Techniques for Extraction of Green Tea Polyphenols: A Review

Food Bioprocess Technol DOI 10.1007/s11947-015-1479-y REVIEW Techniques for Extraction of Green Tea Polyphenols: A Review D. Pasrija & C. Anandharamakrishnan Received: 27 January 2014 / Accepted: 27 January 2015 # Springer Science+Business Media New York 2015 Abstract Tea is the second most consumed beverage all over the world after water. In recent times, tea has stretched the eyebrows of researchers and catching all the attention towards its health benefits. Tea has been proven beneficial by preventing the risk of some diseases like cancer and cardiovascular problems. Green tea is least processed and gives maximum benefits. The main components of the green tea are polyphenols which include the catechins, epicatechins, epigallocatechins, epicatechingallate, epigallocatechingallate, gallic acid, flavanoids, flavanols, and flavonols. Other than polyphenols, caffeine and theophylline are also present. Among which compounds of catechins family has been widely reported to have most beneficial effects on the health. Currently, the extraction of catechins is catching much higher attention and many techniques have been discovered and modified to extract these compounds. But very limited reviews have been reported discussing the impact of various techniques used for extraction of green tea polyphenols. This review focuses on various techniques employed for the extraction of polyphenols from green tea and other sources (pine bark, grape seed, and pomegranate) with their advantages and limitations. The current trends and future prospects are also highlighted. Keywords Green tea . Polyphenols . Microwave-assisted extraction . Ultrasonication Introduction Originating from China, tea has gained the world’s taste in the past 2000 years. Chinese have known about the medicinal benefits of green tea since ancient times, using it to treat evD. Pasrija : C. Anandharamakrishnan (*) Food Engineering Department, CSIR-Central Food Technological Research Institute, Mysore 570 020, India e-mail: [email protected] erything from headaches to depression. Green tea is catching more interest than other beverages due to its healthy beneficial effects and has become the most consumed beverage all over the world, after water. Tea is mainly prepared from leaves and bud of the plant Camellia sinensis member of the Theaceae family. The plant is also found as the large shrub with white flowers in Asia and grown on the commercial basis in Africa, Sri Lanka, Malaysia, and Indonesia. Initially black tea was mainly manufactured as compared to green tea in India. However, recently, a great interest has been seen in manufacturing green tea due its antioxidant activity, anti-carcinogenic, anti-obesity, anti-inflammatory, and anti-bacterial properties (Kumar et al. 2012). Green tea is least processed and, thus, retains all the health ingredients in natural form. The presence of various polyphenols, flavanoids, and flavonols including catechins, epicatechins, epigallocatechins, epicatechin gallate, epigallocatechin gallate, and gallic acid accounts for green tea health benefits. Polyphenols are secondary metabolites with one aromatic ring and one or more hydroxyl groups, involved in chemical defense of plants against predators. Health benefits in relation to cancer, arthritis, cardiovascular diseases, diabetes, obesity, and dental caries are in focus of scientific investigations. Many reviews have reported the health benefits of green tea polyphenols highlighting the basic mechanism involved in the reactions (Scalbert et al. 2005; Gramza et al. 2005a; Lambert and Elias 2010; Kumar et al. 2012) and few animal studies have been conducted by researchers to observe the influence of polyphenols on health (Chen et al. 1997; Yang et al. 2001). To extract secondary metabolites including polyphenols in from various sources, many techniques have been exploited (Starmans and Nijhuis 1996; Arceusz et al. 2013). Though extraction of green tea polyphenols is common in few countries, it still needs to be commercialized. Extraction of these compounds in concentrated form facilitates their incorporation in the food ingredients to increase the nutritional value of the food and also provides medicinal benefits. Food Bioprocess Technol Many techniques have been exploited for the extraction of polyphenols including hot water extraction (Cheong et al. 2005; Liang et al. 2007; Vuong et al. 2011), microwaveassisted extraction (Quan et al. 2006; Nkhili et al. 2009; Nshimiyimana and He 2010), solvent-based extraction (Meterc et al. 2007; Dong et al. 2011), molecular distillation (Tang et al. 2011), and ultrasonication (Koiwai and Masuzawa 2007a). All the above techniques have their own advantages and limitations. However, very few articles have focused on reviewing the different techniques for extraction of polyphenols and other compounds but not exclusively on green tea polyphenols. Thus, this review aims to discuss the various techniques for extraction of green tea polyphenols with their merits and demerits. [α]D of catechin, epicatechin, and epicatechin-3-gallate are 0, 58.3, and 188°, respectively (Shi et al. 2005). Green tea catechins show a variation in the stability with change in the pH of the solution. Catechins show low stability in the solution with alkaline pH and maximum in solution with acidic pH (Kumar et al. 2012). Tea catechins mainly include four compounds, namely, (−)-epigallocatechin-3-gallate (EGCG), that represents approximately 59 % of the total of catechins; (−)-epigallocatechin (EGC) (19 % approximately); (−)-epicatechin-3-gallate (ECG) (13.6 % approximately); and (−)-epicatechin (EC) (6.4 % approximately), chemical structure shown in Fig. 1. Structure transformation in catechins observed due to temperature elevation called epimerization of the molecules. Green Tea Health Benefits Tea can be divided into three categories on the basis of processing, namely, green tea (non-fermented), black tea (postfermented), and oolong tea (semi-fermented) (Cabrera et al. 2006). Green tea provides the higher content of polyphenols in comparison to oolong tea and black tea, notably epigallocatechin-3-gallate (EGCG), which is believed to account for most of the health benefits linked to green tea. However, black tea contains excessive amounts of thearubigins and theaflavins and many complex polyphenols (Mukhtar and Ahmad 2000). The production of green tea is characterized by an initial heating process that inhibits the enzyme polyphenol oxidase responsible for the conversion of the flavanols into the dark polyphenolic compounds. The other important process is rolling in which leaves are cut and twisted to break the cell walls (Cabrera et al. 2006). Chemical composition of green tea includes proteins (15– 20 % dry weight), amino acids (1–4 % dry weight), carbohydrates (5–7 % dry weight), lipids, xanthic bases, polyphenols and flavanoids (30 % dry weight), pigments, volatile compounds, and trace elements including essential oils, riboflavin, niacin, folic acid, ascorbic acid, pantothenic acid, malic and oxalic acids, manganese, potassium, magnesium, and fluoride (5 % dry weight) (Boehm et al. 2009). The health benefits of green tea are mainly due to the presence of catechins. Presence of vitamin C in green tea supplements the antioxidant activity of polyphenols (Cabrera et al. 2006; Kumar et al. 2012). Therapeutic properties of green tea polyphenols include anticarcinogen, anti-inflammatory, and anti-radiation. Physical properties of the polyphenols include their molecular weight, solubility, melting point, absorption of light, and optical rotation (in ethanol). Some polyphenols are water soluble and some are lipid soluble. In general, catechins are water soluble. Melting points of catechin, epicatechin, and epicatechin-3-gallate are 174, 236, and 236 °C, respectively. Absorption of light (λmax) is at 264–280 nm. Optical rotation Many factors including tea to