| With the improvement of living standard, people demand for edible legumes of higher quality. Because moisture content is the main factor affecting quality of edible legumes, it is important to test this criterion accurately and quickly. Traditional testing methods and instruments of moisture content, however, are time consuming, and they are not applied to edible legumes at the same time. In contrast, the moisture content test of dielectric properties of edible legumes has many advantages.This research was a basic work of developing moisture content detector of edible legumes.With mung beans and small red beans as research objects, this study, by adopting the flat electrode technology, has explored the effect of frequency, moisture content, temperature, bulk density upon the dielectric parameters ( including relative dielectric constantεr' and dielectric loss factorεr"), and has established the mathematical model of moisture content test. The model has been verified. The following conclusions have been drawn:(1) Relative dielectric constant and loss factor of mung beans and small red beans decrease gradually as testing frequency increase within the spectrum from 1k to 1MHz. In addition, with the fixed bulk density, the higher moisture content is, the more quickly the dielectric parameters reduce. The dielectric constant and loss factor reduce sharply when f <100 k Hz, the trend is gentle as f >100 k Hz. Compared with the dielectric constant, the decreasing trend of the dielectric loss factor is more conspicuous and the variation range is wider.(2) When frequency and bulk density is fixed, the relative dielectric constant of mung beans and small red beans increases in accordance with increase of moisture content, speed change is faster as moisture content is high. With different testing frequency, the dielectric loss factor of mung beans and small red beans also increases as moisture content increases. Variation trend becomes mild as testing frequency increases.ε"r increases most significantly as moisture contents increases at 2kHz.(3) Given certain frequency and bulk density, the relative dielectric constant and the dielectric loss factor of mung beans and small red beans increases monotonically as temperature increases. The relative dielectric constant increases most significantly when moisture content increases. In contrast to high testing frequency, the relative dielectric constant and loss factor increase is more sensitive to temperature change at low frequency. (4) With fixed temperature, under certain testing frequency and mutative frequency, no regularity was found between bulk density change and relative dielectric properties of mung beans in response to high and low frequency respectively. But the dielectric constant and loss factor of small red beans increases as bulk density increases, and changing trend is more significant at low frequency than that at high frequency.(5) Under testing frequency of 500kHz, cubic regression equation between the dielectric parameters of mung beans ( including relative dielectric constantεr' and dielectric loss factorεr") and moisture content, temperature has been established; simultaneously the mathematical model of the moisture content about mung beans has been established, and the model has been verified. It is accurate and feasible that moisture content of mung beans is tested by using dielectric properties. With testing frequency of 500kHz, mathematical model of the dielectric parameters of small red beans (including relative dielectric constantεr' and dielectric loss factorεr") and moisture content, temperature, bulk density is found; at the same time the mathematical model of the moisture content of small red beans is established, and the model is verified. It shows that small red beans moisture content is tested using dielectric properties accurate and feasible. |
