Refrigeration Flashcards
3 types of Refrigeration
Chilling and freezing
- final temperature
- type of heat removed.
Chilling
0°C to 8°C
sensible heat
Freezing
~ below freezing point often -18°C.
~ crystallisation of water
~ latent heat
~ more energy and time
Refrigeration – Why?
molecular mobility depressed
- chemical reactions slow
- biological processes slow
microorganisms or enzymes
* depresses their activity
Retards spoilage
* But cannot improve initial quality
Not permanent preservation
* definite shelf life
Reliable cold chain and Hurdle principle
Refrigeration and Food Quality
Temperature influence enzymatic spoilage
- Enzyme activity strongly slowed by refrigeration
- but not totally eliminated
- Inactivate enzymes -> blanching
- Enzymatic activity considerable technological
significance - Desirable?
- Undesirable
Temperature influence microorganisms’ growth
[i.e. mesophiles optimal 30 - 45C minimium 5- 10C]
*Effect of storage temperature on microbial load of food
Temperature and biologically active tissue
* Respiration
Respiration rate
estimated using rates of O2 consumption and CO2 evolution. Oxidation of Glucose:
C6 H12 O6 + 6O2 –> 6CO2 + 6H2O +Q
[Q= heat of respiration]
relationship between temperature and biologically active tissue [at 2 stages]
Most important cause of deterioration of fruits &
vegetables during storage:
- ‘Shelf life of fresh produce inversely related to rate of
respiration’ - Rate of respiration closely related to temperature
[10°C increase -> 2- to 4-fold increase rate of respiration] - chill injury
- Post-harvest ripening [Ethylene]
- Control the rate of ripening with refrigeration
- Exothermic process
- Refrigeration load required
respiration of fruit & veg
avacado/ berries/ aspargus/ cauliflower [High respiration]
> Banana/ tomato/ carrot
> Nuts/ grapes/ apple/ citrus [low respiration]
Sensible heat
the heat when added or subtracted from material changes their temperature and it can be sensed
Latent heat
the heat required to change the physical state of materials at constant temperature
[food mostly consists of water and also contains lots of soluble materials]
Soluble materials effects on freezing?
- Soluble materials slow down the movement of water molecules, and the freezing occurs at lower temperature
- 1 mol of soluble matter will decrease (lower) the freezing point by ~1°C
Freezing points for Fruits and vegetables & Meat and fish
Fruits and vegetables = -0.8 to -2.8 °C
Meat and fish = -0.6 to -2.8 °C
Freezing processing principle
change in sensible heat (& heat respiration) to lower the temperature of a food to the freezing point.
A substantial amount of energy is needed to remove latent heat, form ice crystals and hence to freeze foods.
Freezing curve
If the temperature is monitored at the thermal centre of a food as heat is removed, a characteristic curve is btained [slide 19]
- AS – food cooled to below its freezing point θf . At S the water remains liquid: super-cooling may up to 10ºC below freezing point.
- SB - temperature rises rapidly to the freezing point as ice crystals begin to form and latent heat of crystallization is released.
- BC – Heat removed from food at same rate as before, but it is latent heat being removed as ice forms and temperature remains almost constant. The freezing pt depressed by increasing solute concentration in unfrozen liquor & temperature falls slightly. major part of the ice is formed
- CD - One of the solutes becomes supersaturated and
crystallizes out. The latent heat of crystallization is released and the temperature rises to the eutectic temperature for that solute. - DE - Crystallization of water and solutes continues. The total time tf taken (the freezing plateau) is determined by the rate at which heat is removed.
- EF - The temperature of the ice–water mixture falls to the temperature of the freezer. A proportion of the water remains unfrozen at the temperatures used in commercial freezing; the amount depends on the type and composition of the food and the temperature
eutecticum
When the concentration of the solute in the non-frozen portion reaches a certain level, that entire portion solidifies as though it were a pure substance. This new solid phase is called ‘ eutecticum’.
freezing point
temperature at which a minute crystal of ice exists in equilibrium with the surrounding water
Freezing: Theory for Ice Crystal Form
~freezing point
~ Nucleus of water molecules must be present
~ Nucleation
[ Homogeneous nucleation ; heterogeneous nucleation]
~ Supercooling
~High rates of heat transfer
[Large number of small ice crystals]
~ Different for types of food and different pre-freezing treatments.
