Presentation on theme: "ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales"— Presentation transcript:
ADVANCES IN FOOD REFRIGERATION Tuan Pham School of Chemical Engineering and Industrial Chemistry University of New South Wales firstname.lastname@example.org
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 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 m3 of cold-storage facilities and causes big problems! Ozone-depleting effects - Montreal protocol Global-warming effects - Kyoto agreement
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
PART ONE: COMMON PROBLEMS IN FOOD REFRIGERATION EQUIPMENT
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 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 For faster cooling/freezing and higher throughput: Reduce temperature TaTa 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 Surface resistance Internal resistance Freezing time
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 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 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 Re-circulation causes High temperature Moist surface Microbial growth
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 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 Uniform temperature model Network of tank
Accuracy of F.D. & F.E. model for beef chilling heat load (70 tests)
Accuracy of predictions by various models (based on 70 beef chilling tests)
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? 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
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 Optimizer Process inputs: Air temperature Washing, cleaning Product shape, wrap... etc. Process model Results: Product quality Cost Reliability etc...
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 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 chilling A variable temperature regime is the answer:
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! FREEZER (16-h lag) Air T, vCss weight Frozen csses Controller Optimizer Process Model
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. CONCLUSIONS