Milk Drying Systems and Processes [with flow chart]

The systems and process of milk drying can be classified as follows:

Drying Milk by Cold Treatment:

(a) Drying Milk by Freezing Out the Water and Centrifuging:

This system was proposed and patented as early as 1884 and is now obsolete.

(b) Drying Milk by Freezing and Sublimation:

This freeze-drying method, which seems to have been developed by 1945, consists of:

(i) Freezing the product and

(ii) Supplying heat, so that moisture is removed by sublimation (without passing through the liquid phase) by maintaining a vacuum in the vapourizing chamber.

Merits and Demerits of Freeze-Drying Process:

Merits:

(i) Can be designed for continuous operation;

(ii) There is an almost complete absence of air throughout the drying cycle,

(iii) Moisture content of the finished product can be reduced to extremely low values;

(iv) Heat-damage to protein stability, flavour, solubility and colour of the finished product is minimal.

Demerits:

(i) The plant is complicated;

(ii) Operating costs are rather high. (Five to ten times that of conventional heat-drying, according to one estimate.)

Drying Milk by Heat Treatment:

1. Film, Roller or Drum Drying Systems:

Principle:

The milk, preferably concentrated, is applied in a thin film upon the smooth surface of a continuously rotating steam- heated metal drum, roller or cylinder, and the film of dried milk is continuously scraped off by a stationary knife/doctor blade/scraper, located opposite the point of application of milk. The milk-film (in the form of a sheet) has to be ground to obtain the powder.

Advantages and Disadvantages (over Spray-Drying System):

Advantages:

(i) Relatively low capital and operating costs;

(ii) Plant is movable and occupies little floor space;

(iii) Plant is easy to handle;

(iv) Suitable for operating small quantities of milk economically;

(v) Produces milk powder of better keeping quality.

Disadvantages:

(i) Produces milk powder with low solubility;

(ii) Produces a definitely cooked scorched flavour in the reconsti­tuted milk.

Classification of Drum Driers (Adapted from Hall & Hedrick):

(a) Number of Hollow Drums:

(i) Single drum;

(ii) Twin drum;

(iii) Double drum.

(b) Pressure Surrounding the Product:

(i) Atmospheric;

(ii) Vacuum.

(c) Directions for the Turning of Drums:

(i) Turn up at the centre and away at the top—Twin drum;

(ii) Turn at centre and together at top—Double drum.

(d) Method of Placing the Product on the Surface of Drum:

(i) Trough or reservoir above for top feed;

(ii) Spray or splash feed;

(iii) Sump below for dip feeding;

(iv) Trough below for pan feeding.

(e) Method of Obtaining Vacuum (for a Vacuum Drier Unit):

(i) By steam ejector,

(ii) By vacuum pump.

(f) Material for Construction of the Drum:

(i) Steel;

(ii) Alloy steel;

(iii) Stainless steel;

(iv) Cast iron;

(v) Chrome or nickel-plated steel.

Note:

(i) Cast iron is usually used. The wear is excessive on stain­less steel drums. The metal used for the knife should be softer than that used for the drum,

(ii) The double-drum atmospheric drier is most com­monly used in the dairy industry. Vacuum drum driers are essentially the same as atmospheric units except that the drums are enclosed so that a vacuum can be pulled on the product during drying. The single drum with top feed is more commonly used for vacuum drying. A thicker film is obtained with top feed.

Flow of Product:

The product may be placed in its natural form or pre-condensed in a vacuum pan/evaporator before it is fed to the drum drier. Milk is usually pre-condensed for a single-drum unit. The product is usually pre-heated and pumped into the reservoir between the upper portion of the drums to provide a thin film over the turning drums.

The doctor blade, which is a sharp hard flexible knife, scrapes the dried material from the drum. The blade sits at an angle of 15 to 30° to the surface. The film of dry milk forms a continuous sheet from the knife to the auger trough, which is about level with the bottom of the drum.

The auger for each drum discharges the pro­duct into elevators, then to a grinder which pulverizes the product, after which it is sifted. When sifted, the dried product is packaged and stored, or marketed. These steps are carried out in a continuous operation.

Vapour Removal:

Water vapour above the drier has a lower density than the air surrounding the unit and hence rises upwards. A hood is placed over the drums for the vapours to escape.

