Title of the paper: Principles of Food Processing F04PF21 FREEZE DRYING AND ROTARY DRYING PROCESS Dr. P. Suganya

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1 Introduction Freeze drying is a low thermal processing method to preserve food materials. Freeze drying process was initiated during Second World War period in order to preserve the foods. Further this method is applied for commercial processing of foods.the first freeze dried product is coffee but now vegetables, fruits, meats, fish, dairy products can be successfully freeze dried. Though the freeze drying technology is widely used in food processing, recently it takes a major application in emulsion and micro encapsulation technology also.rotary drying process is also used in the food processing sector for the selective food materials. Objectives: After going through this session you will be able to 1. Explain the principle, working and applications of freeze drier and rotary dryer. 2. To interpret the processing and preservation effect of food in freeze and rotary drying process. Preservation of food is achieved by drying of foods under low temperature is called freeze drying. Freeze-drying holds a remarkable place in our multiple advanced technologies, more particularly in the field of food processing.the special characteristic feature of freeze drying process is that it maintains the cell structure, size and shape of the product.removing moisture prevents a product from deteriorating at room temperature. The dried product will be the same size and shape as the original frozen material and found to have excellent stability and convenient to reconstitutewith water. Freeze dried products will retain nutrients, color, flavor, and texture often indistinguishable from the original product.freeze dried products are packed with suitable packaging materialto store upto 25 yearsat ambient temperature Principle of freeze drying Freezing is the unit operation in which the temperature of a food is reduced below the freezing point, and a proportion of the water undergoes a change in state to form ice crystals. Ice crystals are removed by sublimation is called freeze drying. When a frozen liquid is subjected to phase transition from solid to vapour phase sublimation takes place. Drying is achieved by subjecting food material to temperature and pressure below triple point. The major factors that determine the phase which substance takes place depends on the temperature and pressure. This is a multi-stage process and obviously, each step is critical Phase diagram The temperature and pressure at which the solid, liquid, and vapour phases of a pure substance can (water) coexist in equilibrium is called triple point D in Fig.1 and this depicted the phase diagram where the hot air drying occurs in gas and liquid phase, vacuum drying takes place under reduced pressure in gas and liquid phase and freeze drying occurs in the phases of solid and gas by sublimation.

2 Fig 1. Phase diagram 21.3 Components of freeze dryer 1. Drying chamber 2. Heating shelves 3. Vacuum pump 4. Condenser The drying chamber in freeze dryer provides a thermally insulated vessel to accommodate the drying shelves. Heaters are provided to supply sufficient temperaturefor sublimationprocess. The vacuum pump removes all the non condensable vapours to attain maximum vacuum condition. Conduction and radiation type of dryers are used for commercial purpose. For most commercial freeze dryers -65 o C is used as condenser temperature. The condenser collects the water vapour that is released by the product. Coffee is the common drink produced by freeze drying technology. Liquid foods, small fruits and vegetables, green pepper are highly suitable food material for freeze drying. The food material should be contamination free and for selected foods pre treatment is required. For example before freeze drying blanching is the mandatory pre treatment to inactivate the enzyme present in the food materials.food materials such as potato and pepper, blanching is essential to prevent the enzymatic browning and inhibits discolouration. Fig.2 Schematic diagram of freeze dryer Freeze drying process involves the following operation 1. Freezing of food 2. The sublimation phase or primary drying 3. The desorption phase or secondary drying. 21.4Freezing of foods. Almost all food materials consistprimarilyof water that has to be removed to preserve the materials. Since freeze drying is a change in state from the solid phase to the gaseous phase, material to be freeze dried must first be adequately pre frozen. During freezing the food material is hardened by low temperatures. When the aqueous suspension is cooled, changes occur in the solute concentrations of the product matrix and as cooling proceeds, the water is separated from the solutes as it changes to ice, creating more concentrated areas of solute. This process is crystallization of ice in food materials with the consecutive processes of nucleation and growth. These processes are central to the effects of different freezing rates and subsequent effect on food quality. All food materials freeze when its temperature drops below the freezing point and latent heat (i.e heat required during the phase change during crystal formation) has been removed. This is also known as degree of super cooling. The greater the super cooling, the greater the number of nuclei formed. The number of nuclei is greatest in the extracellular space, and they are only formed within the cell when the rate of heat removal is

