Dehydration of fruits and vegetables

Introduction

• Drying is one of the oldest methods of preserving food. Primitive societies practiced the drying of meat and fish in the sun long before recorded history.

• Today the drying of foods is still important as a method of preservation.

• Dried foods can be stored for long periods without deterioration.

The main reasons for this are that the microorganisms which cause food spoilage and decay are unable to grow and multiply in the absence of sufficient water

Many of the enzymes which promote undesired changes in the chemical composition of the food cannot function without water.

Preservation is the principal reason for drying, but drying can also occur in conjunction with other processing.

Losses of moisture may also occur when they are not desired,

Drying of foods implies the removal of water from the foodstuff.

In most cases, drying is accomplished by vaporizing the water that is contained in the food, and to do this the latent heat of vaporization must be supplied.

There are, thus, two important process-controlling factors:

(a) transfer of heat to provide the necessary latent heat of vaporization,(b) movement of water or water vapour through the food material.

Drying processes fall into three categories

1. Air and contact drying under atmospheric pressure. In air and contact drying, heat is transferred through the foodstuff either from heated air or from heated surfaces. The water vapour is removed with the air.

2. Vacuum drying. In vacuum drying, advantage is taken of the fact that evaporation of water occurs more readily at lower pressures than at higher ones. Heat transfer in vacuum drying is generally by conduction, sometimes by radiation.

3. Freeze drying. In freeze drying, the water vapour is sublimed off frozen food. The food structure is better maintained under these conditions. Suitable temperatures and pressures must be established in the dryer to ensure that sublimation occurs.

Air Drying

• In air drying, the rate of removal of water depends on the conditions of the air, the properties of the food and the design of the dryer.

• Moisture can be held in varying degrees of bonding such as free water or bound water.

• In drying, it is expected that the water that is loosely held will be removed most easily.

• Thus it would be expected that drying rates would decrease as moisture content decreases, with the remaining water being bound more and more strongly as its quantity decreases.

CONDUCTION DRYING

• Other methods of drying which are quite commonly encountered are drying by contact with a hot surface; a continuous version of this is the drum or roller dryer where the food is coated as a thin paste over the surface of a slowly revolving heated horizontal cylinder.

In such a case, the food dries for as much of one revolution of the cylinder as is mechanically feasible, after which it is scraped off and replaced by fresh wet material.

DRYING EQUIPMENTS       Tray dryer      Drum dryer      Spray dryer      Tunnel dryer      Fluidized bed dryer      Freeze dryer  

Tray driers

Insulated cabinet fitted with perforated trays

The food is spread out, generally quite thinly, on trays in which the drying takes place.

Hot air is blown to promote uniform air distribution through each tray

Additional heaters placed above the trays to increase the rate of drying

Used for small scale production

Low capital and maintenance costs

Relatively poor control and produce more variable product quality as food dries more rapidly on trays nearest to the heat source

Tray Dryer

Tunnel dryer

• Layers of food are dried on trays which are stacked on trucks

• Trucks move semi continuously through an insulated tunnel

• Typically, a 20 m tunnel contains 12-15 trucks with a capacity of 5000 kg

• Types of air flow– Parallel or co-current type– Counter current– Cross flow

Type of air flow

Advantages Disadvantages

Parallel Rapid initial drying

Little shrinkage

Low bulk density

Less damage

No risk of spoilage

Low moisture content difficult to achieve as cool moist air passes over dry food

Counter current

More economical use of energy

Low final moisture content as hot air passes over dry food

Food shrinkage and possible heat damage

Risk of spoilage from warm moist air meeting wet food

Cross flow Uniform drying

High drying rates

More complex

Expensive to buy, operate and maintain

Drum dryer or roller dryer• Heated surface drying or Contact drying method

• Heat is supplied by conduction

• Not necessary to heat large volumes of air

• Thermal efficiency is high

• Carried out in the absence of O2

• Heat consumption is less compared to hot air driers – i.e 2000-3000 KJ/kg of water evaporated

• Thin layer of food is needed to conduct heat rapidly

• Consists of slowly rotating hollow steel drum

• Heated internally pressurized steam to 120-170°C

• Thin layer of food is spread uniformly over the surface – By dipping– By spraying– By feed rollers

• Time taken to complete one revolution ranges between 20s-3 min

• Dried material is scraped by a doctor blade which contacts the drum surface along its length