water ratio and method of infusion influence the concentration of polyphenols obtained from green tea. However, 90–200 mg of polyphenols is present per cup of green tea (Gramza et al. 2005a; Cabrera et al. 2006). Green tea polyphenols play role against cancer, cardiovascular problems, arthritis, blood pressure, and atherosclerosis (Gramza et al. 2005a; Cabrera et al. 2006; Kumar et al. 2012). Increase in the amount of free radicals is the root cause of many cardiovascular diseases. Green tea catechins are capable of reducing the amount of free radicals. People consuming five cups of green tea per day are found 16 % less prone to cardiovascular diseases (Sano et al. 2004). EGCG blocks inflammation-related compounds like nuclear factor-B (NFkappa-B) and STAT-1 that promotes the oxidative damage and kills heart cells in reperfusion injury. Blocking of STAT1 by EGCG helps in fast recovery after heart attack (Stephanou 2004). Bogdanski et al. (2012) concluded that green tea is beneficial in reducing blood pressure and improving the parameters associated with insulin resistance in obese and hypertensive patients. A threat of atherosclerosis has been reduced by 26–46 % by consumption of 3–4 cups of green tea per day. Polyphenols are capable of decreasing the amount of low density lipoproteins (LDL) cholesterol, triglycerides, lipid peroxides, and fibrinogen in the significant figures and decrease the ratio of LDL to high-density lipoproteins (HDL) cholesterol (Vinson et al. 2004; Basu and Lucas 2007). Green tea polyphenols prevent the formation of pro-inflammatory compounds derived from omega-6 fatty acids, present in vegetable oils such as corn and soy oil. These pro-inflammatory compounds, specifically arachidonic acid from which the inflammatory cytokines thromboxane A2 and prostaglandin D2 are derived, cause platelets to clump together (Liu and Pan 2004). The positive effects of green tea polyphenols against cancer have gained attention and curiosity by the researchers all over Food Bioprocess Technol Fig. 1 Chemical structure of some major compounds: EGCG epigallocatechin-3-gallate, ECG epicatechin-3-gallate, EGC epigallocatechin, EC epicatechin, GA gallic acid CAFFEINE the world. For cancer prevention, the evidence is so overwhelming that the Chemoprevention Branch of the National Cancer Institute has initiated a plan for developing tea compounds as cancer chemopreventive agents in human trials. Anti-cancer activity has been seen in the cases of skin, lung, oral cavity, esophagus, stomach, liver, kidney, prostate, and other organs by the action of either tumor cell apoptosis or decrease in cell proliferation (Cabrera et al. 2006). By blocking the natural process angiogenesis and inhibiting the development of new blood vessels, green tea polyphenols promotes the cancer cell starving. Furthermore, a major mechanism of the anti-carcinogenic activity of green tea polyphenols in animals is the impairment of the interaction of carcinogens with DNA leading to mutations (Fassina et al. 2004). Rosengren (2003) indicated that the green tea catechins reduce the proliferation of breast cancer cells in vitro and decrease breast tumor growth in rodents. Overproduction of the enzyme cyclooxygenase (COX)-2 has been found to play a vital role in many diseases like cancer and arthritis. Green tea anti-cancer effects include its ability to inhibit the overproduction of the enzyme cyclooxygenase (COX)-2. Many anti-inflammatory drugs significantly inhibit the (COX)-2 and its counterpart (COX)-1 but have certain THEOPHYLLINE negative effects. EGCG appears to block only COX-2 without any side effects (Hussain et al. 2004). Caffeine is a stimulant drug and acts on central nervous system. A cup of green tea contains 10–50 mg of caffeine and over consumption can cause irritability, insomnia, nervousness, and tachycardia. Caffeine consumption for pregnant and lactating women as been considered as contraindicated on its possible teratogenic effect, to avoid sleep disorders in infants. Green tea extracts standardized to 80 % total polyphenols are dosed at 500–1500 mg per day (DerMarderosian 1999). A summarized content of medicinal applications of green tea is mentioned in Table 1. Extraction Techniques for Green Tea Polyphenols Extraction of green tea polyphenols depends on the various factors including solubility, pH, extraction time, and temperature. However, the selection of extraction technique is influenced by the niche of the compound of interest and extent of purity required. Furthermore, the use of extraction technique has its impact on the rate, yield, and purity of polyphenols. Food Bioprocess Technol Table 1 A summarized health benefits of green tea polyphenols (Kumar et al. 2012) Application Mechanism References Anti-bacterial Shimamura et al. 2007 Diabetes Inhibition of β-lactamases, reverse transcriptase of HIV, collagenase, fatty acid synthetase, and various other enzymes. Inhibition of mitogen-activated protein kinases (MAPK), growth factor-related cell signaling, activation of activator protein 1 (AP-1) and nuclear factor-B (NF-kappaB), topoisomerase I, matrix metalloproteinases, and other potential targets. Inhibition inflammasome downregulation → decreased IL-1β secretion → decreased NF-κB activities → decreased cell growth. Inhibition of the enzymes catechol-O methyl transferase, acetyl-CoA carboxylase, fatty acid synthase, and impeding absorption of fat via the gut. Increased expression level of glucose transporter IV. Atherosclerosis Reduces LDL oxidizability, improves vascular function. Anti-cancer Anti-inflammatory Anti-obesity Chen and Zhang 2007 Ellis et al. 2011 Thavanesan 2011 Wu et al. 2004 Hodgson et al. 2002; Widlansky et al. 2004 Diarrhea Inhibitory effect on Helicobacter pylori infection. Chacko et al. 2010 Antioxidative Inhibited oxygen consumption and formation of conjugated dienes in AAPH-mediated linoleic acid peroxidative reaction. Osada et al. 2001 Solvent-based extraction gives a higher yield; however, it limits the use of polyphenols for human consumption. Solvent-based extraction needs further purification of polyphenols either by membrane or ultrafiltration. Exposure to a higher temperature for prolonged period of time may lead to the degradation of polyphenols. Advanced techniques like microwave-assisted extraction and ultrasonication can overcome these limitations. Following subsections will discuss in detail the different techniques for the extraction of green tea polyphenols. Solvent-Based Extraction Technique Solvent extraction method was conceived to separate the soluble compounds from a solid matrix (plant tissue) using a liquid matrix (solvent) at lower temperature to prevent deterioration. Solvent extraction of bioactive compounds and antioxidants from plant materials depends on the choice of solvent coupled with heating and/or agitation. Use of different solvents leads to change in composition of the extract (Sharif et al. 2014). Authors studied the effect of different solvents and infusion time on decaffeination of tea and reported dichloromethane as the most promising solvent for decaffeination. However, drying and purification of the product is required at the end of the process. Concentration of extracts is performed after extraction by ultrafiltration or supercritical fluid technique. Process parameters vary with the sample and compound of interest. In the initial stage of extraction, sorption of solvent causes swelling of tissue by the action of capillary and by salvation of the ions in the cells. This stage sometimes causes damage to cells. In next step diffusion takes place within the cells and external diffusion through the outer layers that surround the particles or the solid fragments (EscribanoBailon and Santos-Buelga 2003). Different solvents have been used for the extraction of polyphenols from green tea including water, methanol, ethanol, acetonitrile, CO2, and acetone. i. Hot water-based extraction Hot water extraction is