Food Preservation by Freezing

Principles of Freezing

• Freezing is a method of preservation that involves reducing and maintaining the product temperature below its freezing point.
• It involves the conversion of liquid water into ice crystals, creating obstacles to microbial growth and enzymatic reactions.
• Pure water is frozen at 0°C, but dissolved solids like sugars and acids lower the freezing point.

Water Crystallization and Solute Concentration

• Water crystallization leads to the immobilization of water, reducing the water activity (Aw) and inhibiting microbial growth.
• As water crystallizes, the concentration of dissolved solutes increases, further reducing Aw.

Preserving Effect of Freezing

• Freezing reduces water activity, inhibiting microbial growth and enzymatic reactions.
• Freezing below -12°C inhibits bacterial growth, while deep-freezing below -18°C inhibits the growth of bacteria, yeasts, and molds.

Advantages and Limits of Freezing

• Freezing preserves the microbiological, nutritional, and organoleptic quality of products for several months.
• However, it does not destroy microorganisms, only preventing their growth.
• Particular measures must be taken to ensure the quality of raw materials, hygiene, and temperature control.

Physical Aspects of Freezing

• Formation of ice crystals leads to changes in the product's texture and structure.
• The size of ice crystals affects the product's quality, with smaller crystals resulting in less damage.
• The rate of freezing affects the formation of ice crystals and the final product quality.

Freezing Rate and Time

• Freezing rate and time depend on various factors, including the nature of the product, temperature, pressure, packaging, and refrigerant type.
• Understanding the freezing rate is crucial for determining the time required for batch apparatus or the speed of products for continuous apparatus.

Ice Crystallization and Quality

• Ice crystallization is a key issue in maintaining the shelf-life and quality of frozen foods.
• The rate of freezing affects the quality of frozen foods, with quick freezing resulting in smaller ice crystals and less damage to the product's structure.### Freezing Equipments
• Plate Freezers:
  • Product is pressed between metal plates with channels for refrigerant
  • Slight pressure is applied to improve contact between food surface and plates
  • Increasing product thickness reduces heat transfer at the surface, limiting thickness to 50 mm
  • Two main types: horizontal plate freezer and vertical plate freezer
• Air Blast Freezers:
  • Adapted for bulk products, including beef quarters and fruits for further processing
  • Refrigerant is air or forced air cooled to -30°C to -40°C across the evaporator
  • Food is stacked on trays in rooms or cabinets, or on conveyor belts
  • Includes tunnel freezers, belt freezers, and fluidized bed freezers
• Immersion Freezers:
  • Product is immersed in a cooled freezing medium, such as propylene, glycol, or brine solution
  • Direct contact immersion used for fish freezing at sea
  • Indirect contact immersion used for surface freezing of poultry
• Evaporating Liquid Freezers:
  • LN2 Freezer:
    • Uses liquid nitrogen at -196°C to spray onto a single belt freezer
    • Very high freezing rate, but expensive operation
  • LFF (liquid fluorocarbon freezant) Freezer:
    • Freezant is purified dichlorodifluoromethane with boiling point of -30°C
    • Product is introduced into a container and dropped into a flowing stream of freezant

Types of Freezers

• Plate Freezers:
  • Horizontal plate freezer: product is placed on metal trays and manually pushed in between plates
  • Vertical plate freezer: mainly developed for freezing fish at sea, with vertical freezing plates forming partitions in a container
• Air Blast Freezers:
  • Tunnel freezers: product is placed on trays and passed through a tunnel with refrigerated air
  • Belt freezers: product is placed on a conveyor belt and frozen in a tunnel
  • Fluidized bed freezer: product is frozen individually in an air stream, often used for IQF (Individual Quick Freezing)
• Immersion Freezers:
  • Direct contact immersion: used for fish freezing at sea
  • Indirect contact immersion: used for surface freezing of poultry
• Evaporating Liquid Freezers:
  • LN2 Freezer: uses liquid nitrogen at -196°C
  • LFF Freezer: uses liquid fluorocarbon freezant at -30°C

Advantages and Limitations

• Plate Freezers:
  • Advantages: high freezing rate, low energy consumption
  • Limitations: limited to small products, product must be of uniform thickness
• Air Blast Freezers:
  • Advantages: versatile, can be used for various products, and easy to clean
  • Limitations: slow freezing rate, high energy consumption, and product may dehydrate
• Immersion Freezers:
  • Advantages: rapid freezing, low energy consumption, and no product dehydration
  • Limitations: salty taste into product, and brine contamination
• Evaporating Liquid Freezers:
  • Advantages: very high freezing rate, and improved texture for certain fruits and vegetables
  • Limitations: expensive operation, and may cause cracking of product surface

Examples

• Quick cooling of Foies Gras

• Deep-freezing of great scallop

• Chilling/deep-freezing of meat

• Quick chilling of Salmon### Storage

• Hazards to quality of frozen food during storage include:

  • Low relative humidity in the cold store
  • Retention beyond expected storage life
  • Fluctuation in temperature
  • Physical damage to product or packaging
  • Contamination of product by foreign bodies or vermin

Thawing

• Physico-chemical and microbiological aspects of thawing:
  • Exudation occurs during thawing, resulting in weight loss, creation of favorable medium for microbial development, and loss of nutritional compounds
  • Exudate depends on freezing technique, storage temperature, thawing technique, and presentation of pieces to be frozen

Thawing Techniques

• Air thawing:
  • Electric methods
  • Air blast thawing:
    • Air is ventilated with high humidity to improve convection coefficient and avoid desiccation of product
    • Cool air is forced at 4°C with humidity ~ 70% to limit microbial development
• Water thawing:
  • Not applicable to meat and fish fillets or cut surfaces due to waterlogging and flavor loss
  • Applicable for frozen whole fish with slight loss of pigments
  • Cheap and easy method with clean water supply
  • Methods include water immersion and water spraying
  • Hygiene and intercontamination are important, with T < 20°C to limit microbial development
• Vacuum thawing:
  • Products are placed on racks inside a container with air evacuation
  • Water is allowed to evaporate freely from heated vessels inside the container
  • Condensation of water vapor at 18°C – 20°C on product surface
  • Faster thawing than other surface heating methods for products less than 10 cm thick
• Electrical methods:
  • Three electrical methods: dielectric heating, electrical resistance heating, and microwave heating
  • Microwave heating is quick but can result in cooked products

This quiz covers the method of preservation by freezing, its advantages, and principles. Learn about the effects of freezing on microbiological quality and the benefits of air blast and tunnel freezers.