Drying Technology - PDF - PDF

Document Sample
Drying Technology - PDF - PDF Powered By Docstoc
					Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


    Microwave Drying Technology-Recent Developments and R&D Needs in India

                                   P.P.Sutar and Suresh Prasad

                        Agricultural and Food Engineering Department

                       Indian Institute of Technology, Kharagpur- 721302

Abstract

        This article presents an overview of the microwave drying technology as well as it reviews
the recent developments in microwave assisted drying technologies and future R&D needs in India.
Recently, microwave convective and microwave vacuum drying techniques have been investigated
as potential methods for obtaining high quality dehydrated food products. Microwave drying is
rapid, uniform and energy efficient compared to conventional hot air drying as the microwaves
penetrate to the interior of the food causing water to get heated within the food. This results in a
greatly increased vapour pressure differential between the center and surface of the product,
allowing rapid removal of moisture from the food. Some fruits and vegetables have been
successfully dried by microwave convective and microwave vacuum drying techniques. But a
large-scale industrial application of microwave drying requires the detailed knowledge of various
process parameters and proper design of system and equipment. The article includes the microwave
drying characteristics of several food materials, drying process modeling, problems related to
microwave drying, types of microwave dryers and future research needs in microwave drying in
India. The ultimate objective of this article is to contribute in development and evaluation of precise
microwave drying technology with an effective energy saving system that can result in high quality
dehydrated foods.


Keywords: microwave convective, microwave vacuum, review of microwave drying, dehydrated
            products

Introduction

        Microwaves are electromagnetic waves having wavelength (peak to peak distance)
varying from 1millimeter to 1 meter. Frequency of these microwaves lies between 0.3 GHz
and 3 GHz. Microwaves have greater frequency than radio waves so they can be more
tightly concentrated. Microwaves propagate through air and space at about the speed of
light. Microwaves can also be considered as electromagnetic force fields for better
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


understanding of working of microwave oven. Microwaves interfere inside the microwave
oven to produce high and low energy pockets. Application of microwave energy to dry
food materials is a good approach for coping with certain drawbacks of conventional
drying. Microwaves penetrate to interior of the food causing water to get heated within
food. This results in a greatly increased vapour pressure differential between the center and
surface of the product, allowing fast transfer of moisture out of the food. Hence,
microwave drying is rapid, more uniform and energy efficient compared to conventional
hot air drying. The problems in microwave drying, however, include product damage
caused by excessive heating due to poorly controlled heat and mass transfer (Datta, 1990;
Ramaswamy et al., 1991).


Mechanism of Heating
       In microwave heating or drying, microwave-emitted radiation is confined within the
cavity and there is hardly heat loss by conduction or convection so that energy is mainly
absorbed by a wet material placed in the cavity. Furthermore, this energy is principally
absorbed by water in the material, causing temperature to raise, some water to be
evaporated, and moisture level to be reduced. A domestic microwave oven works by
passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of
12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy
from the microwave beam in a process called dielectric heating. Many molecules (such as
water) are electric dipoles, meaning that they have a positive charge at one end and a
negative charge at the other, and therefore rotate as they try to align themselves with the
alternating electric field induced by the microwave beam. This molecular movement
creates heat by friction as the rotating molecules hit other molecules and put them into
motion.
       Microwave heating is most efficient on liquid water, and much less so on fats and
sugars (which have less molecular dipole moment), and frozen water (where the molecules
are not free to rotate). Large industrial/commercial microwave ovens operating in the
900 MHz range also heat water and food perfectly well. The power generated in a material
is proportional to the frequency of the source, the dielectric loss of the material, and the
square of the field strength within it. The microwave heating rates and potential non-
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


uniformity are functions of oven factors and load characteristics (size, shape, dielectric
properties, etc.).