Drums:

Description:

The drums are normally horizontal, hollow, steel cylinders, 90 to 360 cm. (3 to 12 ft.) in length and 60 to 120 cm. (2 to 4 ft.) in diameter. They are heated internally by steam, usually at 4.2 to 4.9 kg./sq. cm. (60-70 psi) with suitable arrange­ments for steam intake and a condensate outlet.

In the case of double drums, the cylinders are mounted parallel to each other about 0.5 to 0.75 mm. apart; further, one drum is mounted on a stationary bearing, and the other on a flexible one so that it can be moved to provide the desired clearance between the drums.

Drying Particulars:

In atmospheric driers, drying takes place at atmospheric pressure. In vacuum driers, however, the drier is enclosed in a vacuum chamber which is maintained at a vacuum of 68.5 to 73.5 cm. Hg. A high quality product is obtained in vacuum driers, but besides being expensive the process is compli­cated.

The speed of the drums is adjustable, usually averaging 14-19 rpm. The speed is important as it affects the thickness of the film and the time the product is on the roller; the speed may be varied according to the dryness desired. Both drums should turn at the same speed.

The product is removed after the drum has completed ¾ or 7/8 of a revolution. The product is in contact with the drum for 3 seconds or less at a temperature of about 150°C (302°F), depend­ing on steam pressure.

Heat-Transfer through the Drum:

Drums must be carefully machined, both inside and outside, for a difference in thickness will alter heat transfer and prevent uniform drying. The requirement of steam is 1.2 to 1.3 kg. per kg. of water evaporated.

Operation:

The drier is started by:

(i) Lifting the knives;

(ii) Starting the drums;

(iii) Turning on the steam, and

(iv) Placing the product on the drums.

The very first powder is not as good in quality as the remainder because of the tendency to be either over- or under-dried. The drums should be set in motion when the heat is applied by gradually turning on the steam to prevent warping.

Important Operating Points:

(a) Close Control of the Temperature and Time of Drying:

A higher temperature-time of drying tends to produce browning, a scorched flavour and reduced solubility in the powder. On the other hand, a lower heat-treatment may result in incomplete drying and consequently a higher moisture content in the dried product.

Note:

For a given drum size, the moisture content of the powder decreases with increased steam pressure and increases with higher drum speed. Since an increase in steam pressure hastens the drying, it makes possible a more rapid drum speed, thereby shortening the period of heat exposure.

(b) Evenness of Heating over the Drum Surface:

This should be maintained in order to obtain a uniformly dried product of stan­dard quality. The condensate should be promptly removed from inside the drum.

(c) Accurate Alignment of Rollers:

Drums should be properly aligned, particularly if double drum or twin drum units, and should have identical characteristics of speed, heat transfer, wear, etc., for best results.

(d) Accurate Control of the Feed Device:

Only accurate control will ensure uniformity of film thickness on the drum, which is essential for the maintenance of uniformity of moisture content in the finished product through the day’s run. Lack of such uniform­ity also has an unfavourable effect on the solubility, flavour and colour of the finished product.

The level of milk in the reservoir must be uniform and holes kept open in the distribution tubing. Change in the milk level affects the film thickness on the drum and thus the steam requirements.

(e) Pre-Heating of Milk before Drying:

As the milk feed tempera­ture is increased, the rate of drying is proportionately increased (2.2 per cent for each 10°F from 120 to 160°F, with little increase thereafter).

(f) Keeping the Drum Surface Smooth:

The drum surface must be kept smooth and free of rust and pits. Pits in the drum surface get filled with milk, escape the blade and flake off gradually, resulting in scorched particles.

Note:

It may be necessary to resurface the drums after 1000 to 3000 hours of operation. The surface may be smoothened by attaching sand paper beneath the scraper knife. The drums should be coated with oil or paraffin wax when not in regu­lar use.

(g) Keeping Scraper Knives Sharp and Reground Frequently:

The scraper knives should be sharp and true for efficient film removal. The knives must be reground regularly (after approximately every 100 hours) and be uniformly sharp. They must be flexible, machined on both edges, uniformly thick and easily adjustable. A uniform knife pressure against the drum must be maintained.

