LSE 9A (Freezing)

• – subjecting materials to a temperature of -40 C to -15 C; best result is

  achieved from faster freezing effect.

  Freezing
• □ abrupt removal of heat from food retards/inhibits growth of spoilage organisms

  and slows down enzymatic actions

  □ storage temperature should be constant, fluctuation may result in increased size

  of crystals that could greatly mark down the quality of frozen product.

  □ Storage duration: purees and juices,
• Freezing was an obvious preservation method to the appropriate climates. Any geographic

  area that had freezing temperatures for even part of a year made use of the temperature to

  preserve foods. Less than freezing temperatures were used to prolong storage times.

  Cellars, caves and cool streams were put to good use for that purpose.
• Freezing


  • Used for centuries by tribes in cold climates to preserve food
  • Used in shipping poultry since 1885
  • Research on techniques involving freezing has been around since World War II
• Icehouses


  1. Built on estates to store ice and food on ice

  2. Became known as "iceboxes"
• Mechanical refrigeration


  Invented in the 1800's and quickly put to use
• Quick freezing at very low temperatures

  • Discovered by Clarence Birdseye in the late 1800's
  • Made for better tasting meats and vegetables
  • Birdseye perfected his "quick freeze" process and revolutionized this method of food preservation
• Significance of FREEZING

  1.Helps reduce food wastage

  2. Speed and Convenience

  3. Cost-efficiency
• Storage Durations of Frozen Products


  Purees and Juices – stored up to 8 months under 5C or 1 year under 0°C

  precooked food – stored frozen up to 4 months only
• – done at temperatures close to but not reaching freezing point (7 oC)

  □ involves cooling by ice or mechanical refrigeration

  □ enzymatic and microbial

  Chilling
• removal of water vapour from commodities while in a frozen state

  enables the commodity to retain their shape and size.

  freeze drying
• Example for fresh flowers:


  □ Fresh flowers are placed on trays and loaded into the freeze-drying chamber.

  □ Moisture is locked into the cellular tissue of the flowers by freezing them solid.

  □ Air inside the chamber is pumped out creating a vacuum.

  □ Then ice within the flowers slowly changes to vapour.

  □ When these vapours contact the colder refrigerated ice condenser, they again

  change their state to solid ice.

  □ This cycle of solid-vapour-solid is described as sublimation. The end product is

  natural looking in appearance.
• where the products are placed

  freezer
• can be made of plastic or glass

  rigid containers
• Use bags made for freezing in sizes compatible with the amount of your

  food.

  freezer bags
• freezer paper and wraps-protects your food from freezer burn
• Types of freezer:


  1.Upright freezer

  2. Chest freezer

  3. Drawer freezer

  4. Display freezer

  5. Walk-in freezer
• materials should be table ripe and of prime quality

  selection
• – wash and drain;

  – peeled, trimmed, sliced;

  – blanching of materials that are prone to discoloration

  preparation
• for fruits intended for pies, jellies, jams or preserves or

  for people on special diets

  unsweetened dry pack
• best for dessert purposes;

  □ light syrup for mild flavoured fruits to prevent masking of flavour and

  heavy syrup for sour fruits;

  □ 40% sugar solution is recommended for most fruits

  □ syrups should be added cold to prevent cooking of materials; materials

  should be completely immersed in syrup

  Syrup pack
• – best for materials intended for cooking purposes because there is

  less liquid in the product.

  • Mix sugar with material until the juice is drawn out and sugar dissolves,

  place in the container and seal immediately

  Sugar pack
• – should be done immediately

  freezing
• Steps in freezing


  1. Selection

  2. Preparation

  3. Packing

  4.Freezing
• – spots from commodities that are dry, grainy and tough due to

  evaporation of ice crystals.

  freezer burn
• – drying of the surface of commodities due to evaporation

  of surface moisture induced by circulating air.

  surface dessication