a conventional technique which is being used for the extraction purpose. In this technique, dried leaves are simply placed in boiling water bath and temperature is in between 80–100 °C. As per the nature of substrate the extraction period can be prolonged from 1 to 6 h (Shrikande et al. 2002). Though the process is time consuming, it has been comprehensively reviewed and compared with other extraction techniques. Hot water extraction of dried tea leaves was performed by Bharadwaz and Bhattacharjee (2012) at 60 °C followed by the decaffeination to collect the caffeine as a byproduct. The enzymes were deactivated prior to the extraction process. To separate the polyphenols from decaffeinated extract ethyl acetate was used as a solvent. Sample was further analyzed with Folin-Ciocalteau reagent to obtain the total polyphenol content of the extract and different polyphenols present were resolved by HPLC analysis. Total caffeine yield and the total polyphenols reported was 2.56 and 19.33 %, respectively. Effect of temperature and extraction time also influences the total polyphenol content and antioxidant activity of tea leaves. A study was conducted by Liang et al. (2007) to optimize the effect of temperature and ethanol concentration on the extraction efficiency and degradation of green tea polyphenols. Authors observed that treating the green tea at 100 °C leads to change in configuration of catechins to their isomers. However, epimerization of catechins could be inhibited if extraction is performed at 80 °C. Authors reported that epigallocatechingallate (EGCG) and epicatechingallate Food Bioprocess Technol (ECG) were partially epimerized into gallocatechingallate (GCG) and catechingallate (CG), respectively, when tea extract was heated in water solution at 100 °C for 2 h. It was also reported that 50 % ethanol is a better solvent than water to extract polyphenols from dried green tea leaves. A 75 % ethanol has been preferred for the extraction from fresh leaves due to the presence of higher moisture content. A study was conducted by Vuong et al. (2011) and similar results were confirmed. Authors optimized combination of six different parameters including temperature, time, pH of brewing solution, tea-to-water ratio, and tea particle size, for the extraction of catechins from green tea by using hot water extraction. Treating the tea leaves at 80 °C for 30 min gives a maximum yield without degradation with pH of <6 of brewing solvent and a water-to-tea ratio at 50:1 (ml/g). Korean green tea leaves were used by Cheong et al. (2005) for extraction of catechins at different temperatures using hot water extraction technique. It was observed that prolonged exposure at high temperature caused the degradation of polyphenols. Row and Jin (2006) optimized the solvent extraction procedure for Korean tea using water and ethanol as solvents and reported that extraction with water at 80 °C for 40 min gave maximum yield of catechins. The variable dependent nature of two different catechins, namely, EGC and EGCG on time and time/ temperature have been observed in the literature. So, it is feasible to isolate these compounds from green tea extract in a two-step extraction. Bazinet et al. (2007) performed the two-step extraction to fractionate two compounds. The extraction was performed at 50 °C for 10 min in the first step. In the second step, tea leaves were separated and soaked in preheated water at 80 °C. The total EGC content reported was 78.9 % of the total catechin in the first step extract while its concentration decreased to 39.5 % in the second step extract. Conversely, the concentration of EGCG increased from 153.7 g/ml (10.8 % of the total catechin) in the first step to 503.0 g/ml (47.6 %) in the second step (Bazinet et al. 2007). Extraction of polyphenols was performed from crushed fresh green tea leaves by using water as a solvent at different temperatures, and pressure was maintained at 3× 106 Pa. To purify the polyphenols from caffeine and other impurities, water extract was partitioned with water:chloroform in 1:1 ratio. Caffeine containing chloroform was discarded, and another partitioning was made with water:ethyl acetate 1:1 ratio (Goodarznia and Govar 2009). On contrary to the above reports, in order to preserve the flavor, a modified method has been performed by Eri et al. (2008). Authors reported extraction of green tea polyphenols with water as a solvent at a temperature below 10 °C to preserve the green tea flavor. Extraction of green tea polyphenols at lower temperature is followed by warm water extraction at 50 °C and further drying of the extract. Authors reported improved taste sensation, increased green tea flavor, and less bitterness in the drink. Another study by Weizheng et al. (2013) reported preserving the health beneficial elements from green tea by smashing the leaves and extracting the polyphenols at room temperature, followed by low-temperature vacuum concentration. ii. Organic solvent extraction Organic solvent extraction is based on Soxhlet technique. Automation of the process to decrease the duration of process turned out as improved technique known as Soxtec and was comparable with other extraction techniques such as supercritical fluid extraction and microwave-assisted processes (Luque de Castro and Garcia-Ayuso 1998). In organic solvent extraction technique, two aspects put check on the process, namely, equilibrium state and mass transfer rate. Soxhlet technique displaces the transfer equilibrium and increase the mass transfer by repeatedly bringing fresh solvent in the contact of solid matrix (Wang and Weller 2006). A significant highconcentration gradient between substrate and solvent and a large diffusion coefficient or small particle size of substrate can increase the extraction of polyphenols. Increase in temperature promotes the diffusivity of polyphenols into the solvent (Shi et al. 2005; Cacace and Mazza 2002). Factors affecting the extraction efficiency of polyphenols include the nature of solvent, pH of extraction medium, temperature, number of extraction steps, volume of solvent, and particle size and shape. The pH influences the solubility of the compounds and increase in temperature increases the efficiency. Homogenization favors the extraction process and can be carried out in contact with the extraction solvent (Escribano-Bailon and Santos-Buelga 2003). The applications of solvent extraction have been foreseen in a wide area of industry, with better reproducibility and efficiency (Wang and Weller 2006). Solvent extraction can be further categorized into pressurized liquid extraction (PLE) and accelerated solvent extraction (ASE). In pressurized liquid extraction, organic solvents are used at certainly elevated pressure (500– 3000 psi) and temperature (40–200 °C) above their normal boiling points. Sample packed in the extraction cell is allowed to experience these extreme conditions, and extract is collected into the vial by purging with compressed gas. With increased pressure, the extraction cell fills faster and forces the liquid into solid matrix (Wang and Weller 2006). Accelerated solvent extraction is the part of the pressurized liquid extraction, and in the same way as PLE, the sample is exposed to high temperature (50– 200 °C) and pressure (1475–2175 psi) which increases Food Bioprocess Technol the diffusivity of the solvents and promotes the level of the extraction kinetics. The use of such high temperatures and solvents can easily reduce the time and the amount of solvents usage. However, this technique cannot be used for thermolabile compounds (Ajila et al. 2011). Accelerated solvent extraction can be exploited for the extraction of thermolabile organic pollutants from environmental matrices. A limited scope of ASE has been seen in the field of nutraceuticals in the literature. Richter et al. (1996) compared the technique of ASE with Soxhlet method and found ASE is having more potential than Soxhlet method. The