Internal drum view

Type of drum dryer

Advantages

• High drying rates

• High energy efficiency

• Suitable for slurries in which particles are too large for spray drying

Disadvantages

• High capital cost of the machined drums

• Damage to heat sensitive foods – Leads to move to spray drying for many bulk

dried foods

Products

• Potato flakes

• Pre cooked cereals

• Dried soups

• Fruit purees

Types

• Single drum

• Double drum or twin drum– Clearance between is 0.25mm to 3 mm to

regulate the film thickness– Rotates in opposite direction – Efficient operation would not allow any

material to remain on the drum surface– Free from any deposits before operation

Process variables

• Solids content of feed

• Speed of drum – to control residence time

• Temperature of drum surface

• Roughness of the drum surface

• Adjustment of doctor blade

Advantages of single drum

• Greater flexibility

• More surface area available for drying

• Easier access for maintenance

• No risk of damage

Developments

• Use of auxiliary rolls to remove and reapply food

• Use of high velocity air to increase the drying rate

• Use of chilled air to cool the product

• May be enclosed in vacuum chamber to dry food at lower temperature

All the above will improve the sensory and nutritional qualities of food

Fluidized Bed Dryers

In a fluidized bed dryer, the food material is maintained suspended against gravity in an upward-flowing air stream.

Heat is transferred from the air to the food material, mostly by convection

Dryers are compact and good control over drying conditions

High drying rates

Features

• Distributor– To evenly distribute the air at a uniform velocity

around the bed of material

• Plenum chamber– To produce an homogeneous region of air and

prevent localized high velocity

• Cyclone– To separate out fine particles

Developments of fluidized bed drier• Torbed drier

– Fluidized bed of particles is made to rotate around a torus shaped chamber by hot air blown directly from a burner

– High rates of heat and mass transfer

– Reduced drying time

– Suitable for agglomeration

• Spin flash drier

– Drying chamber fitted with rotor at the base– Hot air enters tangentially, causes a turbulent rotating flow of air with the action of rotor

• Centrifugal fluidised be dryer

– Hot air is forced through the bed of food at a velocity high enough to overcome the centrifugal force and fluidize the particles

– Rate of drying is high

Spray drying• Feed material is in the form of a liquid solution, capable of being dispersed

into a fluid like spray

• Fluid is atomized as fine droplets

• Comes in contact with a flow of hot air

• Dryer body is large so that the particles can settle, as they dry, without touching the walls

• Droplets provide an extensive surface area for heat and mass transfer

• Suitable for heat sensitive products

• Sensible heat losses from the hot air provide the latent heat for evaporating liquid from the product

• Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

Advantages

• Maintains quality and functional properties of the product

• Relative simplicity and ease of control of the system

• Preservation of most volatiles

• Energy usage comparable to that of other drying methods is less

Disadvantages

• High initial investment

• Hard to control final particle size

• Requires a pumpable feed

• Problems with product and dust recovery

Spray drying involves…

• Atomization

• Droplet-air mixing

• Evaporation

• Recovery of dried product

Types of atomiser

• Centrifugal atomizer– Liquid is fed to the centre of a rotating disc (90-200 m/s)– Droplets size 50-60um in dia

• Pressure nozzle atomizer– Liquid is forced at high pressure thro’ a small aperture– Droplet size 180-250um

• Two fluid nozzle atomizer– Compressed air creates turbulence which atomizes the liquid– Operating pressure is lower than pressure nozzle– Wider range of droplet size is produced

• Atomization is critical step

• Achieved by pressure nozzle, rotary wheel or pneumatic nozzle

• Atomizer controls droplets size, size distribution and speed that in turn determine the final character of the dry particle

• Contact between drying air and spray droplets can be conducted either a co-current, countercurrent or combined as mixed flow systems

• Cyclones are widely used in product recovery as they are highly efficient, require low maintenance and are easily cleaned

1-reservoir 2- feed pump 3-prodcut feed pipe line 4- atomizer 5- drying chamber 6-air fan 7- air heater 8-hot air duct 9-mixture of dried product &air carrying duct 10-cyclone separator 11-heavy powder 12 – product tank 13-exhaust air

Schematic Diagram of Spray Dryer

Air flow in the chamber

Freeze Dryers• The material is held on shelves or belts in a chamber that is under

high vacuum. In most cases, the food is frozen before being loaded into the dryer

• Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed.

• The pieces of food are shaped so as to present the largest possible flat surface to the expanded metal and the plates to obtain good heat transfer.

• A refrigerated condenser may be used to condense the water vapour.

Lab model freeze Dryer

Purpose

• Application of heat under controlled conditions to remove water from food

• To extend shelf life of foods by a reduction in water activity

• To inhibit microbial growth and enzyme activity

• To reduce transport and storage costs

• To provide greater variety and convenience of the consumer

Pneumatic Dryers

In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension.

Heated air accomplishes the drying and often some form of classifying device is included in the equipment.

In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying

Rotary Dryers• The foodstuff is contained in a horizontal inclined cylinder

through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls.

• In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.