Advances in Drying of Foods :

Presented By : Rupesh Datir

Need for Advanced Drying Technologies

The conventional drying technologies are not necessarily optimal in terms of :

Energy consumption Quality of product Safety in operation Ability to control the dryer in the

event of process upsets, Ability to perform optimally even with large changes in throughput . Ease of control

• Energy and Environmental considerations as well as quality demands are now stringent.

• Thus there is a need of Advanced drying technologies.

• Two evolutionary types of advances in Drying technologies.i) Intensification of drying rates, ii) Multistaging of Convective dryers.

Intensification of Drying Rates :

• Reduction of capital and operating costs of dryers clearly depends on the feasibility to enhance drying rates within the limits of product quality equipements.

• Higher drying rates translate into smaller physical size of the dryer as well as the associated ancillary equipment. Generally, it is also reflected in lower running costs.

• Highly intensified heat and mass transfer results in high volumetric evaporation rates, so that the dryer volume can be reduced significantly as compared to the conventional type of same throughput.

• In general, the feedstock to be dried contains both surface and internal moisture.

• The rate at which the surface moisture can be removed depends only on the external heat and mass transfer rates by,

Increasing the gas velocity, Gas temperature or Reducing gas humidity will lead to increased drying rates for a purely convective (or direct) dryer.• Superheated Steam ( SHS )

Multistage Dryers : • If a material has both surface and

internal moistures, that is, both the so called constant and falling rate periods exists.

• For optimal drying, the drying conditions and even the type of a dryer in some cases, should be different to remove these two distinctively different types of moisture.

• Zoning of the dryers along their length is commonly used in conveyer, continuous fluidized beds, continuous vibrated beds, tunnel dryers, etc. to ensure optimal drying.

• This is especially true for heat-sensitive materials that could be dried under intense conditions only while surface moisture is being removed.

However, for large production rates and for certain materials, it is cost-effective

to employ two different dryer types for removal of surface and internal

moistures. Removal of surface moisture is

generally a more rapid process requiring shorter time in the dryer whereas,

Internal moisture removal is a slower process requiring a longer time and hence a larger dryer.

Dryers suited for surface moisture removal are fluid bed, spray dryers, etc. Whereas,

For longer residence times, one could employ through circulation, fluid bed, continuous tray dryers, etc.

Thus, a spray dryer can be followed with a fluid or vibro-fluidized bed dryer to reduce the overall cost of drying.

Development of Improved Drying Technologies :

New development in any field occurs as a result of either Evolutionary or Revolutionary effect.

New technologies are intelligent combinations of existing technologies necessitated by changes in the market place.

e.g. Evolutionary changes occurred in most commonly used dryers. i.e. Rotary dryers and Flash Dryers.

• Rotary Dryers : Purely convective, Axial gas flow Internal heaters ( Tubes or coils ) or external heating of shell to improve efficiency and capacity. Direct drying by air injection into the rolling bed of particles in the rotating shell through tubes connected to

central header

• Flash Dryer : Single pass, vertical, round, insulated

tube ( Adiabatic ) Single pass, jacketed tube for increasing heat input, faster drying ( Non adiabatic )

Use of SHS as carrier gas – Adiabatic and non adiabatic designs

Fluidized Bed Dryers : Fluidized bed dryers (FBD) are used

extensively for the drying of wet particulate and granular materials that can be fluidized, and even slurries, pastes, and suspensions that can be fluidized in beds of inert solids.

Fluidized bed operation gives important advantages such as good solids mixing, high rates of heat and mass transfer, and easy material transport.

In most cases, spray drying alone is not energy efficient to remove all moisture content inside the solids.

This is because considerable amount of heat and time is needed to remove internal moisture that is trapped inside the solids internal.

Fluidized bed drying can be incorporated as the second-stage drying to remove the internal moisture.

The bed of particles rests on a gas distributor plate. The fluidizing gas passes through the distributor and it is uniformly distributed across the bed.

At a certain gas velocity, the bed is fluidized when the gas stream totally supports the weight of the whole bed.

This state is known as minimum fluidization and the corresponding gas velocity is called minimum fluidization velocity.

