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1.

Introduction

Transforming liquid milk into dry powder requires removal of almost all the water, the amount of which exceeds by many times that of the final product. During this water removal process, significant changes of the properties, physical structure and appearance take place. Milk is a sensitive product and its quality can be seriously affected especially by the influence of heat or bacterial activity. Thus it is obvious that a single water removing process cannot have optimum performance throughout the whole duration of dehydration.

Therefore it is necessary to apply successively several methods, each being chosen with respect to the properties of the material processed at each individual process step, while taking into account both product quality and economy.

Two main water removal processes used in milk powder industry are:

  • vacuum evaporation which removes the first part of water and transforms a thin liquid into relatively high viscous milk concentrate, and
  • spray drying which transforms the concentrate into powder by atomization and dispersion of small droplets into a flow of hot air. This air supplies the heat necessary for evaporation of water from the droplets and carries away the water vapour.

These two main processes can be further supplemented by:

  • membrane processes (reverse osmosis) are, in some cases, suitable for removing the very first portions of water prior to transportation and further evaporation possibly also removing some monovalent salts (nanofiltration). The concentration of high molecular substances like whey proteins is interesting, but not dealt with in this book.
  • fluid bed drying which is used for removing the very last portions of water, i.e. transforming the moist powder leaving the spray dryer into a final dry powder. Modern spray dryers are often equipped with an integrated fluid bed in the bottom of the chamber, for either final after drying/ cooling or for intermediate drying followed by a second fluid bed.

The technology of milk powder manufacture is very complex. There are a vast number of different products and compositions, and each product can be produced according to various quality requirements. Moreover there are a great number of qualitative criteria defining not only the composition and overall quality from the general hygiene and health hazard point of view, but also optimum suitability for a given application - so-called functionality.

As with every industrial production, occasionally some irregularities may occur which become evident as abnormal behaviour of the plant. Such behaviour often occurs without any obvious reason, i.e. without any apparent deviation of the production line set-up or operational parameters. The consequence of that may be lack of capacity, excessive loss of product, high consumption of energy, product quality degradation etc.

If this occurs it is necessary to mobilize all efforts to find the reasons for the problems, to solve them and to re-establish regular operation conditions, i.e. to conduct a troubleshooting operation. The objective of this book is to provide for a milk powder technologist the guidelines for trouble-shooting actions. However one cannot discuss trouble-shooting without knowledge of the fundamentals of evaporation and spray drying and the principles of milk powder manufacturing technology. Even well established and well controlled milk powder production facilities cannot fully avoid some production irregularities. An appreciation of basic principles can, to a great extent, reduce their occurrence, and moreover provide means to facilitate the trouble-shooting action. Therefore this book draws the attention to these aspects.

Even if this book is focusing on dairy products, the principles described herein covevering equipment and processes may be used as well in drying other products whether food, chemical or pharmaceutical.

