Optimization of Preparation Technology for Selenium-Rich Green Tea Powder with a Ball-Grinding Method

Selenium-enriched green tea powder was prepared with Ziyang selenium-rich tea leaves as raw materials by blanching,freeze-drying and ball-grinding methods. The loss rate of selenium in this process was investigated,and the efficiencies of lyophilization and oven drying at 40 ℃ were compared. Grinding time,ratio of big to small ball,and rotation speed were chosen as factors,single factor and optimal orthogonal experiments were adopted to optimize the whole process. Base on the results,the amount of selenium within leaf was decreasing as time under 100 ℃,the loss rate was 7% as the weight changed from 69.58 μg/kg at 0 s to 64.72 μg/kg at 60 s. After 16 h,the water loss rate values were 52.05% in lyophilization and 49.52% at 40 ℃ by oven drying. The factors affecting particle size were following this order:grinding time>ratio of big to small balls>rotational speed. The results showed the optimal parameters of the ball-grinding method were as follows:grinding time 50 min,rotational speed 800 r/min,and ratio of big to small ball:1:6. Laser particle size analyzer and scanning electron microscope were empolyed to characterize the size and shape of the tea powder. The results indicated the morphology of ball milling particles’section was clear,particle size distribution was uniform,D₅₀ was 3.25 μm,and the selenium content in tea powder was(63.18±4.22) μg/kg. In the process condition,the selenium-enriched green tea powder had uniform granules,bright green color and high retention of selenium.