Dried Milk Powders- Processing and Manufacturing.

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%

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 then 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. the amount is tried to be removed in the concentration stage.

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 calendrics in an evaporator to around 40-60% total solids. 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 reused.

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 the drying chamber while the fine particles get collected in the cyclone separator, still, some fine particles remain in the air which is filtered using a filter bag.

In some companies, the spray drying process is typically a two-stage process that involves the spray dryer at the first stage with a static fluid bed integrated into 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.

                               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 the same direction	The spray and feed are introduced at the opposite ends of the drier with atomiser at top and air entering at the 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 in contact with the hot air)	Generally used for grains
Advantage	Evaporation is rapid	More rapid than co-current	Can be used for a wide variety of products
Disadvantage	–	Not suitable for heat-sensitive product	Not suitable for heat-sensitive product
On the basis of the type of Stage:
2.	Single-stage Spray Drier	Two-stage Spray Drier
Working	Moisture is reduced to the target in one go.	The moisture content of the product leaving the chamber is higher (5-10%) and is than further reduced in the 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.

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.

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 are 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 a smooth surface with high bulk density whereas Roller Drying will give compact structure with an irregular shape having very low bulk density.