机械超微粉碎与不同粒度常规粉碎对红茶理化特性的影响

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

为探究机械超微粉碎对茶叶物性及功能成分浸出特性的影响,该文通过超微粉碎和常规粉碎2种方法制备超微红茶粉和4种粒度的粗粉红茶粉,研究了5种茶粉的粒度分布、微观形貌、比表面积、结晶度、表面氧碳元素比、中红外图谱和功能成分的浸出特性。结果表明:常规粉碎不同粒度茶粉间差异较小,而超微粉碎较大程度地改变了茶叶的物料性质。超微粉碎后,茶粉粒度减小至微米级,比表面积增大,茶粉的结晶度降低,表面暴露的纤维素和半纤维素相对含量增多,而中红外谱图显示超微粉碎茶粉的官能团结构并没有改变。沸水浸提过程中茶多酚、咖啡因、可溶性糖和水浸出物的溶出结果表明:5种茶粉功能成分的浸出量都随时间的增加而增加,经过一段时间后达到平衡。在达到浸出平衡时,不同粒度茶粉的茶多酚和咖啡因含量差异不显著,超微茶粉的可溶性糖含量为4种粗粉的2倍左右,水浸出物比粗粉茶粉提高10%以上。上述研究结果表明:超微粉碎能够有效减小茶粉的粒度,破坏植物的细胞壁结构,使内部成分暴露于表面,此外,组织结构和细胞壁的破坏使不溶的纤维成分转化成可溶性成分,可溶性糖和水浸出物显著增多,超微粉碎能够在一定程度上提高茶粉的有效利用率。

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 °C 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.