pH-Dependent radical scavenging capacity of green tea catechins

The effect of pH on the radical scavenging capacity of green tea catechins was investigated using experimental as well as theoretical methods. It was shown that the radical scavenging capacity of the catechins, quantified by the TEAC value, increases with increasing pH of the medium. Comparison of the pKa values to theoretically calculated parameters for the neutral and deprotonated forms indicates that the pH-dependent increase in radical scavenging activity of the catechins is due to an increase of electron-donating ability upon deprotonation. The data also reveal that the radical scavenging activity of the catechins containing the pyrogallol (or catechol) and the galloyl moiety over the whole pH range is due to an additive effect of these two independent radical scavenging structural elements. Altogether, the results obtained provide better insight into the factors determining the radical scavenging activity of the catechins and reveal that the biological activity of green tea catechins will be influenced by the pH of the surrounding medium or tissues.     

Application of far-infrared irradiation in the manufacturing process of green tea

Seven kinds of green tea leaves were manufactured with far-infrared (FIR) irradiation, and the physicochemical characteristics of the green tea were determined. Appropriate FIR irradiation during the manufacturing process significantly increased the polyphenolic content of green tea. FIR irradiation at 90 degrees C for 10 min, replacing the roasting step, and of the fully processed green tea leaves (GTP3) increased the total phenol content of green tea from 475.6 to 811.1 mg/g and the total flavanol content from 175.7 to 208.7 mg/g, as compared to the control. Epigallocatechin and epigallocatechin gallate increased from 57.68 and 9.60 mg/g in a nonirradiated control to 89.88 and 16.33 mg/g in GTP3, respectively. Ascorbic acid, caffeine, and nitrite scavenging activities were also increased in GTP3. However, the overall color change of GTP3 was negligible. These results indicate that the chemical properties of green tea are significantly affected by FIR irradiation at specific stages of the manufacturing process of green tea leaves and that this FIR irradiation results in high-quality green tea.     

Degradation of green tea catechins in tea drinks

Green tea cateachins (GTC). namely (-) epicatechin (EC), (-) epicatechin gallate (ECG), (-) epigallocatechin (EGC), and (-) epigallocatechin gallate (EGCG), have been studied extensively for their wide-ranging biological activities. The goal of the present study was to examine the stability of GTC as a mixture under various processing conditions. The stability study demonstrated that GTC was stable in water at room temperature. When it was brewed at 98 degrees C for 7 h, longjing GTC degraded by 20%. When longjing GTC and pure EGCG were autoclaved at 120 degrees C for 20 min, the epimerization of EGCG to (-) gallocatechin gallate (GCG) was observed. The relatively high amount of GCG found in some tea drinks was most likely the epimerization product of EGCG during autoclaving. If other ingredients were absent, the GTC in aqueous solutions was pH-sensitive: the lower the pH, the more stable the GTC during storage. When it was added into commercially available soft drinks or sucrose solutions containing citric acid and ascorbic acid, longjing GTC exhibited varying stability irrespective of low pH value. This suggested that other ingredients used in production of tea drinks might interact with GTC and affect its stability. When canned and bottled tea drinks are produced, stored, and transported, the degradation of GTC must be taken into consideration.     

Effects of tea catechins on the ERE-regulated estrogenic activity

Tea catechins exert many biological effects, including anticancer and antibacterial activities. Also, it is reported that some plant flavonoids exhibit estrogenic activity. In this study, we investigated estrogenic or antiestrogenic activities of catechins in HeLa cells transiently transfected with an estrogen response element (ERE)-regulated luciferase reporter and an estrogen receptor (ER) alpha or ERbeta expression vector. Catechins alone did not induce luciferase (luc) activity in either of the ERs. Addition of 17beta-estradiol (E2) plus epicatechin gallate (ECG) or epigallocatechin gallate (EGCG) at 5 x 10(-6) M resulted in significant decreases in the ERalpha-mediated luc activity compared with that of E2 alone. On the contrary, lower concentrations significantly increased the E2-induced luc activity. Similar effects were observed with tamoxifen. The ERbeta-mediated estrogenic activities were stimulated by catechins. In conclusion, some catechins, particularly EGCG, were antiestrogenic for ERalpha at higher doses, and co-estrogenic for ERalpha at lower doses and for ERbeta. The lower doses were found in human plasma after tea-drinking. In addition, some catechins may be antiendocrine disruptors because they suppressed bisphenol A-induced luc activities.     