Note:

The knife pressure against the drum is adjusted by screws at each end of the knife, and by numerous screws along the length of the knife for local adjustment. Excessive pres­sure of the blade on the drum increases the operational energy of the drum and the danger of metallic shavings in the finished product. The long one-piece knife may be subs­tituted with a series of independent short knives arranged in staggered positions for effective coverage of all parts of the drum over its entire length.

(h) Using Fresh Sweet Milk:

This will ensure a high quality pro­duct. Any developed acidity will lower the solubility and produce an oxidized or tallowy flavour in whole milk powder.

(i) Using Pre-Condensed Milk:

Normally milk which is drum-dried is pre-condensed to 2: 1, so as to contain 16-18 per cent total solids in order to obtain the following advantages:

(i) To provide satisfactory film thickness on the drum surface, especially with single drum dryers. The fluid sweet milk, because of its low visco­sity, will not do so;

(ii) Greater thermal economy. This is actually due to the use of a double-effect evaporator for pre-condensing;

(iii) Increased capacity of dryers;

(iv) Increased bulk density, and

(v) Increased keeping quality.

Note:

(i) The product may be damaged and scorched if there is an uneven milk supply, incomplete removal of the film, imperfect roller alignment, a rough roller, and an excessively high product-temperature (caused by an ex­cessively high steam temperature or excessively slow drum speed).

(ii) High moisture in the product is due to low temperature, thick film, high total solids, and fast speed (rpm).

Factors Affecting Capacity of Drum Driers:

(a) Steam Pressure inside the Drum:

A higher steam pressure permits greater drum speed consistent with satisfactory completeness of drying.

(b) Temperature of milk feed

(c) Level of milk in the trough

(d) Pre-condensing of milk.

Merits and Demerits of Vacuum Drum Drying over Atmospheric Drying:

Merits:

(i) Higher solubility of powder;

(ii) Better keeping quality of powder.

Demerits:

(i) Higher initial cost of plant;

(ii) Higher operat­ing cost of plant;

(iii) More complicated plant.

Flow Diagram of Drum Drying System:

Spray-Drying System:

Principle:

The basic principle of spray drying consists in atomizing the milk, preferably pre-heated and concentrated, to form a spray of very minute droplets (fog-like mist), which are directed into a large, suitably designed drying chamber, where they mix intimately with a current of hot air. Owing to their large surface area, the milk particles surrender their moisture practically instantaneously and dry to a fine powder, which is removed continuously.

Advantages and Disadvantages (over Roller-Drying System):

Advantages:

(i) Yields milk powder which is markedly supe­rior in appearance, flavour and solubility (and hence commands a higher market price);

(ii) Most economical when large quantities of milk are handled.

Disadvantages:

(i) Involves large capital investment in plant and buildings;

(ii) Plant is complicated.

Classification of Spray Driers (vide Hall and Hedrick):

(a) Method for Atomizing Spray Material:

(i) Hydraulic pressure jet (pressure spray);

(ii) Pneumatic (compressed air);

(iii) Centrifugal disc.

(b) Method of Furnishing Heat:

(i) Steam;

(ii) Gas;

(iii) Fuel oil;

(iv) Electricity.

(c) Method of Heating Air:

(i) Direct (gas or fuel oil);

(ii) Indirect (utilizing heat exchanger plate or coils).

(d) Position of Drying Chamber:

(i) Vertical;

(ii) Horizontal.

(e) Number of Drying Chambers:

(i) One (main only);

(ii) Two (main and subsidiary).

(f) Direction of Airflow in Relation to Product Flow:

(i) Counter-cur­rent;

(ii) Parallel;

(iii) Right-angle.

(g) Pressure in Drier:

(i) Atmospheric (usually a very slight pres­sure);

(ii) Vacuum.

(h) Method of Separation of Powder from Air:

(i) Cyclone,

(ii) Multi-cyclone;

(iii) Bag filter;

(iv) Liquid dust collector;

(v) Electrical dust collector.

(i) Treatment and Movement of Air:

(i) Recirculation of air;

(ii) Dehydration of air;

(iii) Conventional (atmospheric air used and exhausted after use).

(j) Removal of Powder from Drying Chamber:

(i) Conveyor;

(ii) Vibrator;

(iii) Sweep conveyor;

(iv) Air conveyed to cyclone.

