微奈米綠茶之理化特性與抗氧化活性研究

Abstract

綠茶中因含有豐富的兒茶素，但兒茶素水溶性不高，故市售綠茶飲料的加工過程無法將茶葉之兒茶素完全萃取出，同時亦會產生大量廢棄物，如：茶葉渣與茶梗等。據文獻指出經介質研磨的奈米級藥物，可有效提高其於水中的溶解度，並可完全利用物料而無廢棄物之產生。本研究將比較微奈米綠茶、傳統水萃綠茶與微奈米茶渣製成之綠茶產品在理化特性、化學成分與抗氧化活性之差異 。 適當研磨條件為0.3 mm釔鋯珠、3500 rpm轉速，水懸浮液之綠茶粉濃度為0.38% (w/v)，研磨時間90分鐘，粒數平均粒徑降至108 nm、體積平均粒徑降至7.83 μm；以相同操作條件研磨水萃綠茶之茶渣，微奈米茶渣之粒數平均粒徑降至114 nm、體積平均粒徑降至16.87 μm，顯示介質研磨降低綠茶粉與綠茶渣之粒徑均達到微奈米等級。 在理化性質方面，綠茶粉懸浮液、綠茶渣懸浮液經微奈米化後，pH值下降，可能由於酸性物質於研磨過程中釋放；色澤則不若水萃綠茶明亮與黃綠。以每克原料綠茶粉換算有效成分的釋放量，微奈米綠茶可釋放較大量兒茶素與茶胺酸，且介質研磨可能將EGCG轉變為EGC與gallic acid；微奈米綠茶可釋放111.43 mg總酚，顯著較水萃綠茶(80.31 mg)與微奈米茶渣(38.82 mg)為高；微奈米綠茶可利用之總膳食纖維含量為66.14 g/100 g綠茶粉乾重，顯著較水萃綠茶與微奈米茶渣加總(53.49 g/100 g綠茶粉乾重)為高。顯示微奈米綠茶可較傳統水萃綠茶與微奈米茶渣有效利用每克的綠茶粉原料。 以每克綠茶粉可提供之抗氧化能力作為比較基準，微奈米綠茶的抗氧化能力較水萃綠茶佳，其抗氧化能力主要來自酚類化合物，總酚含量與ORAC或Tlag之間的相關性(R2)均達0.88以上。於細胞抗氧化試驗，微奈米綠茶組有無PBS wash處理並不顯著影響防禦細胞被自由基侵襲的能力，顯示微奈米綠茶中化學成分組成較易進入細胞或與細胞膜表面有緊密連接，或是小粒徑的粒子較多，較容易與細胞作用而提供抗氧化能力。 本研究發現介質研磨可增加綠茶粉中有效成分的利用，使微奈米綠茶之總兒茶素、茶胺酸與總酚收率較水萃綠茶為高，而膳食纖維含量亦大幅提高；微奈米綠茶可開發為新型保健產品，作為國人補充抗氧化物質與膳食纖維來源之一。介質研磨方式可有效利用原料綠茶粉，或許將來可發展作為新式提取兒茶素或多酚類物質的方法。

Green tea contains abundant catechins, but the extraction yield of catechins is low due to its poor water-solubility. Large amounts of wastes including tea leaves and stems are produced during the commercial processing of green tea beverages. Recently, nanotechnology via media milling has been reported to improve some poorly water-soluble drugs into stable, biologically active nanoparticlulate dispersion and enable the use of whole raw materials. This thesis aims at preparing micron/nano-sized green tea by media milling and finding the appropriate media-milling conditions. Physicochemical properties, chemical components and antioxidant activities will be compared among micron/nano-sized green tea suspension, green tea extract and micron/nano-sized green tea waste. The appropriate milling condition was to use 0.3 mm Y.Z.T. (yttria-stabilized zirconia) beads and 3500 rpm to mill green tea powder suspension (0.38%, w/v) for 90 min. The particle size of micron/nano-sized green tea was 108 nm of number mean diameter (Dnm) and 7.83 μm of volume mean diameter (Dvm); the particle size of micron/nano-sized green tea waste was 114 nm of Dnm and 16.87 μm of Dvm. The green tea powder suspension and waste suspension were ground to micron/nanoscales. For physicochemical properties, the Hunter L, a, b color results showed that green tea extract was more bright and yellow-green than the media-milled products; the media-milled products had lower pH that might be caused by the release of acidic substances. Comparison of the chemical components (on one-gram basis of dry green tea powder) among the micron/nano-sized green tea suspension, green tea extract and micron/nano-sized green tea waste showed that micron/nano-sized green tea could release more catechins and L-theanine. Some components such as EGCG would be converted into EGC and gallic acid. Micron/nano-sized green tea released 111.43 mg of total phenollics that was significantly higher than those of the green tea extract (80.31 mg) and micron/nano-sized green tea waste (38.82 mg). Total dietary fiber of micron/nano-sized green tea was found to be 66.14 g/100 g of dry green tea powder that was higher than the sum of those from green tea extract and micron/nano-sized green tea waste (53.49 g/100 g green tea powder). These results showed that micron/nano-sized green tea could release higher amounts of effective components than green tea extract. Comparison of antioxidant activities among micron/nano-sized green tea suspension, green tea extract and micron/nano-sized green tea waste on one-gram basis of dry green tea powder showed that micron/nano-sized green tea had higher antioxidant activity such as ORAC and LDL oxidation. The presence of abundant phenolic components might lead to a high antioxidant activity, while the correlations (R2) between ORAC and total phenolics as well as Tlag and total phenolics were above 0.88. In CAA assay, with or without PBS wash treatment did not affect the antioxidant activity of micron/nano-sized green tea significantly. It showed that the chemical components in micron/nano-sized green tea were easily uptake by cells or bound to the surface of cell membrane, or there might be some other smaller particles interacting with cells to present antioxidant activity. This study showed that media-milling process could release high amounts of effective components. The yields of total catechins, L-theanine, total phenolics and dietary fiber from micron/nano-sized green tea were higher than those from green tea extract and therefore, micron/nano-sized green tea was expected to be a new green tea product with rich source of antioxidant and dietary fiber. Media-milling process could help collect effective components from green tea powder, it is also possible develop a new method to collect catechins and other phenolic components.