17 Milk and whey powder

Prévia do material em texto

Dairy Processing Handbook/Chapter 17 375
Milk and whey powder
The method of preserving foodstuffs by drying them, and thereby
depriving micro-organisms of the water necessary for their growth, has
been known for centuries. According to Marco Polo’s accounts of his
travels in Asia, Mongolians produced milk powder by drying milk in the
sun.
Skim milk powder has a maximum shelf life of about three years, while
whole milk powder has a maximum shelf life of six months. This is
because the fat in the powder oxidises during storage, with consequent
deterioration in flavour. However, the shelf life can be extended by using
appropriate packaging technology, i.e. packing the powder under an inert
gas such as N2.
Dairy Processing Handbook/Chapter 17376
Drying
Drying means that the water in a liquid product – in this case milk
concentrate – is removed, so that the product takes on a solid form. The
water content of milk powder ranges between 1,5 and 5 %. Micro-
organisms are unable to reproduce at such a low water content. Drying
extends the shelf life of the milk, simultaneously reducing its weight and
volume. This reduces the cost of transporting and storing the product.
Freeze-drying has been used to produce very high quality powder. In this
process, the product is deep-frozen and the water frozen in the product is
evaporated under vacuum. This ensures high product quality as, among
other things, no protein denaturing takes place. When drying milk at higher
temperatures, the proteins are denatured to a greater or a lesser extent.
Freeze-drying is not widely used for the production of milk powder because
of the high energy demand.
Commercial methods of drying are based on heat being supplied to the
product. The water evaporates and is removed as vapour. The residue is
the dried product – the milk powder. Nowadays, spray drying is the method
primarily used for drying in the dairy industry. Roller drying is also used for a
few special products. In spray drying, the milk is first concentrated by
evaporation and then dried in a spray tower.
During the first stage of drying, the excess water – in free form between
the particles of the dry solids – is evaporated. In the final stage, the water in
the pores and capillaries of the solid particles is evaporated.
The first stage is relatively quick, whereas the last stage demands more
energy and time. The product will be significantly affected by the heat if this
drying is carried out in such a way that the milk particles are in contact with
the hot heat transfer surfaces – as in the case of roller drying. The powder
may then contain charred particles, which impair its quality.
Various uses of milk powder
Dried milk can be used for various applications, such as:
• Recombination of milk and milk products
• In the bakery industry to increase the volume of bread and improve its
water-binding capacity. The bread will then remain fresh for a longer
period of time
• Substitute for eggs in bread and pastries
• Producing milk chocolate in the chocolate industry
• Producing sausages and various types of ready-cooked meals in the
food industry and catering trade
• In baby foods
• Production of ice cream
• Animal feed, calf growth accelerator
Skim milk powder
Skim milk powder is by far the most common type of milk powder.
Table 17.1
Typical properties of spray-dried milk powder
Product Whole milk Skim milk
powder powder
Solubility, ml 0,1 – 0,5 0,1 – 0,5
Scorced particles, ADMI Disc A A
Residual moisture, max. % 3,0 4,0
Bulk density, tapped, g/ml 0,45 – 0,57 0,45 – 0,6
Total spore count, max. n/g 10 000 10 000
Dairy Processing Handbook/Chapter 17 377
Each field of application makes its own specific demands of milk powder.
If the powder is to be mixed with water for direct consumption
(recombination), it must be readily soluble and have the correct taste and
nutritive value. For this application, the product has to be dried very carefully
in a spray dryer. Some degree of caramelisation of the lactose is beneficial
in chocolate production. Here, the powder can be subjected to intensive
heat treatment in a roller dryer. Two types of powder are therefore
distinguishable:
• Spray-dried powder
• Roller-dried powder
Depending on the intensity of the heat treatment, milk powder is classified
into categories related to the temperature/time combinations to which the
skim milk has been exposed prior to and during evaporation and drying.
Heat treatment denatures whey proteins, the percentage denaturated
increasing with the intensity of the heat treatment. The degree of
denaturation is normally expressed by the Whey Protein Nitrogen Index
(WPNI), i.e. in milligrams of undenatured whey protein nitrogen per gram of
powder.
