drying methods

Drying                                                                                                                                  

Drying is one of the oldest methods of preserving food. Primitive societies practised 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 occurring. The principal 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 and 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. For example in the baking of bread, application of heat expands gases, changes the structure of the protein and starch and dries the loaf. 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 that enter into the unit operation of drying:  

(a) Transfer of heat to provide the necessary latent heat of vaporization,

(b) Movement of water or water vapours through the food material and then away from it to effect separation of water from foodstuff.

Methods of drying:

Sun Drying                                                                                                                                  Sun drying works best when the temperature is in the 90s, the humidity is low, and air pollution levels are low. A major advantage to sun drying is its low cost. Drying trays, netting to protect against bugs and food to dry are our only investments. Sun drying makes you dependent on the weather, however if it is sunny one day and cloudy the next, you will have to finish drying by another method. That’s because spoilage can occur while the drying food still has enough moisture for microbial growth. Also, you should bring the food inside on cool nights. Another disadvantage is time. What dries in 6 to 8 hours in an electric dehydrator may take 2 to 4 days in the sun.

Solar Drying

Solar drying is like sun drying, only better. The sun rays collect in a solar box so that, compared with sun drying, drying temperature is higher and drying time is shorter. The shorter drying time gives microorganisms less chance to cause spoilage. If you do not want to buy or build a solar box, you can use the back window ledge of an automobile where the sun shines through. Crack the windows slightly to allow air flow so temperatures do not get too hot. Cover the trays with netting to keep bugs out.

Oven Drying

You can use your oven to dry small amounts of food at one time. You’ll have little or no investment in equipment and you won’t have to depend on the weather. Although oven drying produces a safe, generally tasty product, don’t expect top quality. Oven-dried food is more brittle and usually darker and less flavorful than food dried in a dehydrator. Another disadvantage of oven drying is its energy cost. Oven drying takes two or three times longer than drying in a dehydrator. Before drying in an oven, test the oven temperature with an oven thermometer for about 1 hour. Prop open the oven door as you would when actually drying fruit. The oven should maintain a temperature of 130° to 150°F. If the oven cannot maintain a temperature in this range, you will not have high-quality dried food. If the oven is too hot, your food will begin to cook instead of dry. If it is too cool, your food may not dry fast enough and spoil instead.

Hot air drying

Heat from drying air is absorbed by food and provides the latent heat needed to evaporate water from the surface. The temperature of the air, measured by a thermometer bulb, is termed the dry-bulb temperature. An increase in air temperature, or reduction in RH, causes water to evaporate more rapidly from a wet surface and therefore high drying rates are obtained. The third factor that controls the rate of drying, in addition to air temperature and humidity, is the air velocity. When hot air is blown over a wet food, water vapour diffuses through a boundary film of air surrounding the food and is carried away by the moving air. A water vapour pressure gradient is established from the moist interior of the food to the dry air. This gradient provides the ‘driving force for water removal from the food. In summary, the three characteristics of air that are necessary for successful drying when the food is moist are:

1. A moderately high dry-bulb temperature

2. A low RH

3. A high air velocity.

Freeze drying

In freeze drying the firstly food is frozen in conventional freezing equipment. Small pieces of food are frozen rapidly to produce small ice crystals and to reduce damage to the cell structure of the food. In liquid foods, slow freezing is used to form an ice crystal lattice, which provides channels for the movement of water vapour. The next stage is to remove water during subsequent drying and hence dry the food. If the water vapour pressure of a food is held below 4.58 Torr (610.5 Pa) and the water is frozen, when the food is heated the solid ice sublimes directly to vapour without melting. The water vapour is continuously removed from the food by keeping the pressure in the freeze drier cabinet below the vapour pressure at the surface of the ice, removing vapour with a vacuum pump and condensing it on refrigeration coils. As drying proceeds a sublimation front moves into the frozen food, leaving partly dried food behind it.

Microwave drying

Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.

