The dielectric properties of grains and seeds and their dependence on moisture content of the seeds have been of great importance because of their application in the measurement of moisture content (Nelson, 1977).The moisture content measurement of the seeds is important from the point of view of preservation of stored seeds and their protection from insects. This preservation and protection can be achieved by dielectric heating (Nelson et al., 1972). Therefore a lot of study have been done on cereals grains and many factors that affect, the electrical parameters i.e., dialectic constant, dielectric loss and conductivity have been evaluated from application point of view. Knowledge of these data are also important in the applications like improvement of germination of seed (Nelson et al., 1970), dielectric cleaning and separation of seeds by seed separator (Kazimirchuk et al., 1995) and improvement of nutritional qualities (Norris et al., 1952).
The dielectric properties of seeds are also helpful in studying the viable test (Wendell, 2000), metabolic mechanism of seed (Hunt et al., 1952), hydration mechanism in seed (Knosta et at., 1996) as well as in the designing of equipments for sensing and monitoring of moisture using electrical methods.
The electrical parameters for many agricultural materials are influenced by ionic conductivity (Gerhard, 1978) and water retention of cell (Bennet-Clark, 1959) but with the development of practical application of microwave energy, need of data on dielectric properties of seeds have gained much importance.
Poppy seeds are the main source of edible oil (68%) and its digestibility coefficient is 81% (Pushpanngadan et al., 2000). They are medicinally important and specially used for extracting the Opium, which provides the basic compound of the different medicines. Drying of poppy seed to an optimum moisture level is a pre-requisite for their safe quality storage before germination.
Therefore present study have been undertaken as a comparative study of electrical
properties of Poppy seeds at different moisture level i.e., the dielectric constant
(ε′), dielectric loss (ε′′ and a.c. conductivity (σ = ωxε0xε′′,
where ε0 permittivity of free space and w is angular frequency
has been measured to identify the viable and non-viable seeds and to analyze
their dependence on moisture level for Indian poppy seeds. The measurements
have been done in temperature range of 15 to 45°C and with the varying frequency
from 5 KHz to 10 MHz.
Materials and Methods
The poppy seeds have been obtained from certified seed research institute
(National Botanical Research Institute Lucknow, India). The nutritive value
of poppy seed is reported by (Pushpanngadan et al., 2000) as given below.
Moisture 4.3-5.2%, Protein 22.3-24.4%, Crude fibre 4.8-5.8%, Calcium 1.03-1.45%, Phosphorus 0.79-0.89%, Iron 8.9-11.1mg/100 g, Iodine 6 μg kg-1, Arginine 10.41%, Histidine 2.9%, Lysine 1.5%, Methionine 2.3%, Theonine 4.2%, Valine 7.1%.
The chemical composition of seed oil is also given a Palmitic acid (16:0):
8.90-21.48%, Stearic acid (18:0): 1.40-10.80%, Oleic acid (18:1): 13.22-36.79%,
Linolenic acid (18:2): 41.00-60.00%, Linol enic acid (18:3):0. 00-9.40%, Manganese
(29 mg kg-1), Copper (22.9 mg kg-1), Magnesium (15.6 g
kg-1), Zinc (130.0 mg kg-1).
The capacitances (Cp) and dissipation factor (Dp)
measurements have been done with the help of computer controlled Impedance Gain
Phase Analyzer (Model No.-HP-4194 A) in the frequency range of 5 KHz to 10 MHz,
using a specially designed open ended coaxial probe for permittivity measurement
of agricultural product (Sheen et al., 1999). The sample holder has been
gold plated to reduce dissipation losses. It is calibrated for error removal
in measurements using standard liquid C6H6 and CCl4
and the error was found to be less than 1%. The dielectric parameters and conductivity
have been calculated with the help of following formula-
Where Cp is the capacitance of the sample holder with sample, Co
is the capacitance of empty sample holder, Cg is the geometrical
capacitance of sample holder
where h is height of the sample holder, a and b are the internal and external
radii respectively, εo is the permittivity of free space, Dp
is the dissipation factor for sample holder with sample and Do is
the dissipation factor for empty sample holder, while w represents the angular
frequency. Temperature of poppy seed have been varied by placing the sample
holder in a specially designed double walled glass jacket through which heated
oil has been circulated using a refrigerated circulator of Julabo (F-25 model,
Germany). The accuracy of the temperature measurement is ±0.01°C.
The experiments have been performed at our laboratory in Physics Department,
Lucknow University, Lucknow, India, during September to November 2005.
