Dried milk powders- Processing and Manufacturing- Introduction, Principle, Processing and Manufacturing, Working, Types

With changing lifestyle and increasing demand of the convenience food, this segment of dairy is
becoming extremely essential and it is expected to grow further because of its capability to
solve the problems associated with this perishable product. The manufactured dairy product
i.e. Dried Milk Powder results when the water is removed by boiling the milk under reduced
pressure at low temperature in a process known as evaporation. When we talk of Dried milk
powder we generally talk of Whole milk powder(WMP) and Skim milk powder (SMP).
Drying is a mass transfer process consisting of the removal of water or another solvent by
evaporation from a solid, slurry or liquid. The science behind drying is that dry air comes in
contact with food and absorbs some of the moisture from the food. This air then has to be
blown away and be replaced with dry air so that the process of extracting moisture from the
food can continue until the food is dry.

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Dried Milk Powders- Processing and Manufacturing.


Introduction:
With changing lifestyle and increasing demand of the convenience food, this segment of dairy is
becoming extremely essential and it is expected to grow further because of its capability to
solve the problems associated with this perishable product. The manufactured dairy product
i.e. Dried Milk Powder results when the water is removed by boiling the milk under reduced
pressure at low temperature in a process known as evaporation. When we talk of Dried milk
powder we generally talk of Whole milk powder(WMP) and Skim milk powder (SMP)

S.No WMP SMP
1. Requires “high-heat”
method, heating milk to 85-
88 C and holding at this
temperature for 15 to30
seconds
milk is pasteurized at low
temperature with little or no
holding time required
2. Homogenization is required
in order to stop the fat
separating out after re-
constitution
No homogenization required
because of its low-fat content
3. Should contain 26-40% fat
(by wt.)
Max 1.5% fat (by wt.)
4. Moisture should not be more
than 5%
Moisture should not be more
than 5%

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Principle of Drying:
Drying is a mass transfer process consisting of the removal of water or another solvent by
evaporation from a solid, slurry or liquid. The science behind drying is that dry air comes in
contact with food and absorbs some of the moisture from the food. This air then has to be
blown away and be replaced with dry air so that the process of extracting moisture from the
food can continue until the food is dry.

Processing and Manufacturing:
1.Reception of Milk- Raw milk on arrival at the factory is rapidly tested for its SNF, fat, acidity,
smell, temperature, hygiene, antibiotics, water addition and adulteration. On acceptance the
milk is pumped into a silo storage tank at the processing plant and held at temperatures below
7° C and usually below 5 °C.

2.Standardization -It is done to adjust the ratio of fat and solid-not-fat in raw milk to meet the
legal requirements. When the raw milk arrives at the manufacturing plant it is usually separated
into cream and skimmed milk to enable standardization of the fat content. Good players of
dairy industry can automate this cream fat standardization using an inline “standomat” which
doses cream back into the skim to give the correct fat % in the milk to be processed
3.Pasteurization- The milk is than high temperature short-time pasteurized (HTST)by heating it
to at least 72.3 °C and holding at or above this temperature for at least 15 seconds (as per
requirement an equivalent temperature / time combination can be used). It has been noticed in
industries that (MAP - Mycobacterium avium subspecies paratuberculosis may be capable of
surviving pasteurization.) So, most high-volume liquid milk plants now operate on a higher
holding time of 25 seconds to 35 seconds as a precaution over the possible survival of MAP
which can cause Crohn’s disease in humans. The latest research (2019) indicates that the
optimum time and temperature combination for liquid milk is 72.3C for 26 seconds.
4.Concentration/Evaporation -It is an important stage before drying for reasons of energy
efficiency as it is far cheaper to evaporate the water than to spray dry it directly. It is generally
done by falling film evaporators because of their overall low energy consumption. therefore as
max. amount is tried to be removed in concentration stage.