Information about the various categories of spray dried skim milk powder
is summarised in Table 17.2.
Table 17.2
Categories of spray-dried skim milk powder.
Category Temp/time WPNI
mg/g
Extra low-heat <70 °C *)
Low-heat (LH) powder 70 °C/15 s > 6,0
Medium-heat (MH) powder 85 °C/20 s 5 – 6,0
” 90 °C/30 s 4 – 5,0
” 95 °C/30 s 3 – 4,0
Medium high-heat (HH) 124 °C/30 s 1,5 – 2,0
High-heat (HH) appr. 135 °C/30 s <1,4
High-heat high stable (HHHS) appr. 135 °C/30 s <1,4
(from selected milk)
*) Not measurable
Table by Sanderson N.Z., J. Dairy Technology, 2, 35 (1967)
Whole milk powder
Spray dried whole milk powder is normally produced from standardised
milk. After standardisation of the fat content, the milk need not be
homogenised, provided that it is thoroughly agitated, without air inclusion.
Homogenisation is normally carried out between evaporation and spray
drying.
Fat standardised milk for the production of roller dried powder is
normally homogenised.
Whole milk powder, unlike skim milk powder, is not categorised. Milk
intended for whole milk powder is pasteurised at 80 – 85 °C in order to
inactivate most of the lipolytic enzymes that would otherwise degrade the
milk fat during storage.
Instant-milk powder
Special methods for the production of both skim milk and whole milk
powder with extremely good wettability and solubility – known as instant
powder – are also available. This powder is agglomerated into larger
particles. A number of particles are combined to form a larger grain
(agglomerate). The average grain size of the product increases. This instant
powder, as it is known, dissolves instantly, even in cold water.
Dairy Processing Handbook/Chapter 17378
Bulk density
When powders are shipped over long distances, it is important that they
have a high bulk density as the reduced volume saves on transportation
costs and packaging. However, in some instances, producers may be
interested in low bulk density in order to supply visibly larger amounts of
powder than their competitors. Low bulk density, as influenced by
agglomeration, is also an important characteristic of instant powders.
Definition
Bulk density is the weight of a unit volume of powder; in practice it is
expressed as g/ml, g/100 ml or g/l.
Production of milk powder
In the production of roller-dried powder, the pre-treated milk is fed to a roller
dryer after evaporation and the whole drying process takes place in one
stage.
In the production of spray-dried powder, the milk is first evaporated in a
vacuum evaporator to a DM content of 48 – 50 % and then dried in the
spray tower. Spray-dried skim milk powder is manufactured in two basic
qualities:
• Ordinary product
• Agglomerated (instant) milk powder by various spray drying processes
Following roller or spray drying, the powder is packed in cans, paper bags,
laminated bags or plastic bags, depending on the quality and the
requirements of the consumers.
Raw material
Very strict demands are made of the quality of the raw material for the
production of milk powder.
Since spray powder production involves vacuum evaporation, it is veryimportant to keep heat-resistant bacteria under control so that they cannot
multiply during evaporation. Bactofugation or microfiltration of the milk can
therefore partly be used to remove bacteria spores from the milk, thereby
improving the bacteriological quality of the end product.
Milk for powder production must not be subjected to excessive thermal
impact prior to evaporation and drying. This could denature whey proteins
and thereby impair the solubility, aroma and flavour of the milk powder. The
milk is subjected to a peroxidase test or a whey protein test to determine
the degree of heat treatment.
General pre-treatment of the milk
In the production of skim milk powder, the milk is clarified in conjunction
with fat separation. This is also the case if the fat content is adjusted in a
Strict demands are made on
the quality of the raw material
for production of milk powder.
Table 17.3
Composition of milk powder
Product Whole milk Skim milk
Fat, % solids content 26 – 29 max. 1,25
Protein, % solids content 25 – 27 34 – 38
Lactose, % solids content 35 – 38 48 – 56
pH 6.6 – 6.7 6.6 – 6.7
Acid value, % < 0,16 < 0,16
Total spore count, max. n/ml 10 000 10 000
Dairy Processing Handbook/Chapter 17 379
direct standardisation system. Standardised milk used for producing whole
milk powder is normally homogenised.
