DRYING OF FOOD

BY

Pintu Choudhary Master of Technology

( Food Science and Technology) Indian Institute of Crop Processing Technology

Ministry of Food Processing Industries Government of India Thanjavur - 613005

DRYING

Thermal drying: Drying commonly describes the process of thermally removing volatile substances (moisture) to yield a solid product.

Non-thermal drying 1. As Squeezing wetted sponge2. Adsorption by desiccant (desiccation)3. Extraction.

PURPOSES OF DRYINGIn food technology, drying is carried out for one or

moreof the following reasons:

1. To avoid or eliminate moisture which may lead to corrosion and decrease the product stability.

2. To improve or keep the good properties of a material, e.g. flow ability, compressibility.

3. To reduce the cost of transportation of large volume materials ( liquids)

4. To make the material easy or more suitable for handling.

5. Preservative.6. The final step in: Evaporation, Filtration,

Crystallization

TYPES OF DRYING Direct :Convective Dying Drying is established through direct contact between the

product and the gas heating medium .Material reaches steady state temperature near gas wet temperature

Indirect Drying:Established from heated surface in contact with the

product.The heating medium and product are separated by wall.

Material reaches steady state temperature near liquid boiling point for contact rate drying .

Radiation :Heat transfer establishes by radiation from energy source.

This is no contact from heated surface or medium and product

MECHANISM OF DRYING

Liquid diffusion: if the wet solid is at a temperature below the boiling point of the liquid

Vapor diffusion: if the liquid vaporizes within material

Condensation diffusion : if drying takes place at very low temperatures and pressures, e.g., in freeze drying

Surface diffusion (possible although not proven) Hydrostatic pressure differences: when

internal vaporization rates exceed the rate of vapor transport through the solid to the surroundings

Combinations of the above mechanisms

TRAY DRYING

These types of dryers use trays or similar product holders to expose the product to heated air in an enclosed space.

The trays holding the product inside a cabinet or similar enclosure are exposed to heated air so that dehydration will proceed.

Air movement over the product surface is at relatively high velocities to ensure that heat and mass transfer will proceed in an efficient manner.

In most cases, cabinet dryers are operated as batch systems and have the disadvantage of non-uniform drying of a product at different locations within the system. Normally, the product trays must be rotated to improve uniformity of drying.

TRAY DRYING

Cabinet Type Tray Drier

ADVANTAGES OF TRAY DRYING Simple in handling Lesser capital cost

DISADVANTAGES Non-uniform drying of a product at different

locations within the system. Time required for drying is more The major disadvantages of this type of

dryers are the high labor cost involved during the loading and unloading of the drying materials and the low capacities of the units.

FLASH DRYER

WORKING OF FLASH DRYER Flash dryers utilizing superheated steam as the

drying medium have some unique quality and energy advantages over air drying systems.

Flash dryers consisting of inert media have been employed at pilot scales to dry slurries and suspensions, which are sprayed onto them.

The particles are coated thinly by the slurry and dried rapidly as a thin film.

Attrition due to inter-particle collisions and shrinkage induced breakage of the dried film allows entrainment of the dry powder into the drying gas for collection in a cyclone or bughouse.

ADVANTAGES Short contact time and parallel flow make possible

to dry thermo labile materials. The dryer needs only a very small area and can

be installed outside a building. The dryer is easy to control. The low material

content in the dryer enables equilibrium conditions to be reached very quickly.

Due to small number of moving parts the maintenance cost is low.

The capital costs are low in comparison with other types of dryers.

Simultaneous drying and transportation is useful for materials handling process.

DISADVANTAGES High efficiency of gas cleaning system is required.

Because of powder emission, the dryer cannot be used for toxic materials.

In some cases this disadvantage can be avoided using superheated steam as a drying agent.

For lumped materials difficult to disperse, drying is impossible to carry out in this apparatus.

There is a risk of fire and explosion, so care must be taken to avoid flammability limits in the dryer.

