IGCSE Food & Nutrition (0648) Study Notes: Food Preservation (Chapter 11)

Hello future food scientists! 👋

This chapter is all about keeping food safe and edible for longer. Why is this important? Because preserving food helps us reduce waste, save money, and enjoy seasonal ingredients all year round. Preservation methods are essentially a battle plan to stop the tiny enemies (microorganisms and enzymes) that cause spoilage.

Let's dive in and master the science behind making food last!

1. Reasons for Preserving Food

Why do we bother preserving food? It costs time and energy, but the benefits are huge!

Reasons to Preserve Food:
  • To prevent spoilage: To stop the action of
    bacteria, yeasts, moulds, and enzymes which make food unsafe or inedible.
  • To increase shelf life: Allowing food to be stored for longer periods (e.g., from weeks to years).
  • To maintain nutritional value: While some nutrients are lost, preservation aims to keep as much goodness as possible compared to allowing the food to spoil completely.
  • To provide variety: Allowing us to eat foods that are out of season (e.g., canned fruits in winter).
  • To aid distribution and transport: Preserved foods (like dried or tinned goods) can be transported long distances without needing immediate refrigeration.
  • To save money: Bulk buying when food is cheap and preserving it for later use.

Key Takeaway: Preservation gives us control over when and how we use our food resources, making food supplies safer and more reliable globally.

2. The Principles of Preservation: Stopping Spoilage

Before looking at methods, we must understand the principles. Spoilage organisms (like bacteria) need certain conditions to thrive. Preservation methods work by removing or limiting one or more of these conditions.

The Spoilage Enemies' Needs (Quick Review from Chapter 10):

Microorganisms (M/Os) need:

  • Temperature (Warmth, usually 5°C to 63°C)
  • Moisture (Water activity)
  • Food (Nutrients)
  • PH (Neutral environment)
  • Oxygen (for most moulds and some bacteria)

Memory Trick: T M F P O – Think of it as the recipe for rotten food!

Preservation techniques target these needs by:

  1. Killing or inactivating M/Os (using heat or chemicals).
  2. Slowing down M/Os (using cold temperatures).
  3. Removing the required moisture (drying/dehydrating).
  4. Changing the environment (adding acid/salt/sugar).

3. Methods of Food Preservation

The syllabus requires you to understand the following key methods and the principles behind them:

3a. Heating (Canning and Bottling)

Principle: Heat kills bacteria, yeasts, moulds, and destroys enzymes. Sealing the food prevents recontamination.

  • Canning (Tins): Foods are placed in metal cans, sealed airtight, and heated to very high temperatures (often above 115°C) under pressure. This achieves commercial sterility – meaning virtually all harmful organisms and their spores are destroyed.
  • Bottling (Jars): Often used for home preservation (e.g., jams, chutneys). Food is packed hot into sterilised jars, sealed, and sometimes further processed in a boiling water bath or pressure cooker.

Did you know? The heat used in canning must be high enough to kill spores of
Clostridium botulinum, a dangerous bacterium that produces toxins in anaerobic (no oxygen) conditions.

3b. Removal of Moisture (Dehydrating/Drying)

Principle: Microorganisms cannot grow or reproduce without water. Removing water lowers the water activity (\(A_w\)) below the point where spoilage can occur.

  • How it works: Water is removed using heat (sun or ovens/dehydrators) or sometimes freezing (freeze-drying).
  • Examples: Dried fruit (raisins, apricots), dried herbs, powdered milk, beef jerky, instant coffee.
  • Advantages: Reduced weight and volume make transport easy.
3c. Reduction in Temperature (Freezing)

Principle: Cold temperatures dramatically slow down enzyme activity and the multiplication of microorganisms. (Note: Cold doesn't usually kill the bacteria, it just puts them to sleep!)

  • Freezing: Stored at or below \(–18^\circ\text{C}\). The water turns to ice, making it unavailable to microorganisms.
  • Important Tip: Quick freezing is preferred because it creates smaller ice crystals, which causes less damage to the food's cell structure, thus better preserving texture and flavour upon thawing.

Analogy: Think of freezing like hitting the pause button on your favourite TV show. When you unpause (thaw the food), the action (spoilage) starts right back up again!

3d. Chemical Preservation (Sugar, Salt, Vinegar)

Principle: These chemical agents draw water out of the food and the microbial cells via osmosis, or they create an environment (low pH) that microorganisms hate.

