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ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New So PowerPoint Presentation
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ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales tuan.pham@unsw.edu.au. History of Food Refrigeration. Harrison - ice making (1860), frozen meat export (1873) China 1000BC - ice harvesting

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ADVANCES IN FOOD REFRIGERATIONTuan PhamSchool of Chemical Engineering and Industrial ChemistryUniversity of New South Walestuan.pham@unsw.edu.au

slide2

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
food refrigeration is big

Food refrigeration is BIG

Annual investment in refrigerating equipment: US$170

Annual refrigerated foodstuffs: US$1200 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

plan of talk

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
chillers and freezers

Chillers and Freezers

Chillers and freezers can be classified into

air-cooled

immersion

spray

cryogenic

surface contact chillers.

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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

surface contact chillers freezers

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.

how to have efficient cooling freezing
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
effectiveness of door protective devices
Effectiveness of door protective devices
  • Vertical air curtain: 79%
  • Horizontal air curtain: 76%
  • Plastic strip curtain: 93%
  • Air + plastic strip: 91%
vapour barrier breach

Vapour barrier breach

  • Heat bridge
  • Delamination
  • Collapse
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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)

selection and operation of refrigeration components
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
selection and operation of refrigeration components23
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
selection and operation of refrigeration components24
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.

heat mass transfer in irregular food
Heat & mass transfer in irregular food
  • Re-circulation causes
  • High temperature
  • Moist surface
  • Microbial growth
mathematical simulation
Mathematical Simulation

Objectives: to predict changes in

  • temperature at surface and centre
  • moisture, especially surface moisture
  • heat load
  • quality changes
  • microbial risks
simulation overview of models
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
simulation tank models
Simulation: Tank models
  • Uniform temperature model
  • Network of tank
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Accuracy of predictions by various models

(based on 70 beef chilling tests)

cfd models

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!

why is cfd so difficult
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
other cfd applications
Other CFD Applications
  • Chillers and freezers
  • Cold stores
  • Transport containers
  • Pasteurisation/cooling of liquid foods
  • Design of cooling coils, air curtains
quality physical changes

Quality: Physical changes

Weight loss, dry appearance

Water absorption, bloated appearance

Drip

Crystal growth (ice cream)

Water penetration (bakery products)

quality biochemical changes

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)

quality fungal microbial changes

Quality: Fungal & microbial changes

Mildew, rot (fruit)

Spoilage organisms

Pathogenic organisms

modelling microbial growth
Modelling microbial growth

Growth Rate = Optimum rate

× Temperature Inhibition Factor

× Water Activity Inhibition Factor

× pH Inhibition Factor

× Other Inhibition Factors

growth rate dependence on temperature
Growth rate: dependence on Temperature

Ratskowsky’s square root model:

Zwietering model:

microbial death
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 (?)
the ultimate objective of simulation is to control and optimize
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

search optimisation methods
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

optimising air temperature in beef chilling
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
optimising air temperature in beef chilling62
Optimising air temperature in beef chilling

A variable temperature regime is the answer:

controlling air temperature in lamb freezing
Controlling air temperature in lamb freezing
  • Objective:To freeze all product in exactly 16 hours
  • Problems:
    • Product weight varies (10-24 kg)
    • 16 hour lag time!

Css weight

Air T, v

Controller

FREEZER

(16-h lag)

Process

Model

Optimizer

Frozen csses

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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 maiden’s prayers.
  • In near, computer control and optimisation of refrigeration processes will become more widespread.