Kinetic study of the thermal stability of tea catechins in aqueous systems using a microwave reactor

Tea catechins may undergo complex reactions such as oxidation, polymerization, and epimerization during thermal processing. The thermal stability of tea catechins in an aqueous system, including degradation and epimerization reactions, was investigated using a microwave reactor. Reactions were controlled at high temperatures ranging from 100 to 165 degrees C with various durations up to 120 min. Three sources of tea catechins containing different levels of (-)-epigallocatechin gallate (EGCG), (-)-epicatechin gallate (ECG), and their epimers were studied. Kinetic models for the degradation/epimerization of tea catechins were developed and validated by the reactions at 145 degrees C. It was shown that the epimerization and degradation of tea catechins followed first-order reactions and the rate constants of reaction kinetics followed the Arrhenius equation. Values of the activation energy (E(a)) for the epimerization of EGCG from epi- to nonepi-structures, the epimerization of GCG from nonepi- to epi-structures, and the total degradation of EGCG and its epimer GCG were 117.6, 84.2, and 42.8 kJ/mol, respectively. For ECG and CG, the E(a) values were 119.3, 96.2, and 41.6 kJ/mol, respectively. The mathematical models may provide a useful prediction for the loss of tea catechins during any thermal processing.     

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.     

Interaction of environmental moisture with powdered green tea formulations: effect on catechin chemical stability

Green tea and tea catechins must be stable in finished products to deliver health benefits; however, they may be adversely affected by tea processing/storage conditions and the presence of other components. The objective of this study was to determine the effects of storage relative humidity (RH) and addition of other ingredients on catechin stability in simulated dry beverage mixtures. Samples of green tea powder alone and mixed with sucrose, citric acid, and/or ascorbic acid were prepared and stored in desiccators at 22 degrees C and 0-85% RH for up to 3 months. Epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate were determined by high-performance liquid chromatography (HPLC). Formulation and the interaction of formulation and RH significantly promoted catechin degradation ( P < 0.0001). The chemical degradation of total and individual catechins in green tea powder formulations was significantly increased ( P < 0.0001) by exposure to increasing RH, and the degradation was exacerbated at > or = 58% RH by the presence of powdered citric acid and at > or = 75% RH by the presence of ascorbic acid. Catechins degraded the most in formulations containing both acids. Although catechin chemical stability was maintained at < or = 43% RH in all samples stored at 22 degrees C for 3 months, caking was observed in samples at these relative humidities. These results are the first to demonstrate that addition of other dry components to tea powders may affect catechin stability in finished dry blends and highlight the importance of considering the complex interplay between a multicomponent system and its environment for developing stable products.     

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.     

Epimerization of tea catechins and O-methylated derivatives of (-)-epigallocatechin-3-O-gallate: relationship between epimerization and chemical structure

Epimerization at C-2 of O-methylated catechin derivatives and four major tea catechins were investigated. The epimeric isomers of (-)-epicatechin (I), (-)-epicatechin-3-O-gallate (II), (-)-epigallocatechin (III), (-)-epigallocatechin-3-O-gallate (IV), and (-)-epigallocatechin-3-O-(3-O-methyl)gallate (V) in green tea extracts increased time-dependently at 90 degrees C. The epimerization rates of authentic tea catechins in distilled water are much lower than those in tea infusion or in pH 6.0 buffer solution. The addition of tea infusion to the authentic catechin solution accelerated the epimerization, and the addition of ethylenediaminetetraacetic acid, disodium salt (Na(2)EDTA) decreased the epimerization in the pH 6.0 buffer solution. Therefore, the metal ions in tea infusion may affect the rate of epimerization. The proportions of the epimers to authentic tea catechins [III, IV, V, and (-)-epigallocatechin-3-O-(4-O-methyl)gallate (VI)] in pH 6.0 buffer solution after heating at 90 degrees C for 30 min were 42.4%, 37.0%, 41.7%, and 30.4%, respectively. These values were higher than those of I and II (23.5% and 23.6%, respectively). The O-methylated derivatives at the 4'-position on the B ring of IV and VI were hardly epimerized. These results suggest that the hydroxyl moiety on the B ring of catechins plays an important role in the epimerization in the order 3',4',5'-triol type > 3',4'-diol type > 3',5'-diol type.     