Table of contents

  1. 1.Introduction
  2. 2.Evaporation
    1. 2.1. Basic principles
    2. 2.2. Main components of the evaporator
    3. 2.2.1. Heat exchanger for preheating
    4. 2.2.1.1. Spiral-tube preheaters
    5. 2.2.1.2. Straight-tube preheaters
    6. 2.2.1.3. Preheaters to prevent growth of spore forming bacteria
    7. 2.2.1.3.1. Direct contact regenerative preheaters
    8. 2.2.1.3.2. Duplex preheating system
    9. 2.2.1.3.3. Preheating by direct steam injection
    10. 2.2.1.4. Other means to solve presence of spore forming bacteria
    11. 2.2.1.4.1. Mid-run cleaning
    12. 2.2.1.4.2. UHT treatment
    13. 2.2.2. Pasteurizing system including holding
    14. 2.2.2.1. Indirect pasteurization
    15. 2.2.2.2. Direct pasteurization
    16. 2.2.2.3. Holding tubes
    17. 2.2.3. Product distribution system
    18. 2.2.3.1. Dynamic distribution system
    19. 2.2.3.2. Static distribution system
    20. 2.2.4. Calandria(s) with boiling tubes
    21. 2.2.5. Separator
    22. 2.2.5.1. Separators with tangential vapour inlet
    23. 2.2.5.2. Wrap-around separator
    24. 2.2.6. Vapour recompression systems
    25. 2.2.6.1. Thermal Vapour Recompression – TVR
    26. 2.2.6.2. Mechanical Vapour Recompression - MVR
    27. 2.2.7. Condensation equipment
    28. 2.2.7.1. Mixing condenser
    29. 2.2.7.2. Surface condenser
    30. 2.2.8. Vacuum equipment
    31. 2.2.8.1. Vacuum pump
    32. 2.2.8.2. Steam jet vacuum unit
    33. 2.2.9. Flash coolers
    34. 2.2.10. Sealing water equipment
    35. 2.2.11. Cooling towers
    36. 2.3. Evaporator design parameters
    37. 2.3.1. Determination of heating surface
    38. 2.3.2. Heat transfer coefficient
    39. 2.3.3. Coverage coefficient
    40. 2.3.4. Boiling temperature
    41. 2.4. Evaporation parameters and its influrence on powder properties
    42. 2.4.1. Effect of pasteurization
    43. 2.4.1.1. Bacteriological requirements
    44. 2.4.1.2. Functional properties of dried products
    45. 2.4.1.2.1. Heat classified skim milk powders
    46. 2.4.1.2.2. High-Heat Heat-Stable milk powders
    47. 2.4.1.2.3. Keeping quality of whole milk powders
    48. 2.4.1.2.4. Coffee stability of whole milk powders
    49. 2.4.2. Concentrate properties
  3. 3.Fundamentals of spray drying
    1. 3.1. Principle and terms
    2. 3.1.1. Drying air characteristics
    3. 3.1.2. Terms and definitions
    4. 3.1.3. Psychrometric chart
    5. 3.2. Drying of milk droplets
    6. 3.2.1. Particle size distribution
    7. 3.2.2. Mean particle size
    8. 3.2.3. Droplet temperature and rate of drying
    9. 3.2.4. Particle volume and incorporation of air
    10. 3.3. Single-stage drying
    11. 3.4. Two-stage drying
    12. 3.5. Expansion of air bubbles during drying
    13. 3.6. Extended Two-stage drying
    14. 3.7. Fluid bed drying
  4. 4.Components of a spray drying installation
    1. 4.1. Drying chamber
    2. 4.2. Hot air supply system
    3. 4.2.1. Air supply fan
    4. 4.2.2. Air filters
    5. 4.2.3. Air heater
    6. 4.2.3.1. Indirect: Gas / Electricity
    7. 4.2.3.2. Direct heater
    8. 4.2.4. Air dispersers
    9. 4.3. Feed supply system
    10. 4.3.1. Feed tank
    11. 4.3.2. Feed pump
    12. 4.4. Concentrate heater
    13. 4.4.1. Filter
    14. 4.4.2. Homogenizer/High-pressure pump
    15. 4.4.3. Feed line
    16. 4.5. Atomizing device
    17. 4.5.1. Rotary wheel atomizer
    18. 4.5.2. Pressure nozzle atomizer
    19. 4.5.3. Two-fluid nozzle atomizer
    20. 4.6. Powder recovery system
    21. 4.6.1. Cyclone separator
    22. 4.6.2. Bag filter
    23. 4.6.3. Wet scrubber
    24. 4.6.4. Combinations
    25. 4.7. Fines return system
    26. 4.7.1. For wheel atomizer
    27. 4.7.2. For pressure nozzles
    28. 4.8. Powder after-treatment system
    29. 4.8.1. Pneumatic conveying system
    30. 4.8.2. Fluid bed system
    31. 4.8.3. Lecithin treatment system
    32. 4.8.4. Powder sieve
    33. 4.9. Final product conveying, storage and bagging-off system
    34. 4.10. Instrumentation and automation
  5. 5.Types of spray drying installations
    1. 5.1. Single stage systems
    2. 5.1.1. Spray dryers without any after-treatment system
    3. 5.1.2. Spray dryers with pneumatic conveying system
    4. 5.1.3. Spray dryers with cooling bed system
    5. 5.2. Two stage drying systems
    6. 5.2.1. Spray dryers with fluid bed after-drying systems
    7. 5.2.2. TALL FORM DRYER™
    8. 5.2.3. Spray dryers with Integrated Fluid Bed
    9. 5.3. Three stage drying systems
    10. 5.3.1. COMPACT DRYER™ type CDI (GEA Niro)
    11. 5.3.2. Multi Stage Dryer MSD™ type
    12. 5.3.3. Spray drying plant with Integrated Filters and Fluid Beds - IFD™
    13. 5.3.4. Multi Stage Dryer MSD™-PF
    14. 5.3.5. FILTERMAT™ (FMD) integrated belt dryer
    15. 5.4. Spray dryer with after-crystallization belt
    16. 5.5. TIXOTHERM™
    17. 5.6. Choosing a spray drying installation