Effect of far-infrared irradiation on the antioxidant activity of defatted sesame meal extracts

To determine the effect of far-infrared (FIR) irradiation on the antioxidant activities of sesame meal, half of sesame seeds were FIR-irradiated and then oil was extracted from the seeds. The resulting defatted sesame meal (DSM) was extracted with methanol, and the antioxidant activities of methanolic extract were determined. FIR irradiation of sesame seeds for 30 min increased the total phenol content from 34.0 to 59.0 muM and radical scavenging activity of DSM extracts from 26.40 to 68.76%. The induction time of lipid oxidation of oil added to extracts was also retarded from 0.82 to 0.96 h. According to the gas chromatography-mass spectrometry analysis, several low molecular weight phenolic compounds, such as p-hydroxy benzoic acid, vanillic acid, p-coumaric acid, isoferulic acid, and o-coumaric acid, were frequently detected in FIR-irradiated DSM extracts as compared to unirradiated ones. These results indicated that FIR irradiation of sesame seeds increased the antioxidant activity of methanolic extracts of DSM.     

微波远红外耦合杀青工艺对绿茶品质的影响

为研究微波远红外耦合杀青工艺对绿茶品质的影响,试验分析了杀青时间、杀青功率和投叶量等因素对绿茶香气、滋味品质的影响,通过L9(34)正交试验设计,优化了绿茶微波远红外耦合杀青工艺,并与其他杀青方式进行了比较,结果表明：较佳的耦合杀青工艺为杀青功率微波10 kW +远红外6 kW,投叶量0.5 kg/min,杀青时间先微波1.5 min+后远红外1.5 min。此时杀青的茶叶香气滋味最好,感官品质总分达到91.40,茶多酚、儿茶素、咖啡碱、氨基酸、可溶性糖和芳香油质量分数分别达到17.24%、12.62%、3.10%、3.72%、4.64%、15.40×10-2 mg/g,香气滋味品质好于其他杀青方式,杀青时间是传统杀青的1/4～1/3,能耗成本是传统杀青的1/3～1/2。该杀青工艺克服了单纯使用微波杀青的不足,为绿茶加工生产提供参考。     

Effect of green tea catechins on the postprandial glycemic response to starches differing in amylose content

The effect of tea polyphenols (TPLs), specifically tea catechins, on the postprandial glycemic response to cooked starches differing in amylose contents was investigated. The in vivo test using a mouse model showed a moderate reduction of the postprandial glycemic response to co-cooked normal (containing 27.8% amylose) or waxy corn starch with 10% TPLs (dry weight of starch), while an augmented glycemic response with a delayed blood glucose peak was observed when high amylose corn starch (HAC, containing 79.4% amylose) was used as the starch component. Enzyme kinetics results demonstrated that TPLs noncompetitively inhibit the digestion of waxy or normal corn starch, while the digestion rate of HAC starch was increased in the presence of TPLs, which supports the observed postprandial glycemic responses. Further studies using X-ray powder diffraction showed that the diffraction intensity (area under the diffraction curves) of normal and HAC starch was increased by 45% and 74%, respectively, whereas no change was observed for waxy corn starch. Consistently, dynamic laser light scattering studies using a solution of pure amylose showed an increased hydrodynamic radius of amylose molecules from ～54 nm to ～112 nm in the presence of TPLs. These experimental results indicate that there might exist an interaction between TPLs and amylose, which facilitates the association of amylose molecules to form a special nonordered structure that can produce a high and sustained postprandial glycemic response. Thus, a combination of tea polyphenols and specific starches could be used to manipulate postprandial glycemic response for glycemic control and optimal health.     

Radical scavenging conserves from unused fresh green tea leaves

Green teas were made by inactivating the enzymes present in fresh leaves of coarse/pruned (unused) and normal (used for tea) grades using different sources of thermal energies. Green teas were extracted in a Soxhlet using different solvents. The obtained miscella was subjected to concentration to give the extract. The extract was subjected to solvent-solvent extraction. Solvent extract was concentrated to obtain conserve. The yields of conserves are 17 +/- 0.8 and 15 +/- 0.8% from green teas of normal and coarse tea leaves, respectively. The radical scavenging activity of these extracts was evaluated using a DPPH in vitro model system. The total polyphenol content was also determined and found to be higher in conserves from normal tea leaves. However, radical scavenging activity of conserves from coarse and normal green tea leaves was found to be >90% at 15 ppm concentration. The HPLC profiles of these conserves were used to quantify the total catechin content with the help of calibration curves prepared using authentic samples at known concentrations. The total catechin content is found to be in the range of 55-85%. Results indicated that the extracts from coarse leaves also possess potential biological activity and could be used as nutraceuticals as well as for preservation purposes in food formulations.     