~ Rate controlled by the rate of heat transfer for the
majority of the freezing plateau
Freezing Theory for Solute Concentration
Increase in solute:
- by changes in the pH, viscosity, surface tension, redox
potential of the unfrozen liquor
- Eventually individual solutes reach saturation point
EUTECTIC TEMPERATURE
- Lowest Temp at which a crystals of individual
solute exists in equilibrium with the unfrozen liquor and
ice
[ Meat -50 to -60ºC; Bread -70ºC]
No further concentration of solutes as solution freezes
Lowest eutectic temperature for food
Commercial foods not frozen to such low temperatures so unfrozen water is therefore always present.
Below point E
- glass transition
- glass encompassing ice crystals
- protection
Freezing: Effect on Food
Cell damage by ice crystal growth
Negligible changes to pigments, flavours or nutritionally important components (preparation / storage)
Plant v Animal Origin:
~ Meats -> flexible fibrous structure
~ Fruits & vegetables -> rigid cell structure
~ Extent damage -> size of the crystals
Raw material quality, pre-freezing treatments
Rate of freezing effects on Food
Slow freezing allows large size crystal formed and damaged food cell
rapid freezing allows equal size crystal contribution formed, less damage for food
Frozen Storage: Effect on Food
Enzymes not inactivated
Variable effect on micro-organisms:
[-4ºC to -10ºC]: greater lethal effect on microorganisms
[-15ºC to -30ºC]: lesser lethal effect on microorganisms
Varying resistance:
~ vegetative cells of yeasts, moulds and gram negative bacteria
~ gram-positive bacteria and mould spores
~ bacterial spores
Vegetables blanched
lower temp allow better colour and flavour preservation:
~ colour change detected sooner [for same storage temp] than flavour
~ colour change detected at lower storage temp [for same storage time] than flavour
Main Effect on Frozen Stored Food
Degradation of pigments
- chlorophyll -> pheophytin (Veg)
- Precipitation salts -> pH change -> anthocyanins (fruit)
Loss of vitamins
- Water-soluble vitamins lost at sub-freezing temperatures (fruit/veg)
- Drip loss (meat/fish)
Residual enzyme activity:
- Polyphenoloxidase activity -> browning (Fruit & Veg)
- Lipoxygenases -> off-flavours and off-odours, degrades of carotene (Fruit & Veg)
- Proteolytic and lipolytic activity -> texture and flavour (meat)
Oxidation of lipids:
- slowly at -18ºC
- off-odours and off-flavours
Recrystallisation Effect on Frozen Stored Food
-> Quality loss
Physical changes to ice crystals
~ Shape/ Size / Orientation
Migratory recrystallisation
~ Increase in average size & reduction in the number of crystals
~ Growth of larger crystals at expense of smaller crystals.
↑ Temp -> melts ice crystals -> ↓ size -> ↑ water vapour pressure:
~ moisture moves to area lower vapour pressure
~ Area dehydrated
~ No new nuclei formed – larger ice crystals
!! Avoid unstable temp change !!