In general, especially when recirculation is applied, not all material particles have the same residence time in the dryer .

APPLICATION OF FLASH DRYING Drying of heat-sensitive products in this type of dryers

is very useful. Flash dryers are successfully used in the chemical,

food, pharmaceutical, mining, ceramic, and wood industries.

Magnesium sulfate, magnesium carbonate, copper sulfate, di-calcium phosphate, ammonium sulfate and phosphate, calcium carbonate and phosphate, and boric and adipic acids are common examples of chemicals and by-products.

Cement, aniline dyes, blowing agents, chlorinated rubber, coal dust, copper oxide, gypsum, iron oxide, and silica gel catalyst are typical by-products and minerals that can be dried in a very efficient way in flash dryers.

Flash dryers are widely used in the plastic and polymer industries.

EXPECTED NEW DEVELOPMENTS IN FLASH DRYER Enhancement of product quality Increased efficiency by heat recovery Applications of heat pumps in drying systems Efficient combined processes Development of models for overall systems

that would include mechanical dewatering, drying, heat recovery, powder collection, cooling stages, etc. in order to study interactions between various parts of the system and develop improved design procedures

DRUM DRYER It consists of a drum of about 0.75-1.5 m in

diameter and 2-4 m in length, heated internally, usually by steam, and rotated on its longitudinal axis.

Operation: The liquid is applied to the surface and spread to a film, this may be done in various ways, but the simplest method is that shown in the diagram, where the drum dips into a feed pan. Drying rate is controlled by using a suitable speed of rotation and the drum temperature. The product is scraped from the surface of the drum by means of a doctor knife.

FIG. DRUM DRYER

ADVANTAGES OF THE DRUM DRYER

1. The method gives rapid drying, the thin film spread over a large area resulting in rapid heat and mass transfer.

2. The equipment is compact, occupying much less space than other dryers.

3. Heating time is short, being only a few seconds.

4. The drum can be enclosed in a vacuum jacket, enabling the temperature of drying to be reduced.

5. The product is obtained in flake form, which is convenient for many purposes.

Operating conditions are critical and it is necessary to introduce careful control on feed rate, film thickness, speed of drum rotation and drum temperature.

The uneven scrapping of the doctor’s blade at the rubbery and glassy parts of the sheet forms wrinkles in the sheet which eventually become ‘‘sticks’’ in the final product. The ‘‘sticks’’ reduce the quality of the product by making it very hard to disperse and physically unsatisfactory in appearance

DISADVANTAGES OF DRUM DRYER

APPLICATIONS: The drum dryer has been used extensively to dry

chemicals and food products. Examples of Chemicals are polyacrylamides, and various salts such as silicate, benzoate, propionate, and acetate salts.

Drum dryers have been successfully used in drying sludge The drum dryer is also extensively used to dry and

gelatinize or ‘‘cook’’ starch slurries, such as potato, rice,wheat,maize,corn, soybean,banana,and cowpea slurries to produce pregelatinized starch for instant foods.

Non-starch, low-sugar foods, such as tomato puree, milk, skim milk, whey, beef broth, yeast, coffee, and malt extract, have also been successfully dried on a drum dryer.

Heat-sensitive products such as pharmaceuticals30 and vitamin-containing products can be dried in a vacuum drum dryer.

FOAM MAT DRYING Foam-mat drying is one of the simple methods of

drying in which a liquid food concentrate along with a suitable foaming agent is whipped to form a stable foam and is subjected to dehydration in the form of a mat of foam at relatively low temperature .

Rate of drying in this process is comparatively very high because of an enormous increase in the liquid-gas interface, in spite of the fact that the heat transfer is impeded by a large volume of gas present in the foamed mass.

FOAMING AGENT USE FOR MAKING FOAM

1.Glycerol 2.Monostearate , 3.Egg albumin,4.Ground nut protein isolate,5.Gur gum and 6.Carboxy methyl cellulose (CMC)

Fig. foam mat dryer

ADVANTAGES Reduction in thermal exposure to heat

sensitive foods, dried foams retain a porous structure, allowing rapid rehydration characteristics.