Kinetic characterization of the enzymatic and chemical oxidation of the catechins in green tea

The oxidation of green tea catechins by polyphenol oxidase/O2 and peroxidase/H2O2 gives rise to o-quinones and semiquinones, respectively, which inestability, until now, have hindered the kinetic characterization of enzymatic oxidation of the catechins. To overcome this problem, ascorbic acid (AH2) was used as a coupled reagent, either measuring the disappearance of AH2 or using a chronometric method in which the time necessary for a fixed quantity of AH2 to be consumed was measured. In this way, it was possible to determine the kinetic constants characterizing the action of polyphenol oxidase and peroxidase toward these substrates. From the results obtained, (-) epicatechin was seen to be the best substrate for both enzymes with the OH group of the C ring in the cis position with respect to the B ring. The next best was (+) catechin with the OH group of the C ring in the trans position with respect to the B ring. Epigallocatechin, which should be in first place because of the presence of three vecinal hydroxyls in its structure (B ring), is not because of the steric hindrance resulting from the hydroxyl in the cis position in the C ring. The epicatechin gallate and epigallocatechin gallate are very poor substrates due to the presence of sterified gallic acid in the OH group of the C ring. In addition, the production of H2O2 in the auto-oxidation of the catechins by O2 was seen to be very low for (-) epicatechin and (+) catechin. However, its production from the o-quinones generated by oxidation with periodate was greater, underlining the importance of the evolution of the o-quinones in this process. When the [substrate] 0/[IO4 (-)] 0 ratio = 1 or >1, H2O2 formation increases in cases of (-) epicatechin and (+) catechin and practically is not affected in cases involving epicatechin gallate, epigallocatechin, or epigallocatechin gallate. Moreover, the antioxidant power is greater for the gallates of green tea, probably because of the greater number of hydroxyl groups in its structure capable of sequestering and neutralizing free radicals. Therefore, we kinetically characterized the action of polyphenol oxidase and peroxidase on green tea catechins. Furthermore, the formation of H2O2 during the auto-oxidation of these compounds and during the evolution of their o-quinones is studied.     

Degradation kinetics of catechins in green tea powder: effects of temperature and relative humidity

The stability of catechins in green tea powders is important for product shelf life and delivering health benefits. Most published kinetic studies of catechin degradation have been conducted with dilute solutions and, therefore, are limited in applicability to powder systems. In this study, spray-dried green tea extract powders were stored under various relative humidity (RH) (43-97%) and temperature (25-60 °C) conditions for up to 16 weeks. High-performance liquid chromatography (HPLC) was used to determine catechin contents. Catechin degradation kinetics were affected by RH and temperature, but temperature was the dominant factor. Kinetic models as functions of RH and temperature for the individual 2,3-cis-configured catechins (EGCG, EGC, ECG, and EC) were established. The reaction rate constants of catechin degradation also followed the Williams-Landel-Ferry (WLF) relationship. This study provides a powerful prediction approach for the shelf life of green tea powder and highlights the importance of glass transition in solid state kinetics studies.     

Oxidation of tea extracts and tea catechins by molecular oxygen

Tea polyphenols (PP) are known as potent antioxidants. At the same time, PP have been repeatedly reported to oxidize by molecular oxygen with the formation of active forms of oxygen. In this work, the Clark electrode technique was applied to study the kinetics of the autoxidation of tea extracts and individual tea PP as well as model PP, catechol, gallic acid, and pyrogallol. Aqueous extracts of both green and black teas were found to undergo extensive autoxidation under physiological conditions. The addition of superoxide dismutase (SOD) and milk resulted in a significant decrease in the rate of oxidation. Studied individually, PP were found to autoxidize at a rate, which increased with pH, proportional to PP concentration and nearly proportional to oxygen concentration. The collected data were used for the extrapolation/interpolation of the starting rates of oxidation to the standard conditions (at pH 7.40, 100 microM PP, 200 microM O2). PP oxidizability is basically determined by that of the key PP fragment (pyrogallol > gallate > catechol). Meta-OH groups do not contribute to the oxidation even at pH 13.0. Similar to tea brew, the oxidation of individual PP was inhibited by milk and SOD addition, with catechol being the only exception (the oxidation of catechol was accelerated when SOD was added). Comparison of the autoxidation of PP (o-hydroquinones) with that of p-hydroquinones (Roginsky, V.; Barsukova, T. K. J. Chem. Soc., Perkin Trans. 2 2000, 1575-1582) displays the dramatic difference both in the oxidizability and in the kinetic regularities. The difference in the kinetics has been suggested to be due to the difference in the initiation of the chain process. Whereas for p-hydroquinones the oxidation is initiated by the reaction between hydroquinone and a corresponding quinone, the oxidation of o-hydroquinones is likely started by direct interaction between substrate and molecular oxygen. As the second process is much slower, this may explain the relatively low oxidizability of PP as compared to p-hydroquinones.     