        In conventional heating, heat is transferred to the surface of the material to be
heated by conduction, convection, and/or radiation, and into the interior by thermal
conduction. In contrast, in dielectric heating, heat is generated directly inside the material,
making possible higher heat fluxes and thus a much faster temperature rise than in
conventional heating. However, heat conduction still plays an important role when heating
thick samples by dielectric heating and for equilibrating temperatures when heat generation
is uneven.Depending on water content the depth of initial heat deposition may be several
centimeters or more with microwave ovens, in contrast to grilling, which relies on infrared
radiation, or the thermal convection of a convection oven, which deposit heat shallowly at
the food surface. Depth of penetration of microwaves is dependent on food composition
and the frequency, with lower microwave frequencies being more penetrating. The heat
generated per unit volume of material (Q) is the conversion of electromagnetic energy in to
heat energy. Its relationship with the average electric field intensity (Erms) at that location
can be derived from Maxwell’s equations of electromagnetic waves as shown by Metaxax
and Meredith (1983):

                               Q = 2πf∈0 ε '' E2rms                                   …(1)

        Where, f is the frequency of microwaves, ∈0 is the dielectric constant of the free
space (8.854 × 10−12 A2 s4/kg m3), and ε '' is the loss factor of the food being heated. At a
given frequency, the dielectric loss factor is a function of the composition of the food
materials and its temperature. Penetration depth (Dp) is another important factor in
microwave heating. It is defined as the depth below the surface of the material where the
power density of a plane electromagnetic wave decays by 1/e (37%) from the original value
at the surface. The Dp is calculated as follows:
                                              c
                       Dp =                                                           …(2)
                              2π f 2ε ' ⎡ 1 + ( ε ''/ ε ' ) − 1⎤
                                                         2
                                        ⎣                      ⎦
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


        Where, c is speed of light in free space (3×108 m/s), f is the frequency (Hz), ε '' is
the loss factor of the food and ε ' is dielectric constant.

Microwave assisted drying

Several researchers in the developed and developing countries have done studies on
microwave assisted drying of various fruits and vegetable and reported that the drying by
microwave assisted convective and microwave vacuum methods is more efficient than
conventional drying techniques.

Some researchers (Shivhare et al., 1992; Sharma and Prasad, 2001) have reported
microwave assisted hot air drying of foodstuffs and found considerable improvements in
the drying process and quality of dehydrated products. The simple laboratory microwave
convective dryer for foodstuffs is shown in Fig.1.




Fig. 1 Schematic diagram of microwave convective dryer (Sharma and Prasad, 2001)

In recent years, microwave-vacuum drying has been found as a potential method for
obtaining high quality dried foodstuffs. 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 fast drying at low temperature and thus it has the scope to improve
energy efficiency and product quality. Some foods like grains, fruits and vegetables have
been successfully dried by microwave-vacuum drying techniques. The effect of vacuum in
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


microwave drying operation is system specific, and for successful design and operation of
an industrial microwave-vacuum drying system, the knowledge of the drying
characteristics of the material to be dried under a range of condition is vital (McLoughlin et
al., 2003; Sutar and Prasad, 2007). The schematic diagram of the laboratory microwave
vacuum dryer for foodstuffs is shown in Fig.2




            Fig. 2. Laboratory microwave vacuum dryer (Sutar and Prasad, 2007)
1.Food material 2.Vacuum chamber 3.Fiber optic cables with sensors 4.Signal conditioner 5.Microwave
timer 6.Microwave power on/off switch 7.Turn table controller 8.Electric power switch 9.Manual microwave
power control 10.Microwave power level indicator 11.Remote power control 12.Heat extractor fan switch
13.Moisture condenser 14.Vacuum pump 15.Pressure gauge 16.Pressure control valve in moisture suction
line 17.Heat extractor fan 18.Turn table 19.RS-232 cable 20.Personal computer


