ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales tuan.pham@unsw.edu.au - PowerPoint PPT Presentation

Loading...

PPT – ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales tuan.pham@unsw.edu.au PowerPoint presentation | free to view - id: 6db64-ZDc1Z



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales tuan.pham@unsw.edu.au

Description:

Accuracy of two-tank model for lamb freezing. Simulation: (2-D) finite difference model ... Controlling air temperature in lamb freezing ... – PowerPoint PPT presentation

Number of Views:120
Avg rating:3.0/5.0
Slides: 65
Provided by: tuan5
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales tuan.pham@unsw.edu.au


1
ADVANCES IN FOOD REFRIGERATIONTuan
PhamSchool of Chemical Engineering and
Industrial ChemistryUniversity of New South
Walestuan.pham_at_unsw.edu.au
2
History of Food Refrigeration
  • Harrison - ice making (1860), frozen meat export
    (1873)
  • China 1000BC - ice harvesting
  • Ancient Egypt - (evaporative cooling, ice making)
  • Prehistory - use of caves and ice

3
Food refrigeration is BIG
  • Annual investment in refrigerating equipment
    US170
  • Annual refrigerated foodstuffs US1200 billion
  • (3.5 times USA military budget)
  • 700-1000 million household refrigerators
  • 300 000 000 m3 of cold-storage facilities
  • and causes big problems!
  • Ozone-depleting effects - Montreal protocol
  • Global-warming effects - Kyoto agreement

4
Plan of talk
  • Part I Common industrial problems
  • - Chillers and freezers
  • - Cold stores
  • - Refrigerated transport
  • - Retail display

Part II Simulation of food refrigeration -
Temperature and moisture changes - Quality and
microbial growth
Part III Optimisation of food refrigeration
5
PART ONECOMMON PROBLEMS IN FOOD REFRIGERATION
EQUIPMENT
6
Typical refrigeration system
7
Chillers and Freezers
  • Chillers and freezers can be classified into
  • air-cooled
  • immersion
  • spray
  • cryogenic
  • surface contact chillers.

8
Air Chillers/Freezers
9
  • Immersion and Spray Chillers/Freezers
  • faster than air chilling, especially for small
    products
  • absorption of liquid or solutes by the product,
    leading to undesirable appearance or other
    quality losses
  • cross-contamination between products
  • leaching of food components such as fat
  • effluent disposal problem

10
Surface contact chillers/freezers
  • Include plate chillers/freezers, mould freezers,
    belt chillers, scraped surface freezers
  • High heat transfer rate (similar to immersion
    freezers) - only metal bw refrigerant product
  • No absorption of liquid
  • No liquid effluent.
  • Need products with flat surfaces, such as cartons
    Preferably thin or small products such as fish
    and peas.
  • Labor intensive or need sophisticated automation.

11
How to have efficient cooling/freezing
Freezing time
Surface resistance
Internal resistance
  • For faster cooling/freezing and higher
    throughput
  • Reduce temperature Ta
  • Increase h (high air velocity, use spray/
    immersion/ contact, less packaging)
  • Decrease product size R
  • Biot Number hR/k ( external/internal resistance)
    should be not too far from 1

12
Cold store
13
Cooling coil
14

Air Infiltration through Doors
15
Effectiveness of door protective devices
  • Vertical air curtain 79
  • Horizontal air curtain 76
  • Plastic strip curtain 93
  • Air plastic strip 91

16
(No Transcript)
17
Vapour barrier breach
  • Heat bridge
  • Delamination
  • Collapse

18
Frost heave
19
  • Problems with transport vehicles containers are
    same as in cold rooms, but multiplied
    several-fold (because of high A/V ratio and
    fluctuating ambient conditions)

20
Retail display

21
Retail display
22
Selection and Operation of Refrigeration
Components
  • Reliability
  • Food remains safe and wholesome according to
    specifications.
  • Flexibility
  • Ability to handle different products or
    production rates
  • Capital and Operating costs

23
Selection and Operation of Refrigeration
Components
  • Freezers and chillers
  • Extract heat within a certain time from product
    and other sources
  • Cool product uniformly
  • Avoid surface drying, contamination, microbial
    growth and other quality problems
  • Avoid condensation

24
Selection and Operation of Refrigeration
Components
  • System must be well balanced to give optimal
    performance for given price.
  • An undersized cooling coil or freezer will
    require oversized compressors, condensers etc.

