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.
- Blanching
- Dipping
- Osmotic dewatering
- Microwave treatment
- Ultrasound treatment
- 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
- Product characteristics
- Intensity of the electric field
- Type of pulse waveform
- Number of pulses
- Treatment time
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