Principles and methods of food preservation

Food preservation refers to the techniques used to prevent spoilage and decay of food, allowing it to be stored safely for future use. The primary goal is to maintain the quality, texture, flavor, and nutritional value of food while extending its shelf life.

Purpose of Food Preservation

1. Extension of Shelf Life: Prevents spoilage and increases the availability of food.
2. Year-Round Availability: Ensures seasonal foods can be consumed throughout the year.
3. Dietary Variety: Allows for diverse food choices and meal options.
4. Time and Energy Saving: Reduces preparation time and cooking efforts.
5. Price Stability: Helps in controlling market prices by reducing wastage.
6. Nutritional Improvement: Preserves essential nutrients during storage.

Causes of Food Spoilage

1. Microbial Activity: Growth of bacteria, yeast, and molds.
2. Enzymatic Reactions: Breakdown of food by natural enzymes.
3. Chemical Reactions: Oxidation, rancidity, and discoloration.
4. Physical Changes: Drying, freezing, or mechanical damage.
5. Pests and Rodents: Insects and rodents causing damage.

Types of Food Based on Perishability

1. Non-perishable (Relatively Stable): Low moisture content, resistant to microbial spoilage. Examples: Grains, pulses, nuts, sugar, dry beans.
2. Semi-perishable (Protectable): Moderate moisture content, spoil within a few weeks to a month. Examples: Potatoes, onions, flour, dried fruits, biscuits.
3. Perishable: High moisture content, spoil quickly without preservation. Examples: Milk, fruits, vegetables, meat, fish, poultry.

Principles of Food Preservation

1. Prevention of Microbial Decomposition: Asepsis (keeping microorganisms out). Filtration (removing microorganisms). Hindering growth (low temperature, drying). Killing microorganisms (heat, radiation).
2. Delay of Self-Decomposition: Inactivation of food enzymes (e.g., blanching).
3. Protection from External Damage: Preventing spoilage by insects, rodents, and mechanical causes.

Methods of Food Preservation

1. Preservation by Low Temperature Low temperatures slow down microbial growth and enzymatic activity.

• Freezing:
  • Preserves food for long periods.
  • Sharp Freezing: Slow freezing at -4°C to -29°C.
  • Quick Freezing: Rapid freezing at -32°C to -40°C, forming fine ice crystals, which better preserve food texture and quality.
• Dehydrofreezing: Partially drying fruits and vegetables (up to 50% weight reduction) before freezing.

2. Preservation by High Temperature; High temperatures kill microorganisms and inactivate enzymes.

• Pasteurization:
  Heat treatment below 100°C to kill most but not all microorganisms.
  • Methods:
    1. Batch or Holding Method: Heating milk at 65°C for 30 minutes, followed by rapid cooling.
    2. High-Temperature Short Time (HTST): Heating at 72°C for 15 seconds.
    3. Ultra-High Temperature (UHT): Heating at 135°C for 2–5 seconds, increasing shelf life without refrigeration.
• Blanching: Brief boiling (180–190°F) followed by cooling, used for vegetables to inactivate enzymes and retain color, texture, and flavor before freezing.
• Canning: Food is sealed in sterilized containers and heated to destroy microorganisms. Common for fruits, vegetables, and meat products.

3. Preservation Using Chemical Preservatives Preservatives retard microbial growth and prevent spoilage.

• Class I Preservatives (Natural): Salt, sugar, vinegar, honey, edible oils.
• Class II Preservatives (Synthetic): Benzoic acid, sorbic acid, sulphur dioxide.

Common Applications:

• Sodium benzoate: Used in fruit juices, ketchup, and soft drinks.
• Sulphur dioxide: Used in dried fruits, fruit juices, and wines.
• Sorbic acid: Used in cheese, baked goods, and beverages.

4. Preservation by High Osmotic Pressure Osmosis draws water out of microbial cells, inhibiting their growth.

• High Concentration of Sugar: Used for making jams, jellies, and preserves from fruits like apples, guavas, grapes, and pineapples. High sugar concentration binds water, preventing microbial growth.
• High Concentration of Salt: Used for preserving butter, cheese, vegetables, meats, and fish. Salt reduces water activity and prevents spoilage. In pickles, oil and spices further inhibit microbial growth.

5. Preservation by Dehydration (Drying) Drying reduces moisture content, preventing microbial growth and enzyme activity.

• Examples: Dried milk, eggs, coffee, tea, fruits, vegetables, meat, and fish.
• Moisture Requirements for Microbial Growth:
  • Bacteria: 18% moisture.
  • Yeast: 20% or more.
  • Molds: 13–16%.
• Proper drying ensures shelf-stable products without compromising quality.

6. Food Irradiation; Food is exposed to ionizing radiation (e.g., cobalt-60) to kill microbes and pests.

Applications:

1. Disinfestation of stored grains and spices.
2. Extension of shelf life for meat, poultry, and seafood.
3. Killing foodborne pathogens without altering taste, texture, or nutrition.

Irradiated foods are safe, wholesome, and free from chemical residues.

7. Preservation by Fermentation Fermentation converts sugars into acids, alcohol, or gases, preventing spoilage.

• Examples: Yogurt, cheese, sauerkraut, kimchi, pickles.
• Fermentation not only preserves but enhances the taste and nutritional value.

Summary of Major Preservation Methods