More retention of nutrition .

APPLICATIONS

Several applications of foam drying have been developed, including vacuum puff drying, foam spray drying, and foam-mat drying

FREEZE DRYING

Freeze drying is a process used to dry extremely heat – sensitive materials. It allows the drying , without excessive damage, of proteins, blood products and even microorganisms, which retain a small but significant viability.

In this process the initial liquid solution or suspension is frozen, the pressure above the frozen state is reduced and the water removed by sublimation.

Thus a liquid –to-vapour transition takes place, but here three states of matter involved: liquid to solid, then solid to vapour

THE PHASE DIAGRAM FOR WATER

The diagram consists of three separate areas representing the phases of water, solid, liquid, and vapour . The point O is the only point where all the three phases can coexist, and is known as the triple point .

On heating at constant atmospheric pressure ice will melt when the temperature rises to 0 C . At this constant temperature and pressure it will then change to water. Continued heating will raise the temperature of the water to 100 C where, if heat addition is continued, the liquid water will be converted into water vapour at 100 C.

If , however, solid ice is maintained at a pressure below the triple point then on heating the ice will sublime and pass directly to water vapour without passing through the liquid phase.

This sublimation, and therefore drying, can occur at a temperature below 0 C.

This will only happen if the pressure is prevented from rising above the triple point pressure .

It may be thought that as the process takes place at a low temperature the heat required to sublime the ice will be small.

FREEZE DRYER

STAGES OF THE FREEZE DRYING PROCESS

1. Freezing stage Shell freezing Centrifugal evaporative freezing

2. Vacuum application stage 3. Sublimation stage

Primary drying Heat transferVapour removalRate of drying

4. Secondary drying and5. Packaging

ADVANTAGES OF FREEZE DRYING1. Drying takes place at very low temperatures, so the

chemical decomposition, particularly hydrolysis is minimized.

2. The solution is frozen occupying the same volume as the original solution, thus , the product is light and porous.

3. The porous form of the product gives ready solubility.

4. There is no concentration of solution prior to drying. Hence, salts do not concentrate and denature proteins, as occurs with other drying methods.

5. As the process takes place under high vacuum there is little contact with air, and oxidation is minimized.

INDUSTRIAL FREEZE DRYERS Tray and Pharmaceutical Freeze Dry Multi-batch Freeze Dryer Tunnel Freeze Dryers Vacuum-Spray Freeze Dryers Continuous Freeze Dryers

DISADVANTAGES & USES OF FREEZE DRYING

Disadvantages:

There are two main disadvantages:1.The porosity, ready solubility and complete dryness yield a very hygroscopic product. Unless products are dried in their final container and sealed in situ, packing require special conditions.2.The process is very slow and uses complicated plant, which is very expensive .It is not a general method of drying but limited to certain types of valuable products.

Uses of freeze drying :

The method is used for products that can not be dried by any other heat method. These include biological products, e.g. antibiotics, blood products, vaccines, enzyme preparations and microbiological cultures.

LIMITATIONS OF THE FREEZE DRYING PROCESS.

1. The freeze drying of products such as blood plasma, although simple in theory, presents a number of practical problems.

2. The depression of the freezing point caused by the presence of dissolved solutes means that the solution must be cooled below the normal freezing temperature for pure water (-10-30).

3. Sublimation can only occur at the frozen surface and is slow process (1mm thickness of ice per hour). So, the surface area must therefore be increased .

4. the liquid thickness prior to freezing be reduced in order to reduce the thickness of ice to be sublimated.

5. At low pressure large volumes of water vapour are produced which must be removed to prevent the pressure rising above the triple point pressure.

6. The dry material often needs to be sterile, and it must also be prevented from regaining moisture prior to the final packaging.

