Rapid Detection Of The Moisture Content In Rice And The Polar Compounds Content In Fried Oil Using The Piezoelectric Capacitive Sensor

The piezoelectric quartz crystal sensor attracted many analysts because of its sensitive response, low cost and easy operation. Based on different parameter response, it was divided into two types:quartz crystal microbalance and series piezoelectric quartz crystal. The quartz crystal microbalance is a sensitive tool, which responds to changes in mass loading on electrode surface down to a nanogram level. The response of the piezoelectric sensor is affected by other factors, such as viscosity, density, conductivity and permittivity of the liquid. The series piezoelectric crystal quartz sensor was constructed by combining in series a pair of electrodes immersing in a liquid with a piezoelectric quartz crystal in oscillating circuit. It can sensitively respond to changes of liquid conductivity with excellent frequency stability. Compared with classical conductive methods, the superiority of this method is that it can detect a small conductivity change in the presence of foreign electrolyte. With the rapid development of information technology and computer processing technology, the circuit of the piezoelectric sensor can be miniaturized on a very small chip. The conductivity electrode can also select an interdigital electrode with the width in the micron and even nano level, and it is easy to implement on-site rapid detection. Therefore, we proposed to detect both the moisture content in rice and the polar compounds content in fried oils using the piezoelectric sensor as follows:1. A piezoelectric sensing method was proposed for rapid detection of moisture content in rice based on combination of a cylindrical electrodes and a piezoelectric sensor. The detection is based on the piezoelectric sensor has a sensitive frequency shift response to the changes of dielectric properties between the electrodes. The typical frequency shift responsive curves were obtained and the characteristic frequency values were extracted to detect the moisture content in rice. The calibration equation of rice moisture content was obtained combined with the national standard (GB/T 5497-1985) as follows:y=-75x+9007366, r=-0.98. According to the calibration equation, the moisture content of rice samples was detected quickly using the piezoelectric sensing method. For comparison, the national standard method was carried out at the same time. The results showed that there is no significant difference in measuring precision between two methods. The proposed method is accuracy, rapid and convenient. It is expected for online determination of moisture content in different agricultural products.2. A piezoelectric sensing method was proposed for rapid detection of polar compounds content in fried oil based on combination of an interdigital electrode and a piezoelectric sensor. The detection is based on the piezoelectric sensor has a sensitive frequency shift response to the changes of dielectric properties between the electrodes due to the polar compounds content change during the frying. Firstly, both the relationship between the frying times and the dielectric properties and the relationship between the polar compounds content and the dielectric properties were studied. The calibration equation between the polar compounds content and frequency was obtained combined with the national standard (GB/T 5497-1985) as follows:y=-0.396699x+9007010, r=-0.996. The threshold value 9006898 Hz was obtained to identify the oil according to the polar compounds content 27% proposed by national standard. According to the calibration equation, different brand fried oils were detected quickly using the piezoelectric sensing method. For comparison, the national standard method was carried out at the same time. The results showed that there is no significant difference in measuring precision between two methods. The proposed method is accuracy, rapid and convenient for detection and identification of fried oils.