25
PART TWOSIMULATION OF FOOD REFRIGERATION
26
What happens in the productHeat mass transfer

27
Mass transfer in wrapped food

28
Heat mass transfer in Cartoned food

29
Heat mass transfer in irregular food
  • Re-circulation causes
  • High temperature
  • Moist surface
  • Microbial growth

30
Mathematical Simulation
  • Objectives to predict changes in
  • temperature at surface and centre
  • moisture, especially surface moisture
  • heat load
  • quality changes
  • microbial risks

31
Simulation Overview of models
  • Lumped capacitance (uniform temperature) model
  • Tank network model
  • Product discretization models
  • - finite differences
  • - finite elements
  • - finite volumes
  • Computational fluid dynamics (CFD) model

32
Simulation Tank models
  • Uniform temperature model
  • Network of tank

33
Accuracy of two-tank model for lamb freezing
34
Simulation (2-D) finite difference model

35
Accuracy of F.D. model for beef chilling weight
loss (70 tests)
36
Simulation (2-D) finite element model

37
Accuracy of F.D. F.E. model for beef chilling
heat load (70 tests)
38
Accuracy of predictions by various models (based
on 70 beef chilling tests)
39
CFD Models
  • Can simulate the flow field outside the product
    (air, water, cryogen...) as well as inside
  • Computationally expensive (fast computers, lots
    of memory, days of runtime)
  • Software expensive (especially for non-U)
  • Need lots of expertise to use properly
  • Need lots of time for data preparation
  • Accuracy NOT guaranteed even when all the above
    are satisfied!

40
Why is CFD so difficult?
  • Solve several interacting partial differential
    equations simultaneously (density, v, T, c,
    turbulence parameters)
  • Must discretize the object and its surrounding
    into tens of thousands to millions of volume
    elements

Why is CFD not quite accurate?
  • Calculation of turbulence only approximate
  • Turbulence affects boundary layer and hence heat
    and mass transfer rates

41
(No Transcript)
42
CFD example Beef chilling - model
100,000 nodes
43
CFD example Beef chilling - results
44
CFD model of display case Predicted (color) vs
measured (number) temperatures
45
Other CFD Applications
  • Chillers and freezers
  • Cold stores
  • Transport containers
  • Pasteurisation/cooling of liquid foods
  • Design of cooling coils, air curtains

46
Quality Physical changes
  • Weight loss, dry appearance
  • Water absorption, bloated appearance
  • Drip
  • Crystal growth (ice cream)
  • Water penetration (bakery products)

47
Quality Biochemical changes
  • Tenderness (beef, lamb)
  • Fat rancidity flavour
  • PSE (pale soft exudative) (pork)
  • DFD (meat)
  • Flavour (fish)
  • Colour (meat)
  • Browning, spots, freezing injury (fruit)
  • Tissue breakdown (fruit)

48
Quality Fungal microbial changes
  • Mildew, rot (fruit)
  • Spoilage organisms
  • Pathogenic organisms

49
Modelling microbial growth
Growth Rate Optimum rate Temperature
Inhibition Factor Water Activity Inhibition
Factor pH Inhibition Factor Other
Inhibition Factors
50
Growth rate dependence on Temperature

Ratskowskys square root model
Zwietering model
51
Growth rate dependence on Humidity pH

52
Predictive microbiological modelling

53
Predictive microbiological modelling

54
Predictive microbiological modelling

55
Microbial death
  • Death rate influenced by
  • High temperature
  • Low pH
  • Low water activity
  • Combination
  • Death during freezing
  • high solute concentration (low aw)
  • membrane shrinkage and damage
  • intracellular ice (?)

56
Microbial death during freezing
57
PART THREEOPTIMIZATION OF FOOD REFRIGERATION
58
The ultimate objective of simulation is to
control and optimize
Optimizer
Results Product quality Cost Reliability etc...
Process inputs Air temperature Washing,
cleaning Product shape, wrap... etc.
Process model
59
Search (optimisation) methods
  • Gradient (classical) methods
  • - fast methodical
  • - ends up at nearest local optimum

Stochastic methods (SA, GA...) - methods with
madness - can be time consuming - 100,000
trials? - better at obtaining global optimum -
better at dealing with errors - can perform
multi-objective optimisation
60
(No Transcript)
61
Optimising air temperature in beef chilling
  • Objectives
  • Chill centre to 7C in 24 hours
  • Tenderness score is minimized
  • E. Coli grows less than 8-fold at surface
  • However
  • Fast chilling (low air T) causes toughness (high
    tenderness score) in loin
  • Slow chilling encourages microbial growth on leg
    surface

62
Optimising air temperature in beef chilling
A variable temperature regime is the answer
63
Controlling air temperature in lamb freezing
  • ObjectiveTo freeze all product in exactly 16
    hours
  • Problems
  • Product weight varies (10-24 kg)
  • 16 hour lag time!

Air T, v
Css weight
Controller
FREEZER (16-h lag)
Process Model
Optimizer
Frozen csses
64
CONCLUSIONS
  • Attention to details needed in design and
    operation of refrigeration facilities.
  • Growing computer power allows more precise
    simulation of processes and prediction of product
    quality.
  • CFD is not yet the answer to the maidens
    prayers.
  • In near, computer control and optimisation of
    refrigeration processes will become more
    widespread.
About PowerShow.com