The most important part of the experiment was to prepare the samples at
different moisture levels. The initial percentage of moisture content in 20
g of sample were determined using a standard oven-drying method in which the
moisture contents were determined gravimetrically after drying at 130°C
for 1 hour (ISTA, 1996). Then by adding distilled water to samples and by frequent
agitation to aid uniform distribution of moisture sample was prepared at certain
moisture level and sealed with Para film and with a plastic cap in the jar which
was stored at 4°C for 72 h (Nelson et al., 1976) to allow complete
water uptake and seed moisture equilibration. For moisture level lower than
initial level, the sample were dried in open air for several hours to reach
the lower desired moisture content. The sample is kept in this condition for
about 48 h before the measurements were made. The moisture contents of the sample
have been determined by approved oven method (USDA, 1971) and we have prepared
sample in two parts one of them for measurement of dielectric properties and
other is for germination of seed.
Germination of Seed
We have taken the 10 g of prepared sample having moisture content 5, 10,
20, 30% of dead seeds and normal seeds and kept in the 5 labeled disks and placed
it into the seed germinator at 25°C temperatures for approximate 30 days.
The experimentally determined values of dielectric constant (ε), dielectric loss (ε") and electrical conductivity (σ) of poppy seed, over the frequency range of 5 KHz to 10 MHz at different discrete frequencies of 5, 10, 50, 100, 500 KHz, 1, 5 and 10 Mhz and for moisture levels 5 to 30% between the temperature range 15 to 45°C has been analysed in the present study. The mean temperature coefficient per unit moisture content for dielectric parameter is calculated as per definitions given below:
Mean temperature coefficient per unit moisture content
at given frequency
= [(Value of parameter at T2
- value of parameter at T1
Where, f is a selective frequency and T, M are the temperature and moisture content, respectively.
Behaviour of Dielectric Constant and Dielectric Loss Factor
From the Fig. 1 we have been observed that both the dielectric
constant and dielectric loss of the complex permittivity decreases with increases
in the frequencies. This exhibits dielectric dispersion in the material at different
frequencies. The high values of dielectric constant at lower frequencies (5,
10, 50 and 100 KHz) and high moisture content could be attributed to high mobility
of dipole for free water state and electrode polarization. The high values of
the dielectric loss can be attributed to high mobility of water dipole, electrode
polarization and increase in ionic and surface conductivity. The relation between
dielectric loss and ionic conductivity is given by (Magario et al., 1988):
|| Frequency dependence of dielectric properties of poppy at
indicated moisture content and 25°C
As we go towards the lower frequency side the ionic loss (σi/2xπxfxε0) is inversely proportional to frequency and it becomes almost absent at higher frequencies due to the dipolar energy dissipation, which is the predominant loss and ionic loss become almost absent. The dielectric properties of the seeds and other food materials can be represented as combination of ionic and dipolar polarization losses.
The change in dielectric constant and corresponding variation in dielectric loss at indicated frequencies showed that the changes in loss factor are less regular than the change in dielectric constant. The similar behaviours in Corn for the moisture range 5 to 10% and frequency range 1 to 11 GHz have been reported by Nelson (1979b). In other studies (Jones et al., 1978) on wheat, Corn and Soybean over the frequency range 1 to 200 MHz range similar types of behaviour has also been reported. The complex dielectric relaxation and dispersion phenomena may be one of the causes in the irregularity in loss factor.
From the Fig. 1 it can be observed that the curves divers and the separations increase between the curves for different moisture levels as we move to the lower end of the frequency range but no change in the curve have been observed for the dead seed and it shows nearly a straight line.
||Moisture dependence of dielectric constant of poppy seed at
indicated frequencies and 25°C temperature
||Moisture dependence of dielectric loss of poppy seed at indicated
frequencies content and 25°C temperature
It is clear from Fig. 2 and 3 that the
complex dielectric permittivity increases with increase in the moisture content
at a given frequency and temperature. It can be observed that the rate of increase
in ε′ and ε′′ is high at 5 KHz and 10 KHz. The reason is water
dipoles easily follow the applied field variations, at high moisture level and
more water dipoles contribute to the polarization, due to high water mobility.
At low moisture content, particularly below 5% both ε and ε"
of the complex permittivity are small because the distance between the water
molecule and cell wall is very small and force of attraction is very large in
the case of strong bound water state (monolayer). Therefore, the dielectric
constant and dielectric loss both are small.
When the moisture content increases beyond the 8%, bound water changes state from first (monolayer) to second (multilayer) type and add to the complex permittivity, which shows a sharp increase in ε′ and ε′′ for the moisture content over 10% for all the frequencies. At high moisture content, particularly 30% and frequency below 10 KHz, only live seeds gives high values of ε′ and ε′′. This behaviour could be attributed to transition of bound water state second (multilayer) to third state due to osmotic tension (Bennet-Clark, 1959) or free state of water at high moisture level. At low frequency the ionic conductivity is high therefore for such moisture level and low frequencies, the dielectric losses are considerably high.
