Chapter II: Low Temperature Storage

• Terms associated with low temperature storage:
• Low temperature storage is a method of preserving food
• Low temperature storage can preserve food and may be used as an adjunct to other food preservation methods
• Differentiating freezing and chilling/refrigeration:
• Chilling/Refrigeration:

  • Food stored between 1 - 4°C, just above the freezing point of water
  • Liquid content of the food remains in the liquid state
  • Minimal adverse effect on taste, texture, and nutritional value
  • Short-term method of preserving food
  • Does not kill microorganisms or inactivate enzymes, only slows down their effects
  • Temperature affects storage period and maintenance during storage
  • Every 10°C reduction in temperature means a 50% reduction in metabolic activity and spoilage rate
• Freezing:

  • Food stored lower than 0°C, never more than -10°C
  • Most water in the food is converted to ice
  • Freezing can inhibit microorganism activities without destroying them
  • Freezing rate affects damage to food, faster freezing causes less damage
  • Enzymes maintain activity, blanching is necessary prior to freezing
  • Freezing inhibits microbial growth and enzymatic reactions due to low temperature and lack of water
• Quality changes in frozen foods:
• Changes on long storage of frozen foods include oxidation of fat, growth of microorganisms, enzymatic reactions, and loss of surface moisture (dehydration)
• Chemical, physical, and flavor changes may occur during freezing and subsequent storage

  • Enzymes can cause loss of color, nutrients, and flavor in frozen food
  • Lipase activity is more problematic than proteases during frozen storage
• Treatment of food prior to cold storage influences enzymatic activities

  • Enzymes in fruits need to be controlled to prevent enzymatic browning
  • Oxidative rancidity can develop in frozen products through contact with air, proper packaging is essential
• Textural changes in frozen foods:

  • Water freezing causes expansion and rupture of cell walls, resulting in softer texture after thawing
  • High starch vegetables show less noticeable textural changes due to freezing
  • Sandy texture in ice creams due to temperature fluctuations during storage
• Color changes in frozen foods:

  • Pigment changes in frozen beef and fruits and vegetables during storage
  • Enzymatic action can produce color changes, blanching and chemical compounds can help control enzymatic browning
• Moisture loss and freezer burn:

  • Fast cooling and freezing prevent moisture loss and freezer burn
  • Freezer burn causes off flavors, prevented by thick moisture-proof packaging
• Nutritional value of frozen foods:

  • Proper freezing can preserve the greatest quantity of nutrients
  • Preparation prior to freezing affects nutritional merits, water-soluble vitamins are lost during thawing
  • Blanching inactivates enzymes but can destroy Vitamin C and B-vitamins
• Freezing processes:
• Air freezing:

  • Still air/slow freezing: Large ice crystals formed between cells, slow freezing process
  • Air blast/quick freezing: Small ice crystals formed within cells, quick freezing process
• Fluidized bed freezing:

  • Food placed on mesh belt, cold air forced upwards through the bed to lift particles, resembling a boiling fluid
• Plate freezing:

  • Contact freezing used for products frozen in packaged boxes or blocks, metal plates cooled with ammonia
• Block freezing:

  • Product embedded in a block of ice to reduce vulnerability to temperature fluctuations
• Immersion freezing:

  • Dipping or spraying refrigerant directly over food to be frozen
  • Methods include brine freezing and cryogenic freezing using low-temperature condenser or liquid nitrogen
• Glazing:

  • Applying a coat of ice over frozen food to prevent moisture loss during storage
  • Gel glaze involves dipping in cold gelatin or gelling substance before refrigeration