Inhibition of radical reaction of apolipoprotein B-100 and alpha-tocopherol in human plasma by green tea catechins

(-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECg), (-)-epigallocatechin gallate (EGCg), and Trolox inhibited the decreases of apolipoprotein B-100 (apoB) and alpha-tocopherol in a radical reaction of human plasma initiated by Cu(2+). The concentrations of EC, EGC, ECg, EGCg, and Trolox for 50% inhibition (IC50) of apoB fragmentation were 39.1, 42.2, 14.6, 21.3, and 36.2 microM, respectively. Similar IC50 values were observed for alpha-tocopherol consumption, indicating the close relationship between apoB fragmentation and alpha-tocopherol consumption. These results demonstrate that tea catechins serve as an effective antioxidant in plasma and that the gallate group has a strong antioxidative activity.     

Molecular dynamics study on the biophysical interactions of seven green tea catechins with lipid bilayers of cell membranes

Molecular dynamics simulations were performed to study the interactions of bioactive catechins (flavonoids) commonly found in green tea with lipid bilayers, as a model for cell membranes. Previously, multiple experimental studies rationalized catechin's anticarcinogenic, antibacterial, and other beneficial effects in terms of physicochemical molecular interactions with the cell membranes. To contribute toward understanding the molecular role of catechins on the structure of cell membranes, we present simulation results for seven green tea catechins in lipid bilayer systems representative of HepG2 cancer cells. Our simulations show that the seven tea catechins evaluated have a strong affinity for the lipid bilayer via hydrogen bonding to the bilayer surface, with some of the smaller catechins able to penetrate underneath the surface. Epigallocatechin-gallate (EGCG) showed the strongest interaction with the lipid bilayer based on the number of hydrogen bonds formed with lipid headgroups. The simulations also provide insight into the functional characteristics of the catechins that distinguish them as effective compounds to potentially alter the lipid bilayer properties. The results on the hydrogen-bonding effects, described here for the first time, may contribute to a better understanding of proposed multiple molecular mechanisms of the action of catechins in microorganisms, cancer cells, and tissues.     

Changes in the concentrations of acrylamide, selected odorants, and catechins caused by roasting of green tea

This research aims to optimize roasted green tea (Houjicha) processing by using roasting treatments to achieve acrylamide mitigation without compromising the quality. 2-Ethyl-3,5-dimethylpyrazine and 2-ethyl-3,6-dimethylpyrazine were identified as potent odorants by aroma extract dilution analysis. In preliminary sensory experiments, the desirable Houjicha flavor was produced in products roasted at 160 degrees C for 30 min and at 180 degrees C for 15 min. Under these conditions, potent odorants were formed at levels adequate for contributing to the Houjicha flavor. Acrylamide amounts in tea infusions were 2.0 and 4.0 microg/L by roasting at 160 degrees C for 30 min and at 180 degrees C for 15 min, respectively. Compared to roasting at 180 degrees C, the degradation of tea catechins was suppressed by roasting at 160 degrees C. Hence, roasting at 160 degrees C for is recommended for Houjicha processing for acrylamide mitigation, formation of potent odorants, and suppression of degradation of tea catechins.     

Effect of far-infrared radiation on the antioxidant activity of rice hulls

After far-infrared (FIR) radiation onto rice hull, a methanolic extract was prepared for the determination of antioxidant ability. After 30 min of FIR treatment, the radical scavenging activity and total phenol contents of rice hull extracts increased from 47.74 to 79.63% and from 0.12 to 0.19 mM, respectively, compared to control. Inhibition of lipid peroxidation in extracts was also increased from 41.07 to 47.96%. According to the GC-MS analysis, more phenolic compounds (p-coumaric acid, 3-vinyl-1-oxybenzene, p-hydroxybenzaldehyde, vanillin, p-hydroxybenzoic acid, and 4,7-dihydroxyvanillic acid) were detected in FIR-irradiated rice hull extract. These results indicated that FIR radiation onto rice hull could liberate and activate covalently bound phenolic compounds that have antioxidant activities.     