Plank’s Equation limitation
This equation works well for freezing of pure water but can result in large errors for freezing time of food materials
factors that influence freezing time
↑ ΔT, h, k ↓ freezing time
[ ΔT= temp change
h = convective heat transfer coefficient at air/ ice interface (W m^-2 K^-1)
k= thermal conductivity of the frozen phase (W m^-1 K^-1)]
↑ ρ, L, a ↑ freezing time
[ ρ = density (kg m^-3)
L = latent heat of freezing of the liquid (J. kg^-1 )
a = dimension (normally thickness/diameter) ]
Derivation of Plank’s Equation limitation
Defects in Assumptions:
~ Density independent of temperature
~ K independent of temperature
[ K frozen food greater than unfrozen food]
~ Temperature of the freezing medium is constant
~ Temperature of unfrozen material is at Tf at the start of process
~ Freezing point is fixed
~Gives ball park figure
~ For more accuracy use different equation
~ Planks assumption not valid for food but valid for water
Freezing Systems [2 types]
Direct Contact:
~ More efficient – no barrier
~ Rapid freezing – Individual Quick Freezing (IQF)
Indirect Contact:
(i.e. Food on plate placed on top of refrigerant)
Freezing Systems - direct
Air blast freezing:
~ High air velocities -> high h value
~ Moisture loss
Fluidised freezing:
~ High air velocities -> high h value
~ Good contact between refrigerant and product
~ Suitable for particulate products eg peas
Immersion Freezing (with a pre-cooling step)
~ Liquefied gas sprayed directly onto product
[ Change of phase – evaporation of gas eg CO2 or N2]
~ Very rapid freezing
~ Superior quality product
Freezing Systems - indirect
Plate freezer:
~ Used for block low volume products
~ Pressure applied to lower freezing time
Air blast (Packaged food):
~ Increased freezing time but decreased moisture loss
~ For unusual shaped objects
Scraped Surface Heat Exchanger:
~ Same as used for high viscosity fluids (difficult process
applications)
~ Removed crystals formed on outside
~ Partial freezing of liquid (60-80%)
[Objective to get a frozen ice slurry of ice crystals]
Refrigeration: Low temperatures may be delivered by three types of sources
~ Natural sources (ice, snow, climatic conditions)
~ Cryogenic agents
~ Mechanical refrigeration (Air blast freezers/chillers; Plate freezers)
Refrigeration systems
allow to transfer of heat from the cooling chamber to
a location where the heat can be easily discarded
Refrigeration Cycle
transfer heat by refrigerant suh as: ~ Change of state (liquid to vapor) ~ Boiling point e.g. -33.3°C ~ Latent heat ~ Pressure – boiling point
Refrigeration Cycles [3]
transfer of heat from cooling chamber to a discarded point (external)
~ (Mechanical) vapour compression cycle
~ Absorption refrigeration cycle
~ Ejector refrigeration cycle
Vapour compression cycle
Compression (points 1–2)
• refrigerant is in a gaseous state (point 1)
• Work done by compressor
• pressure and temperature elevated
Condensation (points 2–3)
• High pressure vapour enters heat exchanger (condenser)
• Using air or water cooled atmospheres refrigerant gives up heat to surroundings
• condenses to form a liquid
• heat of condensation rejected to ambient
Expansion (points 3–4)
• Liquefied refrigerant enters expansion engine
• Experiences pressure drop and drop in temperature
• Mixture of liquid and gas leaves this process.
Evaporation (points 4–1)
• Energy received in heat exchanger (evaporator)
• Refrigerant evaporates
• Latent heat of evaporation needed is supplied by the cooling load => generating refrigeration
• enters compressor and continues cycle
Coefficient of Performance of Vapour compression cycle
Ratio refrigeration effect obtained to the work
done in order to achieve it
COP = Refrigeration effect / work done = qe/ qw
Absorption refrigeration cycle composition
Generator/ Condenser/ Evaporator/ Absorber
/ Pump/ Heat exchanger/ Two expansion valves
/ NH3–H2O absorption cycle/ Rectifier/ Dephlegmator
Absorption refrigeration cycle principe
ammonia evaporated off the generator contains some water vapor
~ elevate evaporating temperature
~ water may also freeze along pipelines.
