(9) Food preservation methods

Minimising decay of food from microorganisms like fungus and bacteria

Food preservation techniques are important to

Increase the shelf life of food products and ensure we don't suffer from food poisoning due to contamination from pathogens.

Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. It might refer to whether a commodity should no longer be on a pantry shelf and unfit for use, or just no longer on a supermarket shelf (unfit for sale, but not yet unfit for use).
 

Here, to make the food product as safe as you can, you want to

minimise the risk of contamination, and slow down the rate of decay as much as possible.

Therefore we need to apply our knowledge of the 'rate of decay' factors described in a previous section and the technicalities of food packaging e.g.

you can slow down the decomposition of food by reducing the temperature, minimising water content and keeping out air (oxygen) and potential pathogens.

 

Methods involved to minimise food decay and poisoning from fungi and bacteria

1. Keeping the food at a low temperature to slow down the rate of decay e.g. storing food products appropriately in a refrigerator (~2-4^oC ideally, NOT 0^oC or food freezes!)) or a freezer - the latter is so low in temperature (e.g. -18^oC) it stops decay all together!

The lower the temperature the lower the rate the microorganisms can degrade the food and therefore reproduce - less organisms - less decay- slower rate of decomposition.

This is all about slowing the rate of the enzyme decay chemical reactions.

2. If it is possible, and appropriate, to reduce the water content in food, this removes the medium the microorganisms need to survive and reproduce.

You can also add salt and/or sugar to food causes microorganisms to lose water by osmosis, denaturing the microbial cells and killing them - but note that modern dietary views advise us to minimise our intake of salt and sugar!

3. Storing the food in airtight conditions ensures neither oxygen or pathogens from air can get in and contaminate the food.

Food cans can be filled and sterilised at high temperatures to kill pathogens and the cans sealed under high pressure.

4. Many products are airtight sealed in plastic packages from which all the air (including oxygen) is removed - this is vacuum sealing and completely inhibits any microorganism from respiring aerobically.

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Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key points about food decay from fungi and bacteria and methods of food preservation

Fungal and Bacterial Food Decay

Food decay is caused by microorganisms, primarily bacteria and fungi, which break down organic matter.

This process leads to spoilage, changes in texture, taste, and safety concerns.

1. Causes of Food Decay

• Microorganisms: Certain bacteria (e.g., Salmonella, E. coli) and fungi (e.g., Penicillium, Aspergillus) grow on food, secreting enzymes that break down nutrients.

• Temperature: Warm conditions encourage microbial growth.

• Moisture: Microbes thrive in damp environments.

• Oxygen: Many bacteria and fungi require oxygen for respiration.

• Nutrient Availability: Sugars, proteins, and fats in food provide energy for microbial growth.

2. Food Decay by Bacteria

• Bacterial decomposition involves bacteria feeding on food and secreting enzymes to break down macromolecules.

• Some bacteria produce toxins, leading to food poisoning.

• Examples:

  • Salmonella (found in raw poultry) can cause foodborne illness.

  • Lactobacillus ferments sugars, useful in yoghurt production but can spoil other foods.

3. Food Decay by Fungi

• Fungal decomposition occurs when fungi secrete enzymes externally, breaking down food before absorption.

• Mould growth leads to visible spoilage.

• Examples:

  • Penicillium causes blue mould in fruits and bread.

  • Aspergillus can produce harmful aflatoxins.

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Methods of Food Preservation

Food preservation prevents decay by inhibiting microbial growth using various physical, chemical, and biological methods.

1. Temperature Control

• Refrigeration (0–5°C): Slows bacterial metabolism and reproduction.

• Freezing (-18°C or below): Stops microbial activity by solidifying water molecules, preventing enzyme action.

• Pasteurisation (heating to ~72°C for 15 sec): Kills most bacteria while maintaining nutritional value.

2. Dehydration and Drying

• Removing water inhibits bacterial growth, as they need moisture to survive.

• Examples:

  • Drying fruits and meats (e.g., beef jerky).

  • Powdered milk.

3. Salting and Sugaring

• Osmosis draws water out of microbial cells, dehydrating them.

• Examples:

  • Salting fish and meats.

  • Sugar-preserved jams.

4. Acidification (pH Control)

• Acidic conditions inhibit bacterial growth.

• Examples:

  • Vinegar in pickles and chutneys.

  • Lactic acid in yoghurt fermentation.

5. Vacuum Packing

• Removes oxygen so aerobic bacteria and fungi cannot grow.

• Used for meat and vegetable storage.

6. Radiation (UV and Gamma rays)

• Kills microbes and extends shelf life.

• Used for sterilising packaged foods.

7. Chemical Preservatives

• Sulphur dioxide prevents bacterial growth in dried fruits.

• Sodium benzoate is used in soft drinks.

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Summary of learning objectives and key words or phrases

Be able to explain and describe methods of how to preserve food from fungal and bacteria growth that will decay food i.e. to safely increase the shelf life of food.

Understand the methods of minimising decomposition by microorganisms including the prevention methods of air-tight packaging, low temperature storage and reducing water content.

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