efficiency of ASE was 1.2–1.5 times higher than the Soxhlet extraction and time consumed was very less. There is still a scope of exploiting this technique for the polyphenols as the technique is limited for extracting the contaminants from the environment. The antioxidant activity of solvent extract of dried green tea leaves, using 90 % ethyl alcohol at 50– 60 °C of polyphenols, was compared against standard ascorbic acid, and their synergistic effect was on the antioxidant, and photo-chemoprotective activity of green tea polyphenols was studied by Kaur and Saraf (2011). Authors confirmed the additive synergistic antioxidant and photo-chemoprotective activity of polyphenols along with the ascorbic acid. Polyphenols are quite different from each other in relation to their structures and found in different niche. Some may occur in combination with sugars, proteins, and sometimes get polymerized with each other. Solvent extraction technique is found to be more efficient and provides required conditions in terms of temperature and pH to extract the polyphenols present in derived forms (Druzynska et al. 2007). A study was conducted by Druzynska et al. (2007) to observe the effect of different solvents on the efficiency of the extraction of polyphenols from green tea. The different solvents used to perform extraction were water, 80 % ethanol, 80 % methanol, and 80 % acetone for different durations. The antioxidant activity of the polyphenols was estimated by the method of DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2, 2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid). With the yield of 9.80 g of polyphenols per 100 g dry mass of tea 80 % acetone solution showed the maximum efficiency with respect to 80 % ethanol and 80 % methanol which gave the output of 6.85 and 7.09 g of polyphenols per 100 g dry mass of tea, respectively (Druzynska et al. 2007). A same kind of inference was mentioned earlier by Turkmen et al. (2006) on the effect of solvent on the concentration and antioxidant activity of polyphenols. Six different combinations of solvents were optimized. Total polyphenol content was estimated by ferrous tartrate method and Folin-Ciocalteau method. 50 % acetone extract from mate tea showed the highest polyphenol content of 132.5 mg/g and 120.4 mg GAE per gram by method 1 and method 2, respectively. However, 50 % ethanol extract from mate tea and 50 % acetone extract from black tea had the greatest antioxidant activity. Santos-Buelga et al. (1995) described the process of extraction by using hexane and methanol as the solvent in combination. N-hexane is used on the aqueous extract for the removal of liposoluble material, and the remaining solvent was evaporated. In a study conducted by Jun et al. (2011), a comparison has been made between the different techniques, namely, ultrahigh pressure extraction (UPE), microwave extraction, ultrasonic extraction, and heat reflux extraction to determine total polyphenol content and antioxidant activity of different extracts. Authors reported that ultrahigh pressure extraction (using 50 % v/v ethanol) gave a higher antioxidant activity and higher green tea extract concentration as compared to other extraction techniques. Another solvent with low polarity, ethyl acetate was considered good for polyphenols by Shi et al. (2005). Polyphenols can be extracted from lipid fraction of food by using ethyl acetate, and the main advantage served by using ethyl acetate is low boiling point which eases solvent evaporation. The polyphenol extraction efficiency of different techniques (ultrasonication, heating, and stirring) was studied by Heng et al. (2014). Different combinations of solvents, hydrogen bonds donors (ethylene glycol, glycerol, 1,2-Butanediol, 1,4-Butanediol, 2,3Butanediol, 1,3-Butanediol, 1,6-Hexanediol) with choline chloride (ChCl), were optimized for maximum recovery of polyphenols. Heating of green tea with ethylene glycol and ChCl gave a maximum recovery of catechins. Extraction of green tea polyphenols at optimum temperature and pH during solvent extraction promotes the net content and yield of polyphenols. It also reduces the threat of degradation of polyphenols (Row and Jin 2006). The limitation of the process is post process of extraction. Purifying and drying of polyphenols is costly and exaggerated and can be overcome by the use of CO2 as a solvent. iii. Supercritical fluid extraction technique (SCF) The use of carbon dioxide as solvent for extraction purpose has reduced the cost of using chemicals to a great extent as well as the threat of thermal degradation. From the last three decades, the technique was restricted to a very small range of products due to high capital investment and unawareness of the method. With advances in Food Bioprocess Technol equipment design and realization of the potentially profitable opportunities in the production of high valueadded products, industries are becoming more and more interested in supercritical fluid technology (Mohamed and Mansoori 2002). The main reason for the interest in supercritical fluid extraction (SCF) was the possibility of carrying out extractions at temperature near to ambient temperature, thus preventing the substance of interest from thermal denaturation. The relatively fast extraction due to less viscosity and high diffusivities are associated with supercritical fluids. Any fluid above its critical temperature (TC) and pressure (PC) is known as critical fluid. A critical fluid can also be called as dense gas. Without applying pressure above the critical temperature, a gas cannot be liquefied. The critical pressure is the vapor pressure of the gas at the critical temperature. For a fluid to attain such a stage, the reduced temperature Tr (i.e., T/ TC) should range in between 1.0 and 1.3, and the reduced pressure Pr (i.e., P/PC) can be as much higher as that can be allowed by the system (Karale et al. 2011). The possibility of using supercritical fluids (SFs) as extraction solvents is directly linked to their density by an empirical correlation proposed by Chrastil in 1982,   b s ¼ ρa exp þc T where, s is the solute solubility, ρ is the solvent density, and T is the absolute temperature; a, b, and c are correlation parameters to be adjusted to experimental solubility data in supercritical CO2 (Karale et al. 2011). Different substances with different solubility can be extracted by CO2 at different extraction conditions using a single extractor. Applications of SCF technique has been studied for the extraction of catechins from tea (Chang et al. 2000), carotenoids from vegetables (Prado et al. 2014), functional ingredients from plants, algae, and microalgae (Herrero et al. 2006). Extraction of different green tea catechins by SCF technique has been reported by Chang et al. (2000) using CO2 as solvent followed by the characterization by HPLC. An addition of 95 % ethanol as a co-solvent raised the mean content of extract by 4.4 folds than by using water. In another study, selective extraction of caffeine and epigallocatechin gallate was performed by Kim et al. (2008) with CO2 as a solvent and water as a co-solvent. Different parameters including temperature (40–80 °C), pressure (200–400 bar), water content (4–7 % by weight), and co-solvent (water and ethanol) were optimized to concentrate mainly EGCG in the extract and extract out the caffeine. The maximum yield of caffeine and EGCG was obtained at 40 °C, 400 bar pressure, and 7 % water content. Yield of caffeine and EGCG were 54 and 21 %, respectively. The solubility parameter differences between SCCO2/water and caffeine/EGCG resulted in the maximum yield of caffeine. The selectivity of caffeine/EGCG extraction with water was 0.88, and the selectivity of caffeine/EGCG extraction was 0.24 with ethanol (Kim et al. 2008). Later investigation mentioned the potential use of supercritical CO2 for microbial inactivation of foods and the implementation of technique for the sterilization of