Uses of Fluidized Bed Dryers : They are commonly used in processing

many products such as , Chemicals Carbohydrates Foodstuff Biomaterials Beverage products

Healthcare products Pharmaceuticals in powder or agglomerated form Polymer and resins

Products for calcinations

MODIFIED FLUIDIZED BED DRYERS :

( A ) Fluidized bed spray dryer :

• Spray drying is carried out in the upper part of the chamber followed by fluidized bed drying or agglomeration.

( B ) Hybrid Fluidized Bed Dryers : Hybrid fluidized bed dryers are useful for

through drying of solids that contain surface and internal moistures.

Surface moisture can be removed in the first stage drying using a flash or cyclone dryers.

Second stage drying is then carried out in fluidized bed dryers in which residence time can be easily control led.

( C ) Pulsating Fluidized Bed Dryers : Pulsating fluidized bed dryers are used to

overcome the problems of restricted particle size and size distribution, as well as aggregative fluidization and channelling that occur in a conventional fluidized bed dryer.

By pulsating the fluidizing gas stream, the fluidized bed either the whole bed or part of the bed is subjected to variable fluidizing gas velocity

( D ) Vibrated Fluidized Bed Dryers :

• Vibration combined with upward flow of air in an aerated bed enables particles to pseudo fluidize smoothly.

• The gas velocity required for minimum fluidization is considerably lower than the minimum fluidization velocity in conventional fluidized bed dryer.

• Attrition due to vigorous actions between particle–particle and particle wall is thus minimized appreciably.

• Hence, application of fluidized bed can be extended to fragile, abrasive, and heat-sensitive materials.

• The problem of fine particle entrainment is also avoided.

( E ) Fluidized Bed Freeze Dryer :

• Freeze drying is suitable for drying of highly heat-sensitive materials such as drugs, pharmaceutical, biological and food products.

• Advantages : ( Of FBD )

High drying rates due to excellent gas

particle contact Higher thermal efficiency

Lower capital and maintenance costs Easy to control

Disadvantages : ( Of FBD ) Particles should be isometric as flakes,

strips, and fibers cannot be fluidized High power consumption due to the need

to suspend the entire bed in a gas stream leading to a high pressure drop

Entrainment of fine particles, attrition or pulverization of particles , agglomeration of fine particles

Size distribution should be as narrow as possible to avoid excessive carryover.

Spray Freeze Drying : A step forward in producing materials

of unique properties is spray freeze-drying (SF-D), which offers free-flowing, ultrafine, and uniform particles with high surface area, porosity, and enhanced solubility.

Thus, SF-D has a great potential for producing drugs, nutraceuticals, functional foods, advanced materials, and other high-value products.

• SF-D can broadly be categorized as SF-D into liquids (SFL), SF-D into gases (SFG), and SF-D into gases over a fluidized bed (SFG-FB).

• Each technology can be carried out at atmospheric pressure or under vacuum.

Freeze drying of food and biological materials also has the advantage of little loss of flavour and aroma.

The low processing temperatures, the relative absence of liquid water, and the rapid transition of any local region of the material dried from a fully hydrated to a nearly completely dehydrated state minimize the degradative reactions that normally occur in ordinary drying processes, such as non enzymatic browning, protein denaturation, and enzymatic reactions.

The liquid feed is atomized by a pressure nozzle located at the top of a spray dryer.

The droplets are frozen instantly by a stream of air or nitrogen that is cooled to approximately

-90°C and blown to the fluidized-bed dryer. In a continuous operation, the liquid spray

would be frozen by the cold gas stream from cooler I and fed at the top of the spray dryer, whereas the bed of fluidized frozen droplets would be dried by another gas stream supplied to the fluidized-bed dryer from cooler II.

• SF-D into liquids (SFL): Here, a solution is atomized into a cryogenic medium such as liquid nitrogen to freeze the droplets. The dispersion of frozen droplets in the cryogenic medium is then separated and dried in a freeze-dryer at atmospheric pressure or under vacuum.

SFL

• SF-D into gases (SFG): Here, the solution is atomized into refrigerated air (gas), so that the droplets freeze. The frozen microspheres are then freeze-dried in a classical freeze-dryer or in atmospheric freeze-dryers.

• SF-D into gases over a fluidized bed (SFG-FB): Here, the particles frozen in cooled gas are then transported to the drying chamber and freeze-dried in a fluidized bed.