Water must be removed
Vapor driven off at generator flows countercurrently to
incoming solution in rectifier
Passes through dephlegmator and condenses some
water-rich liquid, which drains back to the rectifier
High-pressure refrigerant vapor 1 generated by generator–> condenses into liquid 2 in the condenser,
–> heat of condensation rejected
Condensed liquid passes through valve –> maintains pressure difference between condenser and evaporator
Low pressure liquid enters evaporator 3 to evaporate
–> heat required for evaporation provided by cooling load
Vapor 4 absorbed by the liquid strong solution 10 coming
from the generator in the absorber
Heat of absorption rejected to environment
The pump receives low-pressure liquid weak solution 5 from the absorber, elevates the pressure of the weak solution 6, and delivers 7 to the generator
Generator:
heat drives off refrigerant vapor 1 –> strong solution 8
returns to absorber 9 through throttling valve 10.
Coefficient of Performance of Absorption refrigeration cycle
COP = Refrigeration effect / work done = qe/ (qg + Wm)
Advanced Cooling Techniques challenges
EU guidelines for cooling cooked meat
~ cooled within certain time limits post-cooking
~ Meat joints should not exceed 2.5 kg and 100 mm in thickness
~ should be chilled from 74ºC to 10ºC within 2.5 hours after the conclusion of the cooking process
Conventional cooling methods
~ air blast, cold room and immersion cooling
~ depend on heat conduction for cooling inside of joints
~ relatively low thermal conductivity of meat
~ must maintain a temperature of the cooling fluid above 2ºC (to avoid surface freezing)
~ difficult to significantly increase the cooling rate.
Vacuum cooling
~ Rapid evaporative cooling technique
~ Moisture in foods boiling under vacuum conditions
~ The products to be cooled are loaded into a closed chamber
~ Vacuum pumps are then used to evacuate air from the chamber
~ Pressure reduced and water starts to evaporate
~ Latent heat of evaporation supplied by product
~ Sensible heat reduced and cooling occurs
Vapour generated in Vacuum cooling is removed by
- vacuum pump
- condensation
Products suitable for vacuum cooling
~ free water
~ structure not damaged by water removal
~ porous structure
e.g. Cooked meat/ Bakery/ Fishery/ Viscous food processing
Pre-cooling treatments needed in vacuum cooling for?
leafy vegetables: field heat and prolong product shelf life
Advantages of Vacuum cooling
Speed and efficiency:
- boxed or palletised products -> 30 min
- 0.5°C/min
- ↓ product hold up time ↑ production throughput
- Tight delivery schedules
- strict cooling requirements
Reduce postharvest deterioration:
* Prolonging storage life
More uniform internal temperature
distribution
Rate not directly affected by sample size
* large dimensions
Precise product temperature control possible
↓ energy consumption
Disadvantages of Vacuum cooling
cannot replace established cooling techniques
Weight:
- In vegetables ~ 3–4% of original weight
- Add water
Vacuum cooling: Baked products
Integration into baked bread lines
modulated vacuum cooling
For cooling baked products from oven
* immediately after removal from the oven before packaging
avoid vapour condensation in the wrapping
* plastics bags
Bread rolls, crusty breads, baked biscuits
* Rack cooler/ In-line cooler
Benefits:
- precise control over cooling rates
- increased product stability
- humidity distribution
- shape and texture
- baking cycle reduced by 2hrs
temperature range: 98 to 30°C
weight loss:
~ 6.8% (conventional 3 and 5%
* Compensate: reduce baking time, spray with
sterile water
Vacuum cooling: Cooked Meat
Temperature should be rapidly reduced
retention of nutrients
Conventional cooling
* heat removed from core by conduction & to cooling
medium by convection.
* ↑surface heat transfer ↑ the velocity of the cooling
medium
* surface temperature approaches cooling medium
temperature quickly but not so for the core
* poor thermal conductivity and large dimensions
To meet cooling guidelines the shortest dimension of the meat should not exceed a certain value
Not feasible for cooked meat processors and caterers
Weight loss
- 72–75°C to 3–8°C results in 10–12% weight reduction
- Conventional 6-7%
Compensate weight loss
* brine injection level
Quality
* Texture, Juiciness, Colour
Immersion vacuum cooling
Vacuum cooling: fishery products
At sea: frozen in brine immediately
Canning plants:
- thawed
- steam cooked to 65°C
- Cooled 35 and 40°C
- 3–4% weight loss
