Soluble Solid Content Determination of Limau Madu Using Microwave Sensing Technique at 2.0 -2.6 GHz

Orange is a fruit that essentially made up of a lot of nutrients. It is rich in vitamin C and folate. These are the qualities that must be maintained. Maintaining the qualities of this fruit is necessary. One of the prominent techniques is by measuring the solid soluble content of these local oranges non-destructively. In this study, the monopole sensor has been selected as the medium to find out the sweetness. The relationship of reflection coefficient, S11 and dielectric constant is observed. It is given by coefficient of determination, R2 = 0.9103 at 2.4 GHz. The statistics show a promising result of frequency at 2.4GHz


Introduction
The prominent type of orange in Malaysia is citrus suhuiensis. Or well known to the local as limau madu. Sometimes they are being called limau manis due to their sweetness taste. Orange generally is known as important sources of vitamin C and folate [1]. Besides these nutrients, it also has significant amount of antioxidants, such as beta-carotene and flavonoid compounds [2]. These nutrients are simplified in the tables below.

Literature review
Dielectric studies gained interest in learning the penetration of electric fields can be utilized in product quality determination non-destructively [3]. Microwave techniques also have been widely used in various studies due to its penetration [4]. The internal quality that is most important for all type of fruits is solid soluble content (mostly sugar), which can be determined only by destructive method. Hence, in order to preserve the condition of the fruit several non-destructive methods are proposed. Microwave measurement is potential in providing information related to permittivity.
Permittivity is the magnitude of dielectric properties which stimulus electromagnetic waves reflection and attenuation within materials [5] . Relative permittivity of a material can be expressed as: An optimal monopole probe is designed to measure the moisture content of mangosteen [4]. This study used the Misra-Blackham model for permittivity calculation. The magnitude of reflection coefficient is highest at 2.45 GHz. As the primary results, this study is able to prove that microwave techniques by open-ended coaxial probe can be effectively used in grading mangosteen non-destructively.
R. Rosman et. al observed the effect on reflect coefficient, Г when an electromagnetic wave is emitted from the coaxial line port through a microwave monopole antenna [6]. On top of it, the idea of antenna monopole is to investigate the sweetness level of Malaysian local oranges non-destructively. The significance of this study is the author designed a unique holder for sample that is made up of nylon.

Materials and Methods
This experiment will be conducted into two separate forms. The first experiment is non-destructive, where the reflectance coefficient of each sample is measured at frequencies of 85MHz to 5 GHz. The second experiment is on destructive which will measure pH, brix, acidity and moisture content.

Sample
There are 40 samples of oranges that are bought at local market. These samples are being kept at temperature of 26C. They are weighed by an electronic balance. Diameter of the oranges is measured using a digital caliper. These oranges were numbered accordingly and their weight is recorded every day.

3.2.1.
Microwave. This method is used to determine the reflection coefficient S11 of local oranges fruit. It is done by connecting the cabanR54 reflectometer to a personal computer. The sample is then touched at the end of the pole which connected to the CABAN R54 reflectometer.

Results and Discussion
Initially, it was necessary to investigate the linearity of the relationship between the permittivity of the material and the resulting microwave attenuation. During the process it was essential to ensure the consistency of the results. Figure 3 shows the reflection coefficient of local oranges at different days. As the day increases, the magnitude of S11 also increases.  Figure 3. S11 is being measured using CABANR54

Conclusion
The paper focuses on reflection coefficient, S11 . This measurement is obtained open-ended coaxial line probe technology and a network analyzer over the frequency range from 85 MHz to 5 GHZ. At increasing S11, permittivity, brix or SSC(mostly contain sugars) and pH also increases. Only percentage of acidity decreases. This proves that as the fruit gain maturity it is less acidic. During the decline in S11, the percentage of moisture content increases. It is observed that at 2.6 GHz, S11 is at the highest magnitude of -42.84 dB. However, it does not comply with other measurements. Averagely, frequency of 2.4 GHz shows an interesting relationship with dielectric constant, solid soluble content and also moisture content. Hence, it can be said that 2.4GHz is the best operating frequency.