INDUSTRIAL SPRAY DRYING SYSTEMS

INDUSTRIAL SPRAY DRYING SYSTEMS :

• The demand for tailor-made agglomerated powdered foodstuffs throughout the industrial world is of great today. And among all other drying technologies, spray drying offers the best solutions.

• By combining atomization, fluidization and agglomeration technologies in advanced spray drier designs, it is possible to meet the end product quality specifications that ensure nutritive advantage and consumer acceptance within a safe, hygienic and environmentally friendly process. 

Uses of spray drying technologies :

• Baby food Lactose• Butter Buttermilk• Carbohydrates Casein• Caseinates Cheese• Coconut milk• Coffee and coffee whitener• Corn syrup Dextrose• Eggs Fructose• Fruit juices with filler High fat powders• Milk: Flavoured, Sweetened condensed, whole,

skim, etc.

• Ingredients / ready mixes• Ice cream mix Malt extract• Maltodextrin Milk replacer• Mother liquor Sorbitol• Soup mixes Soy isolate• Soy sauce Tea• Starch / sweeteners Tomato paste• Total sugar Vegetable protein• Vegetable purees with filler• Whey : acid, sweet Whey permeate

Principle :

• The spray drying process transforms a pumpable fluid feed into a dried product in a single operation.

• The fluid is atomized using a rotating wheel or a nozzle and the spray of drop lets immediately comes into contact with a flow of hot drying medium, usually air. The resulting rapid evaporation maintains a low droplet temperature so that high drying air temperatures can be applied without affecting the product.

Spray Drying Process

• Spray drying consists of three process stages:

1. Atomization2. Spray–air mixing and moisture evaporation3. Separation of dry product from the exit air

• Atomization: Atomization is the most important operation in the spray drying process.

• The type of atomizer not only determines the energy required to form the spray but also the size and size distribution of the drops and their speed, on which the final particle size depends.

• Three general types of atomizers are available. The most commonly used are the rotary wheel atomizers and the pressure nozzle single-fluid atomizers. Pneumatic two-fluid nozzles can also be used in very special applications.

• Wheel atomizer :

Liquid is fed into the centre of a rotating wheel

The spray angle is about 1800

Linear peripheral speed ranges from 100 to 200 m/s

angular speeds between 10,000 and 30,000 rpm

• Pressure Nozzle:

Sometimes called a single-fluid nozzle, creates spray as a consequence of pressure of 5-7MPa.

Angle varies from 40 to 1400

Orifice diameter is usually 0.4 to 4 mm,

• Pneumatic atomizers:

Also known as two-fluid nozzles as they use compressed air or steam to atomize the fluid.

Spray angle ranges from 20 to 600

Approximately, 0.5 m3 of compressed air is needed to atomize 1 kg of fluid

• Air droplet contact system:

• very important in determining the final product quality and design must prevent the local overheating due to reverse flow of wet particles into the hot air area.

• Three main types :a) co-currentb) counter currentc) mixed flow

Hot air distribution layout

Temperature Data in the Drying Chamber

Powder Separators :

• Dry powder:Cyclones, bag filters, electrostatic precipitators.

• Wet powder: Wet scrubbers, wet cyclones, irrigated fans.

Dry separators are used for the principal dry product separation and collection while wet separators are used for the final air cleaning and hence are situated after dry collectors.

• Cyclone: • centrifugal force is

employed to move the particle toward the wall and separate them from the air core around the axis.

• Air and particle swirl in a spiral down the cyclone, where the particles collect and leave the cyclone

• Bag Filter:• The airflow containing dry particles passes

through a woven fabric, powder is collected on one side of the fabric and the air leaves on the other.

• A modern unit consists of several bags installed in a bag house. The fabric used in a bag filter is selected in accordance with the characteristics of the dry product and the air temperature.

Cyclone efficiency curve (Theoretical and actual)

Bag filter efficiency curve

Wet scrubber efficiency curve

• Wet Scrubber:

It is used following dry collectors.

The particles are separated from air by contacting it with a liquid, usually water.

Mostly venturi wet scrubber (See Fig.) is preferred because it offers easy cleaning and maintenance.

New Developments in spray drying:

(A) Superheated stem spray drying :Here superheated steam is used as heating medium.

