Abstract:
Harvested fresh rice kernels would encounter compress load with various intensities in the sequent drying, storage and milling processes. Excess compress load which exceed the fracture load of the rice kernel may cause rice cracking, which reduces the head rice yield and its economical value of the rice kernel. The compress fracture load is an important property parameter of paddy rice in sequent processing and it is necessary to know exactly the value of this parameter. In this paper, the compression fracture load of rice kernel was determined using the statistics analysis method. A series of mechanical compression experiments were conducted on the rice kernels with various moisture contents of 14%-21% and temperatures of 16-60 ℃ and the value of the compression fracture load in each experiment was recorded. Through the large sample statistics analysis, it was found that under the same temperature and moisture content, the compression fracture load of rice kernels had different values which may be caused by the various growing environment and the statistical distribution characteristics was demonstrated for the compression fracture load. In the view of statistics, the concept of the median and large compress fracture load were proposed and used to express the compression fracture load statistically. In the case of the moisture content of 14% and temperature of 16 ℃, the rice kernels was determined to have the median fracture load F50 of 63 N and the large fracture load F90 of 80 N. The effects of two important process parameters of the rice temperature and moisture content, on the compression fracture load of rice kernel were also investigated and it was found that the compression fracture load decreased with the increase of the rice kernel temperature, and increased with the decrease of the moisture content of the rice kernel, and the moisture content had a greater impact on the fracture load than the temperature. When the moisture content of the rice kernel was decreased from 21%, to 18%, 16% and 14% at the temperature of 16 ℃, the median compression fracture load F50 was 47, 51, 58, 63 N and the large compression fracture load F90 was 60, 65, 74, 80 N. When the moisture content of the rice kernel was decreased from 21%, to 18%, 16% and 14% at the temperature of 60 ℃, the median compression fracture load F50 was 27, 33, 42, 49 N and the large compression fracture load F90 was 35, 42, 52, 60 N. The glass transition theory played an important role in explaining the change of the fracture load of rice kernel. In this paper, when the rice kernel from a high temperature and high moisture content (60 ℃, 21%) status changes into a low temperature and low moisture content (16 ℃, 14%) one, it transits correspondingly from the rubber state into the glass state, and the large fracture load F90 increased from 35 to 80 N. The fracture load of rice kernels obtained in the view of statistics have more accurate physical meaning and was helpful to rice process optimization and improvement of rice quality.