Low Temperature Preservation Flashcards
Where was food stored before any kind of low temperature preservation technologies?
Cold cellars
Holes in the ground
Natural caves
When did ice became a widespread means of cold preservation?
Middle 1800s
What was the first refrigerator called? Also, describe.
Ice box. A close wooden box with two compartments. Top compartment has the ice. Lower compartment has the food.
When was mechanical refrigeration introduced?
late 1800s
Chilling/refrigerated storage
Storage of foods above freezing and below 15 C (59 F)
Chilling is only a short-term preservation because of
Growth of microorganisms
Moisture loss
Post Harvest/-slaughter metabolic activities of intact plant and animal tissues
Deteriorative chemical reactions - autolysis of fish, nutritional loss, color degradation
What causes loss of moisture in chilling?
Low humidity inside the refrigerator
Does psychrophilic bacteria cause spoilage?
Yes.
Chilling injury
When fruits and vegetables develop physiological disorder because they are exposed to temperatures below optimum storage but above their freezing points.
Types of cooling methods
Blast chillers
Hydro coolers
Vacuum coolers
Blast chillers
Operate by passing cold air over food at high speed
Blast chillers
Air speed:
Air temperature:
Blast chillers
Air speed: 4 m/s
Air temperature: 4 C
Hydro coolers
Use of chill water, by spraying or immersion
In hydro coolers, there is no risk of ?
Freezing
Hydro coolers sometimes use
Fungicides
Hydro coolers are suitable for products…
Not affected by water
Vacuum coolers are mainly applied for
Pre-packed leafy vegetables with large surface area
Vacuum coolers
Food is placed in a vacuum chamber and the pressure is reduced to ~0.5 kPa.
Sous vide meaning
French - under vacuum
How does sous vide work?
Raw food is vacuum sealed in heat-stable, food-grade plastic pouches. These are cooked using precisely controlled heat (temperature and time).
Benefits of vacuum sealing in sous vide
Inhibits off-flavors due to oxidation
Prevents the loss of flavor volatiles and moisture during cooking
Reduces aerobic bacterial growth
Process of sous vide
Raw or par-cooked food
Vacuum sealing in plastic pouches
Heat treatment (controlled cooking)
Rapid chilling
Reheated for service
Dangers of sous vide
Pasteurization does not reduce pathogenic spores to a safe level. If not chilled rapidly enough or refrigerated for too long —> grow in dangerous levels
Chilled storage period of sous vide
21 days at 0-3 C
Minimum recommended thermal process for sous vide is
90 C for 10 mins or its t-T equivalent
Freezing
Unit operation in which the temperature of food is reduced below the freezing point and a proportion of water undergoes phase change to firm ice
Principle of freezing
1) Lowering of temperature by the removal of heat
2) Change from liquid to solid
For a product to freeze, it must be cooled…
Below its freezing point
Explain the process of freezing.
Water component freezes —> dissolved solids are left in a more concentrated solution —> this solution requires a lower temperature to freeze
True or false. In freezing, all water content is frozen.
False.
Fast freezing
Produces small crystals
For ice cream, fruits, and carrots, peas, and corn.
Main difference between slow and fast freezing.
Slow freezing result in shrunk cells and destructed tissue. Meanwhile, fast freezing maintains integrity of the cell and there are less damages to the tissue.
Quality changes due to freezing are encompassed by two phenomena
Freezing concentration effect
- loss of consistency
- syneresis
- chemical rxns
Large ice crystal damage and recrystallization
- physical rupture of cell walls and membranes and separation of plant and animal cells
- disruption of emulsion, foams, and gels
When does freezer burn occur?
There is headspace in the packaged food
Subjected to fluctuating temperatures
In freezer burns, there is _____ ______ because of fluctuating temperatures.
Moisture loss
Types of freezing equipment | cooled air freezer
Chest/still air cabinet freezer
Blast freezer
Belt freezer
Fluidized bed freezer
Still Air Cabinet freezer
Chest or cabinet systems
Chest vs cabinet freezer
Chest
- retain cold air
Cabinet
- lose cold air when door is opened
Advantage of still air cabinet freezer
Relatively simple, versatile, cheap
Used as storage after freezing
Disadvantage of still air cabinet freezer
Slow freezing
Blast freezer
Air speed:
Air temperature:
Blast freezer
Air speed: 400 m/min —> rapid heat transfer
Air temperature: -25 to -36 C
Advantage of blast freezing
Many types (spiral, cabinet, belt)
Fast freezing but slower than plate freezing
Disadvantages of blast freezing
High capital cost
Moisture loss can be a problem
Thawing
Application of heat until the ice present has reverted to water
True or false. Thawing can be more damaging than freezing.
True
Main considerations in thawing
Avoid overheating
Minimize thawing times
Avoid excessive dehydration
Thawing vs freezing
1) Thawing is slower
2) Thawing has less temperature differential
3) Thawing is usually less controlles
4) Thawing’s time temperature pattern may be more detrimental
5) Refreezing may be done in thawing
Thawing is microbiologically okay if
Temperature is low
How is thawing product dependent?
Quality must not suffer when planning to refreeze
How to ensure that refreezing after thawing is still microbiologically okay?
Remanufacture
Cook and refreeze
Test for microorganisms if not sure
Thawing time of frozen egg pulp using a microwave
15 mins