Effects of heat processing and storage on flavanols and sensory qualities of green tea beverage

This research was conducted to understand the effects of heat processing and storage on flavanols and sensory qualities of green tea extract. Fresh tea leaves were processed into steamed and roasted green teas by commercial methods and then extracted with hot water (80 degrees C) at 1:160 ratio (tea leaves/water by weight). Green tea extracts were heat processed at 121 degrees C for 1 min and then stored at 50 degrees C to accelerate chemical reactions. Changes in flavanol composition and sensory qualities of green tea extracts during processing and storage were measured. Eight major flavanols (catechin, epicatechin, gallocatechin, epigallocatechin, epicatechin gallate, catechin gallate, epigallocatechin gallate, and gallocatechin gallate) were identified in the processed tea extract. Among them, epigallocatechin gallate and epigallocatechin appeared to play the key role in the changes of sensory qualities of processed green tea beverage. The steamed tea leaves produced a more desirable quality of processed green tea beverage than the roasted ones.     

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).     

Kinetics of the stability of broccoli (Brassica oleracea Cv. Italica) myrosinase and isothiocyanates in broccoli juice during pressure/temperature treatments

The Brassicaceae plant family contains high concentrations of glucosinolates, which can be hydrolyzed by myrosinase yielding products having an anticarcinogenic activity. The pressure and temperature stabilities of endogenous broccoli myrosinase, as well as of the synthetic isothiocyanates sulforaphane and phenylethyl isothiocyanate, were studied in broccoli juice on a kinetic basis. At atmospheric pressure, kinetics of thermal (45-60 degrees C) myrosinase inactivation could be described by a consecutive step model. In contrast, only one phase of myrosinase inactivation was observed at elevated pressure (100-600 MPa) combined with temperatures from 10 up to 60 degrees C, indicating inactivation according to first-order kinetics. An antagonistic effect of pressure (up to 200 MPa) on thermal inactivation (50 degrees C and above) of myrosinase was observed indicating that pressure retarded the thermal inactivation. The kinetic parameters of myrosinase inactivation were described as inactivation rate constants (k values), activation energy (Ea values), and activation volume (Va values). On the basis of the kinetic data, a mathematical model describing the pressure and temperature dependence of myrosinase inactivation rate constants was constructed. The stability of isothiocyanates was studied at atmospheric pressure in the temperature range from 60 to 90 degrees C and at elevated pressures in the combined pressure-temperature range from 600 to 800 MPa and from 30 to 60 degrees C. It was found that isothiocyanates were relatively thermolabile and pressure stable. The kinetics of HP/T isothiocyanate degradation could be adequately described by a first-order kinetic model. The obtained kinetic information can be used for process evaluation and optimization to increase the health effect of Brassicaceae.     

Hydroxymethylfurfural and furosine reaction kinetics in tomato products

The reaction kinetics of two heat damage indices, HMF and furosine, were examined in four tomato products with different dry matter contents (10.2, 25.5, 28.6, and 34.5%) over a temperature-time range of 80-120 degrees C and 0-255 min. The reactions followed pseudo-zero order kinetics. E(a) and z-value were, respectively, 139. 9 kJ/mol and 19.2 degrees C for HMF, and 93.9 kJ/mol and 28.4 degrees C for furosine. The analyses of both indices in several samples of commercial and industrial tomato products showed very low levels of HMF (from 1 to 42 ppm) and a lack of correlation between HMF and furosine mainly because of the different evolution of the two indices during storage. The HMF level of a tomato paste sample stored at 25 degrees C decreased from 609 to 17 ppm after 98 days, while furosine increased from 458 to 550 mg/100 g of protein.     

Characterization of anthocyanin-rich waste from purple corncobs (Zea mays L.) and its application to color milk

Pigment production from anthocyanin-rich purple corncobs generates a deeply colored waste precipitate. Our objectives were to characterize this anthocyanin-rich waste (ARW) and to find a suitable application in a food matrix. Composition and solubility characteristics of ARW were evaluated. Color (CIELAB) and pigment (monomeric anthocyanin and HPLC profiles) stability of ARW in milk (35 mg/100 mL) were evaluated using an accelerated test at 70 degrees C and phosphate buffer as a control. ARW provided milk an attractive purple hue (324-347 degrees ). Monomeric anthocyanin degradation followed zero-order kinetics in skim and whole milk and second-order kinetics in the control, with half-lives of 173, 223, and 44 min at 70 degrees C, respectively. ARW shows potential as a natural colorant for a pH range unusual for anthocyanin applications. A protective effect of matrix constituents on the stability of anthocyanins was evident. Anthocyanins may interact with different compounds in biological systems when the pH values are close to neutral.     