thermally and pressure-sensitive materials (Spilimbergo et al. 2002). The process of supercritical fluid extraction with carbon dioxide (CO2) was not feasible initially though having a critical temperature of 31.06 °C and critical pressure of 73.81 bar. As CO2 has two oxygen molecules attached with same carbon atom, makes it a non-polar solvent, an additional polar solvent is required for the decaffeination process or removal of any selective polar compound. The order of the catechins were found to be EGCG>EGC>ECG>EC after HPLC analysis (Gudala 2008). An addition of a small percentage of cosolvents is required, since the selectivity of carbon dioxide is restricted to low molecular weight solutes. Polar solvents like methanol or ethanol as co-solvents or modifiers change the polarity of supercritical fluid and enhance its capacity towards solute. However, co-solvents add the complexity and cost to the process. After recovery of the product, the solvent must be recycled and pumped back to the extractor, in order to minimize operating costs (Karale et al. 2011). Solvent extraction is the basic technique used for the extraction of metabolites. It offers a wide range of solvents to extract different compounds with maximum recovery, based on polarity and solubility of the compounds. However, the use of organic solvent is not considered safe for consumption. In contrast, after SCF extraction of green tea, liquid CO2 can be converted to gas by releasing the pressure and giving solventfree polyphenols. Moreover, solvent extraction requires a long time with huge amount of solvent for extraction and post process purification which limits this technique. The limitation of a long extraction process can be overcome by Microwave-assisted and ultrasonication extraction techniques. Polyphenols and plant bioactives have been extracted from a number of sources (grape seeds, pomegranate peel) with solvent extraction (Murga et al. 2000; Cam and Hisil 2010). Hydroxycinnamic derivatives, flavonols, anthocyanins, and catechins have been extracted with solvent extraction method from various sources, and similar inferences have been made with respect to temperature, time, and recovery of the bioactive compounds. Microwave-Assisted Water Extraction Technique Microwave-assisted water extraction (MWE) is a technique with higher efficiency requiring less time. The sample under microwaves get heated up by dual mechanism of ionic conduction of electromagnetic (EM) waves and dipole rotation. Food Bioprocess Technol Microwaves are positioned in the EM spectrum with frequency between 300 MHz to 300 GHz. Sample can be heated up by either mechanism or by both simultaneously (Tatke and Jaiswal 2011). Traces of moisture in plant leaves are the target for heating by microwaves that exerts the pressure on the cell wall and rupture it. Authors observed the SEM images and found more rupturing in microwave-assisted sample than untreated sample (Jassie et al. 1997). This releases the active compounds from the cell into the outer environment. Enhancement in the release of active compounds is increased by the temperature or soaking the plant matrix with solvents with a higher dissipation factor, which can be defined by the equation: . tan δ ¼ ε” ε where, ε” indicates the efficiency of converting microwave energy to heat, i.e., the dielectric loss, and ε is the measure of the ability to absorb microwave energy, i.e., the dielectric constant (Mandal et al. 2007). For MWE, mainly two types of instrumentation designs are available, namely, closed extraction vessels and focused microwave ovens. The closed vessel performs the extraction under controlled pressure and temperature. The focused microwave ovens is also named as focused microwave-assisted Soxhlet or solvent extraction (FMASE), in which only a part of the extraction vessel containing the sample is irradiated with microwave. Some advantages of closed vessel systems include less time requirement as the temperature is higher due to a higher pressure, no loss of volatile compounds, no evaporation leads to less use of solvents, no risk of contamination, and there is an easy handling. Limitations of closed-vessel systems include high pressure, processing of limited amount of sample, coat material of vessel limits the temperature, addition of solvents between the processes is not possible, and cooling of vessel should be taken care of to avoid the loss of volatile compounds (Tatke and Jaiswal 2011). Factors affecting MWE are choice of solvent (must be with a higher dissipation factor), volume of extracting solvent, extraction time (increase in time increase the extraction of compounds, but susceptible to degradation of polyphenols), microwave power (low power for a longer period is feasible), matrix characteristics (effect elucidation of compounds), and temperature (Tatke and Jaiswal 2011). A solvent with high dielectric constant and high dielectric loss is having a more potential to absorb more energy from microwaves and, hence, get heated up at faster rate. The use of various combinations of different solvents results in the new improved characteristics of the solvent. But, during the selection of solvent, the sensitivity of the product towards solvent should also be taken into account. For thermolabile products, a solvent with low dielectric constant is preferred (Routray and Orsat 2012). Volume of the solvent is another critical factor and must be enough to immerse the sample completely for proper extraction (Tatke and Jaiswal 2011; Mandal et al. 2007). An increase in temperature shows a direct relation with the extraction of the product by MWE. But after a certain stage saturation point is attained, the gradual increase in time does not show a significant increase in the yield of extracted product (Routray and Orsat 2012). Microwave power and exposure time are interrelated factors which affect the extraction process. Higher power is also used to increase the heating effect and to reduce the time of the extraction. But sometimes, increase in power results in rupturing of the cell wall. Hence, a combination of the moderate power with longer exposure is always suggested and preferred (Mandal et al. 2007). Higher temperature of the process is the inducer of several intermolecular interactions between the solvent and sample and a relatively higher velocity of the molecules which gradually increase the rate of extraction. Higher temperature leads to increase in the solubility of the product in the solvent. Sometimes, this elevated temperature is also responsible for the increased pressure on the cellular matrix of the sample which causes rupturing of the cell, and product can be eluted out quickly (Routray and Orsat 2012). Two different modes of ovens are available with monomode and multimode cavities. In multimode, the incident wave is able to affect the several modes of resonance (Routray and Orsat 2012). Microwave-assisted water extraction of green tea polyphenols was performed by Nkhili et al. (2009). Authors optimized two parameters, temperature, and extraction time to estimate the higher extraction of polyphenols and confirmed polyphenols chemical composition by HPLC-MS analysis. Nkhili et al. (2009) concluded that MWE of polyphenols was better and gave 97.46 mg of catechins equivalent per gram against only 83.06 mg of catechins equivalent per gram by conventional extraction method at 80 °C. For the rapid extraction and fast analysis of catechin and epicatechin from green tea, the method of MWE was also performed by Li et al. (2010), followed by capillary electrophoresis. It was concluded that MWE takes just 1 min with 400 W for the complete extraction of catechins and epicatechins, whereas the ultrasonic extraction took 60 min for 1 g of tea sample with 15 ml of deionized water as solvent. The proposed method showed good recoveries, which are 118 % for catechin and 120 % for epicatechin with respect to ultrasonication extraction. Hence, MWE was concluded