Whole milk and skim milk are pasteurised at various temperatures
depending on the powder quality requirements.
Roller drying
In roller drying, the milk concentrate is distributed as a film on rotating,
steam-heated drums or rollers. The water in the concentrated milk
evaporates, and the vapour is drawn off. The high temperature of the
heating surfaces denatures the proteins, solubility deteriorates and brown
discolouration may occur.
On the other hand, this intensive heat treatment increases the water-
binding properties of the powder. This characteristic is useful in the
production of ready-cooked meals, sausages and bread, cakes and
pastries. Roller powders are used in the chocolate industry in particular due
to their special properties.
The pre-treated milk is applied to the hot roller surface as a thin film.
The dried milk is scraped off the rollers and removed by means of a screw
conveyor, at which point the milk is broken down into flakes. The flakes are
then transferred to a grinder which is used to create the desired grain size.
After that, hard and burned particles are sifted off.
Depending on the desired capacity, the roller dryer is 1 – 6 m long and
has a roller diameter of 0,6 – 3 m. Its performance is dependent on film
thickness, roller surface temperature, roller speed and the DM content of
the supplied milk.
Spray drying
Powder production is carried out in two phases. In the first phase, the pre-
treated milk is evaporated to a DM content of 48 – 52 %. Whey is
concentrated to a DM content of 58 – 62 %. In the second phase, the
concentrate is turned into powder in a spray tower. Likewise, drying is also
a multiple-stage process:
• Atomisation of the concentrate into very fine droplets in a hot air stream
• Water evaporation
• Separation of the powder from the drying air
Evaporation is necessary to produce high-quality powder. Without prior
concentration, the powder particles will be very small and have a high air
content, poor wettability and a short shelf life. The process would then also
be extremely uneconomical.
Falling-film tubular evaporators are generally used for concentration,
which is carried out in one or more effects. See Chapter 6.5.
Fig. 17.1 Principle of the trough-fed
roller dryer.
Milk
Heating medium
Air for pneumatic
transportation and cooling
Fig. 17.2 Conventional spray dryer (one-
stage drying) with conical base chamber.
1 Drying chamber
2 Air heater
3 Milk concentrate tank
4 Feed pump
5 Atomiser
6 Main cyclone
7 Transport system cyclone
8 Air suction fans and filters
1
2
3
4
5
6
7
8
Concentrated milk
Air
Powder
Dairy Processing Handbook/Chapter 17380
Basic drying installations
Single-stage drying
The simplest installation for producing a powder consists of a drying
chamber with an atomisation system, the air heater, a system for collecting
the finished powder from the dry air and a fan which sucks the necessary
amount of air through the entire system. An installation of this type is known
as single-stage dryer as the entire drying process takes place in a single
unit, the drying chamber. A powder with a small grain size and a high fines
content is produced here.
Two-stage drying
The system described above is extended by means of a fluid bed dryer. The
powder leaves the drying chamber with a higher residual moisture. In the
fluid bed, drying ends at relatively low temperatures, and the powder may
also be cooled. In terms of energy, this installation is better than a single-
stage dryer as it is possible to work with considerably lower air exit
temperatures. The quality of the powder can be improved by separation of
the fine powder in the fluid bed.
Three-stage drying
Three-stage drying is an extension of the two-stage concept, developed to
achieve greater savings in process costs and to meet various product
quality demands more effectively.
Operating principle of spray drying
In all configurations, most of the air required for drying is sucked through
the installation by centrifugal fans. The air for building up the fluidized layer
in an integrated and/or external fluid bed and the air for optional
recirculation of the fine powder is brought into the system via fresh air
ventilators.
The temperature of the outgoing air is the reference variable for the
residual moisture in the powder. Attempts must be made to avoid high
outgoing air temperatures and thus high product temperatures which have
an adverse effect on the quality of the powder, e.g. deterioration of
solubility.