  6. 6.Technical calculations
    1. 6.1. Evaporation and product output
    2. 6.2. Heating of atmospheric air
    3. 6.3. Mixing of two air stream
    4. 6.4. Dry air rate, water vapour rate and air density
    5. 6.5. Air velocity in ducts
    6. 6.6. Air flow measurements
    7. 6.7. Barometric distribution law
    8. 6.8. The heat balance of a spray dryer
  7. 7.Principles of industrial production
    1. 7.1. Commissioning of a new plant
    2. 7.2. Causes for trouble-shooting
    3. 7.3. Production documentation
    4. 7.3.1. Production log sheets
    5. 7.3.2. General maintenance log book
    6. 7.3.3. Product quality specification
    7. 7.3.4. Operational parameter specification
    8. 7.4. Product quality control
    9. 7.4.1. Process quality control
    10. 7.4.2. Final quality control
  8. 8.Dried milk products
    1. 8.1. Regular milk powders
    2. 8.1.1. Regular skim milk powder
    3. 8.1.2. Regular whole milk powder
    4. 8.1.3. Whole milk powder with high free fat content
    5. 8.1.4. Butter milk powder
    6. 8.1.4.1. Sweet butter milk powder
    7. 8.1.4.2. Acid butter milk powder
    8. 8.1.5. Fat filled milk powder
    9. 8.2. Agglomerated milk powders
    10. 8.2.1. Agglomerated skim milk powder
    11. 8.2.2. Agglomerated whole milk powder
    12. 8.2.3. Instant whole milk powder
    13. 8.2.4. Agglomerated fat filled milk powder
    14. 8.2.5. Instant fat filled milk powder
    15. 8.3. Whey and whey related products
    16. 8.3.1. Ordinary sweet whey powder
    17. 8.3.2. Ordinary acid whey powder
    18. 8.3.3. Non-caking sweet whey powder
    19. 8.3.4. Non-caking acid whey powder
    20. 8.3.5. Fat filled whey powder
    21. 8.3.6. Hydrolysed whey powder
    22. 8.3.7. Whey protein powder
    23. 8.3.8. Permeate powders
    24. 8.3.9. Mother liquor
    25. 8.4. Other Dried Milk Products
    26. 8.5. Baby food
    27. 8.6. Caseinate powder
    28. 8.6.1. Coffee whitener
    29. 8.6.2. Cocoa-milk-sugar powder
    30. 8.6.3. Cheese powder
    31. 8.6.4. Butter powder
  9. 9.The composition and properties of milk
    1. 9.1. Raw milk quality
    2. 9.2. Milk composition
    3. 9.3. Components of milk solids
    4. 9.3.1. Milk proteins
    5. 9.3.2. Milk fat
    6. 9.3.3. Milk sugar
    7. 9.3.4. Minerals of milk
    8. 9.4. Physical properties of milk
    9. 9.4.1. Viscosity
    10. 9.4.2. Density
    11. 9.4.3. Boiling point
    12. 9.4.4. Acidity
    13. 9.4.5. Redox potential
    14. 9.4.6. Crystallization of lactose
    15. 9.4.7. Water activity
    16. 9.4.8. Stickiness and glass transition
  10. 10.Achieving product properties
    1. 10.1. Moisture content
    2. 10.2. Insolubility index
    3. 10.3. Bulk density, particle density, occluded air
    4. 10.4. Agglomeration
    5. 10.5. Flowability
    6. 10.6. Free fat content
    7. 10.7. Instant properties
    8. 10.7.1. Wettability
    9. 10.7.2. Dispersibility
    10. 10.7.3. Sludge
    11. 10.7.4. Heat stability
    12. 10.7.5. Slowly dispersible particles
    13. 10.7.6. Hot water test and coffee test
    14. 10.7.7. White Flecks Number (WFN)
    15. 10.8. Hygroscopicity, sticking and caking properties
    16. 10.9. Whey Protein Nitrogen Index (WPNI)
    17. 10.10. Shelf life
  11. 11.Analytical methods
    1. 11.1. Moisture content
    2. 11.1.1. Standard oven drying method (IDF Standard No.26-1964 [32])
    3. 11.1.2. Free moisture
    4. 11.1.3. Total moisture
    5. 11.1.4. Water of crystallization
    6. 11.2. Insolubility index
    7. 11.3. Bulk density
    8. 11.4. Particle density
    9. 11.5. Scorched particles
    10. 11.6. Wettability
    11. 11.7. Dispersibility
    12. 11.8. Other methods for determination of instant properties
    13. 11.8.1. Sludge
    14. 11.8.2. Slowly dispersible particles
    15. 11.8.3. Hot water sediment
    16. 11.8.4. Coffee test
    17. 11.8.5. White flecks number
    18. 11.9. Total fat content
    19. 11.10. Free fat content
    20. 11.11. Particle size distribution
    21. 11.12. Mechanical stability
    22. 11.13. Hygroscopicity
    23. 11.14. Degree of caking
    24. 11.15. Total lactose and α-lactose content
    25. 11.16. Titratable acidity
    26. 11.17. Whey Protein Nitrogen Index (WPNI)
    27. 11.18. Flowability (GEA Niro [31])
    28. 11.19. Lecithin content
    29. 11.20. Analytical methods for milk concentrates
    30. 11.20.1. Total solids
    31. 11.20.2. Insolubility index
    32. 11.20.3. Viscosity
    33. 11.20.4. Degree of crystallization
  12. 12.Troubleshooting operations
    1. 12.1. Lack of capacity
    2. 12.2. Product quality
    3. 12.3. Deposits in the system
    4. 12.4. Fire precaution
    5. 12.5. Principles of good manufacturing practice
    6. 12.6. The use of computer for quality control and trouble-shooting
  13. References
Reference: Schlünder,E.U.:Dissertation Techn.Hochschule Darmstadt D 17, 1962.