A microwave oven consists of:

Ø  A high voltage power source, (a simple transformer or an electronic power converter )

Ø  A cavity magnetron, which converts high-voltage electric energy to microwave radiation

Ø  A magnetron control circuit (usually with a microcontroller)

Ø  A waveguide (to control the direction of the microwaves)

Ø  A cooking/drying chamber

Mechanism of Microwave Drying

Microwave drying is based on so called dielectric heating, in which energy is absorbed by ions or molecules that are either induced or permanent dipoles. As the electromagnetic wave travels through the medium the electric and the magnetic field oscillates about zero at each location. The periodic variation of the electric and the magnetic field causes a stress on ions, atoms and molecules, which is converted to heat. When exposed to the microwave field, polar molecules and ions align themselves with the rapidly changing direction of the field, during which heat is generated throughout the material. The energy conversion occurs by two mechanisms ion conduction and dipole rotation, the latter being dominant in most materials. Ion conduction designates the phenomena that ions are accelerated by the electric field. When the ions move they collide with other bodies by which kinetic energy is transferred. The collisions are repeated and extended to other bodies, in the field that is changing polarity many millions of times per second. The electric energy is converted to an ordered kinetic energy which is in turn converted to disordered kinetic energy that can be regarded as heat. In general, polar substances absorbs more energy than nonpolar, however the intensity of vibration depends on the molecular structure, viscosity, temperature and intermolecular bonding. This result in the temperature to rise, some water gets evaporated. Internal heating and evaporation of moisture inside the food materials generates significant pressure. Moisture is then pumped to the surface due to the pressure gradient. Resulting pressure-driven flow becomes an additional mechanism of internal moisture transport.

Microwave vacuum drying

Vacuum-microwave drying is a novel alternative method of drying, allowing to obtain products of acceptable quality. Microwave-vacuum drying combines the advantages of both microwave heating and vacuum drying. The low temperature and fast mass transfer conferred by vacuum combined with rapid energy transfer by microwave heating generates very rapid, low temperature drying and thus it has the potential to improve energy efficiency and product quality.

It permits a shorter drying time and a substantial improvement in the quality of dried materials, in relation to those dried with hot air and microwaves drying methods. Its main features are

Ø  Reduced evaporation temperature

Ø  Shorter drying time

Ø  Better product quality

PEF most suitable Drying pretreatment

Pretreatment

Thermal dehydration (drying) is the most popular and efficient way to reduce moisture content and preserve foods. However, drying is a high energy consuming process. Besides, the product quality depends notably on its texture, colour, and flavour and deteriorates on drying. Pretreatments are recommended techniques used to reduce energy consumption, time of operation and to make quality products. Pretreatments not only prevent darkening and improve quality; they also cause the destruc­tion of pathogens that could cause food borne illness, like Escherichia coli O157:H7, Salmonella species, and Listeria monocytogenes. Pretreatments include dipping, blanching, cooking, or candying. Some other techniques, such as mechanical expression (pressing) and osmotic dehydration, can be used separately or combined with drying in order to remove moisture at lower energy consumption and with better quality retention. Unfortunately, moisture transfer is slow through the semi- permeable membranes of cells that remain intact.

Following pretreatments have been applied successfully to accelerate drying.

1. Blanching
2. Dipping
3. Osmotic dewatering
4. Microwave treatment
5. Ultrasound treatment
6. Pulsed electric field

1.                           Blanching: Blanching is a method of pretreating fruits and vegetables by heating them before drying. Blanching is one of the most widely used pretreatment techniques in the food industry. It involves heating the product to a high temperature, below water boiling point, for some minutes in order to inactivate enzymes, cause tissue softening, and minimize color and flavor loss during drying and storage. However, loss of some nutrients such as water soluble vitamins and solids like sugars, amino acids, and minerals could occur during blanching.. There are two methods of blanching.

(a)                Water Blanching- In this vegetables are submerged in boiling water for a set amount of time.

(b)      Steam Blanching- In this, vegetables are suspended over the boiling water and steamed   

      for a certain length of time.

2.               Dipping: In this treatment products are dipped in salt or sugar or some other solutions. This helps in preserving the sensory and nutritional qualities of product during the convective drying of products like fruits and vegetables.

3.               Osmotic dehydration: Osmotic dehydration is a complex dynamic mass transfer process. By putting vegetables and fruit pieces into highly osmotic solution, the water in the cells of the materials permeates into the osmotic solution through the cell membrane due to the high osmotic pressure and low water activity of the osmotic solution. This osmotic dehydration process can reduce the water content of vegetables and fruits by 50%. Since osmotic dehydration is more economical than thermal drying, it is often used as a pretreatment for drying of biological materials. Osmotic dehydration process is affected by the concentration and the temperature of osmotic solution, the type of osmotic agent, the size of the bio-material and the mixing intensity, etc. In the experiments with apple and kiwi, water loss and solute gain were found to increase with the concentration of osmotic solution.

