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Fast and Accurate Technique for Determination of Moisture Content in Oil Palm Fruits using
Open-Ended Coaxial Sensor

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2005 Jpn. J. Appl. Phys. 44 5272
(http://iopscience.iop.org/1347-4065/44/7R/5272)
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Japanese Journal of Applied Physics
Vol. 44, No. 7A, 2005, pp. 5272–5274

Brief Communication

#2005 The Japan Society of Applied Physics

Fast and Accurate Technique for Determination of Moisture Content in Oil Palm Fruits using Open-Ended Coaxial Sensor
Zulkifly A BBAS, You Kok Y EOW, Abdul Halim SHAARI, Azmi ZAKARIA1 , Jumiah H ASSAN,
Kaida K HALID1 and Elias S AION
INSPEM, Universiti Putra Malaysia, 43400 UPM Serdang, Malaysia
1
Department of Physics, Universiti Putra Malaysia, 43400 UPM Serdang, Malaysia
(Received March 30, 2004; accepted September 3, 2004; published July 8, 2005)

A simple, fast and accurate technique employing an open-ended coaxial sensor for the determination of the moisture content in oil palm fruit is presented. For this technique, a calibration equation has been developed based on the relationship between the measured moisture content obtained by the oven drying method and the phase of the reflection coefficient of the sensor for
21 fruits. The moisture content predicted by the sensor was in good agreement with that obtained using the standard oven drying method within Æ5% accuracy when tested on 145 different fruits samples. [DOI: 10.1143/JJAP.44.5272]
KEYWORDS: coaxial sensor, reflection coefficient, network analyzer, oil palm fruit, moisture content

Palm oil is now one of the world’s most popular edible oils. It is extracted from the flesh of oil palm fruits by a series of processes performed at a mill. The quality and quantity of the oil are functions of the ripeness of the fruit.
Moisture content is one of the most important factors that influence the ripeness of oil palm fruit.1) The measurement of moisture content using a microwave method is widely known to be accurate and rapid. However, not all microwave techniques are suitable for single fruit measurements due to the small sample size. It has been pointed out that a microstrip sensor2) offers distinct advantages in this respect since only a small part of the sample interacts with the sensor and there is no restriction on the sample size.
Unfortunately, use of the sensor requires laborious sample preparation in which the fresh mesocarp of the oil palm fruit has to be separated from the nut, cut into small pieces and crumbled to form a semi solid sample. In this article, we propose the use of an open-ended coaxial sensor to realize a simple, fast and accurate technique for the determination moisture content in oil palm fruits. In the near future, the sensor shall be incorporated in the development of a costeffective portable microwave moisture meter operating at frequencies below 5 GHz.
The open-ended sensor, as shown in Fig. 1, was fabricated from a 12.7 mm by 12.7 mm square flange, sub-miniature
A type (SMA) coaxial stub contact panel. The dielectric material separating the gold-plated, stainless-steel inner and outer conductors of the coaxial line is PTFE. The diameters of the inner and outer conductors are 1.3 and 4.1 mm, respectively. The PTFE stub was machined flat and polished.
The reflection coefficient of the oil palm fruits was measured using the sensor with a HP8720B Vector Network Analyzer
(VNA). All calibrations and measurements were performed for 801 points in the frequency range between 1 and 5 GHz at room temperature in a stepped continuous-wave (CW) mode for which a synthesized frequency could be obtained for each data point. A full one-port calibration technique was implemented on the BB0 plane using an HP 85052D 3.5 mm calibration kit. Under the assumption of a quasi-TEM mode, the measured reflection coefficient ÀAA0 of the fruit sample on the plane AA0 can be de-embedded to the connector end of the probe which coincides with the calibration plane BB0 to give a reflection coefficient ÀBB0 by the relation3)

ÀBB0 ¼ ÀAA0 expð jDÞ;

ð1Þ

where  is the propagation constant of the coaxial line section and D is the physical length of the coaxial line.
Sampling was performed on fruits of various degrees of ripeness from twenty bunches from different 12-year-old oil palms of the tenera variety. Part of the fresh mesocarp of each fruit was sliced in the longitudinal direction to ensure good contact between the surface of the mesocarp and the sensor. The nondestructive testing of fruits can be accomplished in the future by using an open-ended coaxial sensor with inner and outer conductors of small diameter. The true moisture content of the fruit samples was determined using the standard oven drying method.4) A set of 21 fruits was used to establish the calibration equation relating the true moisture content to the phase of the reflection coefficient.
The reliability of the calibration equation was tested using another set of 145 different fruits.
Figure 2 clearly shows a linear relationship between the true moisture content (mc) of the oil palm fruits and the phase of the reflection coefficient, . Due to the large number of frequency points, only representative frequency points, namely 1, 2, 3, 4 and 5 GHz, are shown in the graph.
The coefficients A and B along with their R-squared values found using the linear regression model, mc ¼ A þ B;

