The solid curds are salted, pressed into shapes, and left to "ripen" or "mature."
Analogy: Think of rennin like glue for protein. It sticks the protein molecules together so tightly that they separate from the water!
Key Takeaway: Cheese production relies on bacteria (to create acid) and enzymes (specifically rennin) to turn liquid milk into solid curd.
Summary Checklist
Before you move on, make sure you can answer these questions:
1. Can you name three ways heat is used to process milk? (Pasteurisation, Sterilisation, UHT)
2. What is added to condensed milk to help preserve it? (Sugar)
3. Which living organism is responsible for making yoghurt? (Bacteria)
4. Which enzyme is used to clot milk when making cheese? (Rennin)
5. What are the two parts milk separates into during cheese making? (Curds and Whey)
Important Tip for the Exam: Always remember the difference between bacteria and enzymes. Bacteria are living microorganisms that produce acid; enzymes (like rennin) are biological catalysts that cause the physical change of clotting.