3 higher. As soon as the nuclei form, they began to grow by accumulation of molecules at the solid/ liquid interface. However, the way they grow depends on the microgeometry and the temperature distribution ahead of the freezing front (Fig. 3) Fig. 3 Ice crystal formation The sublimation phase or primary drying The frozen food is placed in the drying chamber where the chamber pressure is reduced to initiate the sublimation process in the primary drying stage. The drying progresses gradually from the surface to the centre of the products. The channels formed by the sublimation ice between the ways of vapour to escape. The margin between drying layer and frozen layer is known as the sublimation interface. The temperature of the sublimation interface is the critical parameter to be controlled in primary drying process. In the frozen layer, the water vapour released by the frozen ice crystal in the product during sublimation and the non frozen water present in the dried layer travels by diffusion through the pores (Fig. 5) in the dried layer and get in to the drying chamber. The absence of frozen layer represents the end of primary drying process and 90% of the moisture is removed at this stage. Fig. 4 Freeze drying of foods Secondary drying The secondary drying stage involves the removal of unfrozen water. The secondary drying processes starts immediately after the primary drying process. The water bounded by food material is removed by heating under vacuum condition in the secondary drying stage. In general conduction, convection and radiation modes of heat transfer is supplied to the product. The following product temperatures are usually employed 1. between o C for heat sensitive product or more for less heat sensitive materials. At the end of the secondary drying stage, two critical factors affecting the shelf life of the products are 1. The residual moisture content in the dried material. 2.The temperature at which the dried material is kept in storage

4 At the beginning of the secondary drying, the decrease in water content is rapid and the drying speed increases with increasing specific surface of the product.heat transfer occurs in freeze drying process in the sublimation front through the frozen layer and dried layer. The rate of freezing is one the influencing factor on the ice crystal configuration and hence the final porous structure of the freeze dried product.in the primary drying process maintenance of low temperature will reduce the aroma losses. Fig. 5 Porous structure of freeze dried food Types of freeze dryers Different types of dryer are used depending on the materials to be dried.the dryers are classified as 1. Pilot freeze drying. 2. Industrial freeze drying a. Multi batch freeze dryers b. Continuous freeze dryers c. Tunnel freeze dryers d. Vacuum spray freeze dryers Pilot freeze drying Pilot freeze dryers mainly used for food laboratories and pharmaceutical industries.since they are used to explore possibilities for the preservation of biological materials, they are highly demanded. This type of freeze dryer can dry the materials in batches consisting of from 2 to 20 kg of frozen product. Fig.5 Batch freeze dryer Industrial freeze dryers For industrial applications, large capacity dryer with continuous production is required. The following industrial freeze dryers are employed in food processing industries. a.multi batch freeze dryers Utilization of resources is minimum in batch type dryers. So as to increase the utilization of available resources the multi batch freeze dryers are introduced with number of batch cabinets. In this type of dryers different types of products are dried simultaneously and this is in practice in most of the industries to dry the food materials. b. Continuous freeze dryers

5 To overcome the disadvantages in batch type freeze dryers, multiple freeze dryers are used. The continuous type of dryers is economic in terms of labours, large capacity and to get uniform drying. Two types of multiple freeze dryers are available. 1. Static or tray in which the freeze dried products is stationary on trays. 2. Dynamic or trayless dryer, in which product is moved with belt conveyors. The main disadvantage of this continuous method is equipment is huge, complex and difficult to manufacture. Fig.6 Continuous freeze dryer c. Tunnel Freeze Dryers A tunnel type freeze dryer consists of large vacuum cabinet in which tray carrying trolleys are loaded at one end. The end is discharging end where the dried products are unloaded. Vacuum locks are fixed in each end of the dryer. Low pressure steam is used to heat the plates.in this type of dryers it is impossible to dry different type of food materials at the same time. Cottage cheese and coffee are the best examples for this method of drying. Fig.7 Tunnel freeze dryers d. Vacuum spray freeze dryers This type of dryers is specially designed for liquid foods such as coffee and milk. Liquids are sprayed at the top of the cylindrical vacuum tower at very low pressure. The liquid product with 30% pre concentration is sprayed at the top of the drying chamber. The droplets freeze in the vacuum cabinet into very small sized particle around 150 micrometers and collected at the bottom of the dryer Applications of freeze dryer The foods products which are freeze dried have a superior keeping quality and high nutritional profile with appropriate packaging. No case hardening occurs in freeze dried product; less shrinkage hence the texture of the product is retained Coffee products

