Finals FP2: Chapters 7 to 10

• Refrigeration and Freezing of Food

  • Most forms of food preservation in use today.
  • In case of refrigeration, the idea is to slow bacterial action to a crawl so that it takes food much longer to spoil.
  • In case of freezing the idea is to stop bacterial action altogether.
  • Frozen bacteria are completely inactive.
  • Used on almost all foods, meats, fruits, vegetables, beverages etc.
• Freezer


  -18 to -35°C (Frozen food)
• Refrigerator


  1 to 4°C (Fresh Food)
• The storage life of fresh perishable foods such as m e ats, fish, vegetables, and fruits can be extended by several days by storing them at temperatures just above freezing, usually between 1 and 4°C.
• Refrigeration

  • slows down the chemical and biological processes in foods.
  • also extends the shelf life of products.
• Freezing


  A method of food preservation whereby:

  • The heat is removed (heat of fusion)
  • Temperature of the food is reduceed below its freezing point (T<Tf)
  • a portion of water in food undergoes a change in state to form ice crystals (aw lowered)
• Preservation by Freezing achieved by:

  • Low temperature
  • Reduced water activity due to ice formation & high concentration of solutes in unfrozen water
  • Blanching of some foods
• Goals of Freezing

  1. To prevent the growth of microorganisms
  2. To lower temperature enough to slow down chemical reactions
  3. Physical, biochemical and microbiological degradation of food controlled by heat removing process.
• 3 Stages of Freezing

  1. Cooling to the freezing point (removing the sensible heat)
  2. Feezing (removing the latent heat)
  3. Further cooling to desired subfreezing temperature (removing the sensible heat of frozen food)
• Principles of Freezing

  • Temperature at the thermal curve centre of a food when heat is removed -> characteristic curve: FREEZING CURVE

  
• Freezing Curve


  1. Food is cooled to below its freezing point (sensible heat)

  2. Water remains liquid, although the temperature is below the freezing point (supercooling)

  3. Temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallization is released

  4. Heat is removed from the food at the same rate as before, latent heat is removed and ice forms, but temperature almost constant (freezing point is depressed by the increase in solute concentrations in the unfrozen liquor)

  5. Temperature of the ice-water mixture decreases to the temperature
• Supercooling


  • Partly determines the crystal size
• Major part of the ice is formed
• Specific Heat

  • is the quantity of heat that is gained of lost by a unit of mass of products to accomplish a unit change in temperature w/out the change in state (kJ/kg C)
• Sensible Heat

  • When an object is heated, its temperature rises as heat is added. the increase in heat is called sensible heat. Similarly, when heat is removed from an object ans uts temperature falls, the heat removed is also called sensible heat. (No change their state)
• Latent Heat

  • Pure substances in nature able to change their state. Solids can become liquids (ice to water) and liquids can become gases (water to vapor). These require the addition or removal of heat. The heat changes with state changes is called latent heat.
• Supercooling

  • Going below freezing point w/out the formation of ice crystals (crystallization)
  • It yields better quality food than if not present
  • This shoes that the undersirable effects of freezing are due to ice formation rather than reduction of temperature
• Crystallization occurs

  • at point B of the freezing curve
  • consist of nucleation and crystal growth
• Nucleation


  occurs by combining molecules into an ordered particle of a size sufficient to survive and serve as a rate for crystal growth
• Types of Nucleation

  1. Homogeneous nucleation : in pure systems
  2. Heterogeneous nucleation : nucleus formation around suspended particle or at a cell wall, in food systems, takes place during supercooling.
• Slow Freezing

  • Ice crystals grow in intercellular spaces
  • deform and rupture adjacent cell walls
  • Ice Crystals have a lower water vapor pressure than regions within the cells
  • water moves from the cell to growing crystals
  • cells: dehydrated and permanently damaged due to the increase of solute concentration
  • On thawing,
  • cells do not regain their original shape & turgidity
  • food is softened and the cellular material leaks out
• Fast Freezing

  • Smaller ice crystals form within both cells and intercellular spaces
  • little physical damage to cells, and water vapor pressure gradients are not formed
  • minimal dehydration of the cells
  • texture of the food is retained to a greater extent
• The localization of the crystals is determined by the freezing rate, the cellular structure and the temperature
• Effects of freezing on plant tissues

  a.) Slow Freezing

  b.) Fast Freezing

  
• Thawing

  • Freeze food in normal atmosphere, surface ice melts to form a layer of water.
  • Water has a lower thermal conductivity and a lower thermal diffusivity than ice.
  • Therefore, reduces teh rate of heat is conducted to the frozen food interior called "Insulating effect".
  • Foods are heated immediately to a specific temperature which is sufficient to destroy pathogenic micro-organisms
• Improper Thawing

  • Cold point effect - drip loss
  • contamination by spoilage and pathogenic micro-organisms
• How to overcome Cold Point Effect -Drip Loss

  • food is thawed by microwave or dielectric heaters
  • heat is generated within the food
• Volume Changes

  • Volume of ice is 9% higher than that of pure water
  • expansion of foods after freezing
  • the degree of expansion varies depend in factors
• The factors degrees of expansion

  1. Moisture content
  2. Cell Arrangement
  3. The concentration of solutes
  4. Freezer temperature
  5. Crystallized components
• Moisture Content

  • higher moisture content -> greater changes in volume
• Cell Arragement

  • plant materials have intercellular air spaces
  • absorb internal increases in volume without large changes in overall size
• Examples of Cell Arrangement

  Both are frozen to -20 degrees Celsius

  1. whole strawberries increase in volume by 3.0%
  2. coarselt ground strawberries increase by 8.2%
• The concentrations of solute

  • High Concentrations reduces the freezing point
  • No freeze or Expand
  • Commercial use freezing technology
• Freezer Temperature


  -determines the amount of unfrozen water and degree of expansion
• Crystallized components:

  • ice, fats, and solutes - when they cooled : reduces the volume of food
  • The volume changes leads to ; internal tensions -> mechanical damage
  • Firm vegetables tissues damage easier than flexible membrane of muscle tissue
• Volume change and Solute concentration


  main causes of damage of frozen food