(k) Method of Heat Transfer:

(i) Convection;

(ii) Radiation.

(l) Kind of Atmosphere in Drying Chamber:

(i) Nitrogen;

(ii) Air;

(iii) Other (usually inert gas).

(m) Position of Fan Provided:

(i) Pressure in chamber;

(ii) Suction in chamber.

(n) Direction of Air-Flow in Chamber:

(i) Updraft;

(ii) Downdraft;

(iii) Horizontal;

(iv) Mixed.

(o) Shape of Drying Chamber:

(i) Silo or cylindrical;

(ii) Box-like;

(iii) Square cross section;

(iv) Tear-drop.

(p) Product Being Dried:

(i) Milk;

(ii) Other milk products;

(iii) Other food products.

Atomization:

Object:

To reduce the milk to particles so small in size that due to their large surface area they surrender their moisture practi­cally instantaneously.

Purpose:

To obtain many small particles, preferably uniform in size, generally ranging from 50 to 150 microns in diameter.

Uniform particles provide:

(i) A superior instantizing product;

(ii) Reduced product losses;

(iii) Less over- and under-drying, and

(iv) More efficient drying (large drops are more difficult to dry and require a longer time or a higher temperature, or both).

Influence of Type and Efficiency:

This affects the desired design (size, air temperature, exposure time, evaporation rate and efficiency). The atomization also affects the product properties, such as air content, moisture, bulk density, particle size (range and average) and reconstitutability.

The pattern produced by the atomizer must be so directed that the particles will be dried before hitting the surface of the drying chamber, else there will be an accumulation of partially dried product on the drier.

Methods/Systems:

There are three major methods/systems of atomizing.

These are:

(i) Pressure spray jet/nozzle;

(ii) Centrifugal (spinning) disc;

(iii) Compressed air.

(i) Pressure Spray Nozzles:

These include the swirl nozzle (called whizzer or centrifugal pressure nozzle), the solid cone spray nozzle, and the fan nozzle. The pressure spray jet is the most common in the USA for milk and food product atomization for spray drying. Pressures from 1500-5000 psi are used.

A high pressure pump, such as a three or five piston homogenizer pump, is commonly employed to ensure uniform fineness of spray. The milk is thus forced under high pressure through a small office and is atomized instantly. The higher the pressure or the smaller the orifice, the finer the particle size of the mist, and vice versa.

To control the spray powder satisfactorily, and ensure uniformity of moisture content, the orifice must be of the right size and shape and its edges must be smooth. On the other hand, if the milk stream flows at high velocity under high pressure, the spray discs wear out rapidly due to the intensive abrasive action of the milk solids. The new-alloy (tungsten-carbide) nozzles of today are much more wear- resistant.

Note:

(a) The purpose of the swirling motion is to widen the angle of the milk fog and intensify the atomizing action,

(b) A recent development is the use of sonics or ultrasonics to vibrate the nozzle and thereby increase the uniformity of the droplets from the spray nozzle.

(ii) Centrifugal Spinning Discs:

The atomizing device consists of a radial vaned disc (vanes placed between two discs), multiple discs (three or more discs), and a bowl or hemispherically shaped liquid chamber through which the product moves. The atomizing device may revolve at 50,000 rpm for a small diameter disc (approxi­mately 5 cm.) or at 3500 rpm for a large diameter disc (approxi­mately 75 cm.).

The product may be ejected from the spinning disc/slotted basket over a lip or through a slot, hole or other opening. The disc atomizer permits considerable variation in capacity, about + 25 per cent of the design capacity. The pattern produced by the disc is umbrella-shaped although for very fine droplets a mist of cloud is formed. The spinning disc is particularly useful for viscous materials and for materials in suspension. The centrifugal unit is used in a vertical drying chamber.

Advantages:

1. Absence of small orifices that are subject to clog­ging;

2. Permits spray drying of highly concentrated milk (contain­ing as much as 50 per cent solids), which results in high thermal economy;

3. No pump pressure is required;

4. Capable of continuous operation over prolonged periods without special attention;

5. Free from abrasive action.

Disadvantage:

Requires upkeep of high-speed bearings.