Degradation kinetics of the antioxidant additive ascorbic acid in packed table olives during storage at different temperatures

The kinetics of ascorbic acid (AA) loss during storage of packed table olives with two different levels of added AA was investigated. Three selected storage temperatures were assayed: 10 degrees C, ambient (20-24 degrees C), and 40 degrees C. The study was carried out in both pasteurized and unpasteurized product. The effect of pasteurization treatment alone on added AA was not significant. In the pasteurized product, in general AA degraded following a first-order kinetics. The activation energy calculated by using the Arrhenius model averaged 9 kcal/mol. For each storage temperature, the increase in initial AA concentration significantly decreased the AA degradation rate. In the unpasteurized product, AA was not detected after 20 days in samples stored at room temperature and AA degradation followed zero-order kinetics at 10 degrees C, whereas at 40 degrees C a second-order reaction showed the best fit. In both pasteurized and unpasteurized product, the low level of initial dehydroascorbic acid disappeared during storage. Furfural appeared to be formed during storage, mainly at 40 degrees C, following zero-order kinetics.     

Comparative study on pressure and temperature stability of 5-methyltetrahydrofolic acid in model systems and in food products

A comparative study on the pressure and temperature stability of 5-methyltetrahydrofolic acid (5-CH(3)-H(4)folate) was performed in model/buffer systems and food products (i.e., orange juice, kiwi puree, carrot juice, and asparagus). Effects of pH and ascorbic acid (0.5 mg/g) on 5-CH(3)-H(4)folate stability in buffer systems were studied on a kinetic basis at different temperatures (from 65 to 160 degrees C) and different pressure/temperature combinations (from 100 to 700 MPa/from 20 to 65 degrees C). These studies showed that (i) the degradation of 5-CH(3)-H(4)folate in all model systems could be described by first-order reaction kinetics, (ii) the thermostability of 5-CH(3)-H(4)folate was enhanced by increasing pH up to 7, (iii) 5-CH(3)-H(4)folate was relatively pressure stable at temperatures lower than 40 degrees C, and (iv) ascorbic acid enhanced both the thermo- and barostabilities of 5-CH(3)-H(4)folate. In food products, temperature and pressure stabilities of 5-CH(3)-H(4)folate were studied at different temperatures (70-120 degrees C) and different pressure/temperature combinations (from 50 to 200 MPa/25 degrees C and 500 MPa/60 degrees C). 5-CH(3)-H(4)folate in orange juice and kiwi puree was relatively temperature (up to 120 degrees C) and pressure (up to 500 MPa/60 degrees C) stable in contrast to carrot juice and asparagus. Addition of ascorbic acid (0.5 mg/g) in carrot juice resulted in a remarkable protective effect on pressure (500 MPa/60 degrees C/40 min) and temperature degradation (120 degrees C/40 min) of 5-CH(3)-H(4)folate.     

Stability of isoflavones in soy milk stored at elevated and ambient temperatures

Soy isoflavones are widely recognized for their potential health benefits. The increased use of traditional and new food products calls for the assessment of their stability during processing and storage. The present study examines the stability of genistein and daidzein derivatives in soy milk. Soy milk was stored at ambient and elevated temperatures, and the change in isoflavone concentration was monitored with time. Genistin loss in time showed typical first-order kinetics, with rate constants ranging from 0.437-3.871 to 61-109 days(-1) in the temperature ranges of 15-37 and 70-90 degrees C, respectively. The temperature dependence of genistin loss followed the Arrhenius relation with activation energies of 7.2 kcal/mol at ambient temperatures and 17.6 kcal/mol at elevated temperatures. At early stages of soy milk storage at 80 and 90 degrees C, the 6' '-O-acetyldaidzin concentration increased, followed by a slow decrease. The results obtained in this study can serve as a basis for estimating the shelf life of soy milk as related to its genistin content.     