Single-stage drying
Figure 17.2 shows the arrangement of a single-stage spray drying
plant. The concentrate is fed via a high-pressure pump (4) to an
atomisation system (5) integrated centrically in the roof of the
chamber. This system produces very small droplets, Ø 40 – 125
µm. The drying air is normally sucked through a pre-filter and fine-
filter and then passes an air heater. Depending on the desired air
temperature, the air is steam-heated up to 190 – 200 °C. New
installations are mostly provided with an indirect air heater which can
be operated by means of a combined fuel burner for natural gas or, in
an emergency, with light fuel oil.
An indirect heat recovery system can be provided to improve energy
economy. Residual heat from the outgoing air and the flue gas from the
heater are used to pre-heat the incoming air.
Depending on the product, the incoming air is heated to a temperature
of 150 – 210 °C. The hot air flows through a distributor which ensures that
the air is travelling at a uniform speed into the drying chamber, where it
is mixed with the atomised product in the straight flow.
The free water evaporates immediately when the atomised
product enters the drying chamber. Surface water evaporates
very quickly, as does the moisture from the inside of the
droplets which quickly reach the surface by capillary action. Then
heat is transferred into the particles by convection. This results in the
evaporation of bound water, diffusing it onto the surface of the particles.Fig. 17.3 Multi-stage dryer set up. (CPS)
Dairy Processing Handbook/Chapter 17 381
Because the heat content of the hot air is continuously consumed by
evaporation of the water, the product heats up to a maximum temperature
of only 15 – 20 °C less than the temperature of the air when it exits the
drying chamber; under normal conditions 60 – 80 °C.
The evaporation of the water from the droplets leads to a considerable
reduction in weight, volume and diameter. Under idealdrying conditions,
weight will decrease by about 50 % and the volume by about 40 %. The
diameter is reduced to 75 % of the droplet size after leaving the atomiser,
Figure 17.4.
During the drying process, the powder settles in the bottom cone of the
chamber and is discharged from the system. It is conveyed to a silo or
packing station by a pneumatic conveyor which uses cold air to cool the
hot powder. The powder is then separated from the transport air by means
of a cyclone.
Small, light particles may be sucked out of the drying chamber, mixed in
with the air. This powder is separated in one or more cyclones (6, 7 in
Figure 17.2) and fed to the main flow of powder.
Atomisation
The more finely the product is atomised, the larger their specific area will be
and the more effective the drying process. One litre of milk in a spherical
shape has a surface area of about 0,05 m2. If this quantity of milk is
atomised in the spray tower, each of the small droplets will have a surface
area of 0,05 – 0,15 mm2, i.e. atomising increases the specific area by a
factor of about 700.
The type of atomisation depends on the product, the desired particle
size and the properties required of the dried product. These may include
texture, grain size, bulk density, solubility, wettability and density. There is an
important functional differentiate between nozzle and centrifugal
atomisation. A stationary nozzle which sprays the milk in the same direction
as the flow of air is shown in Figure 17.5. A centrifugal disc for atomisation
is shown in Figure 17.6.
The pressure at the nozzle determines the particle size. At high
pressures, up to 30 MPa, (300 bar), the powder will be very fine and have a
high density. At low pressures, 5 – 20 MPa, (50 – 200 bar), larger particles
will be formed and the fines content will be lower. The pressure is built up
by means of multi-plunger high-pressure pumps. These are mostly
homogenisers, which are needed for many products and can also operate
as high-pressure pumps with by-passed homogenisation devices.
The centrifugal atomiser consists of an electric drive which rotates a disc
with a number of horizontal passages. The product is fed into the middle of
the disc and forced through the passages at high speed by centrifugal
force. The discs rotate at speeds of 5 000 to 25 000 rpm depending on
their diameter. Peripheral speeds of between 100 – 200 m/s are achieved.
The flow of product is atomised into very fine droplets upon its exit from
the passage due to the high speed. The size of the droplets – and thus also
the grain size of the powder – can be influenced directly by changing the
atomiser speed. A centrifugal pump is normally sufficient to feed this type of
atomiser.
Essentially, a larger grain size can be achieved by means of nozzle
atomisation, 150 – 300 µm as compared to 40 – 150 µm with centrifugal
atomisation. However, centrifugal atomisation is straightforward to operate
and not sensitive to variations in product viscosity and quantity supplied.