Modeling and simulation of temperature and moisture in foods

The successful design of industrial microwave application can be done with the aid of
modeling techniques which relate electrical and physical properties of foods (Van Remmen
et al., 1996). The non-uniformity in temperature and moisture distributions is the main
reason affecting the food quality of microwaved products. Some of the key factors that
influence the uniformity of temperature distribution are the dielectric and thermo physical
properties of the product, frequency and power of the incident microwave energy, and the
geometry and dimensions of the product (Jun and Puri, 2004). The literature shows the two
approaches of modeling of power deposition patterns: (1) making use of Maxwell’s
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


equations for electromagnetic field, and (2) using Lambert’s law in which power is
attenuated exponentially as a function of distance of one dimensional penetration into the
material. However, the Lambert’s law can be used for samples thicker than about three
times the characteristics penetration depth of microwaves, but the law fails for thinner
samples. It turns out that Lambert’s law is inapplicable for most foods prepared in home
microwave ovens. Therefore, Maxwell’s equation must be used to accurately describe the
propagation and absorption of radiation (Ayappa et al., 1991). Generally finite difference,
finite element and boundary element methods are used to solve the Maxwell’s equations to
obtain power deposition patterns in slabs, cylinders and spheres (Van Remmen et al.,
1996).

Future research needs in microwave assisted drying in India

Microwave heating has been established in a number of industrial sectors in the other
countries. India needs immediate attention towards the following issues:


To determine the effects of food geometry and formulation on heating and drying patterns
and assess their impact on overall process effectiveness To determine the effect of
equipment design factors, including frequency of microwaves (for examples, 915 MHz is
sometimes proposed instead of the commonly used 2450 MHz for better uniformity of
heating) To develop variable frequency ovens and assess their usefulness in food
applications for improved uniformity of heating Understanding the factors affecting heating
patterns, including qualitative changes occurring with frequency changes To understand the
food physics in microwave environment Determination of Dielectric Properties of fruits
and vegetable of various varieties available in India.
                                         References


Ayappa, K.G., Davis, H.T., Davis, E.A. and J. Gordan (1991) Analysis of microwave
     heating of materials with temperature dependent properties. AIChE Journal, 37(3):
     313-322
Datta, A.K. (1990) Heat and mass transfer in the microwave processing of food. Chemical
     Engineering Progress, 86(6): 47-53.
Sutar PP and S Prasad. (2008) Microwave Drying Technology-Recent Developments and R&D Needs in
     India. In proceedings of 42nd ISAE Annual Convention, during February 1-3.


Jun, S., and V. M., Puri. (2004) Development of interactive computational model of
     temperature and moisture distributions of microwaved foods. Applied Engineering in
     Agriculture. 20 (5): 677-682
McLoughlin, C.M., McMinn, W.A.M., and T.R.A., Magee (2003) Microwave-vacuum
      drying of pharmaceutical powders. Drying Technology, 21(9): 1719-1733.
Ramaswamy, H.S., Wil, T.P. and M. Fakhauri (1991). Distribution and equalization of
     temperature in a microwave heated food model. ASAE paper No. 91-3518, ASAE, St.
     Joseph, MI 49085.
Sharma, G.P., and S. Prasad (2001). Drying of garlic (Allium sativum) cloves by
      microwave-hot air combination. Journal of Food Engineering, 50: 99-105.
Shivhare, U.S., Raghavan, G.S.V., and R.G. Bosisio (1992) Microwave drying of corn II.
      Constant power, continuous operation, Transaction of the ASAE, 35(3): 951-958
Sutar, P. P., and S., Prasad (2007) Modeling Microwave Vacuum Drying Kinetics and
      Moisture Diffusivity of Carrot Slices', Drying Technology, 25:10, 1695 – 1702
Van Remmene, H. H., J. Ponne, C. T. Nijhuis, H. H. Bartels, P.V., and P. J.A.M. Kerkhof
    (1996) Microwave Heating Distributions in slabs, spheres and Cylinders with relation
    to food processing. Journal of Food Science, 61(6): 1105-1114

				
DOCUMENT INFO
Description: Drying Technology document sample