Feed tank Atomized of feed Drying of feed Powder food Fig. 1 Principle of spray drying process.

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1 Introduction In last two decades, consumer needs and demands leads to an intensive developments in new products in food processing industries.the expansion of these innovative product increases the new techniques like spray drying. Spray drying process meets the demand in many ways in our day to day life. Objectives After going through this session you will be able to 1. Explain the principle, working and applications of spray and drum drying process. 2. To interpret the processing and preservation effect and applications of spray and drum drying processes. Spray-drying has been widely used so far in medicinal, chemical, food and cosmetic industries by rapid drying method. The conversion of liquid feed into a dried particulate material by spraying the aqueous feed in a hot air drying medium is the basic working concept of spray drying.in food industries spray drying is a common method used to dry aqueous or organic solutions, emulsions etc. For example skim milk powder, fruit juice powder and pigment are few spray dried products available in the market.dairy industry is the major food process sectorto converting liquid milk and other milk based products into powder. The unique characteristic of spray drying process is that it can produce powder ranges from few hundreds grams per hour to several tons per hour.powder particles, granular materials, agglomerates and encapsulated products can also be produced by using spray drying process.it also provides the advantage of weight and volume reduction. 20.1Principle The aqueous feed mixturebeing sprayed can be an emulsion, suspension, dispersion or solvent. This feed material is converted to water droplets using atomizer inside the spray dryer where each droplet is evaporated by means of hot air medium. The drying medium is usually air. The drying process continues until the desired moisture content is attained in the sprayed particle then powder particle is collected from the air.fig. 1 shows the principle involving in spray dryer. Feed tank Atomized of feed Drying of feed Powder food Fig. 1 Principle of spray drying process Spray drying system Feed concentrate is pumped to the atomizer device where it is broken into small droplets. These droplets are in contact with hot air and the moisture present in the droplet is evaporated and the powder particle still in the suspension. The dry powder is recovered by using cyclone separator and removed through a discharging device which is connected with the cooling system Components of spray dryer 1. Feed tank. 2. Atomizer

2 3. Drying chamber 4. Separation unit Fig. 2 Components of spray dryer Feed The homogenous, pumpable, clear suspension or paste food material should be the feed liquid for spray drying process. Feed composition, solid concentration, physical properties like density, viscosity, and surface tension play a major role in spray drying process. The feed is transferred from feed tank to the atomizer through pump. Pre concentration of feed Pre concentration of feed is necessary to increase the capacity, particle size and particle density. This will also help in efficient powder separation, improved dispensability of the product, and economizing of operation Atomization Atomization is the process which turns a liquid feed into small droplets immediately prior to drying (Fig.3). Size of the dryer and rate of drying will be controlled by atomization process. The size of the droplet formed controls the size of the powder particles coming out of a spray dryer. The selection of the atomizer type depends upon the nature and amount of feed and the desired characteristics of the dried product. Atomizer is a device used for liquid atomization to form droplets.the selection and operation of the atomizer is of supreme importance in achieving economic production of a premium quality product.shape of orifices, consistency of the fluid, mode of operation and energy utilized for atomization are the important factors for classification of an atomizer.

3 Fig.3 Atomization of liquid feed with different viscosity. The usual classification is based on the type of the energy utilized for disintegration of droplets from the bulk liquid. The viscosity of the fluid prevents breakup of fluid and results in larger particle size droplets. Fig.3 depicts the average droplet size for low, medium and high viscous fluids. High viscous materials need more pressure to produce droplets. Liquid foods usually concentrated to minimize the amount of water to be evaporated during drying. The concentrated liquid foods are pumpedto an atomizer. The atomizer is operated so that a spray of droplets of the desired size distribution is produced based upon the usage, the atomizers are classified as 1. Rotating wheel atomizer (centrifugal energy 2. Pressure nozzle atomizer (pressure energy) 3. Pneumatic or two fluid nozzle atomizer (pressure and gas energy) 4. Ultrosonic nozzle (sonic energy) Wheel atomization The feed is passed through a rotating disk is called wheel atomization (Fig 4a). The wheel atomizer features the feed being fed into centre of a wheel rotating at high speed. The wheel contains radial vanes which direct the feed to the periphery. This enables the liquid, now as a thin film on the vane surface, to accelerate and acquire the peripheral speed of the wheel. Automation occurs immediately as the thin film leaves the wheel edge. Spherical droplets are formed as the thin film disintegrates because of the influence of the imparted energy, the physical properties of the liquid. Smaller droplets with the feed temperature of areformed with the high speed of the wheel. The majority of spray dryers for food stuffs operate in the medium speed wheel range and which can be generally works for longer periods of time Pressure nozzle atomization The pressure nozzle atomizer has a device to create liquid rotation within the nozzle head and an orifice through which the liquid is discharged as a conical spray(fig.4b). The breakup at the orifice and discharged at the spray angle of o. Both pressure nozzles and wheel atomizer have their own advantages and disadvantages. Wheel atomizers are usually preferred for high feed rates and where standard powders of small particle size are required. a.rotating Wheel b.pressure nozzle