Two-stage drying
Two-stage drying in the fluid bed dryer, which is divided into a number of
zones, ensures that the desired residual moisture is achieved and that the
powder is cooled. Final drying in the integrated fluid bed dryer ensures that
the desired residual moisture is achieved. After final drying the powder is
cooled in a pneumatic cooling and conveying duct.
The installations can be operated with both nozzle and centrifugal
atomisers.
100
90
80
70
60
50
40
30
20
10
0
50 60 70 80 90 10045
D
W
V
4 % H20
Percentage of initial value
Droplet solid content (%)
D = Diameter
W = Weight
V = Volume
Fig. 17.4 Weight, volume and diameter
decrease of droplet under ideal drying
conditions down to 4 % H2O.
Fig. 17.5 Stationary nozzles for atomis-
ing the milk in a spray drying chamber.
Fig. 17.6 Rotating disc for atomising
milk in the spray drying chamber.
Dairy Processing Handbook/Chapter 17382
The products consist mainly of individual particles but have a slightly
lower fines content. The solubility index and content of air included are
smaller in the case of powders dried using the two-stage method on
account of the lower thermal impact overall, although the bulk density is
higher.
Air and the fine powder are transported out of the chamber via a vertical
duct placed in the center of the integrated fluid bed.
The biggest advantage over single-stage drying is the improved
efficiency achieved by increasing the temperature difference between
supply air and outgoing air. The energy required for drying is about 10 –
15 % lower than in the single-stage process.
Table 17.4 shows a comparison between single-stage and two-stage
drying systems.
Three-stage drying
If the pneumatic cooling and conveying duct in a two-stage dryer with
integrated fluid bed is replaced with an external fluid bed with a drying and
a cooling section, this is known as a three-stage dryer. A large number of
agglomerated and non-agglomerated dairy and food products can be
manufactured using this process. If agglomerated products are produced
the fine powder is recirculated to the wet atomisation zone. This
configuration allows the supply air temperature to be increased and the
outgoing air temperature to be lowered. This reduces the specific energy
requirement by a further 10 – 15 % compared with two-stage drying. A
large number of plants installed work to this principle. Mass-produced
products such as skim milk powder and whey powder are manufactured
cost-effectively in this way.
Multi-function dryers
This system is based on two-stage or three-stage drying. Transporting the
drying air out through the roof of the chamber is characteristic of this
structure, which represents the current state of the art in drying technology.
This optimised air conduction carries the fine powder directly back into the
atomisation zone, so recirculating large amounts of fine powder which
would otherwise remain in the loop between the chamber and the cyclone
or filter, is no longer necessary in order to achieve good agglomeration.
Nozzle atomisation systems in particular are used which are made up of
a number of pressure nozzles, one central one and the others in a circle.
The central pressure nozzle is provided with an outer tube through which
the fine powder can be aimed into the core atomisation zone. Suitable
selection of a nozzle structure and correct adjustment of the movable
Fig. 17.7 Two-stage dryer with
integrated fluid bed. (CPS)
1 Balance tank for concentrated milk
2 Feed pump
3 Air heater
4 Atomiser
5 Filter for fines recovery
6 Air fans and heaters
7 Transport system cyclone
8 Drying chamber
1
5
4
2
3
7
6
6
8
Concentrated milk
Powder
Cold air
Hot air
Steam
Dairy Processing Handbook/Chapter 17 383
Table 17.4
Comparison of one-stage and two-stage drying systems.
Drying system One-stage Two-stage
Inlet temp. Inlet temp. Inlet temp.
200 °C 200 °C 230 °C
Spray dryer (First stage)
Evaporation in chamber, kg/h 1 150 1 400 1 720
Powder from chamber:
6,0 % moisture, kg/h – 1 460 1 790
3,5 % moisture, kg/h 1 140 – –
Energy consumption,
spray drying total, Mcal 1 818 1 823 2 120
Energy/kg powder, kcal 1 595 1 250 1 184
Fluid Bed (Second Stage)
Drying air, kg/h – 3 430 4 290
Inlet air temperature, °C – 100 100
Evaporation in fluid bed, kg/h – 40 45
Powder from fluid bed
3,5 % moisture, kg/h – 1 420 1 745
Energy consumption, kW – 20 22
Energy consumption,
total in fluid bed, Mcal – 95 115
Total plant
Energy consump. total, Mcal 1 818 1 918 2 235
Energy/kg powder total, kcal 1 595 1 350 1 280
Energy relation 100 85 80
Basis: Same drying chamber size with inlet air flow = 31 500 kg/h.
