1 July 2007
New wave spices
Microwave sensing can really spice up moisture content measurements
By T. Jayanthy and P.E. Sankaranarayanan
Cardamom is the queen of spices in India and is one of the most valued spices in the world. Moisture measurement is a critical factor affecting the physical and chemical properties related to quality, harvest time, and the potential for safe storage of this valuable food product. It is also an important factor in determining market price because the dry matter of spice has more value than the water it contains.
Manufacturers have considered microwave techniques for a long time for moisture sensing in food processing and agriculture-related industries. They are suitable for online real-time monitoring and control. It is possible to determine moisture content of spices such as cardamom directly from microwave dielectric properties by analyzing dielectric properties of wet samples.
When exposed to microwave radiation, dielectric materials in food items polarize under the action of an external electric field. The magnitude of polarization is proportional to the permittivity or the dielectric constant of the material. Also, when microwave radiation interacts with wet material, the dielectric loss or attenuation of microwave energy is mostly due to the total amount of water in the wet material. We can compute the percentage of moisture from the attenuation we measure.
Two of the most common methods of measuring moisture content include resistance type and RF capacitance type moisture meters. The resistant type moisture meters are low cost, but crush and waste spice samples. Moisture meters measure the capacitance of the sample between two electrodes, thus sensing the dielectric constant of the cardamom. You can use this nondestructive method with a wide range of moisture contents, but changes in bulk density could cause errors. In the microwave frequency measurement, we employ transmission lines using wave guides or coaxial lines and resonant cavity. A disadvantage of these methods is the need for precise sample holder fabrication and sample confinement.
Our method is to calculate the dielectric properties of moist cardamom from the measured attenuation and phase shift, using two horn antennas and a detector. The dielectric constant and loss factor are functions of measured attenuation and phase shift.
The attenuation depends only on the dielectric properties of spice samples when the wavelength and sample thickness are constant. It is possible to measure the moisture content of moist spice by detecting the peak volt-age of the microwave signal from the receiving antenna. To determine the dielectric properties of cardamom with various moisture contents, we computed the attenuation and phase shift of the microwave signal through the cardamom samples at X-band frequency range. Dielectric constant and loss factor of different moist cardamom samples at X-band increased with moisture content.
Dielectric properties of materials usually come from measurement, reflection, or transmission coefficients and in some instances from both. We can determine the transmission coefficient by computing the attenuation and phase shift, which a sample between two antennas introduces. We can calculate the dielectric properties of moist spice from the above two parameters with two horn antennas and a detector. The dielectric properties of spices are of interest mainly because of their correlation with grain moisture content and their consequent usefulness for determining rapid moisture content.
Electrical and electronic grain moisture meters sense the electrical conductivity or the dielectric constant of grain, and calibrate to read moisture content. The essence of microwave dielectric methods for moisture sensing is based on coupling between the electromagnetic energy of the incident wave and the material under consideration.
Dielectric properties of material depend on the concentration and activity of permanent electric dipole molecules and ionic conduction and on the degree of dipole alignment with the time varying electric field applied. Therefore, when the sample holder fills with moist spice, the dielectric properties of the spice are highly affected by water concentration in mass per unit volume and temperature, which affects molecular movement. When the electromagnetic field applies to a dielectric material, electromagnetic energy dissipates in the dielectric material as a result of a dielectric relaxation process. The interaction of the applied electromagnetic field with moist spice depends on the complex dielectric permittivity, relative to free space: ε = ε’- jε”
ε’ = Dielectric constant
ε’’ = Dielectric loss factor
The dielectric constant describes the ability of a material to store electromagnetic energy while the dielectric loss factor represents the loss of electromagnetic field energy in the material. To minimize the undesirable effects of bulk density fluctuations, measure attenuation and phase shift, which are the functions of the loss factor and the dielectric constant, respectively, for microwave energy transmitted through spice.
Moisture content expresses in percentage as: M = mw/ mw+md *100
mw = Mass of water
md = Mass of dry material
Cardamom is conditioned to higher moisture levels in environmental chambers in which the temperature and relative humidity could adjust in small increments to avoid checking and cracking of the spice. After conditioning samples to desired moisture levels, we sealed them in glass jars and stored them at 4ºC before using them for measurements. You can use different measurement techniques to determine the dielectric constant ε’ and the loss factor ε’’ of dielectric materials. In this instance, we used a free space technique, which is ideal for nondestructive and non-contact moisture measurement in cardamom. It is based on the method of analyzing dielectric properties of wet samples. When a plane wave incident normally takes place on a dielectric medium in free space, part of it transmits to the medium, and part reflects at the interface between free space and the dielectric medium. We placed a material layer of known bulk density, moisture content, and temperature between two horn antennas. Then we measured the attenuation (A) and phase shift (φ) for each sample. The dielectric constant and loss factor are functions of the measured attenuation and phase shift. The components of the relative complex permittivity are:
ε” =AλO(ε’)1/2/ (8.686πt)
λO= Free space wavelength
Φ = Phase shift difference with and without spice samples in degrees
A = Difference between the attenuation with and without spice samples in decibels
t = Thickness of the grain layer in centimeters
In a block diagram of the experimental setup used to measure the attenuation and phase shift of the microwave signal through the spice sample, a 10.5 GHz microwave signal at 60 mw generated by an oscillator transmits through an isolator, attenuator, frequency meter, slotted section and radiates from a transmitting horn antenna in to the sample holder.