4 Fig. 4 Types of atomizer Two-fluid nozzle atomization (pressure and gas energy) In this type of atomization, the feed is mixed with a compressed gas to produce spray particles.this is a least energy efficient method and useful for making extremely fine particles Ultrasonic nozzle (sonic energy) Sonic atomizers use high-frequency sound energy created by a sonic resonance cup placed in front of the nozzle to disintegrate droplets from the bulk liquid. Sonic atomizers are even now at developmental stage which is being utilized for pilot scale production for dispersing liquids Drying chamber The drying chamber consists, most commonly, of a verticalcylindrical section with a conical bottom. The large volumeof the drying chamber is required in order to provide the residence time necessary for complete drying. In spray dryers with centrifugal atomizers,the droplets are ejected in a radial direction. In this case, a large diameter isrequired in order to prevent the droplets from reaching the chamber walls whilethey are still wet and sticky. Dryers with pressure nozzles are tall butless wide, because of the narrower spray angle. The conical bottom serves tocollect the major part of the dried product.a spray dryer of a different model has a nearly flat bottom and uses a rotatingsuction duct (air broom) for collecting the product Spray air Contact The spray particle has to be in contact with hot air to initiate drying process. Depending upon the characteristics of the feed, the air temperature is pre-defined and heated by the heating element which is placed outside the chamber. The hot air is passed in the spray drying chamber to dry the spray particles. The hot air flows in the following airflow patterns. 1. Cocurrent airflow 2. Counter current airflow 3. Mixed air flow In general,cocurrent air flow pattern is used for food materials because the temperature of the product is lower than the inlet air temperature.the counter current air flow pattern is applied for heat sensitive materials. In counter current, the direction of air flow is opposite to the product flow. In most of the cases mixed flow pattern is used in the economical spray dryers whereas it is not suitable for heat sensitive materials.

5 a. Co current flow b. Counter current flow c.mixed current flow Fig. 5 Airflow pattern

6 Drying of droplet The moisture present in the spray is evaporated in two stages. At the initial stage of drying, due to very high velocity of spray, circulation of liquid in the droplet occurs. This circulation greatly enhances transfer of heat and mass. The temperature gradient between the hot air and the droplet the moisture present in the droplet is evaporated from the surface of the drop and evaporation takes place at a relatively constant rate. The second stage begins when there is no longer enough moisture to maintain saturated conditions at the droplet surface, causing a dried shell to form at the surface. The evaporation rate falls rapidly in this stage of drying. Rate of evaporation and characteristics of particle formation varies depends on the nature of the food product Powder collection In general cyclone separator, bag filters or wet scrubbers are used to collect fine particles. Cyclones otherwise known as dust separators are used to recover particles from the gas stream in most of the industries. The air stream enters the cyclone separator and forms a spiral path moving downwards along with the particles. Due to centrifugal force, particles hit the cyclone wall and drop off by gravity and due to differential pressure, the clean air moves up from the bottom to the top of the cyclone 20.4 Dispersion Characteristicsof Spray dried powder Many spray dried powders do not readily disperse in water.the exterior of the powder absorbing water very rapidly and forming lumps, which will not allow to solubilize the inner most part of the powder particle intern covered with a viscous layer through which water penetrates very slowly which cause the problems. This problem arises particularly in products which contain soluble protein; the following properties are desired for good dispersing ability viz., a wettable surface, sinkability(must not float on the surface),solubility and resistance to sedimentation. The wettability is vital and depends upon the surface area and surface properties of the powder particle Applications of spray drier Dairy and Egg products Spray drying process is widely used in dairy industries. Commonly available dairy products are baby food, ice cream mix, skim milk, whole milk, whey protein etc. Egg products like egg white powder, yoke powder and whole egg powder is produced using spray dryer Fruit and vegetable products Spray drying process is applicable for a wide range of fruits and vegetables. The most extensively used spray dried product is tomato. When comparing to fruits, this technology is easy for vegetable products due to the presence of sugar content. Since the sugar content is more in case of fruits, fillers are added for successful drying process. Fruit juices, pulps and paste can be spray dried. Vegetable powder is used in dry soup mixes Carbohydrates and Starch materials Shell is removed from the corn and is soaked in water for 40 hours. The steep water contains all the nutrients is drained and concentrated. This concentrate is spray dried. Gum Arabic, pectin, starches, sweeteners, corn starch, corn syrup solid, corn gluten, glucoseetc are produced using rotary atomizer. Pressure nozzles or rotary nozzle are used for the production of wheat gluten Fish products The perishable by products from slaughtered house such as blood glands and guts can be processed by spray drying into stable form. The blood contains 18% solids which is rich in nutritive value. The whole blood or after separation into plasma and albumin can be spray dried. To prevent coagulation blood products are spray dried at low temperatures.the fish soluble contains scales, vitamins and protein which are the by-product of fish processing plant. Fish pulp, fish meal, fish flour, gelatine and tissues can be spray dried Coffee