simple, fast, and reliable method for catechin extraction (Li et al. 2010). In another study conducted by Tang et al. (2011), applications of MWE have been cited. Authors used this technique for the extraction of tea polyphenols with microwave intensity 600 W for 3 min, followed by the technique of molecular distillation and spray drying for the concentration of Food Bioprocess Technol polyphenols. Nshimiyimana and He (2010) compared the radical scavenging activity of green tea and black tea polyphenols by extracting from two different methods. Authors opted for hot water and microwave-assisted extraction and concluded that microwave-assisted extraction at atmospheric pressure gives a better yield and, hence, a better radical scavenging activity than hot water extraction and green tea contains a higher concentration of polyphenols than black tea. High intensity of waves, dual heating mechanism, and use of wide range of solvents at high temperature under high pressure during the microwave-assisted extraction collectively aids in reducing the time of extraction to a great extent. However, additional filtration step is required to remove the solid residues which limit the process. Process optimization and modifications for MWE in recent times has allowed a wide exploitation of the technique for the extraction of various compounds from different sources. MWE has been widely exploited for the extraction of large number compounds other than polyphenols including isoflavonoids and saponins from Radix astragali (Xiao et al. 2008), phenolic acids (gallic, protocatechuic, p-hydroxybenzoic, chlorogenic, vanilic, caffeic, syringic, p-coumaric, ferulic, sinapic, benzoic, mcoumaric, o-coumaric, rosmarinic, cinnamic acids), Hypericum perforatum and Thymus vulgaris (Sterbova et al. 2004). All these studies have concluded MWE as a better technique than all other conventional techniques with a better recovery of bioactives, less degradation, and higher product integrity in terms of purity. Ultrasonication Extraction Technique Ultrasonic-assisted extraction possesses enough potential than conventional techniques for the extraction of plants secondary metabolites. It has been least exploited in comparison to other techniques. Ultrasonication overcomes all the disadvantages of high cost, post process concentration, low recovery, and others. The use of ultrasonication technique for extraction of polyphenols increases the efficiency of process and circumvents the degradation of polyphenols. It increases the mass transfer kinetics and quasi-equilibrium can be achieved by using this technique (Both et al. 2014). The technique is based on the acoustic cavitation phenomenon which allows the formation, growth, and burst of the bubbles (micro size) inside the liquid phase (Chemat et al. 2008). Koiwai and Masuzawa (2007a) proposed an experimental setup for the extraction of the polyphenols compounds from the green tea. Tea leaves were placed in Teflon bottle being inert in nature. The bottle was placed in a water tank and ultrasound waves were provided under the tank. A comparison was made by giving different range of frequencies: 25, 45, 100, and 130 kHz (Koiwai and Masuzawa 2007b). The applications of ultrasonication are being exploited commercially at a large extent. Both et al. (2014) compared the extraction efficiency of polyphenols with a conventional and ultrasonication technique. Authors reported ultrasonication extraction as a promising technique with 15 % in polyphenol content. Other than polyphenols, many beneficial compounds, namely, anthocyanins, aromatic compounds, polysaccharides, and functional foods can also be extracted by using this technique. Ultrasonication can enhance the extraction yield and quality of compounds by opening new opportunities with other extraction techniques (Vilkhu et al. 2008). A study was performed by Xia et al. (2006) to investigate the chemical changes and sensory quality of tea by applying the ultrasonic-assisted extraction. Authors concluded ultrasonication as a better technique as compared to hot water extraction, with better extraction efficiency at low temperature decreasing the threat of degradation of compounds. An ultrasonic cleaning bath (40 kHz, 250 W) was used for a 3-g tea sample, mixed with 300 ml of distilled water in a 500-ml plastic flask in this work. Koiwai and Masuzawa (2007a) presented their work on the extraction of catechins. To reduce the risk of contamination in the food items and maintain the quality of catechins, ultrasonication was opted by authors. Six ultrasonic transducers with frequency of 25 kHz and varied ultrasonic pressure at a constant temperature were used. Authors (Koiwai and Masuzawa 2007a) concluded that it is possible to extract more catechins with irradiation of sound waves at low temperature. A comparison study between the different methods of extraction (hot water extraction, Soxhlet extraction, and ultrasonication technique) of polyphenols was performed by Mo et al. (2008). The authors concluded that ultrasonication gave the maximum extraction yield. Ultrasonication is the simplest technique, very easy to scale up, requires low capital investment, and the least exploited with respect to extraction of green tea polyphenols. It provides the application for thermolabile compounds and prevents the risk of degradation of polyphenols due to a higher temperature. However, dispersion medium decreases the intensity of the waves with time that influence the extraction efficacy. The lack of uniformity in the process limits the use of this technique for the wider application of extraction of bioactive compounds. Ultrasonication has been exploited for the extraction of polyphenols from apple pomace (Virot et al. 2010), saponin from ginseng (Wu et al. 2001), and lycopene from tomatoes (Lianfu and Zelong 2008). It has been concluded from the literature that ultrasonication gives a better recovery of bioactives than other conventional extraction techniques. The technique needs process optimization and modifications for wide applications. Chemical Extraction Technique A novel and advanced technique for the extraction of tea polyphenol by ammonium chloride precipitation has been Food Bioprocess Technol recently reported by Zhihui et al. (2013). Precipitation of coarse-crystalline tea polyphenols has been done by addition of ammonium chloride followed by sodium bicarbonate (1 mol/L) to regulate the pH to the tea extract. Authors reported an improved extraction efficiency of tea polyphenols by 5 % as compared to traditional extraction techniques. Different techniques for extraction of green tea polyphenols are summarized in detail in Table 2. Comparison of Different Polyphenol Extraction Techniques Polyphenols have been extracted widely from a large number of sources. Different techniques have been employed for the polyphenols extraction, with their respective advantages and limitations (discussed in Table 3). Solvent-based extraction is a primary technique and widely exploited for extraction of number of compounds from various sources. However, the product extracted is not food grade until purified. To avoid the risk of contamination of solvent, carbon dioxide can be used as solvent but, due to limited polarity, cannot be exploited universally. Solvent extraction is cost effective and easy to set up and, hence, is used widely. However, for food grade application of purified polyphenols, microwaveassisted (MWE) and ultrasonication extraction are the most promising techniques and possess enough potential for the maximum recovery of the polyphenols. High intensity of waves along with the temperature during MWE and ultrasonication has significantly increased the recovery of polyphenols and reduced the extraction time. However, MWE serves the advantage of reduced solvent usage and reduced risk of degradation/epimerization of polyphenols to a greater extent than ultrasonication technique as the time required for extraction