  • Sugar: Used in high concentrations (e.g., jams and marmalades). The high sugar level pulls water out of the cells of spoilage organisms, inhibiting their growth.
  • Salt (Curing): Used for meat and fish. Salt acts via osmosis, drawing moisture out of the food. (e.g., salted fish, pickles).
  • Vinegar (Pickling): Vinegar is acidic (low pH). Most spoilage bacteria cannot survive in highly acidic conditions. (e.g., pickled vegetables).
3e. Modern Techniques
Modified Atmosphere Packaging (MAP)

Principle: Changing the composition of the air surrounding the food inside the package to slow down chemical changes (like rancidity) and microbial growth.

  • The air is often replaced with a mixture of gases, typically higher levels of carbon dioxide and/or nitrogen, and lower levels of oxygen (since oxygen speeds up spoilage).
  • Uses: Fresh meat, salads, prepared fruit.
Irradiation

Principle: Using controlled amounts of ionizing radiation (like gamma rays) to kill insects, parasites, and microorganisms (e.g., Salmonella) without significantly raising the temperature of the food.

  • This process is heavily regulated and controversial in some countries, but it is an effective way to extend shelf life, especially for spices and certain fresh produce.

4. Milk Processing and Preservation

Milk is highly nutritious but spoils quickly. Specific heat treatments are essential for safety and preservation.

Key Points of Milk Preservation:

1. Pasteurisation

  • Process: Heating milk to a temperature (e.g., 72°C) for a short time (e.g., 15 seconds), followed by rapid cooling.
  • Effect: Kills all pathogenic (disease-causing) bacteria, improving safety. It does not kill all spores or non-pathogenic bacteria, so the shelf life is still limited (a few days under refrigeration).

2. Sterilisation

  • Process: Heating milk to very high temperatures (around 113°C or higher) for a longer period (around 15–30 minutes).
  • Effect: Kills all bacteria and spores. Results in a very long shelf life, but it causes significant changes in flavour and nutritional value (often giving a 'cooked' flavour).

3. Ultra High Temperature (UHT)

  • Process: Heating milk to extremely high temperatures (around 135°C) for just 1–4 seconds, then cooling quickly and packaging aseptically (in sterile conditions).
  • Effect: Combines the benefits of sterilisation (long shelf life, often 6+ months) with better flavour and nutritional retention than traditional sterilisation. UHT milk does not need refrigeration until opened.
Preserved Milk Products:
  • Evaporated Milk: About 60% of the water is removed, and the milk is then sterilised and canned. It is thick and creamy.
  • Condensed Milk: Water is removed, and a large amount of sugar is added (the sugar acts as the main preservative). It is sweet and very thick.
  • Dried Milk (Powdered Milk): Nearly all water is removed by spraying the milk into a chamber of hot air. Extremely long shelf life and very light for transport.

Quick Review Box: Heat treatments increase safety and shelf life, but the higher the temperature (Sterilisation > UHT > Pasteurisation), the greater the change to the milk’s taste and nutrients.

5. Preservation by Fermentation (Cheese and Yoghurt)

Not all microorganisms are enemies! Some bacteria are useful for preservation.

Principle: The controlled growth of specific, harmless bacteria (often Lactic Acid Bacteria) converts lactose (milk sugar) into lactic acid. This acid acts as a preservative by lowering the pH (increasing acidity) to a level where spoilage organisms cannot grow.

Yoghurt Manufacture:
  1. Milk is pasteurised and cooled.
  2. A starter culture (specific bacteria like Lactobacillus bulgaricus) is added.
  3. The mixture is incubated (kept warm).
  4. The bacteria ferment the lactose into lactic acid.
  5. The rising acidity causes the milk proteins to thicken (coagulate), forming the characteristic texture of yoghurt.
Cheese Manufacture:
  1. Milk is pasteurised.
  2. Starter culture (lactic acid bacteria) is added.
  3. An enzyme called rennet (or rennin) is added. This enzyme causes the milk protein (casein) to coagulate rapidly, separating the milk into solid curds and liquid whey.
  4. The curds (protein and fat) are drained, pressed, and salted (salt also helps preservation).
  5. The cheese is then left to ripen or mature, where further enzymes (natural to the milk or added) develop the flavour and texture.

Key Takeaway: Fermentation uses beneficial bacteria to create an acidic, low-pH environment that naturally inhibits harmful bacteria, extending the product's life.