The Fig. 4 and 5 shows temperature dependence
of ε′ and ε′′ in the temperature range of 15-45°C.
A different frequencies and moisture levels dielectric constant and dielectric
loss increase with increase in the temperature at all moisture levels and frequencies
with a slight non-linearity at high moisture content and low frequency, particularly
at 10 KHz. The nature of the slope of the linear curve decreases with the increasing
frequency and is becoming insignificant as we go to the higher end of the frequency
range. The similar variations are also seen in a recent study on temperature
dependence of dielectric properties of pecan (Lawrence et al., 1992)
and fruits (Sipahioglu et al., 2003) in the range of 0-40°C.
||Temperature dependence of dielectric constant of poppy seed
at indicated frequencies and moisture content
||Temperature dependence of dielectric loss poppy seed at indicated
frequencies and moisture content
||Moisture dependence of mean temperature coefficient for dielectric
constant of poppy seed at indicated frequencies
The energetic status of the molecules and their aptitude to rotate with electric field (Bennet-Clark, 1959) changes to the permittivity due to the increase in the temperature and hence change in effectivecomplex permittivity results in increase or the decrease of the water molecules contribution to the polarization of the medium.
||Moisture dependence of mean temperature coefficient of dielectric
loss for poppy seed at indicated frequencies
As the temperature increases the molecular mobility increases and relaxation frequency, which is strongly related to the molecular mobility, increases (Barrow, 1988). Therefore, the peaks of both, the dielectric constant (ε) and dielectric loss (ε") shifts to higher frequency. Increase in temperature also increases to the ionic conduction, leading to increase in dielectric loss. Thus, as temperature increases both dielectric constant ε and ε" increases. The increase in dielectric parameters with temperature at lower frequency and higher moisture content indicate the predominant effect of ionic conduction as well as molecular mobility. Therefore, under these conditions rate of increase of ε and ε with temperature are high and might be non-linear. The dielectric constant is less affected by the temperature than that of the dielectric loss because of increase in ionic conduction gives additional effect on dielectric loss factor, whereas dielectric constant is less or not at all affected by the ionic conduction.
Figure 6 and 7 shows mean temperature coefficients
for both ε′ and ε′′ increases with increasing moisture content
at a given frequency and decreases with increase in frequency. The magnitude
of mean temperature coefficient is comparatively high at higher moisture content.
This indicates that moisture has an appreciable influence on the mean temperature
coefficient. In general it can be assumed that the mean temperature coefficients
of dielectric constants are positive at all moisture level and frequencies.
These results are in agreement with the finding of (Nelson, 1979b), they reported
positive temperature coefficients at radio frequencies for wheat, Corn and Soyabean
over temperature range of 2-40°C.
The natures of these curves are linear increasing after 5% moisture level having a large variation at 20% moisture level. This could be due to the varying molecular mobility at different moisture content and dispersion mechanism.
Behaviour of Electrical Conductivity
Figure 8 shows that the electrical conductivity for all
the poppy seeds increases with increase in frequency and the moisture content
at 25°C temperature. Figure 9 and 10
shows the electrical conductivity behaviour for poppy seed at different moisture
content and temperature.
||Moisture dependence of electrical conductivity of poppy seed
at indicated frequencies content and 25°C temperature
The higher conductivity values in the given frequency and moisture range are
observed for the normal poppy seed in comparison to dead seed.
Variation of the electrical conductivity with the temperature of poppy seed at a given moisture content and frequency is shown in the Fig. 10. From figure it is observed that the variations with temperature are almost linear in general for all moisture levels at higher frequencies, however non-linearity is present at lower frequencies.
|| Dielectric constant verses time period of germination of
poppy seed at 25°C temperature
Germination Period Dependence on Electrical Parameter
Figure 11 shows a bar diagram of germination period verses
dielectric constant for viable and non viable seeds (dead seed). The observed
fact is that for non viable seed the dielectric constant is least in comparison
to dielectric constant of viable seed at different moisture content. The germination
period is maximum for 5% moisture level and minimum for the 30% moisture level
whereas the dielectric constant is extreme for 30% and minimum for 5%.Thus we
can conclude that the electrical parameters are definitely an indicator of germination
period as well as viability of seeds (Repo et al., 2002) in the case
of poppy. A general theory can be derived by studying a number of seeds.
It can be concluded that moisture level affects the electrical properties of seeds up to a large extent. The electrical properties can be used to measure moisture level, which is directly related with germination of seeds and their viability.
Hence we can say that dielectric properties can be used to determine the seed quality and germination period.