Model studies on the stability of folic acid and 5-methyltetrahydrofolic acid degradation during thermal treatment in combination with high hydrostatic pressure

Stability of folic acid and 5-methyltetrahydrofolic acid in phosphate buffer (0.2 M; pH 7) toward thermal (above 65 degrees C) and combined high pressure (up to 800 MPa)/thermal (20 up to 65 degrees C) treatments was studied on a kinetic basis. Residual folate concentration after thermal and high pressure/thermal treatments was measured using reverse phase liquid chromatography. The degradation of both folates followed first-order reaction kinetics. At ambient pressure, the estimated Arrhenius activation energy (E(a)) values of folic acid and 5-methyltetrahydrofolic acid thermal degradation were 51.66 and 79.98 kJ mol(-1), respectively. It was noticed that the stability of folic acid toward thermal and combined high pressure thermal treatments was much higher than 5-methyltetrahydrofolic acid. High-pressure treatments at room temperature or higher (up to 60 degrees C) had no or little effect on folic acid. In the whole P/T area studied, the rate constant of 5-methyltetrahydrofolic acid degradation was enhanced by increasing pressure, and a remarkable synergistic effect of pressure and temperature on 5-methyltetrahydrofolic acid degradation occurred at temperatures above 40 degrees C. A model to describe the combined pressure and temperature effect on the 5-methyltetrahydrofolic acid degradation rate constant is presented.     

Formation of strecker aldehydes from polyphenol-derived quinones and alpha-amino acids in a nonenzymic model system

Fruits and vegetables contain naturally occurring polyphenolic compounds that can undergo enzyme-catalyzed oxidation during food preparation. Many of these compounds contain catechol (1,2-dihydroxybenzene) moieties that may be transformed into o-quinone derivatives by polyphenoloxidases and molecular oxygen. Secondary reactions of the o-quinones include the Strecker degradation of ambient amino acids to form flavor-important volatile aldehydes. The purpose of this work was to investigate the mechanism of the polyphenol/o-quinone/Strecker degradation sequence in a nonenzymic model system. By using ferricyanide ion as the oxidant in pH 7.17 phosphate buffer at 22 degrees C, caffeic acid, chlorogenic acid, (+) catechin, and (-) epicatechin were caused to react with methionine and phenylalanine to produce Strecker aldehydes methional and phenylacetaldehyde in 0.032-0.42% molar yields (0.7-10 ppm in reaction mixtures). Also, by employing l-proline methyl ester in a reaction with 4-methylcatechol, a key reaction intermediate, 4-(2'-carbomethoxy-1'-pyrrolidinyl)-5-methyl-1,2-benzoquinone (7), was isolated and tentatively identified.     

Design, semisynthesis, and evaluation of O-acyl derivatives of (-)-epigallocatechin-3-gallate as antitumor agents

The partially purified catechin fraction isolated from green tea extract was treated with a variety of acylating agents (acyl anhydrides/chloride) to obtain (-)-epigallocatechin-3-gallate (EGCG) O-acyl derivatives in 20-25.4% yields. The (-)-EGCG O-acyl derivatives were characterized by physical data and spectral studies. These compounds were evaluated for their antitumor activity by use of a two-stage carcinogenesis model in 7,12-dimethylbenz[a]anthracene (DMBA)/12-O-tetradecanoylphorbol 13-acetate (TPA)--induced cancer in Swiss albino mice. The study showed that there was a significant decrease in the antitumor activity with the increase in size and branching of the chain length of acyl groups. The results indicated that these O-acyl derivatives of (-)-EGCG have the potential to be developed as cancer chemopreventive agents. Keywords: Green tea; catechins; (-)-EGCG O-acyl derivatives; antitumor activity.     

Thermal stability of anthocyanin extract of Hibiscus sabdariffa L. in the presence of beta-cyclodextrin

The thermal stability of anthocyanin extract isolated from the dry calyces of Hibiscus sabdariffa L. was studied over the temperature range 60-90 degrees C in aqueous solutions in the presence or absence of beta-cyclodextrin (beta-CD). The results indicated that the thermal degradation of anthocyanins followed first-order reaction kinetics. The temperature-dependent degradation was adequately modeled by the Arrhenius equation, and the activation energy for the degradation of H. sabdariffa L. anthocyanins during heating was found to be approximately 54 kJ/mol. In the presence of beta-CD, anthocyanins degraded at a decreased rate, evidently due to their complexation with beta-CD, having the same activation energy. The formation of complexes in solution was confirmed by nuclear magnetic resonance studies of beta-CD solutions in the presence of the extract. Moreover, differential scanning calorimetry revealed that the inclusion complex of H. sabdariffa L. extract with beta-CD in the solid state was more stable against oxidation as compared to the free extract, as the complex remained intact at temperatures 100-250 degrees C where the free extract was oxidized. The results obtained clearly indicated that the presence of beta-CD improved the thermal stability of nutraceutical antioxidants present in H. sabdariffa L. extract, both in solution and in solid state.     