Product: skim milk, 48 % solids in concentrate.
Source: Evaporation, Membrane Filtration, Spray Drying - NorthEuropean Dairy Journal, 1985 Copenhagen, Denmark.
ISBN No. 87-7477-000-4.
1
7
9
9
9
5
2
3
4
6
9
8
10
Concentrated milk
Powder
Cold air
Hot air
Steam
Fig. 17.8 Three-stage dryer with filter for
fines recovery and integrated external
fluid bed. (CPS)
1 Balance tank for concentrated milk
2 Feed pump
3 Tubular heat exchanger for pre-
heating
4 Air heater
5 Circulation pump for heat recovery
water circuit
6 Atomiser
7 Filter for fines recovery
8 Fluid bed
9 Air fans and heaters
10 Drying chamber
nozzle heads allows optimum agglomeration for practically every product.
Difficult products, even those with a very high fat content, are possible to
produce. The result is a practically fines-free powder with a high degree of
mechanical agglomerate stability. Centrifugal atomisers can also be used.
Dairy Processing Handbook/Chapter 17384
Appropriate design of the roof of the chamber means that an exchange
takes only a short time, making rapid product changes possible.
The structure of the overall plant permits very compact installation with
short powder transport paths.
Additional equipment for spray dryers
Powder separation
The classic system for the separation of powder and drying air is a cyclone
or an arrangement of a number of cyclones in series or in parallel. The
residual fines content in the outgoing air is very high, 100 – 200 mg/Nm3.
Introducing bag filters allowed the residual fines content to be lowered to
less than 10 mg/Nm3. The filtering bags are dedusted by means of regular
pulses of compressed air. The residual fines accumulated in the filter are
waste as far as food is concerned.
The use of washable filters, which are linked to the CIP system, are the
current state of the art. Regular cleaning ensures that the residual fines are
not contaminated and remain suitable as food, which leads to an increase
in yield and thus a reduction in powder manufacturing costs.
Most dairy powders can be produced without the cyclone or cyclones if
the surface area of the filter is dimensioned appropriately. This means a
considerably lower pressure loss in the air system, which is why the
capacity of the main fan can be reduced by up to 20 %. The economic
efficiency of a drying plant of this type is very clearly improved.
Systems for avoiding deposits
A lot of drying plants have to process difficult products which can cause
deposits in the cylindrical and/or conical part of the drying chamber. These
deposits are not only unpleasant due to any contamination they cause:
because of the temperatures in the chamber, they may possibly lead not
only to burnt particles, but they could also form dangerous smoulder spots
which could cause fires, or – in a worst-case scenario – explosions.
To avoid this, slot nozzles can be incorporated into the wall of the
chamber in the lower cylindrical part and/or the cone which produce a hot
air curtain with a high radial velocity directly on the wall of the chamber,
thereby preventing or at least delaying deposits.
Another design element which prevents deposits is an air broom. It is
used in particular in multifunction dryers. This is a tube arranged centrically
in the integrated fluid bed which is driven by electricity and rotates along the
chamber wall at about 1 rpm. It is fitted very close to the wall and coats the
cone and cylinder wall of the chamber with air, which is blown into the tube
from below and emerges from the tube at high speed through overlapping
slot nozzles.
Air conditioning
Depending on the ambient conditions, the water content of the drying air
may be so high that it is unable to absorb any more moisture, particularly in
the case of final drying at low temperatures in the integrated and external
fluid bed. At this point, dehumidification of the air is necessary. This can be
carried out by cooling the air flow to below dew point by means of chilled
water. The water condenses and is separated from the air flow via a mist
collector. The air is then heated up to the desired temperature again. This
also applies in particular to product cooling air.
