1
Freezing and refrigeration
2 Semester 2020
Ranil Coorey
Refrigeration
Refrigeration
• Generally above 00C to –10C
• Minimal sensory change
• Minimal nutritive change
• Extend shell-life
• Together with
– Pasteurisation
– Fermentation
– Packaging
Refrigeration
• Temperature control
– Prevent spoilage
– Rate of chemical and bio-chemical reactions
– Rate of microbial activity
• High risk food and processing facilities
– Prevent contamination
– Prevent cross contamination
– Positive pressure production facility
– All other facilities
Refrigeration
• Must go through 600C to 50C quickly
• Monitor temperature
– Product
– Air / chamber
– Where in the product ?
Freezing
2
History
• Ice – salt systems
• 1842 Mechanical refrigerator
• Clarence Birdseye – quick freezing
In food
• Cells contain
– Dissolved solids
– Organic and inorganic solvents
– Salts
– Gasses
– Proteins
– Carbohydrates
– Etc
In freezing
• Remove heat in food to freezer
• Keep heat away
• Insulated packaging
– Difficulty in removing heat
• Frozen chamber
– For freezing and storage
– Insulated to keep heat away
– Accessibility
– Electrical energy = heat
– Persons = heat
Freezers
• Still air
• Forced air
• Contact
– Refrigerant
• Cryogenic
– Plates
Freezing curves Freezing Curves
0
-10
Critical zones
3
Freezing curves
• Lower in food than in pure water
• Sugars, salts reducers the VP
– Lowers the freezing point
Freezing preservative
• A unit operation
• Maintain the temperature differential
• Very low temperature (below 00C)
– Best below –340C
– But can do around –180C
• Greater the change in free water greater the
stability
• Water frozen reducers A
w
Freezing preservative
• De-hydro freezing
– Lower or lowering free water
• Electrolyte concentrations increase
– Salts, dissolved solutes, sugar
• Greater binding of water
Freezing effects
• Slow freezing
– Large crystals
– Mainly inter-cellular
Freezing effects
• Quick freezing
– Small crystals
– Intra-cellular
– Inter-cellular
Freezing effects
4
Freezing effects
• Small crystals
– Less damage
– “Smoother” crystal structure
• Large crystals
– “Sharp” crystal structure
– Greater cell damage
– Sandy texture
• Melt and refreeze
– Crystal grow
Freezing effects
• Expansion
– Burst containers or caps lost
– Consideration in package designing
• Freezer burn
– Water vapour in the chamber freezers on the coils
– Food goes into equilibrium with the chamber
• Water in food lost to the chamber
– Dehydration by sublimation
Freezing effects
• Cold injury
– Change in colour
– Banana turning black
– Chloroplasts and chromoplasts broken
– Chlorophyll to brown pheophytin
– Anthocyanin colour – pH changes
– Imbalance of metabolic activity
• Metabolites becoming toxic
Freezing effects
• Ammonia injury
– Black to greenish brown discolouration
– Tissue damage
• Shrivelling
– Loss of moisture
Freezing effects

• Larger the value faster the freezing
• Time to go through a temperature range
• Use freezing temperature
– Critical temperature
Freezing effects
• Velocity of the ice front
• Velocity =
• Rate it moves in from the surface
• Faster the velocity faster the rate
x

Time = 
x

5
Freezing effects
• Faster the rate of heat of crystallisation removal
– Faster the freezing
• Quick freezing
– Thick products crack
– Larger temperature difference between surface and
centre
Effects on microbes
• Not a method of microbial inactivation
• Slow freezing
– damage microbial cells
• Yeast that grow at –90C
– Unfrozen water
• Most susceptible – vegetative cells
• Thaw and re-freeze
– Spoilage
– Greater killing effect
Effects on protein
• Electrolyte concentration increases
• Structural damage
• Denaturation
– Water holding capacity
• Visual
• Enzymes MAY still be active
– Proteolysis
– Thermal abuse will increase effects of enzyme activity
Effects on protein
• For enzymes
– Active but slow rate at –700C
– Effects of alerted pH and solutes
– Concentration effects
– Above –90C
• More non-crystalline / super cooled water
• Greater activity
– Blanching prior to freezing
Effects on fat
• Rancidity on animal tissue
– Depend on species
• Faster / greater on pork
• Slower on beef
• Not so much in plant
Effects on vitamin
• Freezing itself is not the problem
• Preparation
– Blanching
– Cutting – drip loss
– Washing
– Thaw – drip loss
• Exposure – oxidation
• Packaging
6
Effects on parasites
• Most parasites killed
• Lower the temperature greater the kill
Thawing
• Surface ice melts – water on surface
• Lower thermal conductivity and diffusivity
• Acts as an insulator
• Insulation layer increases
• Longer process
Thawing Thaw damage
• Freeze – thaw – freeze
– Freeze point increases in beef
• Not in lamb
– Lamb
• Limited structural damage
• Fluid loss little
• Slow melt
– Crystals grow – greater cell damage
– Water not reabsorbed
Thaw damage
• Quick melt
– Limited to no cellular damage
– Most water reabsorbed
• Prevention of moisture on surface
– Staling
– Microbial and biochemical spoilage
Considerations
• Aging of meat
– Yes in beef, not in poultry
• Prevention of drip at final thaw
• Prevention of cellular damage – texture impacts
• Non-destruction of harmful bacteria
• Ingredient hazards / product
– Single component
– Multi component
– Simple process
– Multi process – assembly
7
Considerations
• Thermal abuse
– At manufacture
– At delivery
– On shelf
– At Purchase and consumer
• Effects on shelf-life
• What determines the shelf-life
– Know the quality factor
– When does it go off
– What goes off first
Cooking and chilling
• Food cooked / pasteurised
• Cooked food packaged
– Rapid chilling
• Packaged in controlled / clean condition
– Aseptic packaging?
• Semi-cooked or raw – chilled until served
• Heat treated within the package
• Don’t forget logarithmic reduction in the cook
– Need to kill pathogens
Cooking and chilling
• Reduce spoilage
• Reduce re-growth
• Stop enzyme
• Initial microbial load
• In production
– Balance the line
– Don’t stack (cooked food waiting to chill)
– Cook to package to chilling time
Cooking and chilling
• In production
– Control of stock – FIFO
– Final kill step
• Cook / reheat before serving
– Is it available
– Final microbial reduction