Advantages:• No fire and explosion hazards• No oxidative damage• Ability to operate at vacuum and high operating

pressure conditions• Ease of recovery of latent heat supplied for evaporation• Better quality product under certain conditions• Closed system operation to minimize air pollution

Limitations of steam drying:

• Higher product-temperature• Higher capital costs compared to hot-air

drying• Possibility of air infiltration making heat

recovery from exhaust steam difficult by compression or condensation.

(B) Two stage horizontal spray dryer:

• To reduce difficulties in scale-up, a horizontal spray dryer has been suggested as an alternative to the conventional vertical types.

• Commercial applications for egg, albumin, whole egg powder, cheese powder, skim milk, whey protein, etc. Other applications that are feasible includes variety of dairy products, fish products, meat products and vegetable products (soya milk, soya protein, chocolate, enzymes, glucose, etc.).

Two stage horizontal spray dryer

(C) Low Humidity Spray Drying:

In case of spray drying it uses high temperature gas as the drying medium and there are chances of the deposits on the wall in practical conditions.

So, chances of degradation by overheating. Thus sometimes users have to select freeze

dryers rather than spray dryers, especially for drying biochemical and other heat sensitive materials.

• But, Freeze drying costs 5–10 times higher than hot-air drying, such as spray drying.

• So, to reduce degradation of active components in dried powders, it has been proposed a new type of spray dryer, i.e., low dew point (LDP) spray dryer, which uses LDP air at near ambient temperature to 800 C, but with very low humidity.

Low dew point spray dryer

(D) Encapsulation :

Microencapsulation is defined as a process by which one material or a mixture of materials is coated or entrapped within another material or system.

This process is commonly used to protect a core material from degradation, to control the release of a core material, or to separate reactive components within a formulation.

It is simple and similar to the one-stage spray drying process.

Coated material is called the active or core material, and the coating material is called the shell, wall material, carrier or encapsulant.

The active material to be encapsulated such as an oil or flavour in an oil base is dispersed in a hydrocolloid carrier, e.g., gelatin, modified starch, dextrin or Maltodextrin, or gum arabic.

After the emulsifier is added, the mixture must be homogenized to form an oil-in-water emulsion and then it is fed to the atomizer for spray drying.

In the dryer chamber, the aqueous phase dries and the active material is entrapped as particles within the hydrocolloid or protein film.

The active material from the capsule is released under specified conditions. The main controlling factors of its release are temperature, moisture, and pressure.

SONIC DRYING

Characteristic of sound :

• Sound is a special form of energy transmitted through pressure fluctuations in air, water, or other elastic media. Any displacement of a particle of this elastic medium from its mean position results in an instantaneous increase in pressure.

• When levelling, this pressure peak not only restores the particle to its original position but also passes on the disturbance to the next particle.

• The cycles of pressure increase (compression) and decrease (rarefaction) propagate through the medium as a sound wave. Sound is characterized by pressure and particle velocity. The product of these two parameters is called sound intensity.

• Sound is primarily characterized by the frequency (f) which relates the speed of wave propagation (u) (sound velocity) to the wavelength (λ).

Sound generation :• In industrial applications, sound waves are

generated by a transducer which converts the original form of energy to the energy of oscillatory motion. It is done by,

• Piezoelectric, Electrostatic, Magnetostrictive, electromagnetic, Mechanical & miscellaneous such as Thermal, chemical, optical and other phenomena are there.

• Because the sound pressure depends on the distance from the source generating given sound power (energy per unit time) and on the acoustic environment (sound field), it is then frequently quantified in terms of the sound intensity.

• One of the possible designs of industrial sound generators is shown in next slide. When rotated at 4000–9000 rpm and fed with 0.138 m3 /s of compressed air at 0.3–0.5 MPa, this dynamic siren (d = 0.2 m) emits sound up to 180 dB and 8 kW of acoustic power.

Design of Pneumatic sound generator

Branson sound generatorDynamic siren sound

generator

• In processing of liquid systems such as dewatering of slurries and pasty materials, piezoelectric or magnetostrictive generators are used.

• The active part of such a generator (called a driver) generates and transmits mechanical vibrations through a solid rod (a booster) to the metal profile (a horn).

• The horn serves as an amplitude transformer to amplify the displacement of the driver and to match the transducer impedance with the impedance of the material to be processed (as shown in below figure).