of polyphenols is much lower in MWE (1–3 min for MWE and 30–40 min for ultrasonication). Ultrasonication extraction can be scaled up easily and has comparatively low investment cost than MWE. However, the limitation is the nonhomogenous distribution of wave energy in the system and continuous reduction in power during the extraction. Hence, the utilization of different elements from different techniques simultaneously can further increase the recovery of green tea polyphenols. As discussed above, application of ultrahigh pressure with solvent extraction results in increased polyphenol content than individual techniques. Application of both micro- and ultrasonic waves together can be exploited for improved recovery of polyphenols than individual techniques (Lianfu and Zelong 2008). Effect of Different Solvents and Extraction Techniques on Composition of Green Tea Extract Different composition of polyphenols in tea is mainly attributed to climate, species, region, and processing (extent of fermentation) of the leaves (Meterc et al. 2007). However, the composition and concentration of polyphenols in the extract vary with mode of extraction and selection of solvent. Various techniques and solvents have been reported in literature for the extraction of green tea polyphenols and resulted in various compositions of total polyphenols (Jun et al. 2011; Huang et al. 2013; Lee et al. 2013). Optimization of ultrasonic extraction of green tea polyphenols has been done by Lee et al. (2013) by considering different concentration of solvent, temperature, and extraction time. Response surface methodology was used to optimize the extraction process, and results were further compared with the conventional technique (shaking extraction). Authors reported different concentration of total polyphenols with different parameters. However, the predicted optimum level for maximum polyphenolic content and minimum caffeine level were found at 9.7 % ethanol, 26.4-min extraction time, and 24.0 °C extraction temperature. Furthermore, it has been stated that caffeine level decreases significantly during ultrasonic extraction of green tea polyphenols. In aforesaid instance under solvent extraction techniques, a difference in total polyphenol content has been reported by Jun et al. (2011) by using different extraction techniques including ultrahigh pressure extraction (UPE), microwave extraction, ultrasonic extraction, and heat reflux extraction. Authors reported a maximum concentration of total polyphenols (572±4.02 mg/g) and the highest antioxidant activity with UPE. Furthermore, different concentrations of EGCG, C, EC, ECG, and EGC have also been reported with a maximum recovery by UPE. Tea extracts were prepared by Sharma et al. (2005) using different solvents (acetonitrile, water, methanol, aqueous methanol (15 and 70 %) and acetone), and extraction times. Qualitative and quantitative analysis of extract was done by HPLC technique. The authors reported a maximum concentration of total polyphenols and different components with the use of 70 % methanol. With the use of 15 % methanol, similar concentration of total polyphenols has been reported; however, theophylline was not detected with lower concentration of methanol. Gallic acid and theophylline were not detected with water as a solvent. In the study performed by Meterc et al. (2007), extraction of green tea polyphenols has been optimized with the use of different solvents at different temperatures with solid-to-solvent ratio. The authors reported maximum polyphenols with the use of methanol giving 7.01 % of EGCG and 2.78 % of EGC content at 60 °C. Qualitative and quantitative estimation of phenolic content was performed by Rusak et al. (2008) using different solvents Different techniques for the extraction of green tea polyphenols Extraction technique Source Purpose Hot water extraction Dried tea leaves Green tea 60 °C Yield—2.56 % caffeine 19.33 % total Extraction, further analysis regarding polyphenols decaffeination and antioxidant activity For water as solvent—80 °C gave a Optimization of water temperature and 80 and 100 °C, ethanol concentration—50 and 75 % (v/v) maximum yield, for dried leaves 50 % ethanol concentration to diminish the ethanol, and for fresh leaves 75 % ethanol epimerization for 10 min Optimization of various parameters Temperature, extraction time, water- Maximum yield at 80 °C for 30 min, pH<6, to-tea ratio, particle size of green with particle size of 1 mm and a tea-totea, pH of extraction solution, and water ratio at 50:1 (ml/g) multiple extraction steps Variable temperatures—100, 80, Degradation of catechins by prolonged Extraction of catechins along with 60 °C exposure at a higher temperatures optimization of boiling temperature followed by their concentration Two-step extraction to concentrate EGC Initially at 50 °C in first step, then at EGC 78.9 % of the total catechin in the first and EGCG 80 °C step and 39.5 % in the second step. EGCG, 153.7 g/ml (10.8 % of the total catechin) in the first step and 503 g/ml (47.6 %) in the second step Optimization of extraction process by Different temperatures of 50, 80, and Maximum yield at 80 °C for 40 min using ethanol as solvent 100 °C for 4 h, 40 min, and 15 min, respectively Maximum yield (0.18 g) obtained at 130 °C Extraction of polyphenols and Different temperatures 100, 110, development of two-phase model 120, and 130 °C at constant when solution treated for 10 min pressure 3×106 Pa Cold water extraction to preserve the Extraction at 10 °C Improved taste sensation, less bitterness flavor To preserve health beneficial elements Extraction at room temperature Enhanced content of nutrients Green tea Optimization with different solvents Green tea Green tea Korean green tea leaves Dried green tea leaves Korean tea Fresh green tea leaves Green tea Organic solvent extraction Mate tea and black Examined the effect of solvent on antioxidant activity tea Green tea Green tea Supercritical fluid extraction Green tea Green tea Conditions Outcomes Reference Bharadwaz and Bhattacharjee 2012 Liang et al. 2007 Vuong et al. 2011 Cheong et al. 2005 Bazinet et al. 2007 Row and Jin 2006 Goodarznia and Govar 2009 Eri et al. 2008 Weizheng et al. 2013 Temperature—water, 80 % ethanol, Maximum yield—9.80/100 g dry mass of tea Druzynska et al. 2007 80 % methanol, and 80 % with 80 % acetone acetone; time—15, 30, and 60 min Ethanol, methanol, acetone and N,N- For black tea 50 % DMF showed a maximum Turkmen et al. 2006 yield and for mate tea, 50 % acetone gave a dimethylformamide (DMF) at maximum yield with maximum antioxidant different concentrations activity 50–60 °C Yield—18.25 % (w/w) polyphenols Kaur and Saraf 2011 To observe the antioxidant and photochemoprotective activity of green tea Optimization of solvents for maximum Ethylene glycol, glycerol; 1,2Ethylene glycol with choline chloride gave Heng et al. 2014 polyphenol recovery Butanediol; 1,4-Butanediol; 2,3maximum recovery Butanediol; 1,3-Butanediol; 1,6Hexanediol with choline chloride To examine the effect of adding co95 % ethanol as solvent Increased mean content by 4.4 folds by using Chang et al. 2000 solvent 95 % ethanol Kim et al. 2008 Food Bioprocess Technol Table 2 Table 2 (continued) Extraction technique Microwave-assisted water extraction Source Green tea Green tea Ultrasonication Purpose Conditions Outcomes Selective extraction of caffeine and epigallocatechingallate using supercritical CO2 as solvent and water as co-solvent. Compared with conventional liquid solvent extraction Comparison between microwaveassisted and conventional extraction and optimization temperature and extraction time Optimization of parameters 40–80 °C temperature, pressure 200–400 bar Water found to be better co-solvent. Selectivity of caffeine/EGCG with water was 0.88 and with water 0.24 with ethanol 100 °C Yield—97.46 mg of catechins equivalent per Nkhili et al. 2009 gram Variable power—100, 200, 400 W Maximum yield obtained at 400 W power and