Türkiye Kökenli Yeşil Çayın Antioksidan Kaynağı Olarak Değerlendirilmesi

Abstract

Serbest radikal zincir reaksiyonu şeklinde gerçekleşen yağların otooksidasyonu, günümüzde gıda ve sağlık sektöründe önemli bir problem olarak karşımıza çıkmaktadır. Otoksidasyon reaksiyonu sonunda doymamış yağ asitleri içeren yağlar ve bu yağlan içeren gıda maddeleri bozularak besleyici özelliklerini kaybederken, canlı organizmalarda da yaşlanma kalp hastalığı ve kanser gibi oluşumlar başlamaktadır. Antioksidanlar yağlarda serbest radikal zincir reaksiyonunu engellemek ve yağların oksidasyon stabilitelerini arttırmak amacıyla, özellikle gıda sektöründe geniş uygulama alam bulmuşlardır. Ucuzlukları ve yüksek stabilkeleri nedeniyle BHT, BHA, TBHQ gibi sentetik antioksidanlar gıda maddelerinde yaygın olarak kullanılmaktadır. Ancak son yıllarda sentetik katkı maddelerinin zehirli olmaları ve yan etkileri konusunda şüphelerin oluşmasından dolayı doğal kaynaklardan antioksidan eldesi yollan araştırılmaya başlanmıştır. Doğal antioksidan kaynaklan; baharatlar, şifalı bitkiler, çay, enzimler, proteinler, çeşitli meyve ve sebzelerdir.. Bu çalışmada, Türk kaynaklı çaylardan çeşitli çözücüler kullanılarak antioksidan ekstraksiyonu yapılmış, çay ekstraktlannın toplam polifenol içerikleri tespit edilmiş ve hızlandırılmış oksidasyon testleri ile de antioksidan aktiviteleri incelenmiştir. Çaylardan antioksidan ekstraksiyonunda suyun çaydan çok miktarda madde çözdüğü, metanolun ise seçimli olarak polifenolleri çözdüğü görülmüştür. Deneylerde kullanılan kurutulmuş yeşil ve paket yeşil çaylar antioksidan aktivitesi göstermişler, ancak siyah çay hiç bir aktivite göstermemiştir.