VACUUM DRYING

This equipment is a good example of conduction drier. The vacuum oven consists of a jacketed vessel to withstand vacuum within the oven. There are supports for the shelves giving a larger area for conduction heat transfer. The oven can be closed by a door. The oven is connected through a condenser and liquid receiver to a vacuum pump. Operating pressure can be as low as 0.03-0.03 bar, at which pressures water boils at 25-35 C.

FIG. VACUUM DRYER

ADVANTAGES OF VACUUM OVEN: Drying takes place at a low temperature. Maximum retention of nutrition There is little air present, so there is minimum risk

of oxidation. LIMITATIONS: Sometimes burning onto trays in vacuum shelf

driers, Shrinkage in food which reduces the contact

between the food and heated surfaces of both types of equipment. They have relatively high capital and operating costs and low production rates and

Used mainly to produce puff-dried foods.

FLUIDIZED BED DRYING The product pieces are suspended in the heated

air throughout the time required for drying. The movement of product through the system is enhanced by the change in mass of individual particles as moisture is evaporated.

The movement of the product created by fluidized particles results in equal drying from all product surfaces.

FLUIDIZED BED DYING

WORKING PRINCIPLE

Good contact between the warm drying air and wet particles is found in the fluidized – bed drier.

Principles of fluidization: The particulate matter is contained in a vessel, of which is perforated, enabling a fluid to pass through the bed of solids from below. If the air velocity through the bed is increased gradually and the pressure drop through the bed is measured, a graph of the operation shows several distinct regions, As indicated in the fig. Effect of air velocity on pressure drop through a fluidized bed........

EFFECT OF AIR VELOCITY ON PRESSURE DROP THROUGH A FLUIDIZED BED

ADVANTAGES OF FLUIDIZED-BED DRYING

Efficient heat and mass transfer give high drying rates, so that drying times are shorter than with static-bed convection driers. Economic, heat challenge to thermo-labile materials is minimized.1. The fluidized state of the bed ensures that drying occurs from the surface of all the individual particles and not just from the surface of the bed. Hence, most of the drying will be at constant rate and the falling –rate period is very short.2. The temperature of a fluidized bed is uniform and can be controlled precisely.3. The turbulence in a fluidized bed causes some attrition to the surface of the granule. This produces a more spherical free-flowing product.

DISADVANTAGES OF FLUIDIZED-BED DRYING

1. The turbulence of the fluidized state may cause excessive attrition of some materials, with damage to some granules and the production of too much dust.

2. Fine particles may become entrained in the fluidizing air and must be collected by bag filters, leading to segregation and loss of fines.

3. The vigorous movement of particles in hot dry air can lead to the generation of static electricity charges. The danger is increased if the fluidized material contains a volatile solvent such as isopropanol. Adequate electrical earthling is essential.

4. The free movement of individual particles eliminates the risk of soluble materials migrating, as may occur in static beds

Cond…

NEW DEVELOPMENT IN FLUIDIZED BED DRYER

In fluidized bed drying is the idea of applying microwave energy field continuously or intermittently in a fluidized or spouted bed.

Use of superheated steam will probably become more popular in some applications in the future

SPRAY DRYER The spray dryer provides a large surface

area for heat and mass transfer by atomizing the liquid to small droplets. These are sprayed into a stream of hot air, so that each droplet dries to a solid particle.

The drying chamber resembles the cyclone ensuring good circulation of air, to facilitate heat and mass transfer, and that dried particles are separated by the centrifugal action.....

