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ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New So

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 REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New So

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  1. ADVANCES IN FOOD REFRIGERATIONTuan PhamSchool of Chemical Engineering and Industrial ChemistryUniversity of New South Walestuan.pham@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: 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

  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 ONE:COMMON 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. Vapour barrier breach • Heat bridge • Delamination • Collapse

  17. Frost heave

  18. 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)

  19. Retail display

  20. Retail display

  21. 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

  22. 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

  23. 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.

  24. PART TWO:SIMULATION OF FOOD REFRIGERATION

  25. What happens in the productHeat & mass transfer

  26. Mass transfer in wrapped food

  27. Heat & mass transfer in Cartoned food

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

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

  30. 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

  31. Simulation: Tank models • Uniform temperature model • Network of tank

  32. Accuracy of two-tank model for lamb freezing

  33. Simulation: (2-D) finite difference model

  34. Accuracy of F.D. model for beef chilling weight loss (70 tests)

  35. Simulation: (2-D) finite element model

  36. Accuracy of F.D. & F.E. model for beef chilling heat load (70 tests)

  37. Accuracy of predictions by various models (based on 70 beef chilling tests)

  38. 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!

  39. 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

  40. CFD example: Beef chilling - model 100,000 nodes

  41. CFD example: Beef chilling - results

  42. CFD model of display case: Predicted (color) vs measured (number) temperatures

  43. Other CFD Applications • Chillers and freezers • Cold stores • Transport containers • Pasteurisation/cooling of liquid foods • Design of cooling coils, air curtains

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

  45. 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)

  46. Quality: Fungal & microbial changes Mildew, rot (fruit) Spoilage organisms Pathogenic organisms

  47. Modelling microbial growth Growth Rate = Optimum rate × Temperature Inhibition Factor × Water Activity Inhibition Factor × pH Inhibition Factor × Other Inhibition Factors

  48. Growth rate: dependence on Temperature Ratskowsky’s square root model: Zwietering model:

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