7 The first spray dried food product is coffee. The coffee is cleaned, roasted and grounded. From the grounded coffee the liquor is extracted that is subsequently spray dried. Decaffeinated coffee, tea extract are the spray dried products available in the market Other food products Cake mixes, cocoa mixes, fish protein, soup mixes, protein from soybean, peanuts, sunflower seeds, malt extract, food colouring Agglomeration In order to increase the dispersing ability of food powder, particles are agglomerated. In agglomeration process the particlesare enlarging by means of increasing adhesion force between food particles. In this process, binder agents are incorporated in order to aid the particles stick together.adhesion is strongly depending on the physic chemical material properties of food particle. Water and starchare commonly used binding agents. The binding agents soften the surface of the food particles and help them to stich each other and allow them to grow. Agglomeration process is also referred to as granulation or instantizing or rewetting agglomeration. Agglomerated particles have good solubility, high bulk density, goodflowability and create less dust. The applicable products are milk, coffee, cocoa, baby foods, soups, sauces, soft drinks, sugar mixtures, minerals etc Encapsulation A process to entrap active agents (core material), within a carrier material (wall material) using spray dryer is called encapsulation.in encapsulation process the droplets or tiny particles are surrounded by a wall material in a homogeneous and heterogeneous matrix. Encapsulation process provides a physical barrier in between the core material and the other components of the product. A spherical spray with size ranges from 50nm to 2mm, containing active components is called as microcapsule. Spray drying of core material is achieved by dissolving or dispersing the active component with wall material followed by atomization and drying in the drying chamber. During drying process the active component is entrapped by carrier material whereas the water droplets are evaporated. The food product can be protected from environmental conditions such as moisture, oxygen, heat and light with encapsulation process.improving the stability, enhancing the shelf life and controlling the release of encapsulated materials are the major advantages of encapsulation process. Flavouring agents, sweeteners, colouring materials, preservatives, antioxidants, food additives etc can be encapsulated using spray drying process Drum dryer Drum dryers are used to dry the liquid foods before spray drying came into usage. Like no other dryer, the drum dryer can dehydrate high viscous and sticky and slurry food materials. This type of dryers has direct contact with the food materials. Nowadays, drum dryers are used for drying food products such as baby foods, cereal flakes, fruit pulp, cooked starch etc. These types of driers are highly suitable for viscous liquid and pureed foods and this type of drying is energy efficient Principle The drum body of the drum dryer is heated on the inside by steam and turns continuously. The product is feed in a thin film on the outer side of the revolving drum and the food product is dried immediately. After the 3/4 th of the revolution, from the feeding point, doctor knife are attached to scraps the dried product from the drum surface as a film or as flakes. Drum dryers are continuous, layer type and conductive type of dryers Components of drum dryer The typical drum dryer consists of product feeding system, one or two hollow cylinder (horizontally mounted), supporting frame and a scrap.typical drums range from 0.5 to 6 meter in diameter and 1-6 meter in length. Steam is applied at the temperatures up to 200 o C which heats the inner surface of the drum. The high viscous food material is uniformly feed as a thin

8 layer of around mm thickness on to the outer surface of the drum. Drum dryer takes very less residential time to attain less than 5% of moisture content with energy efficiencies of about 60% 90%. Fig. 6 Single drum dryer Fig.7 Double drum dryer Feeding methods Depending upon the drum arrangement, concentration of solids, consistency of viscous liquid and wettability of the product the surface and feeding method varies. Roll feeding, nip feeding, dipping, spray feeding and splashing are the different methods of feeding system used in drum dryers Applications of drum dryer Products that are suitable for drying on a drum dryer are viscous liquids, slurries, suspensions and pastes. Drum dryer is also extensively used to dry starch slurries such as potato, rice, wheat, maize, corn, banana and cowpea slurries. Sugar containing slurries such as fruit puree, vegetable sauce, citrus pulps and other fruit juices. Summary 1. Spray drying involves evaporation of moisture from an atomised feed by mixing the spray and the drying medium. 2. Spray dryer consists of a feed tank, atomizer, drying chamber and separator. 3. Four types of atomizer and three types of air flow pattern are used in the spray drying system. 4. Spray dryer is applicable for milk, fruit and vegetable, egg and other plant materials processing industries. 5. Agglomeration and encapsulation are the major advantages of spray drying system. 6. Single and double drum dryers are used in food processing industries and five different feeding methods such as roll feeding, nip feeding, dipping, spray feeding and spalishing are implemented. 7. Drum dryer is applicable for high viscous materials food products such as starch, fruit pulp etc.