The receiving horn collects the microwave signal, attenuated by spice sample, and a detector converts input RF signal to dc voltage. The sample holder is a rectangular container with 1mm wall thickness. The dimensions in this experiment are 2.5 cm thick, 7 cm wide, and 10 cm high. We measured the output power and the first minimum distance without the spice samples. The sample holder fills with different moist spice samples, and measures the power and the first minimum distances for each sample. From the above observations, you calculate the attenuation ‘A’ and phase shift ‘Φ.’ We calculated the phase shift ‘Φ’ by measuring the distance (D) of first minima with the spice sample, the distance (DR) without the spice sample and the length (LE) of the sample, and then multiplied this value with twice the phase constant (β) below:
The phase shift ‘Φ’ is given by Φ = 2β ( D – DR – LE ).
We determined the dielectric properties of cardamom with various moisture contents by measuring the attenuation and phase shift of the microwave signal by using cardamom samples at 10.5 GHz. We found the dielectric constant and loss factor to be a function of attenuation and phase shift respectively. Both the dielectric constant and loss factor increased with moisture content. The attenuation and phase shift also vary with moisture content. The output power decreases with increased moisture content.
ABOUT THE AUTHORS
T. Jayanthy is a research scholar at Sathyabama Institute of Science & Technology in Chennai, India. Dr. P.E. Sankaranarayanan is dean of academic research at the Institute.
Microwave sensing for food moisture
Knowing the bulk moisture content of some food materials is an economic must in manufacturing and processing. Most dried foods are low-risk for food poisoning as they rely on drying to a low moisture content to prevent multiplying micro-organisms. Yet herbs and spices are an exception. Contamination can occur during harvesting, washing, and sun drying on small farms, or in primitive conditions. In some cases, processing is restricted to low temperature drying, grading, cleaning, and grinding.
Larger manufacturers usually determine moisture by manually checking samples through determining moisture loss-on-drying in an oven. While such long-standing and trusted methods are economical for limited short-term operations, they become labor-intensive, time-consuming, and expensive for large continuous operations. They can also miss spikes in moisture. Another option for monitoring the moisture content of solids, particulates, powders, and liquids includes microwave instruments.
Principles of microwave sensing
Electromagnetic fields and waves are very sensitive to the dielectric properties of the material in which they exist. In turn, these dielectric properties are sensitive diagnostic indicators of numerous physical properties of the material, such as moisture content, density, concentrations of constituents, porosity, chemical reactions, and state of cure. There are two dielectric parameters of main interest: the dielectric constant (ε’), which characterizes the material’s ability to store and release electromagnetic energy; and the dielectric loss factor (ε”), which characterizes the material’s ability to absorb (attenuate) electromagnetic energy to create heat. By monitoring the electromagnetic fields/ waves that are interacting with the material, it is possible to simultaneously and independently determine these two dielectric parameters (ε’, ε”), and/or two independent physical parameters, such as moisture content and density, while all other physical properties are held constant.
Microwave sensing benefits
An important benefit of microwave moisture sensing is it determines bulk moisture as opposed to surface or near-surface moisture from infrared (IR) or near infrared (NIR) techniques. This is important when monitoring drying operations where moisture gradients exist in the material. Variations in moisture can exist within a few microns of the surface, even within microscopic particles. Such micro-gradients also affect moisture determination with low frequency techniques, such as capacitance and resistance moisture meters. Micro-gradients can take hours to disappear, but their effects are greatly reduced or insignificant at microwave frequencies.
Logistical flexibility in installation is another advantage. You can place a wide variety of sensors on conveyors, or in hoppers, shakers, pipes, and chutes. Measurement time ranges from a few milliseconds to one second, depending on the sensor, so there is ample time and data available to perform statistical averaging, even when the material is moving at one meter per second or faster. Time averaging eliminates short-term fluctuations incurred when sensing particulates. In all but exceptional salty materials, the effects of dissolved (ionic) salts are generally negligible at microwave frequencies, whereas at lower electromagnetic frequencies even small salt concentrations make reliable two-parameter determination of the moisture content difficult if not impossible.
Microwave radiation is non-contaminating and environmentally safe at power levels typically used for online sensing. Human exposure is usually less than that from common consumer electronic devices such as cordless and cellular telephones. Because electromagnetic radiation is so small, microwave moisture meters are free from government and environmental regulations. These sensors are also insensitive to environmental conditions such as dust, color, or ambient light, vapors, and machine vibrations, in contrast to IR and NIR techniques.
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