Lipids that are rich in polyunsaturated fatty acids play an important role in human health and nutrition. The autoxidation of polyunsaturated fatty acids in fats and oils leads to the formation of hydroperoxides, which can damage the quality of foods and are considered to be detrimental to the health. Degradation of highly unsaturated fatty acids, via a free-radical chain mechanism, results in changes in odor and flavor of fats and oils and/or lipid containing foods. The steps of mechanism are depicted in Figure 1. oxidation site in oil molecule (RH) tatty free radical (R) peroxide free radical (ROO) + H- r *\ HHH I I I -c-c=c- I o I o I ^ H J hydroperoxide (ROOH) Figure 1. Free Radical Oxidation of Vegetable Oil. The first step in free-radical chain mechanism is the formation of a fatty free radical. This occurs when hydrogen is lost from the a-methylenic carbon in the fatty acid group. The fatty free radical is readily suspectible to attack by atmospheric oxygen, resulting in the formation of peroxides and hydroperoxides. As a final step, hydroperoxides may split into smaller organic compounds, such as aldehydes, ketones, alcohols and acids, which give the obnoxious odors and flavors characteristic of oxidative rancidity in vegetable oils. Antioxidants are used to preserve lipids and lipid-containing foods, by retarding rancidity, discoloration or deterioration due to autoxidation. Hence, the addition of antioxidants to foods has become increasingly common as a means of increasing the shelf-life and improving the stability of lipids and lipid-containing foods. Ingold classified all antioxidants into two groups, namely primary or chain- breaking antioxidants, which can react with lipid radicals to convert them to more stable products and secondary or preventive antioxidants which reduce the rate of chain initiation by a variety of mechanisms. Primary antioxidants are those substances which function by inhibiting or interrupting the free radical mechanism of glyceride autoxidation. Their ability to do this is based on their phenolic structure or the phenolic configuration within their molecular structure. As depicted in Figure 2 in a very simplified fashion, the primary antioxidant or phenolic substance functions as a free radical acceptor, thus terminating the oxidation at the initiation step. The antioxidant free radical which forms is stable and importantly, will not propagate further oxidation of the glyceride. OH O O O R- + J\ _ /\ /K A/h _^ RH + V \/ V N/ +-*? ?*-*? fatty phenol oil antioxidant free radical free radical molecule (stable resonance hybrids) Figure 2. Phenolic antioxidant mechanism İn vegetable oil. Synthetic antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ) are phenolic antioxidants with only one hydroxyl group in their chemical structures, act as free radical scavengers or chain-breakers. Synthetic antioxidants are widely used because of their high stability, low cost and efficacy. However, the safety and toxicity of them have been important concerns. Plants synthesize the well-known antioxidants tocopherols, ascorbic acid and carotenoids. Although tocopherols are considered as safe natural antioxidants, they do not always provide effective protection against oxidation, especially when oils are contaminated with trace amounts of metal ions such as Fe** or Cu1^. In the early 1980s, consumers began looking for natural ingredients in their foods. This trend has promoted extensive research in the area of natural antioxidants. Sources of natural antioxidant compounds are spices, herbs, tea, oils, seeds, cereals, grains, fruits, vegetables, enzymes and proteins. Naturally occurring antioxidative components in foods include flavonoids, phenolic acids, lignans, terpenes, tocopherols, phospholipids and polyfunctional organic acids. Flavonoids are phenol derivatives synthesized in substantial amounts and widely distributed in plants. They can be subdivided into six classes of flavonoids and flavonoid-related compounds: flavones, flavanones, isoflavones, flavonols, flavanols and anthocyanins. The flavanols, epicatechin polyphenols from tea, are the group of compounds containing the most powerful antioxidants. The activity increases with an increase in the number of o-dihydroxy groups. In recent years, tea has attracted much attention because of its high polyphenol content that show potent antioxidant activity. XI Tea is the most widely consumed beverage worldwide. While a wide variety of herbs are currently used for making tea, for regular tea there is only one kind of tea plant, known as Camellia sinensis. The tea plant is an evergreen shrub (occasionally growing to tree size) with many branches. The leaves are dark green and roundish, with the young leaves being rather hairy. Its flowers are white and usually solitary. The part of the tea plant used in making any tea is the leaf bud and the two adjacent young leaves along with the stem, broken between the second and third leaf. Older leaves are considered inferior and are often the stuff of instant teas. The young leaves and leaf bud are called "tea flush". There are three basic types of regular tea; green, black and oolong. All three types of tea come from the same tea plant ; the differences in the teas are a result of processing methods, degree of fermentation and the oxidation of the polyphenols present in tea leaves. Black tea is made by withering, rolling, fermenting and drying the leaves. During the production of black tea leaves, there is extensive enzymatic oxidation of the leaf polyphenols to form dark products such as theaflavins and thearubigens. These products are responsible for the characteristic color of tea brews, their astringency and unique taste. Green tea is made by steaming and drying. The leaves aren't permitted to ferment and oxidize as in black tea, thereby preserving many more nutrients - particularly the polyphenols known as catechins. Oolong tea is allowed to partially oxidize. Green tea is a major beverage in Asian countries such as China and Japan, while black tea is more popular in North America and Europe. Taste, rather than nutrient content, is credited for the higher demand for black tea. Polyphenols have a strong taste and as black tea hasn't got high poyphenol content, it also has a milder flavor. Green tea contains many nutrients, but the primary nutritious constituents are the polyphenols. The term polyphenol denotes the presence of multiple phenolic rings in the chemical composition. The total content of polyphenols in tea flush is 25-35% on a dry weight basis. The chief polyphenols in green tea are flavonoids such as catechin with the four major polyphenols being epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate. Of the four, epigallocatechin gallate is regarded as the most significant active constituent. Other compounds found in green tea include amino acids, caffeine, organic acids, proteins, sugars and chlorophyll. In this study, the total polyphenol contents of different Turkish tea extracts and their antioxidant activities were investigated. Three kinds of tea were used in the experiments. The dried green tea was prepared from the fresh tea leaves of Rize in 1997. The processed green tea was obtained from Çay Kurumu and black tea was obtained from the market. Different solvents; water, methanol, ethanol and ethyl acetate were used to extract antioxidants from teas. 20 grams of sample from each tea were extracted with 200 mL of solvent for one and a half hour. After filtering, the residues were extracted with 100 mL of solvent for 30 min and filtered again. The filtrates were combined and evaporated in a rotary evaporater to 100 mL. 10 mL of this mixture was taken and evaporated to a constant weight. Dry weight of various tea extracts are given in Table 1. xn Table 1. Dry weight of tea extracts. Through the extracts, water extracts were containing the higher amounts of soluble matter. The extractable matter increased with increasing of the solvent's polarity. The total phenolic contents of tea extracts were tested by Method (9. 110) of the Association of Official Analytical Chemists (AOAC). Briefly, lmL tea extract was added into a 100 mL volumetric flask containing 75 mL distilled water. 5 mL Folin- Ciocalteu reagent and 10 mL saturated sodium carbonate solution were added to the flask and diluted to 100 mL with distilled water. The mixture was then shaken for 1 min and allowed to stand at room temperature for 30 min. The absorbance of the solution was viewed in a spectrophotometer at 760 nm. A standard curve was prepared with caffeic acid at concentrations ranging from 0-lOOjig. The quantity of total polyphenolics in the sample was calculated as caffeic acid equivalents by using the standard curve. The total polyphenolic contents of tea extracts are given in Table 2. Although tea was much more soluble in water than other solvents, methanol extracts had the higher content of polyphenols. Table 2. The total polyphenolic contents of tea extracts. The efficiency of the solvent extraction was determined by testing the effect of different mixtures of water and methanol on total phenolic contents of the extracts. The efficiency of extraction as measured by total phenols increased with decreasing methanol content of different water-methanol mixtures. xm Figure 3. Effect of methanol contents of water-methanol mixtures on extraction of total polyphenols in packet green tea The antioxidant activities of methanolic and ethanolic extracts of teas were determined in accelerated oil oxidation which was examined by peroxide value. The antioxidant activities of tea extracts were compared with BHT, all at 200 ppm, in sunflower oil, corn oil and soybean oil. Black tea extracts didn't show any antioxidant activity. But both green tea extracts showed antioxidant activities in oils. Ethanol extracts of green teas showed better activity than methanol extracts. Ethanol could serve as a good carrier to make tea extracts soluble in oil. Peroxide values of ethanol extracts of teas compared with BHT in sunflower oil at 70°C is shown in Figure 4. Green tea extracts showed an antioxidant activity but it was not as high as sage and rosemary extracts that exploited commercially. Also tea extracts had a dark- green color which will be a problem when adding to oils. But green tea polyphenols are very soluble in water and can be taken by drinking green tea. So it is better to make a habit of drinking green tea instead of black tea. xiv 140 10 -Control -Dried Green Tea -Packet Hack Tea -BHT -Packet Green Tea Figure 4. Changing of the peroxide values of sunflower oil in which the ethanolic extracts of teas were added at 70°C.