4.               Microwave treatment:  Application of microwave heating as pretreatment for drying operation has been reported in the literature. The unique heating mechanism of microwave, which involves heating a food material inside out based on its dielectric properties, makes it applicable in drying operation. The application could be at various stages of drying such as pre, post, and during drying. Subjecting a moist material to microwave energy causes the inner moisture to get heated up and migrate to the surface due to pressure differential for subsequent drying by other methods such as vacuum or hot air drying. Structural modification has also been reported in microwave heating for drying. The added advantage of this is quick drying and reduction in shrinkage.

5.       Ultrasound treatment: High power ultrasound represents a means for food dehydration without affecting the main characteristics and quality of the product. The application of ultrasonic energy can be made alone or in combination with other kind of energies such as hot-air. In this latter case ultrasound helps in reducing temperature or treatment time. The application of high power ultrasound for dehydration of porous materials may be very effective in processes in which heat-sensitive materials such as foodstuff have to be treated. In fact, high-intensity ultrasonic vibrations are capable of affecting mass transfer processes with the result of increasing the drying rate of materials. In this way the ultrasonically assisted hot-air drying process may permit the use of lower temperatures or shorter treatment times. As a consequence, this process may be useful for vegetal dehydration without affecting their main characteristics and quality.

   6.      Pulsed Electric Field Processing

Pulsed electric field (PEF) processing is a non-thermal method of food preservation that uses short bursts of electricity for microbial inactivation and cell membrane breakdown but causes minimal or no detrimental effect on food quality attributes. PEF processing offers high quality fresh-like liquid foods with excellent flavor, nutritional value, and shelf-life. Since it preserves foods without using heat, foods treated this way retain their fresh aroma, taste, and appearance

How does PEF work in drying?

PEF processing involves treating foods placed between electrodes by high voltage pulses (usually for a couple of microseconds). The electric field may be applied in the form of exponentially decaying, square wave, bipolar, or oscillatory pulses and at ambient, sub-ambient, or slightly above-ambient temperature. The applied high voltage results in an electric field that causes microbial inactivation and cell membrane breakdown. When the intensity of electric field exceeds a critical value (U₀), irreversible damage of cells occurs. This phenomenon results in increased porosity either by enlargement of existing pores and/or by creation of new ones. Resulted increased permeability can be exploited to increase drying rates as area available for mass (moisture) transfer is more.

Fig: 1- The above diagram shows the phenomenon of how PEF generates pore space in a plant cell.

Experimental Setup: The pulsed electric field treatment of was carried  out by using an exponential decay pulsed electric field generator (circuit diagram shown in Fig. 1. A variable autotransformer AT was used to supply voltage to the circuit. The input voltage is regulated by the autotransformer to obtain a pulse frequency of 1 Hz. The voltage is then stepped up by a high-voltage transformer T. The resistor is used to limit current that passes through the capacitor Ci. The initial treatment voltage supplying the treatment chamber depends on the distance between the spheres of the discharger. The discharger was made from 15-mm-diameter stainless steel spheres. The break voltage V0 for this diameter can be adjusted. The treatment chamber shown in Fig. 2 was used for pulsed electric field treatment of the samples. The samples were placed between the two electrodes, where one of the electrodes is assembled on the base plate A and other under the top plate to hold the sample in place. The electrodes were made from stainless steel and the plates from Teflon. The weight on top of the plate was used to keep the same force applied to all the samples. A frame  was used to fix the two plates intact to prevent movement.

Product

Fig: 2- Electric circuit diagram for exponential decay pulsed electric field generator.

Process variables:

Ø  Electric field: The space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. This electric field exerts a force on other electrically charged objects. The electric field is a vector field with SI units of Newton per coulomb (N C⁻¹) or, equivalently, volts per metre (V m⁻¹).

Ø  Electric Potential: At a point in space, the electric potential (also called the "electrostatic potential") is potential energy divided by charge that is associated with a static (time-invariant) electric field. It is a scalar quantity, typically measured in volts.

Ø  Capacitance: Capacitance is the ability of a body to hold an electrical charge. Capacitance is also a measure of the amount of electric charge stored (or separated) for a given electric potential. The SI unit of capacitance is the farad; 1 farad = 1 coulomb per volt

Degree of electroporation and cell disintegration depends on following factors

1. Product  characteristics
2. Intensity  of the electric field
3. Type  of pulse waveform
4. Number of pulses
5. Treatment time