Note:

Some European milk-drying factories (LUWA Ltd., Zurich) use centrifugal spray dryers that are equipped with twin spray discs, attached to the same shaft but at different levels. The fresh milk is separated and only the skim milk is pre-condensed.

The condensed skim milk feeds one disc and the fresh cream the other. The two liquids reach their respective spray discs through separate pipes simultaneously. The resulting powder is an automatically produced mechanical mixture of the dried particles from each disc.

(iii) Compressed Air Spray/Pneumatic:

A jet of high-temperature compressed air moves at high velocity through a stream/s of pre­heated milk. The milk streams issue from a battery of simple noz­zles. The high-velocity jet of very hot air strikes the milk stream s at a right angle/s, atomizing the milk instantly.

Heating Air:

A fresh supply of air is drawn through a filter by the intake fan, which blows it over the heater.

(a) Air Supply:

Invariably, fresh new air from the outside atmos­phere is used. Air from the drying chamber is not re-used, since the cost of dehydrating the exhaust air is uneconomical. The supply of air, which may be derived from outside or inside the plant, should be pure, free from visible extraneous material, dust, soot and objec­tionable odours. Air intake may take place from the top of the buil­ding or ground level.

(b) Air Filter:

In order to ensure purity, the air to be used for drying is filtered. This removes suspended impurities and as many micro-organisms as possible. Different types of air filters may be used. Close weave filters are unsuitable. Filtering is normally done by mechanical means.

Those normally used are:

(i) Water Spray Type:

The air is washed by drawing it through a spray of water, or through a series of screens of wire mesh or gauze over which a film of water trickles.

Drawback:

Does not remove greasy turbidity.

(ii) Oil Film Type:

The air is purified by drawing it through a film of oil contained in multiple cells of expanded metal. This is most commonly used. The oil used should be odourless and suffi­ciently heavy to prevent it being blown off by the interior air.

(c) Air Intake Fan:

This is normally installed between the air filter and air heater. Draws cool air and forces it through the air heater into the drying chamber. (The next best location for the fan is at the exit of the drying chamber.)

(d) Air Heater:

The air may be heated directly, as for instance by the furnace flame, or semi-directly by other means such as the furnace with hot air flues; or by indirect means, as by air passing over banks of fin-type coils through which are circulated steam, hot oil, etc.

The direct (and semi-direct) heaters are more efficient, for less heat is lost during heat-transfer. Fuels for direct-fired units are limited to gas and light oil to avoid soot formation; with these units, the selection of fuel is based primarily on the cost and the effect of combustion products of the fuel on the drying product. Where electricity is cheap, or in laboratory spray driers, electrical heating may be utilized. It is easier to control a uniformly low temperature with steam, which provides a relatively non-hazardous source of heat.

Products Moisture Removal:

The atomized product is brought into intimate contact with heated air in the drying chamber, for moisture removal. The temperature and volume of heated air that will evaporate a given amount of water or that will dry a given amount of milk, vary with the temperature and humidity of the atmospheric air, the temperature and concentration of the milk to be dried and the temperature and degree of saturation of the mois­ture-laden air emerging from the drying chamber.

(a) Temperature of Heated Air:

The higher the temperature of heated air, the lower the volume required, and vice versa. However, lower temperatures necessitate large drying chambers. It is econo­mically advantageous to use the highest temperature that the liquid to be dried will stand without injury to its quality.

Heat damage also depends on the rapidity of evaporation. Damaging overheating is avoided by mixing the milk particles with the hot air while violently agitating it, thus providing the maximum rapidity of evaporation.

The inlet-air entering the drying chamber now usually ranges in temperature from 149 to 260°C (300 to 500°F) for drying milk and milk products; the outlet-air exhausted from the drying cham­ber usually ranges from 100 to 105°C (212 to 221°F). The relative humidity of the drying air is also quite low, e.g. 3 to 4 per cent.

(b) Volume of Heated Air:

The temperature of heated air being the same, a smaller volume of air is required in cold weather than m hot weather. This is because the former is denser than the latter. But a smaller quantity of cold air requires more calories to heat it than does the larger quantity of warmer air in order to attain the same temperature.