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5. Spray Drying
It is a continuous process for transforming liquid feed into dried particulate form by spraying
the feed into a hot medium
The milk is fed from the top and is concentrated in a series of calandrias in an evaporator to
around 40-60% total solid. Milk when passed down the tubes wetting the surface whilst steam
is passed from on the other side of the tube and the vapours extracted from the centre by
vacuum. Vapours are normally recompressed in a vapour re-compressor making evaporators
very efficient. Water from evaporators can be recovered and re used.
Working:
Concentrated milk and hot air (180-200°C) is sprayed into a drying chamber which is usually a
stainless-steel jacket with a conical bottom by the help of an atomizer. The drying chamber is
usually a stainless-steel jacket with a conical bottom, the drying gas dries the particle before it
touches the chamber. Heavy particles get collected at the bottom of drying chamber while the
fine particles get collected in the cyclone separator, still some fine particles remain in the air
which are filtered using a filter bag.
In some companies spray drying process is typically a two-stage process that involves the spray
dryer at the first stage with a static fluid bed integrated in the base of the drying chamber. The
second stage is an external vibrating fluid bed. Product is moved through the two-stage process
quickly to prevent overheating of the powder.

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Fig:1.2 Two
stage spray drier

Types of Spray Drier System:
On the basis of type of flow:
1. Co-current Flow
Drier
Counter-current Flow
Drier
Mixed Flow Drier





Diagram
Representation
showing
directions of
flow.







Direction In this feed and
the hot air are in
same direction
The spray and feed are
introduced at the
opposite ends of the
drier with atomiser at
top and air entering at
bottom
It is a combination of co-
current and counter-
current flow
Usage In dairy and for
heat sensitive
systems
Soaps and detergents
(driest particle remain
Generally used for grains

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in contact with the hot
air)
Advantage Evaporation is
rapid
More rapid than co-
current
Can be used for wide
variety of products
Disadvantage

- Not suitable for heat-
sensitive product
Not suitable for heat-
sensitive product
On the basis of type of Stage:
2. Single stage Spray
Drier
Two-stage Spray Drier

Working
Moisture is
reduced to the
target in one go.
Moisture content of the product leaving the
chamber is higher (5-10%) and is than further
reduced in second stage that can be a fluidised bed
drier
Advantage Energy efficient Used for Heat Sensitive products
More energy efficient than single stage
Diagram Refer figure 1.1 Refer figure 1.2

Another drying system that is generally used for drying milk is-
Roller Drum Drying- It involves direct contact of a layer of milk with the hot surface of
rotating drums, which is heated by the steam on the inside. As the drum rotates and is heated
from inside, the product dries on the outside surface. The dried product than finally reaches the
knife blade, it is scrapped off.
Two main advantages over hot-air drying:
1. It is not necessary to heat large volumes of air before drying commences and the thermal
efficiency is therefore high.
2. Drying may be carried out in the absence of oxygen to protect components of foods that are
easily oxidized.

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Freeze-Drying:
Freeze drying or lyophilisation is a drying method where the solvent is frozen prior to drying
and is then sublimed, i.e., converted to the gas phase directly from the solid phase, below the
melting point of the solvent, though this type of drying keeps the biological properties of
proteins, and retains vitamins and bioactive compounds it is not widely used for the production
of milk powder because of the high energy demand.

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6. Packaging and Storage:
Milk powders are immensely more stable than fresh milk but protection from moisture, oxygen,
light, and heat is needed in order to maintain their quality and shelf life. Milk powders readily
take up moisture from the air, leading to a rapid loss of quality and caking or lumping. The fat in
WMPs can react with oxygen in the air to give off-flavours, especially at higher storage
temperatures (> 30 °C) which is typical of the tropics. Milk powder is packed into either plastic-
lined multi-wall bags (25 kg) or bulk bins
Note:
1. Similar to raw milk, powdered milk is loaded with nutrients; it is a good source of
essential minerals and vitamins such as magnesium, calcium, zinc, potassium as well as
vitamins A, D, E and K. During the evaporation process, it is ensured that these
beneficial nutrients are not lost. Milk powder also meets your daily dose of other
important nutritional elements such as amino acids which are responsible for a number
of functions such as cellular growth, stimulating the body’s immune system, aiding in
blood clotting, calcium absorption and so on.
2. Spray Drying gives spherical particles with smooth surface with high bulk density
whereas Roller Drying will give compact structure with irregular shape having very low
bulk density.