ð2Þ

for each frequency are listed in Table I. Furthermore, plotting variations in moisture content mc with respect to frequency and applying curve fitting techniques allow eq. (2) to be written in the form mc ¼ ðÀ0:3332 f 4 þ 4:9564 f 3 À 27:8671 f 2 þ 72:4739 f À 100:4739Þ þ ðÀ0:0566 f 3 þ 0:5524 f 2 À 5:2945 f þ 33:3487Þ; ð3Þ where f is the operating frequency of the sensor. The sensitivity of the sensor in the prediction of mc with respect to changes in the measured phase of the reflection coefficient can be calculated directly from eq. (3), i.e.,

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Jpn. J. Appl. Phys., Vol. 44, No. 7A (2005)

Z. ABBAS et al.

5273

(a)

(c)

(b)

Fig. 1. An open-ended coaxial line probe radiating in tested medium: (a) schematic (b) sensor and (c) sample measurement.

Table I. The values of A and B for 1 GHz, 2 GHz, 3 GHz, 4 GHz and
5 GHz.
80

R2

B

1
40

1GHz

40

-1

-0.8

-0.6

-0.4

-0.2

20

-1.5

28:5500 Æ 1:5073

0.9543

À32:6747 Æ 1:2211

24:5283 Æ 1:2602

0.9741

3

-1

-0.5

60

À27:0243 Æ 0:9186

20:9291 Æ 1:2599

0.9785

4
5

À24:5485 Æ 0:9623
À23:5009 Æ 1:1726

17:4531 Æ 1:5802
13:7320 Æ 2:1859

0.9716
0.9548

80

60

2HGz

À51:2439 Æ 2:5738

2

80

Moisture Content, %

A

60

60

20

80

75

40

20

40

3GHz

-2

-1.5

-1

-0.5

20
-2.5

4HGz

-2
-1.5
-1
Phase Reflection Coefficient, rad

80

Moisture Content, %

Regression model, mc ¼ A þ B

Frequency
(GHz)

60

40

20

5GHz

-2.5
-2
-1.5
-1
Phase Reflection Coefficient, rad

Predicted moisture content (Polynomial equation), %

Moisture Content, %

80

70

65

60

55

50

45

40

35

30
30

35

40

45

50

55

60

65

70

75

80

Measured moisture content (Oven drying method), %

Fig. 2. Variation in moisture content, mc, using oven drying method with phase , for each frequency.

Fig. 3. Comparison between predicted and measured moisture content.

5274

Jpn. J. Appl. Phys., Vol. 44, No. 7A (2005)



@mc
@



¼ À0:3332 f 4 þ 4:9564 f 3 À 27:8671 f 2

Z. ABBAS et al.

ð4Þ

þ 72:4739 f À 100:4739
The reliability of the sensor in predicting the mc in oil palm fruits was validated by comparing the values of mc estimated using eq. (3) with the true mc obtained using the oven drying method on 145 different fruit samples as shown in Fig. 3. It was found that the predicted mc was in good agreement with that obtained using the standard oven drying method within Æ5% accuracy.
A calibration equation relating the phase of the measured reflection coefficient of an open-ended coaxial sensor to the

moisture content in oil palm fruits has been established. The technique allows the fast determination of the moisture content in oil palm fruits and is suitable for quality checking fruit ripeness at both palm oil plantations and mills.
1) A. Ariffin: Proceedings of the Symposium on Impact of Pollination
Weevil on the Malaysian Oil Palm Industry (Palm Oil Research Institute of Malaysia, Kuala Lumpur, 1986).
2) K. Khalid and Z. Abbas: J. Microwave Power Electromagn. Energy 27
(1992) 3.
´ ´
3) D. Berube, F. M Ghannouch and P. Savard: IEEE Trans. Microwave
Theory Tech. 44 (1996) 1928.
4) S. W. Lin, T. T. Sue and T. Y. Ai: PORIM Test Methods (Palm Oil
Research Institute of Malaysia, Kuala Lumpur, 1995) p. 147.