Improving milling quality of brown rice with compound enzyme solution humidification

Generally, lots of broken rice is generated and high energy consumption occurs during the rice milling processing, which bring adverse effect to the economic benefits of rice milling enterprises. Therefore, milling quality improvement is of important significance in practice for rice processing industry. Moisture conditioning treatment for the brown rice, which is an advanced technology, can increase head rice yield and reduce the milling energy consumption to some extent. It reduces the maximum compressive strength and maximum tensile strength with the increase of moisture content and generated moisture gradient within the rice by gradual penetration from the exterior into the interior of kernels. However, it does not fundamentally solve the problem caused by the resistant cortex of brown rice. The brown rice's tough cortex is mainly caused by the coarse fiber structure, whose major components are cellulose and araboxylan. This study proposes a new process with composite enzyme of cellulase and xylanase instead of the conventional humidifying method. This method substantially decreases broken rice in the milling process and reduces the milling energy consumption to improve milling quality. The humidification of compound enzyme solution selectively degrades the cortex of brown rice, which can not only reduce the hardness of brown rice kernel and decrease the whitening pressure, but also facilitate the moisture penetration, which would boost cortex softening and increase the grain's toughness. Moreover, the synergy of compound enzyme can improve the efficiency of the enzymatic hydrolysis. A central composite rotatable orthogonal experimental design of response surface methodology with 4 factors and 5 levels was employed. Taking brown rice in storage with moisture content below 15% as raw material, the influences of the 4 parameters including cellulose-to-xylanase mass ratio, concentration of compound enzyme solution, adding liquid proportion and enzyme treating time on the head rice yield and the milling energy consumption in the humidification process of compound enzyme solution were investigated, as well as the optimal parameters of this process. Mathematical models for the influences of various parameters on the head rice yield and the milling energy consumption were established. The test data were processed with Excel, Matlab and Design Expert. Experiment results showed that the parameters of compound enzyme solution humidification had significant impact on the head rice yield and the milling energy consumption. The optimized pretreatment parameters were as follows: the cellulose-to-xylanase mass ratio was 1.3:1 g/g, the concentration of composite enzyme solution was 65 mg/mL, the adding liquid proportion was 1.25% and the enzyme treating time was 102 min. Under the optimized parameters, the head rice yield was 80.07% and the milling energy consumption was 90.72 kJ/kg. To verify the optimization results, 3 groups of rice milling tests were made under the optimal combination of parameters, and the experiment results showed that there was no significant difference between predicted values and observed values. Contrast validation experiments indicated that the head rice yield and the milling energy consumption were respectively 3.98% higher and 13.06% lower than the values from the moisture conditioning treatment, and 0.98% higher and 5.48% lower than the values from the cellulase solution humidification. The cortex crude fiber degradation of brown rice proved to be the main reason for the improvement of rice milling quality through the optics electronic microscope analysis. This research can provide a theoretical basis for the new technology of rice milling pretreatment.