SPRAY DRYER

Working Mechanism of spray dryer

COMPONENT OF SPRAY DRYER

• Centrifugal atomizer. Liquid is fed to the center of a rotating disc or bowl having a peripheral velocity of 90–200ms1. Droplets, 50–60 m in diameter, are flung from the edge to form a uniform spray.• Pressure nozzle atomizer. Liquid is forced at a high pressure (700–2000103 Pa) through a small aperture to form droplet sizes of 180–250 m. Grooves on the inside of the nozzle cause the spray to form into a cone shape and therefore to use the full volume of the drying chamber.• Two-fluid nozzle atomizer. Compressed air creates turbulence which atomizes the liquid. The operating pressure is lower than the pressure nozzle, but a wider range of droplet sizes is produced.• Ultrasonic nozzle atomizer. A two-stage atomizer in which liquid is first atomized by a nozzle atomizer and then using ultrasonic energy to induce further cavitation

CHARACTERIZATION OF SPRAY DRIED PRODUCTS

The products are uniform in appearance and have characteristic shape, in the form of hollow spheres with a small hole. This arises from the drying process, since the droplet enters the hot air stream, and dries on the outside to form an outer crust with liquid still in the center. This liquid then vaporizes, the vapour escaping by blowing a hole in the sphere.

This method of drying allows a dry product to retain some properties of feed , e.g., a drop from an emulsion dries with continuous phase on the outside. When reconstituted, the emulsion is easily re-formed.

ADVANTAGES OF THE SPRAY DRYING PROCESS1. Product properties and quality are more effectively

controlled.2. Heat-sensitive foods, biological products,

pharmaceuticals can be dried at atmospheric pressure and low temperatures. Sometimes inert atmosphere is employed.

3. Spray drying permits high-tonnage production in continuous operation and relatively simple equipment .

4. The product comes into contact with the equipment surfaces in an anhydrous condition, thus simplifying corrosion problems and selection of materials of construction.

5. Spray drying produces relatively uniform, spherical particles with nearly the same proportion of nonvolatile compounds as in the liquid feed.

6. As the operating gas temperature may range from 150 to 600C, the efficiency is comparable to that of other types of direct dryers .

DISADVANTAGES OF SPRAY DRYING

1. Spray drying fails if a high bulk density product is required.

2. In general it is not flexible. A unit designed for fine atomization may not be able to produce a coarse pro duct, and vice versa.

3. For a given capacity larger evaporation rates are generally required than with other types of dryers. The feed must be pump able.

4. There is a high initial investment compared to other types of continuous dryers.

5. Product recovery and dust collection increase the cost of drying

APPLICATIONS OF SPRAY DRYER

1. Drying of any substance in solution or in suspension form.

2. It is most useful for drying of thermo labile materials e.g. antibiotics.

3. Suitable for large quantities solution.

4. Suitable for both soluble and insoluble substances e.g. citric acid, gelatin, starch.

5. It can produce spherical particles in the respiratory range e.g. dry powder inhalers.

6. Drying of milk, soap and detergents which is pharmaceutically related compounds.

NEW DEVELOPMENTS IN SPRAY DRYING

Superheated steam spray drying Two-stage horizontal spray dryer Low humidity spray drying Spray freeze drying Encapsulation Energy efficiency enhancement

THE GENERAL PRINCIPLES FOR EFFICIENT DRYING 1. Large surface area for heat transfer.

2. Efficient heat transfer per unit area (to supply sufficient latent heat of vaporization or heat of sublimation in case of freeze-drying)

3. Efficient mass transfer of evaporated water through any surrounding boundary layers, i.e. sufficient turbulence to minimize boundary layer thickness.

4. Efficient vapour removal , i.e. low relative humidity air at adequate velocity.

5. It is convenient to categorize food driers according to the heat transfer method they use, i.e. convective, conductive or radiant.

NEW DEVELOPMENT AND EMERGING DRYING TECHNOLOGY 1. New technologies are needed for:2. Drying of new products and/or processes3. Higher capacities than current technology

permits4. Better quality and quality control than currently

feasible5. Reduced environmental impact, use of renewable

energy6. Reduced fire, explosion, toxic hazards, safer

operation7. Better efficiency (resulting in lower cost)8. Lower cost (operating, maintenance cost and

capital)9. Shorter processing time while maintaining high

product quality