Inhibitory effects of green tea polyphenols on the production of a virulence factor of the periodontal-disease-causing anaerobic bacterium Porphyromonas gingivalis

The effect of polyphenolic compounds isolated from green tea (Camellia sinensis) on the production of toxic end metabolites of Porphyromonas gingivalis was investigated. Green tea polyphenols completely inhibited the production of n-butyric acid and propionic acid at a concentration of 1.0-2.0 mg/mL in general anaerobic medium (GAM). (-)-Epigallocatechin gallate (EGCg), which is a major component of tea polyphenols also inhibited the production of phenylacetic acid at 0.5 mg/mL in GAM broth. In the experiment using resting cells of P. gingivalis, phenylacetic acid was produced from l-phenylalanine and phenylpyruvic acid, but this reaction was also inhibited by EGCg, (-)-epicatechin gallate, and (-)-gallocatechin gallate. However, (+)-catechin, (+)-gallocatechin, (-)-epicatechin, and (-)-epigallocatechin did not inhibit those reactions. These results indicate that the inhibitory effect on the production of toxic end metabolites of P. gingivalis can be attributed to the presence of the galloyl moiety, which is ester-linked with the 3-OH of the catechin moiety in the polyphenolic compounds. This study shows that continuous application of tea polyphenols on a daily basis can be considered as a useful and practical method for the prevention of periodontal diseases.     

Quality assessment and quantitative analysis of flavonoids from tea samples of different origins by HPLC-DAD-ESI-MS

Components of green tea ( Camellia sinensis) have been of considerable interest in recent years because of their potential utility as pharmaceutical agents, particularly for their antioxidant and anticarcinogenic activity. Responding to the increasing scientific validation of numerous health benefits of tea, a comprehensive approach was adopted to carry out analysis for the quality assessment of flavonoids in tea samples of different origins. For this purpose, extraction, separation, and mass spectrometric parameters were optimized. Extraction methods evaluated include reflux extraction, a modified accelerated solvent extraction (ASE), namely, Aquasolv extraction, and microwave-assisted extraction (MAE) using different percentages of solvents. Separation was performed by a specifically developed reversed phase high-performance liquid chromatography (RP-HPLC) method using different C18 and C8 stationary phases. Optimization of extraction techniques clearly proved the performance of MAE, which delivered highest yields in a very short time. Additionally, the comparison with Aquasolv extraction provided new insights, as variations in quantified amounts of target compounds between the extracts could be explained on the basis of thermal degradation and epimerization phenomena. Especially the epimerization phenomenon for catechin/epicatechin oligomers, that is, of procyanidins P 2 and P 3, was observed for the first time. Finally, an optimized extraction and separation system was used for qualitative and quantitative investigations of compounds from different green tea samples from Ceylon (cultivated under biologically controlled conditions), Japan, India, and China as well as from one black tea sample from India.     

Kinetics and mechanism of imazosulfuron hydrolysis

Knowledge of the kinetics and pathways of hydrolytic degradation is crucial to the prediction of the fate and transport mechanism of chemicals. This work first describes the kinetics of the chemical hydrolysis of imazosulfuron, a new sulfonylurea herbicide, and evaluates the results to propose a degradation pathway. The hydrolysis of imazosulfuron has been studied in aqueous buffers both within the pH range 1.9-12.3 at ambient temperature (thermostated at 25 +/- 2 degrees C) and at pH 3.6 within the temperature range of 15-55 degrees C. The hydrolysis rate of imazosulfuron was characterized by a first-order kinetics, pH- and temperature-dependent, and accelerated by acidic conditions and higher temperatures. The calculated half-lives at pH 4.5 and 5.9 were 36.5 and 578 days, respectively. At pH 6.6, 7.4, 9.2, and 12.3 no significant change in imazosulfuron concentration was observed after 150 days. Half-lives were much lower at pH <4 (= imazosulfuron pK(a)), at which they ranged from 3.3 to 6.3 days. Moreover, a change in temperature from 15 to 25 degrees C in acidic conditions (pH 3.6) decreased the half-life of imazosulfuron by a factor of approximately 4.0; in any case, a 3-5-fold increase in the rate of hydrolysis was found for each 10 degrees C increase in temperature. In acidic conditions the only hydrolysis products were the two molecules resulting from the cleavage of the sulfonylurea bridge.