Antioxidative activity of green tea polyphenol in cholesterol-fed rats

This study investigated the effects of green tea polyphenol on the serum antioxidative activity and cholesterol levels of cholesterol-fed rats and compared them with those of probucol, an antioxidant hypocholesterolemic agent. To evaluate the antioxidative activity, the susceptibility to oxidative modification of low-density lipoprotein (LDL) isolated from the serum of cholesterol-fed rats was measured, as was the serum antioxidative activity using the spontaneous autoxidation system of brain homogenate. Administration of green tea polyphenol effectively inhibited LDL oxidation and elevated serum antioxidative activity to the same degree as probucol. However, higher amounts of polyphenol than probucol needed to be administered to reduce the total, free, and LDL cholesterol levels. Furthermore, green tea polyphenol increased the levels of high-density lipoprotein (HDL) cholesterol, leading to dose-dependent improvement of the atherogenic index, an effect that was not seen with probucol. Thus, green tea polyphenol may exert an antiatherosclerotic action by virtue of its antioxidant properties and by increasing HDL cholesterol levels.     

Anticarcinogenic activity of selenium-enriched green tea extracts in vivo

Both selenium and green tea have been shown to have potential antitumor effects. Here we have investigated the anticarcinogenic effect of the selenium-enriched green tea extract (Se-TE) in a Kunming mice model transplanted with human hepatoma cells HepG2. Mice were assigned to 8 groups consisting of 10 mice each after tumor cell inoculation. The control group received only water, whereas the remaining groups received regular green tea extract (RT), Se-TE which was produced by fertilization with selenite on tea leaves, selenite, and RT + selenite. After the mice were fed intragastrically with these agents for 8 days, tumor growth in RT-, Se-TE-, and selenite-fed mice was significantly suppressed, compared with that in control mice (P < 0.001). Supplementation with Se-TEs and selenite was able to elevate mice blood and liver Se concentrations, but did not significantly enhance selenoprotein glutathione peroxidase and other antioxidant enzyme superoxide dismutase activity in mice blood and liver. These results suggest that the antitumor function of Se-TEs may be attributed to the oxidative stress induced by selenium and green tea components in a suitable selenium supplementation pathway.     

Stability of tea catechins in the breadmaking process

A green tea extract (GTE) was incorporated into bread as a source of tea catechins. The stability of tea catechins in the breadmaking process including unfrozen and frozen dough was studied. A method was developed for the separation and quantification of tea catechins in GTE, dough, and bread samples using a RP-HPLC system. The separation system consisted of a C18 reversed-phase column, a gradient elution system of water/methanol and formic acid, and a photodiode array UV detector. Tea catechins were detected at 275 nm. GTEs at 50, 100, and 150 mg per 100 g of flour were formulated. The results obtained showed that green tea catechins were relatively stable in dough during freezing and frozen storage at -20 degrees C for up to 9 weeks. There were no further detectable losses of tea catechins in bread during a storage of 4 days at room temperature. It was also revealed that (-)-epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC) were more susceptible to degradation than (-)-epicatechin gallate (ECG) and (-)-epicatechin (EC). (-)-EGCG and (-)-ECG were normally selected as the quality indices of green tea catechins, and their retention levels in freshly baked bread were ca. 83 and 91%, respectively. One piece of bread (53 g) containing 150 mg of GTE/100 g of flour will provide 28 mg of tea catechins, which is approximately 35% of those infused from one green tea bag (2 g).     

Effect of green tea supplementation on insulin sensitivity in Sprague-Dawley rats

Epidemiological observations and laboratory studies have shown that green tea has a variety of health effects, including antitumor, antioxidative, and hypolipidemic activities. The aim of this study was to examine whether it had an effect on glucose tolerance and insulin sensitivity in Sprague-Dawley rats. In experiment 1 (in vivo study), rats were divided into two groups: a control group fed standard chow and deionized distilled water and a "green tea" group fed the same chow diet but with green tea instead of water (0.5 g of lyophilized green tea powder dissolved in 100 mL of deionized distilled water). After 12 weeks of green tea supplementation, the green tea group had lower fasting plasma levels of glucose, insulin, triglyceride, and free fatty acid than the control rats. Insulin-stimulated glucose uptake of, and insulin binding to, adipocytes were significantly increased in the green tea group. In experiment 2 (in vitro study), a tea polyphenol extract was used to determine its effect on insulin activity in vitro. Green tea polyphenols (0.075%) significantly increased basal and insulin-stimulated glucose uptake of adipocytes. Results demonstrated that green tea increases insulin sensitivity in Sprague-Dawley rats and that green tea polyphenol is one of the active components.