• The mechanical vibrations from the horn are further coupled to the processed material either directly (the horn is inserted into a liquid or pasty material) or indirectly through a membrane.

Ultrasonic generator for operation in liquid and pasty materials

Mechanism of sonic drying:

Alternating compression and expansion due to high-frequency pressure pulsation create surface cavitations that break the boundary layer and allows liquid to evaporate under partial vacuum.

Intensive circulation flows (induced by sound pressure) on the drying surface promote surface evaporation.

A pulsating partial vacuum transmitted into the material affects water vapor transport, possibly by decreasing or overcoming the attraction forces between the water and solid molecules.

Expansion of the vapor bubbles inside capillaries yields a migration of the water filament (sonic diffusion current).

An increase in moisture diffusivity and a decrease in viscosity.

Sound assisted spray dryer

• A standard spray dryer with a tangential air inlet is equipped with two sirens situated at the spray cone level.

• The combined action of the sound energy and the air streams from the sirens activated by compressed air not only improves the dispersion of the solid–liquid spray but also enhances heat and mass transfer rates, which finally allows for the product of required quality.

• Sound energy is used to homogenize the mixture of the liquid biomaterial and the solid sorbent-carrier is shown in figure.

Sound Assisted Fluid Bed DryerSound Assisted Fluid Bed Dryer

• Fluidization of a particulate material takes place due to the combined action of the sound energy from the static siren located below the supporting grid and the air stream that excites the siren.

• An interesting feature of this design is the use of two additional sirens located at the sidewall and at the top of the drying chamber to reduce carryover of fines.

FREEZE DRYING

• Used for heat sensitive products.• In freeze drying, the water or another solvent

is removed as a vapor by sublimation from the frozen material in a vacuum chamber. After the solvent sublimes to a vapor, it is removed from the drying chamber where the drying process occurs.

• Freeze drying of food and biological materials also has the advantage of little loss of flavour and aroma.

• The low processing temperatures, the relative absence of liquid water and the rapid transition of any local region of the material dried from a fully hydrated to a nearly completely dehydrated state minimize the degradative reactions that normally occur in ordinary drying processes, such as non enzymatic browning, protein denaturation, and enzymatic reactions.

• Uses:

Nonliving matter, foodstuffs, superconducting materials.

Living cells destined to remain viable for longer periods of time such as bacteria, yeasts, and viruses.

Freeze Drying Process :

•  The freeze drying separation method (process) involves the following three stages:

(A) The freezing stage, (B) The primary drying stage and (C) The secondary drying stage.

(A) Freezing Stage:

The material system to be processed (e.g., gel suspension, liquid solution or foodstuff) is cooled down to a temperature (this temperature depends on the nature of the product) that is always below the solidification temperature of the material system.

At the end of the freezing stage about 65–90% of the initial (at the start of the freezing stage) water is in the frozen state and the remaining 10–35% of the initial water is in the sorbed (non frozen) state.

(B) Primary Drying Stage:

The frozen solvent is removed by sublimation, this requires that the pressure of the system (freeze dryer) at which the product is dried must be less than or near to the equilibrium vapor pressure of the frozen solvent.

If, for example, frozen pure water (ice) is processed, then sublimation of pure water at or near 00 C and at an absolute pressure of 4.58 mm Hg could occur. But, since the water usually exists in a combined state or a solution, the material must be cooled below 00 C to keep the water in the frozen state.

(C) Secondary Drying Stage:

It involves the removal of solvent (water) that did not freeze (this is termed sorbed or bound water).

In the secondary drying stage, the bound water is removed by heating the product under vacuum and the heat is supplied to the product usually by conduction, convection, or radiation.

The following product temperatures are usually employed:

(a) Between 100 C and 350 C for heat-sensitive products and (b) 500 C or more for less-heat-sensitive products.

INDUS(A) Tunnel Freeze Dryer:

The freeze drying process takes place in a large vacuum cabinet into which the tray-carrying trolleys are loaded at intervals through a large vacuum lock at one end of the tunnel and discharged similarly at the other end.

(B) Vacuum Spray Freeze Dryers:

The product is sprayed from a single jet upward or downward in a cylindrical tower of 3.7 m diameter and 5.5 m high. The liquids solidify into small particles by evaporative freezing.