variable duration—n, 4 mins when solution kept for 1 min Tea leaves Extraction of polyphenols followed by 600 W power for 3 min concentration through spray drying Green tea and black Comparison in radical scavenging 77 °C Microwave-assisted extraction gave more tea activity after extraction with two yield than conventional method different methods, hot water and microwave-assisted extraction Tea To investigate the chemical changes and 40 kHz, 250 W Reduced degradation of compounds sensory quality of tea by applying the ultrasonic-assisted extraction Green tea Extraction with ultrasonication to 25 kHz and varied the ultrasonic Increased recovery of catechins reduce contamination pressure Tea Comparison of conventional and Increased extraction of polyphenols by 15 % ultrasonication technique with ultrasonication Reference Li et al. 2010 Tang et al. 2011 Nshimiyimana and He 2010 Xia et al. 2006 Koiwai and Masuzawa 2007a Both et al. 2014 Food Bioprocess Technol Food Bioprocess Technol Table 3 Comparison of extraction techniques Techniques Advantages Limitations Solvent extraction • Wide range of solvents are • Prolonged exposure available (water, to solvents and methanol, ethanol, ethyl high temperature acetate, hexane) • High risk of • Easy to set up and low degradation of installment cost compounds • Purified product is obtained • Huge amount of with SCF (CO2 as solvent) solvent is required • Post process purification is a must Microwave-assisted • Maximum recovery • Batch processing extraction • Short exposure time • Post processing is (1–3 min) required • Less degradation of compounds (here polyphenols) • Reduced solvent usage Ultrasonication • Simple set up and easy to • Long extraction extraction scale up time • No post processing • Lack of uniformity of wave required distribution • Low temperature application; can be used for heat-labile compounds (water, water + lemon juice, and 10, 40, 70 % ethanol) at different extraction times, and a direct correlation of phenolic content was established with antioxidant activity of extracts. The authors reported higher polyphenol content (115 mg/g EGCG and 38.5 mg/g EGC) in green tea extract with the use of 40 % ethanol than water as a solvent. It can be concluded from aforesaid studies that efficiency and composition of polyphenols extracted depends on the extraction technique and polarity of solvent used. Extraction efficiency varies with the polarity of solvent used and extraction time. However, methanol has shown enough potential for maximum extraction of green tea polyphenols. Green tea polyphenols are rich source of minerals like zinc, copper, iron, magnesium, nickel, and many others. Concentration of these minerals in brew varies with solvent used for green and black tea extraction. A comparison study for iron and copper concentration in aqueous and ethanol green tea extract has been performed by Gramza et al. (2005b). It has been reported that ethanol has preserved more amount of iron and copper in the extract. Furthermore, green tea possesses more minerals than black tea. The use of pesticides during the cultivation leaves the traces (of pesticides) in green tea extract. Pesticide concentration in the extract varies with the source of extraction, water solubility, partition coefficient, and brewing duration. The concentration of pesticides in extract increase with increases in brewing temperature, and 60 °C is recommended as optimum temperature for brewing Impact on the quality of compounds Applications • Selective compounds can be • Green tea (Liang et al. 2007; Weizheng obtained by SCF extraction, et al. 2013; Kaur and Saraf 2011) without post process • Grape seeds (Murga et al. 2000) techniques • Pomegranate peel (Cam and Hisil 2010) • More prone to epimerization than other techniques • Solvent extracts (other than water) need to be purified • Traces of solvent (if other than water is used) with compound • Less degradation due to less exposure to heat as compared to other techniques • May prone to epimerization if temperature is near 100 °C • Green tea (Tang et al. 2011; Li et al. 2010) • Radix astragali (Xiao et al. 2008) • Hypericum perforatum and Thymus vulgaris (Sterbova et al. 2004) • Green tea (Koiwai and Masuzawa 2007a; Both et al. 2014) • Apple pomace (Virot et al. 2010) • Saponin from ginseng (Wu et al. 2001) (Cho et al. 2014). It has been reported by Wheeler et al. (1982) that pure acetonitrile is less efficient for extracting dieldrin than pure ethanol and methanol. The presence of water influences the polarity of solvent which consequently effects the extraction of pesticides. Current Status and Future Prospects Extracted polyphenols are very sensitive to the environment and prone to epimerization at a higher temperature and exposure to light. Hence, it is necessary to protect them from degradation. Encapsulation of the polyphenols serves this need with the additional advantage of masking the flavor, control and targeted release, simultaneously increasing the bioavailability of the polyphenols. Various techniques for encapsulation are available, namely, emulsification, coacervation, inclusion, complexation nanoprecipitation, emulsification–solvent evaporation, and supercritical fluid to encapsulate bioactive compounds and pharmaceuticals, which are to be used on the basis of conditions and characteristics of the core material. These techniques do affect the physicochemical properties of the material (Ezhilarasi et al. 2012). Drying techniques involve spray drying and freeze drying and are considered to be very promising techniques for microencapsulation (Choi et al. 2004; Abdelwahed et al. 2006; Ezhilarasi et al. 2012). A study has been performed to check the viability and stability Food Bioprocess Technol of Bifidobacteria by co-encapsulation with green tea. The result confirmed the increased viability of bacteria with 10 % co-encapsulated green tea during storage at 4 °C (Vodnar and Socaciu 2012). Recent research is focused on the fortification of foods (eg. bread, cake, biscuits) with green tea polyphenols as a means of improving its nutritional value without compromising the organoleptic properties. Conclusion Green tea polyphenols are well known for their substantial potential as anti-cancer and antioxidant agents. Different techniques for the extraction of green tea polyphenols and antioxidants have emerged with certain merits and demerits. From aforesaid studies, it can be concluded that various factors influence the polyphenol extraction in terms of content, efficiency, composition, and purity of polyphenols. These factors include technique used for extraction, duration of extraction, solvent used, solvent to solid ratio, and intensity of waves. Microwave-assisted and ultrasonication extraction techniques seem to be more promising and have shown a greater potential and better efficiency for the extraction of polyphenols as compared to other techniques. Furthermore, green tea polyphenols are prone to epimerization and degradation at a higher temperature and alkaline pH. Encapsulation technique serves the need to avoid the degradation of polyphenols. Techniques such as spray drying, freeze drying, emulsification, coacervation, inclusion, complexation, and nanoprecipitation are mainly used for encapsulation purpose. Each extraction technique has certain unique operating factors which affect the concentration and antioxidant activity of the extract and need to be optimized. However, there is a need to reduce or avoid the use of organic solvents for extraction and exploit other techniques with better efficiency. Acknowledgments The authors wish to thank Prof. Ram Rajasekharan, Director, CSIR-CFTRI, Mysore, India, for his support and help. We gratefully acknowledge the Ministry of Food Processing Industries (MoFPI) for providing financial support to carry out this work. References Abdelwahed, W., Degobert, G., Trainmesse, S., & Fessi, H. 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