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Food Processing > Refrigeration > Flashcards

Refrigeration Flashcards

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1
Q

3 types of Refrigeration

A

Chilling and freezing

  • final temperature
  • type of heat removed.
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2
Q

Chilling

A

0°C to 8°C

sensible heat

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3
Q

Freezing

A

~ below freezing point often -18°C.
~ crystallisation of water
~ latent heat
~ more energy and time

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4
Q

Refrigeration – Why?

A

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

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5
Q

Refrigeration and Food Quality

A

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

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6
Q

Respiration rate

A

estimated using rates of O2 consumption and CO2 evolution. Oxidation of Glucose:

C6 H12 O6 + 6O2 –> 6CO2 + 6H2O +Q

[Q= heat of respiration]

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7
Q

relationship between temperature and biologically active tissue [at 2 stages]

A

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
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8
Q

respiration of fruit & veg

A

avacado/ berries/ aspargus/ cauliflower [High respiration]
> Banana/ tomato/ carrot
> Nuts/ grapes/ apple/ citrus [low respiration]

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9
Q

Sensible heat

A

the heat when added or subtracted from material changes their temperature and it can be sensed

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10
Q

Latent heat

A

the heat required to change the physical state of materials at constant temperature

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11
Q

[food mostly consists of water and also contains lots of soluble materials]

Soluble materials effects on freezing?

A
  • 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
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12
Q

Freezing points for Fruits and vegetables & Meat and fish

A

Fruits and vegetables = -0.8 to -2.8 °C

Meat and fish = -0.6 to -2.8 °C

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13
Q

Freezing processing principle

A

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.

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14
Q

Freezing curve

A

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
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15
Q

eutecticum

A

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’.

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16
Q

freezing point

A

temperature at which a minute crystal of ice exists in equilibrium with the surrounding water

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17
Q

Freezing: Theory for Ice Crystal Form

A

~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

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18
Q

Freezing Theory for Solute Concentration

A

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
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19
Q

Freezing: Effect on Food

A

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

20
Q

Rate of freezing effects on Food

A

Slow freezing allows large size crystal formed and damaged food cell

rapid freezing allows equal size crystal contribution formed, less damage for food

21
Q

Frozen Storage: Effect on Food

A

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

22
Q

Main Effect on Frozen Stored Food

A

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
23
Q

Recrystallisation Effect on Frozen Stored Food

A

-> 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 !!

24
Q

Plank’s Equation limitation

A

This equation works well for freezing of pure water but can result in large errors for freezing time of food materials

25
Q

factors that influence freezing time

A

↑ 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) ]

26
Q

Derivation of Plank’s Equation limitation

A

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

27
Q

Freezing Systems [2 types]

A

Direct Contact:
~ More efficient – no barrier
~ Rapid freezing – Individual Quick Freezing (IQF)

Indirect Contact:
(i.e. Food on plate placed on top of refrigerant)

28
Q

Freezing Systems - direct

A

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

29
Q

Freezing Systems - indirect

A

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]

30
Q

Refrigeration: Low temperatures may be delivered by three types of sources

A

~ Natural sources (ice, snow, climatic conditions)
~ Cryogenic agents
~ Mechanical refrigeration (Air blast freezers/chillers; Plate freezers)

31
Q

Refrigeration systems

A

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
32
Q

Refrigeration Cycles [3]

A 
transfer of heat from cooling chamber to a
discarded point (external)

~ (Mechanical) vapour compression cycle
~ Absorption refrigeration cycle
~ Ejector refrigeration cycle

33
Q

Vapour compression cycle

A

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

34
Q

Coefficient of Performance of Vapour compression cycle

A

Ratio refrigeration effect obtained to the work
done in order to achieve it

COP = Refrigeration effect / work done = qe/ qw

35
Q

Absorption refrigeration cycle composition

A

Generator/ Condenser/ Evaporator/ Absorber
/ Pump/ Heat exchanger/ Two expansion valves
/ NH3–H2O absorption cycle/ Rectifier/ Dephlegmator

36
Q

Absorption refrigeration cycle principe

A

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.

37
Q

Coefficient of Performance of Absorption refrigeration cycle

A

COP = Refrigeration effect / work done = qe/ (qg + Wm)

38
Q

Advanced Cooling Techniques challenges

A

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.

39
Q

Vacuum cooling

A

~ 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

40
Q

Vapour generated in Vacuum cooling is removed by

A
  • vacuum pump

- condensation

41
Q

Products suitable for vacuum cooling

A

~ free water
~ structure not damaged by water removal
~ porous structure

e.g. Cooked meat/ Bakery/ Fishery/ Viscous food processing

42
Q

Pre-cooling treatments needed in vacuum cooling for?

A

leafy vegetables: field heat and prolong product shelf life

43
Q

Advantages of Vacuum cooling

A

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

44
Q

Disadvantages of Vacuum cooling

A

cannot replace established cooling techniques

Weight:

  • In vegetables ~ 3–4% of original weight
  • Add water
45
Q

Vacuum cooling: Baked products

A

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

46
Q

Vacuum cooling: Cooked Meat

A

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

47
Q

Vacuum cooling: fishery products

A

At sea: frozen in brine immediately

Canning plants:

  • thawed
  • steam cooked to 65°C
  • Cooled 35 and 40°C
  • 3–4% weight loss

Food Processing (5 decks)

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