Effects on physicochemical properties of black tea by mechanical superfine and general grinding

Superfine grinding technology is widely applied in food industry nowadays. Tea is one of the most popular non-alcoholic beverages around the world. The deep processing of tea with superfine grinding has been carried out in order to make full use of tea by improving solubility and dispersity. To investigate the effects on particle characteristics and extraction properties of tea by mechanical superfine grinding, superfine tea powders were prepared by CJM-SY-B ultrafine vibration grind mill by grinding for 8 hours, while coarse tea powders (grinded under 1.00, 0.50, 0.25 and 0.12 mm meshes) were produced by Retsch ZM 200 centrifugal mill as control. The physicochemical properties of superfine tea powders and coarse tea powders were compared. Particle size distributions were determined by Malvern particle size analyzer. Considering the plant cell size of 8-90 μm, superfine grinding could achieve cell pulverization by decreasing the average particle size to 13.67 μm. However, the average particle sizes of coarse powders were 228.67, 161.00, 140.67, 79.07 μm, respectively. According to the scanning electron microscopy, intact tissue structures could been observed in coarse tea powders but only small cell debris were shown in superfine powders which indicated that tea leaves were effectively pulverized by breaking down the plant structure and tissues. Determinations of the specific surface area were performed by gas adsorption using the Brunauer-Emmett-Teller (BET) method. Superfine tea powders increased 2-7 times compared to coarse tea powders due to the reduction in particle size. Furthermore, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the crystallinity index and surface element ratio of oxygen and carbon (O/C) for tea powders. For particles <1.00, <0.50, <0.25 mm and superfine powder, the crystallinity indexes were 17.70, 20.95, 18.80, 19.85 and 12.68, respectively, and the ratios of oxygen and carbon were 0.40, 0.41, 0.41, 0.41 and 0.50, respectively. The significant decrease in crystallinity for superfine powders was in consequence of the destruction of cell wall while the O/C indicated more exposure of cellulose and hemicellulose on the surface for micron-sized particles. However, Fourier transform infrared spectroscopy suggested no change in functional group within the five powders. Particle characteristics indicate that four coarse grinding powders have similar particle properties while superfine powder makes a tremendous difference compared with coarse powders. Superfine grinding can disrupt the cell structure. In addition, extraction properties of tea polyphenols, caffeine, water soluble sugar and water soluble solids were measured at different time with boiling water for the different tea powders. Tea polyphenols and soluble sugar were determined with an ultraviolet and visible spectrophotometer. Caffeine was analyzed by high performance liquid chromatography. Water soluble solids were determined by oven drying of the filtered residue at 105℃ to constant weight. In brief, all the ingredients gradually dissolved as time went on until equilibrium. No obvious difference was shown among the powders for tea polyphenol and caffeine due to the sufficient solubility of micromolecule at high temperature. However, water-soluble sugar in superfine powder was as twice as those in coarse powders. Besides, water-soluble solids of superfine powders increased by 10%. The increase in water soluble components could be related to the cell wall breakage, where insoluble ingredients could be converted to soluble materials to some extent, which was in accord with the particle characteristics for superfine powders. In conclusion, this study shows that superfine grinding of tea can effectively reduce the particle size and crystallinity index, increase the specific surface area, expose more inner fibers on the surface and increase water-soluble contents by pulverising plant tissues and breaking down the cell wall. With better solubility, superfine grinding is beneficial for the utilization of tea powders. © 2016, Chinese Society of Agricultural Engineering. All right reserved.