About 40 years ago milk powder quality was evaluated using the same criteria as for liquid milk products. The aim of such evaluation was:
a) to ensure that the final product met the specified composition i.e. fat content, total solids content and possibly content of other ingredients, if any (sugar etc.).
b) To ensure that the product during processing was not affected by some undesirable microbiological or chemical processes.
The only properties related specifically to milk powder were solubility index and content of scorched particles. Later on it was found that it was possible to influence the propertiesof the final product by certain pre-treatment processes, by choosing certain conditions for evaporation and spray drying, by dividing the water removal process into spray drying and fluid bed drying and by applying various after-treatment processes. This resulted in the development of many new products with properties tailor-made for a special end-use, i.e. having special functional properties.
The properties of the final products are influenced by a number of factors involving quality and composition of the raw milk and operating conditions applied. As some of the factors are subjected to both seasonal and daily variations, it is necessary to frequently control those properties, which might be affected by those variations and to make the appropriate
correction to the operation parameters.
Moisture content
The moisture content of the final product is a property, which is required by the product specification, defining the permissible maximum level (for instance max 3 %). From the point of view of functionality, too high a final moisture may result in inferior shelf life due to Maillard reaction, creation of lumps, and possibly bacteriological problems or growth of yeast and mould. Thus the moisture contents for individual products and that required by legal specification have been chosen with respect to the above.
The final moisture content is important from the point of view of final powder quality and achieving a standard product. From the economical point of view, it is important to operate as close as possible to the limit. In large spray drying installations each 0.1% of moisture can represent a great sum of money on a yearly basis. However, not less important is the intermediate moisture content of a product leaving the individual processing steps during two
stage or three stage drying. The intermediate moisture content has great influence on such properties as solubility index, bulk density, particle density, agglomeration (i.e. particle size distribution) and also on overall drying economy.
In a single-stage dryer the final moisture is influenced by combination of factors involving properties of the feed (concentration, temperature and viscosity), conditions of atomization (rotating wheel atomizer speed or atomization pressure with pressure nozzles) and conditions of the drying air (inlet and outlet temperature and absolute humidity). The magnitude of influence of some factors on the moisture levels ex-drying chamber is known. For instance,
as shown in Fig. 10.1, an increase of inlet temperature by 10°C, ambient air absolute humidity by 2.8 g/kg or total solids of the feed by 1% and reduction of the outlet temperature by 1°C will result in an increase of powder moisture by 0.2% with skim milk and by 0.16% with whole milk. The direction of change for other factors is indicated by the ±-symbols but not the exact magnitude.


In a two stage drying system the intermediate moisture should be kept reasonably constant because, as mentioned above, it influences several other properties. The importance of the second drying stage is the fine adjustment of the
final moisture below the rejection level from the standard quality point of view, but at the same time it should be as close as possible to that level from the economical processing point of view. Furthermore, it is also important during the second drying stage and cooling stage to ensure continuous reduction of moisture, as the powder passes through the drying/cooling fluid bed system. As mentioned in section 3.6, the milk powders, when cooled down to low final temperatures, can pick up moisture from the cooling air.
Insolubility index

Bulk density, particle density, occluded air







Agglomeration

d) forced secondary agglomeration is obtained by classifying the agglomerated powder, i.e. separating the non-agglomerated particles and re-introducing them back into the atomizing cloud.
Agglomeration is a complex process and its effect on mean particle size and powder bulk density depends on the equilibrium between several partial processes as shown schematically in Fig. 10.12. Agglomeration in this model means exclusively the formation of agglomerates in the atomizer cloud, and agglomeration efficiency means here the percentage of agglomerates of total powder after this stage. As mentioned in the above scheme, the efficiency of agglomeration depends on a number of factors, an important one being the amount of recycled fines.


Flowability
Free fat content


Instant properties
Wettability
Dispersibility
Sludge
Heat stability
Slowly dispersible particles

Property or condition |
Unit |
Min. |
Max. |
Correl. |
---|---|---|---|---|
SDP 85 | SDP 85 | coef | ||
WPNI SDP 85 |
mg N/g A=0 |
2.43 0.00 |
2.00 3.04 |
0.82 |
Titratable acidity SDP 85 |
ml l.a. A=0 |
0.10 0.51 |
0.12 3.92 |
0.75 |
Hot water sediment SDP 85 |
ml A=0 |
0.20 1.23 |
3.95 4.13 |
0.57 |
Coffee test SDP 85 |
ml A=0 |
0.39 0.84 |
1.26 3.70 |
0.36 |
SFB-Fluidizing vel. SDP 85 |
m/s A=0 |
0.97 0.00 |
0.88 3.23 |
0.73 |
Outlet temperature SDP 85 |
°C A=0 |
67.20 0.00 |
63.60 3.18 |
0.60 |
Atomization pressure SDP 85 |
kPa A=0 |
28.10 0.56 |
21.30 3.01 |
0.51 |
Property | Result | Property | Result |
---|---|---|---|
Titr. acidity | 0.11 | Hot water test | 0.30 |
Sludge 25 | 0.04 | Coffee test | 1.06 |
SDP 25 | 1.43 | Free fat | 0.72 |
Dispersibility | 5.00 | WPNI | 3.37 |
Sludge 85 | 0.04 | Fraction >500µm | 17.70 |
SDP 85 | 0.29 | Fraction <125µm | 9.95 |
Hot water test and coffee test

White Flecks Number (WFN)

Hygroscopicity, sticking and caking properties


Whey Protein Nitrogen Index (WPNI)

Shelf life



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