The product passes into a hopper that feeds a vacuum lock, permitting intermittent removal of the product for packing.

• The whole plant operates under a vacuum of about 67 Pa.

• Frozen particles obtained by spraying into a vacuum are about 150 μm in diameter and lose about 15% moisture in the initial evaporation.

• There is no sticking of these particles.

SLUSH DRYING

• Fresh fruit juices are complex aqueous slurries (80–90% by weight of water) containing numerous organic compounds.

• Most juices are very heat-sensitive because of the potential for enzymatic reactions and the volatilization of flavour and aroma components.

• To handle the dehydration of fruit juices, a technology called slush drying was proposed and tested with apple juice for the potential loss of volatile flavour and aroma substances.

• The principle of this method stems from the dependence of the freezing point on the concentration of dissolved solids.

• The principle of this method stems from the dependence of the freezing point on the concentration of dissolved solids.

• Slush drying is somewhat analogous to freeze concentration except the stage in which separating the ice from the supernatant liquid in slush drying is accomplished by sublimation of the ice along with evaporation of some of the water from the liquid concentrate.

• An interesting phenomenon observed in slush drying of apple juices was the formation of stable and coarse foam over the surface of the slush bed.

• The amount and durability of the foam increased with increasing initial content of the dissolved solids.

• The formation of sugar-rich foam may explain the selective retention of aroma in slush drying, as sugars are known to trap volatile flavours.

• In slush drying we can get relatively higher drying rate while it is high in Freeze drying of fruit juices and other sugar containing substances because of the need for holding very low frozen-zone temperatures to achieve adequate solidification during drying.

GRAIN DRYING

• Crop Conditioning:

Crop moisture reduction methods usually called “crop conditioning”, as they are sometimes used in place of or in combination with heated-air drying.

There are basically four methods.

1)Aeration2)Natural air drying3)In-Storage drying with Supplemental heat4)Multistage Drying :

(a) Dryeration and (b) Combination Drying

1) Aeration:

Aeration consists essentially of moving small amounts of unheated air through a pile of grain to equalize the grain temperature and to prevent moisture migration in bins exposed to drastic changes in ambient temperature.

The recommended airflow rate for normal aeration of shelled corn, soybeans, and small grains at 125 Pa (0.5 in. of water) is 5 m3 /h per m3 of grain.

2) Natural Air Drying:

Natural-air drying employs a similar setup as aeration but higher airflow rates than those used for aeration.

Typical rates for a storage depth of 1.2 to 1.8 m (4 to 6 ft) of small grains, peas and beans, and shelled and ear corn are respectively, 150 to 250 m3 /h per m3 of grain (3 to 5 cfm/bu) and 250 to 500 m3/h per m3 of grain (5 to 10 cfm/bu).

3) In-Storage drying with Supplemental heat :

In-storage drying with supplemental heat involves drying of a relatively large batch of grain in situ (i.e., in the storage bin).

Ventilation is accomplished by blowing slightly heated air, 4–120 C (7–228F) above ambient temperature through a duct system or through one centrally placed cylinder as is the case for batch drying.

4) Multistage Drying :

The term multistage drying refers to any process that uses high-temperature drying in combination with aeration or natural-air drying.

(a) Dryeration and (b) Combination Drying

(a) Dryeration:

Dryeration is the term referring to the two-stage process by which grain is dried in a heated-air dryer to within about 2% of its ‘‘dry’’ moisture content and then moved to and stored in an aerating bin for about 10 h. This allows time for moisture within the kernels to move to the outside for easier removal

Advantages of Dryeration:

The ability to use higher drying temperatures as the grain does not remain in the high-temperature dryer until it is completely dry.

Capacity increases of up to 60% of the grain drying system are possible as no cooling time in a high-temperature dryer is required.

The grain quality is improved by cooling the grain immediately after it comes out of the dryer.

(b) Combination Drying:

Combination drying is an extension of the dryeration process and is used primarily for drying grains with very high harvest moisture (>2 5%).

A high temperature dryer is used to reduce the grain moisture content to about 19–23%.

The grain is then moved to a bin dryer in which drying is completed using natural air or supplemental heat.

• With this method, the output of the high-temperature dryer is increased to two or three times that obtained when it is used for complete drying. In addition, energy requirements may be reduced by as much as 50%.