6 The brewed coffee from ground beans is pre concentrated by evaporation and frieze dried immediately to prevent the large ice crystal. Then heat is applied for sublimation process and further drying. The entrapped vapour is remixed into the freeze dried coffee to enhance the flavor Fruits and vegetables Most of the fruits and vegetable are suitable for freeze drying process. For example strawberries should be sliced then loaded directly into trays, and dried with a temperature of 50 o C. A drying time of 2hours is possible. White varieties of mushrooms are preferred for freeze drying, pigmented varieties tending to shrink more. They should be peeled, sliced and blanched before being loaded into trays. With a platen temperature of 82 o C, a drying time of 3 hours is possible Shrimp The pink varieties of shrimp are preferred to the brown for freeze drying. After cooking for 3 min, the shrimp is placed in the drying chamber in trays between platens. Best results are obtained with the platens at 52 o C, which gives a drying time of 10h. Higher drying temperatures give a product which dries in 8 hours but rehydrates less well. The product quality is much lower at the higher temperature.precooked salmon steaks 13 mm thick dry in about 9 hours with a temperature of 100 o C Egg Liquid egg is mixed with stabilizers to a thick cream. The mix is then filled into trays to a depth of 10 mm and the product temperature reduced to -25 o C. When this temperature is reached, cooling is stopped. Evacuation continues to less than 0.07 kpa. Then the platens are heated. The complete drying stage is of the order of 9-10 hours and egg is left in the trays in the form of a crystalline porous cake. The egg is broken up and then sieved to a powder and packed in sacks 21.9 Packaging of freeze dried foods Freeze-dried food materials should be sealed in airtight containers to prevent the diffusion of moisture and oxygen. Laminated foil pouches, metal and plastic cans are used for bulk packaging. Vacuum packaging and inert gas packaging is also applicable for freeze dried food materials to prevent oxidation and spoiling of foods Limitations of freeze dried foods 1. Expensive due to the specialized equipment. 2. Require more storage space. 3. More fragile due to its porous nature. 4. Suitable for selected food materials only Rotary dryers The rotary dryer is another type of secondary dryer, often used as a finisher for granulated products. The inside of the drum is fitted with flights on the inner circumference, designed to turn the product as it passes through from the feed end and the product is agitated by the flights and slowly moved in a helical path inside the drum Principle Rotary dryers are generally used for drying wet granules or grains which do not have tendency to stick. It is a continuous dryer, and consists of rotating horizontal cylindrical drum which is slightly inclined to the outlet. Wet material is fed in the upper end and the dry material is discharged from the other end. The cylindrical drum contains lifting flights as shown in Fig.9. As the drum rotates, the flights lift the material to the top from where it tumbles down. Hot air flows the bottom end upwards, whereas the material flows counter currently from the upper side to bottom side. Thus, the material is heated by direct contact with hot air by convective heating. In some arrangements the hot air also passes through the drum. With this arrangement the drum is also heated and provides conductive heat to the solids when they are in contact with the cylindrical drum, convective heat is also provided when the solids are tumbling down.

7 Fig.9 Schematic diagram of rotary dryer Construction and working The rotary dryer is long rotating cylinder having slope where the feed end is slightly elevated and the discharge end is lower to ease the conveying of the product during drying process. High moisture material is fed in the feed hopper through conveyor or elevator, then the feed pass into the feed pipe in which slope is provided greater than the natural inclination of the materials, so that the feed material can flow into the rotary dryer smoothly. The cylinder has internal flights or cascades (Fig.10) that direct the movement of the product and ensure good mixing and rapid drying. Fig. 10 Cascading inside the rotary dryer Usually the solids are discharged at the outlet through a screw conveyor. The dryer drum rotates on drum supporting rolls. Hot air can be blown from the discharge end. In some arrangements the hot air is sucked from the upper end, here the whole dryer is maintained under slight vacuum. This arrangement help material not to dust, and hence is more useful to those materials which have a tendency to dust. The air temperatures are usually used for C and the air velocities ranging from ,000 kg/m 2 h for coarse particles.in general two types of air and feed pattern is preferred according to the properties of the food materials are concurrent and counter-current. The agitation of the food and the large area of food exposed to the air produce high drying rates and a uniformity dried product. The method is especially suitable for foods that tend to mat or stick together in belt or tray dryers. However, the damage caused by impact and abrasions in the drier restrict this method to relatively few foods likes sugar crystals and cocoa beans Features of rotary driers Applicable for high moist and bulk materials. Highest possible drying temperatures Drying and cooling also applicable Thermal efficiency is high.

8 These dryers are not particularly successful as a drying medium for large particles, as the tumbling action tends to distort the shape of the cut, and the residence time in the drum is normally insufficient to affect any significant degree of evaporation. Summary 1. Freeze drying is the low temperature processing method to dry the food materials by sublimation and the most successful method to preserve food materials. 2. Pre frozen food materials undergoes primary and secondary drying phases under vacuum conditions to get freeze dried food materials 3. Depending up on the products,many types of freeze dryers are available to get quality products and with suitable packaging materials freeze dried foods are stored for longer time period with good keeping quality. 4. Freeze dryers are highly suitable for coffee, fruits and vegetables, fish and egg. 5. Low temperature preservation ensures the quality of food materials and freeze dryer products retains the shape, colour and almost all the nutritional qualities. 6. Rotary dryers are highly suitable for granulated food material which requires proper mixing during drying 7. Rotary dryers are applicable to continuous process and larger production capacity products. 8. Rotary dryers are not suitable for sticky and sensitive materials such as sugar crystals and perishables.