(c) Velocity of Air:

For economical and proper heating of the air and satisfactory performance in the drying chamber, the air should move at the right velocity. This is made possible by correctly correlating the dimensions of the various components of the drying unit.

(d) Drying Chamber Designs:

All drying chambers are intended to completely mix the product droplets with the hot air, and then dry them as rapidly as possible in a space of reasonable dimensions. In practice, size and shape vary considerably, as designs are made according to the type of atomizer used and the method of recover­ing the powder from the outgoing air.

The common ones are:

(i) Rectangular, Horizontal Type:

The concentrated milk is spray­ed in at one end of a long, rectangular chamber as a conical spray, usually from a pressure-jet type atomizer. The hot air is also admit­ted as a parallel stream to the spray and is ejected at the opposite end of the chamber.

The (coarse) powder particles fall gently to the floor of the chamber, from which they are removed continu­ously by mechanical sweeping. The inlet air temperature is around 130 to 140°C (266 to 284°F). A large volume of drying air is used.

(ii) Vertical, Cylindrical Type:

These comprise a tall cylindrical chamber with either a flat or conical base. In most designs, the concentrated product is sprayed in from the top and mixed with a strong spiral and downward stream of hot air which shakes the pro­duct violently. The exposure time is very short and high tempera­tures of 150 to 170°C (302-338°F) can be used with a smaller volume of air.

Note:

The horizontal driers are commonly used in the USA, while the more flexible vertical ones are more common in Europe.

(e) Flow of Air:

The air must be properly and uniformly direct­ed into and through the drying chamber, else the heat in the air will be inefficiently utilized and a partially dried product may accumulate on the inside edge of the drier. In general, the product and air either enter at opposite sides or from the top and bottom respectively and flow in counter-current directions towards each other; or they both enter in the same region, mingling together in a parallel or co-current flow.

The heated air is forced through an insulated duct at the end of which is the air disperser. This is especially designed, since on its correct shape depends the proper functioning of the entire plant. It actually spreads or distributes the hot air stream. In one design the air disperser is placed just below the atomizer, while in another it is behind it, leading to product-air co-current or parallel, or coun­ter-current flow as the case may be.

After milk-dust recovery, the stale air is removed from the system by means of the Exhaust Air Fan and goes out to the atmosphere through the outlet duct furnished with an especially designed Exhaust Air Hood, which acts as a protecting cover in preventing dirt, dust, rain drops, etc., from falling through the duct into the system.

Product Recovery:

(a) Separation of Air and Powder:

As the product is dried it is necessary to separate the dried product from the air, to prevent a major portion of it being carried away by the moist air being eject­ed from the drier (thereby resulting not only in lower yields, but also in air-pollution surrounding the drying plant).

The powder may be separated from the air primarily either inside the drier (internally) or outside the drier (externally). In both cases, it is necessary to use an additional device outside the drier to remove the fines/dust or small particles, which will not normally settle inside the drier.

The particle size of spray-dried milk (or milk product) depends on several factors. Yet regardless of the type of spray drier, a cer­tain proportion of the powder is much finer than the remainder. It has been estimated that the powder particle size may range from 10 to 100 microns.

The relatively large/coarse particles, which may constitute approximately 80 per cent or more of the yield, respond to the force of gravity and get deposited on the sides or drop to the floor of the chamber. The milk-fines/dust, which constitute the remaining 20 per cent or less, become entrained in the air currents and are swept by them to the exit of the drying chamber, where they are more or less completely removed by mechanical means.

(b) Recovery of Larger Particles:

The larger powder particles, which are separated within the drier, can be removed from the drier by:

(i) An air-brush;

(ii) A mechanical sweeper;

(iii) A screw conveyor, or

(iv) A gravity system.

In the air brush system, air from outside the drier, either at room temperature or conditioned to a lower temperature, is used to direct a jet of air. A mechanical sweeper consists of a rake or broom which is pulled across the bottom of the drier.

The screw conveyor helps to remove the pow­der particles by forcible ejection through screw movement, while the gravity system employs the principle of gravity settling. Vibrators are often attached to the sides of the drier to prevent, or minimize, the quantity of powder which sticks to the surface and to move the product rapidly from the drier.