Fire and explosion protection
The powder-air mixture in the drying chamber may be inflammable or it
may, as mentioned above, lead to fires due to smoulder spots. Fires can be
extinguished by installing fire extinguishing nozzles in the chamber roof, the
fluid beds and the bag filter. Carbon oxide (CO) detectors or photoelectric
cells are used to detect fires.
Destruction and thus long-term loss of plant function can be avoided by
Dairy Processing Handbook/Chapter 17 385
installing pressure relief apertures or rupture discs which keep the pressure
on the wall of the chamber low in the event of explosions. Ducts and filters
can be protected by extinguishing barriers which are triggered by means of
pressure detectors. These include fire extinguishers pre-filled to about 50
bar and mounted permanently at strategically important locations and filled
with inert powder (sodium hydrogen carbonate). In the event of an
explosion, a detonator blows up the partition disc between the
extinguishing container and the dryer. The extinguisher is blown into the
plant instantaneously and brings the area where it was blown in into a non-
flammable state by changing the powder to air ratio. The explosion front
thereby stops from spreading. Depending on the product, plant safety must
be prioritised as early as the planning stage. Relevant regulations are
available to plant constructors.
Heat recovery
A large amount of the heat is lost with the outgoing air in the drying
process. Some of this heat can be recovered in suitable heat exchangers.
The outgoing air from the dryer and the flue gas from the air heater are the
main sources for heat recovery. The heat transfer is carried out indirectly, i.e.
the heat in the hot air is transferred to the air heater via a liquid cycle with its
own circulation. This can improve the thermal efficiency of a spray dryer by
10 – 15 %.
Concentrate heating
The thermal efficiency of the dryer is improved if the temperature of the feed
product is increased. However, this increase in temperature has to take
place very carefully to avoid unwanted thermal impact and an increase in
viscosity. Heating from 55 °C to 75 – 80 °C can generally take place, but
must be performed immediately before atomisation. It is appropriate to
mount the concentrate heater on the roof of the chamber near to the
atomiser. The heater, which can be supplied for high-pressure and low-
pressure atomisation, is designed as twin tubular heat exchangers in a
shared casing so that cleaning can be carried out without having to
interrupt operation.
The product is heated very carefully by means of vacuum steam. If the
chamber size and peripheral equipment remain the same, an increase in
performance of about 5 % is possible. This is also true of the retrofitted
equipment in existing plants.
1
2
4 5
6
8
7
109
11
12
14
13
16
15
3
Concentrated milk
Milk powder
Hot air
Cold air
Fig. 17.9 Spray dryer with integrated
belt, Filtermat (three-stage drying).
1 High pressure feed pump
2 Nozzle arrangement
3 Primary drying chamber
4 Air filters
5 Heater/cooler
6 Air distributor
7 Belt assembly
8 Retention chamber
9 Final drying chamber
10 Cooling chamber
11 Powder discharge
12 Cyclone arrangement
13 Fans
14 Fines recovery system
15 Sifting system
16 Heat recovery system
Dairy Processing Handbook/Chapter 17386
Spray belt dryer
A spray belt dryer is shown in Figure 17.9. It consists of a main drying
chamber (3) and three smaller chambers for crystallisation (when required,
e.g. for the production of whey powder), final drying and cooling (8, 9 and
10).
The product is atomised by nozzles located above the main chamber of
the dryer. The feed is conveyed to the nozzles bya high pressure pump.
Atomisation pressure is up to 200 bar.
Agglomeration in the fluid bed
To obtain the correct porosity, the particles must first be dried so that most
of the water in the capillaries and pores is replaced by air. The particles are
then humidified so that the surfaces of the particles swell quickly, closing
the capillaries. The surfaces of the particles will then become sticky and the
particles will adhere to form agglomerates.
One efficient method of instantisation is rehumidifying agglomeration in a
fluid bed as shown in Figure 17.10. The fluid bed is connected to the
discharge opening of the drying chamber and consists of a casing with a
perforated bottom. Air at a suitable temperature is blown in through the
floor at a velocity which is sufficient to suspend the powder and fluidise it.
The casing is mounted in a spring bearing and can be vibrated by a
motor. The holes in the bottom plate are shaped as nozzles in the product
flow direction and the product is fed in the direction of the outflow. The
vibration supports fluidisation and conveys the powder. Weirs between the
individual sections and at the outlet determine the height of the fluidised
powder layer, and the length of the fluid bed determines the residence time.
