# Rapid-Detection Sensor for Rice Grain Moisture Based on NIR Spectroscopy
## 1 Introduction  
Rice is a staple cereal crop in many countries, and quality assessment and process control is very important for rice grain due to the production seasonality and the huge demand [1; 2]. The variation of moisture level not only affects the content of starch, fat and protein, but also affects the quality of rice grain to a certain extent [3; 4; 5]. Therefore, it is an indispensable prerequisite for safe crop storage to determine the rice grain moisture accurately and quickly. Generally, the method for determining moisture content can be divided into direct methods and indirect methods. The precision of direct methods is extremely high, but the complicated and time-consuming operation cannot meet the demands of effective and non-destructive determination. Thus, direct methods, represented by the oven-drying method, are often used as the standard to evaluate the precision of indirect methods. In recent years, researchers have put forward several indirect methods for rapid and non-destructive measurement of rice grain moisture or other cereal crops [6; 7; 8; 9]. Commonly used methods include the resistance method, capacitance method, spectroscopic method and dielectric loss angle method. [PERSON] et al. [10] put forward a moisture detection method for single rice grains using a-film open-ended coaxial probe, which was suitable for the nondestructive measurement of moisture values in the rice grains ranging from 9.5% to 26%. [PERSON] et al. [11] proposed a method for measuring grain moisture content (13-26%) based on dielectric loss tangent and the maximum error was less than 0.5%. [PERSON] et al. [12] designed a paddy rice moisture content meter based on the principle of electrical capacitance properties and the detection accuracy reached 97.83%. Compared with the detection methods mentioned above, NIR spectroscopy has emerged as a more viable means, incorporating the advantages of real time, high precision and non-destructive detection [13]. The information of NIR spectra is mainly derived from O-H, C-H, N-H and other hydrogen-containing groups' internal vibration and absorption in overtone and combination bands. Furthermore, the O-H bond, the only bond in water, has several characteristic absorption peaks in the NIR region [14; 15]. Hence, the moisture content of some seeds and grains can be detected effectively and non-destructively using NIR spectroscopy. [PERSON] et al. [16] determined multiple physicochemical properties of rice grain using a visible and NIR reflectance spectrometer. The results of correlation coefficient of the validation set using partial least squares (PLS) with full cross-validation was higher than 0.8. [PERSON] et al. [17] measured the moisture content (3-34%) of single seed samples and bulk seed samples of Scots pine using NIR spectroscopy. PLS and ordinary least squares (OLS) regression were applied to deal with the spectral data and the prediction accuracy in different spectral range was evaluated. The results suggested that the lowest prediction error was 0.8% for bulk seed samples in the 850-1048 nm spectral range modeled by PLS. [PERSON] et al. [18] monitored the maturation process of wheat seed by NIR spectroscopy. The results pointed out several characteristic bands of water, carbohydrates and proteins, where Water I (1890-1920 nm) and Water III (1150-1165 nm) bands were more sensitive to the variation of water in the maturing wheat seed than the Water II (1400-1420 nm) band, while all three water absorption bands could be used to monitor the changes of water and define the development stage of the wheat seed. [PERSON] et al. [19] applied NIR spectroscopy to analyze the protein, oil, carbohydrate and ash content in maize grain.