(c) Recovery of Fines:

There are principally three types of devices in use for separation and recovery of milk-dust/fines:

(i) Filter Bag Dust Collector:

This consists of a series of filter bags equipped with mechanical shakers. The material of the bags may be cotton or wool (or plastic), of very fine weave. The bags are 180 to 300 cm. high and 20 to 25 cm. wide. The entire series of bags present a large screening surface.

The air is drawn through these bags and deposits the entrained milk particles in their mesh­es. Intermittent shaking helps drop the powder to the bottom of the vault in which the bags are suspended. This device needs close supervision to maintain it in an approved sanitary condition. Losses generally range from 0.2 to 0.5 per cent.

(ii) Cyclone/Cyclonic Separator/Centrifugal Milk Dust Separator:

The cyclone or multi-cyclone is most commonly used today. It is usually a stainless steel cylindrical container with a cone-shaped bottom. The air leaving the drying chamber at a high velocity enters the cyclone tangentially. It thus assumes a rotary motion forming a cyclonic vortex. Centrifugal force throws the solids to the peripheral wall of the cylinder, along which the material works down into the hopper.

In the past, large cyclones (diameter ranging from 150 to 600 cm.) were used, generally supplemental to filter bags and preceding the latter. However, the modern trend is to use the multi-cyclone with a smaller diameter, which increases the efficiency of milk-dust recovery.

The efficiency of separation with a cyclone unit is based on pro­duct, cyclone design and on the size of the particle to be removed, but losses range from 0.5 to 3.0 per cent (average 1 per cent). The cyclone is normally used for separation of material between 5 and 200 microns. As the size of the particle decreases, the efficiency of the cyclone also decreases.

(iii) Liquid Dust Collector:

This consists of a closed tank into which the incoming fresh milk is pumped through a series of suit­ably distributed spray jets. The exhaust air escaping from the dry­ing chamber passes through the fresh milk spray in the collector tank. By this method, the milk dust entrained in the spent air from the drying chamber is deposited in the incoming fresh milk.

Merits:

1. High recovery of product;

2. Pre-condenses fresh milk;

3. Utilizes a considerable portion of the heat-units contained in the exhaust air;

4. Economical;

5. Highly advantageous for skim milk, butter, whey, etc.

Demerits:

Less suitable for whole milk, since oxygen and other gases removed in the drying chamber are re-introduced in the fresh incoming milk, thereby enhancing the possibilities of product dete­rioration and contamination.

Cooling the Powder:

The dried milk/milk product should be removed from the drier as quickly as possible to minimize the effect of heat-damage on the product. The product and air may be re­moved together from the drier and separated outside the drier to reduce heat effect.

Prolonged exposure of the dried milk/product to high temperatures jeopardizes its flavour, colour and keeping quality, besides increasing the tendency towards clumping and sticking. Hence it is desirable to cool the product soon after pro­duction.

Some cooling of the product takes place in the drier when using an air-brush supplied with cool air to remove the dried product from the sides and bottom of the drier.

Cooling powder outside the drier may be done by:

(i) Conduction cooling, in which the product is cooled when moving through a water-jacketed screw conveyor;

(ii) Convection cooling, by using room or refrigerated air to cool it to 38°C (100°F), or by moving conditioned air over the product or through the conveyor handling the product, and

(iii) Surrounding the outlet of a cyclone separator by a chamber through which cold air is moved to cool the product.

Flow Diagram of Spray Drying System:

Foam-Spray Drying:

Common dairy products such as skim milk, whole milk, butter­milk, sweet and sour cream, whey and emulsified cheese slurry can be foam-spray dried. This is done by forcing the ‘gas’ into the liquid product after the pump but before the atomizer. Air is commonly used as the added gas for making foam-spray non-fat dry milk, and nitrogen for foam-spray dried whole milk.

Foam-spray drying permits the use of most items of conventional spray drying equipment for:

(i) Drying liquids up to a maximum of 60 per cent total solids as compared to 50 per cent on a parti­cular drier;

(ii) For drying special products, such as malted milk, cottage cheese and whey, and

(iii) For obtaining an instant powder.

The particles of the dried product obtained by foam-spray dry­ing are more uniform in size. However, foam-spray non-fat dry milk has a low bulk density (about 0.35 g./ml. or less) and hence higher packaging costs.