The powder is conveyed from the spray tower into the first section,
where it is humidified by steam. An air flow and vibrations convey the
powder through the individual drying sections, where hot air at decreasing
temperatures is fed through the powder bed. Agglomeration takes place in
the first stage of drying when the moistened particles adhere to each other
to form agglomerates. The water is evaporated from the agglomerates
during their passage through the drying sections. The powder is then
Milk powder
Steam
Hot air
Cold air
Fig. 17.10 Fluid bed for instantising milk powder.
cooled and leaves the fluid bed with the desired residual moisture.
Large grains and fine grains are screened. The screened and instantised
powder is then conveyed to filling by a gentle transport system. The
outgoing air from the fluid bed, containing a certain amount of fine powder,
is blown to the cyclone or filter of the main air system of the drying plant.
Agglomeration in the drying chamber
A simpler method of rehumidifying agglomeration is to carry out
instantisation immediately in the drying chamber. This is particularly effective
in the multifunction dryer described above in Figure 17.8. Due to the air
conduction, pre-dried fine powder is constantly circulating about the
atomisation zone, where it coats particles which are still moist and forms
agglomerates. However, the essential part of agglomeration is achieved by
Dairy Processing Handbook/Chapter 17 387
deliberately blowing in fine powder out of the filter in the centre of a nozzle
atomisation spray zone, Figure 17.11.
The entirely dry fine powder immediately attaches itself to wet particles
and moist air, consequently resulting in sticking the particles together. This
results in stable agglomerates, with almost no fines content.
Fig. 17.11 Nozzle atomiser designed for
production od instant powder. (CPS
Patent pend.)
1 Fines return pipe.
Packing milk powder
The types and sizes of packages vary from one country to another. The
powder is often packed in paper bags with an inner bag made of
polyethylene. This polyethylene bag is welded, and so this package is
relatively impervious to oxygen and steam. The most common bag sizes are
15 and 25 kg, although other sizes are also used.
Milk powder for households and similar small-scale consumers is
packed in tin cans, laminated bags or plastic bags which, in turn, are
packed in cartons.
Changes in milk powder during storage
The milk fat in whole milk powder oxidises during storage. The shelf life can
be extended by special pre-treatment of the milk, i.e. by the addition of
antioxidants or by filling under inert gas.
Milk powder should be stored in a cool, dry place. All chemical reactions
in milk powder, at room temperature and with a low water content, take
place so slowly that the nutritive value is not affected, even after several
years of storage.
Dissolving milk powder
One part of a spray-dried milk powder is mixed with about ten parts of
water at a temperature of 30 – 50 °C and stirred. The dissolving time can
take from a few seconds to a few minutes.
If instantised powder is used, the required quantity of water is poured
into a tank and the powder is then added. The powder dissolves after very
brief stirring, even if the water is cold. The milk is then immediately ready for
use.
The water quality is very important for dissolving. It must be noted that
during drying, including the first concentration phase (evaporation), pure
(distilled) water has been evaporated off from the milk. Read more about
water quality in Chapter 18, Recombined milk products.
11
Milk powder
Concentrated milk
Hot air
Dairy Processing Handbook/Chapter 17388
	Milk and whey powder
	Drying
	Various uses of milk powder
	Skim milk powder
	Whole milk powder
	Instant-milk powder
	Bulk density
	Definition
	Production of milk powder
	Raw material
	General pre-treatment of the milk
	Roller drying
	Spray drying
	Basic drying installations
	Single-stage drying
	Two-stage drying
	Three-stage drying
	Operating principle of spray drying
	Single-stage drying
	Atomisation
	Two-stage drying
	Three-stage drying
	Multi-function dryers
	Additional equipment for spray dryers
	Powder separation
	Systems for avoiding deposits
	Air conditioning
	Fire and explosion protection
	Heat recovery
	Concentrate heating
	Spray belt dryer
	Agglomeration in the fluid bed
	Agglomeration in the drying chamber
	Packing milk powder
	Changes in milk powder during storage
	Dissolving milk powder