Feng XIE, Lingli JIN, Juan TU, Meiwang LE, Fan WANG

Institute of Horticultural Sciences, Jiangxi Academy of Agricultural Sciences, Nanchang 330200, China

Tea (Camellia sinensis O. Kuntze. L) waste is the general name of waste dominated by tea biomass and it is produced during the cultivation, processing, deep processing and consumption processes of tea[1]. Tea waste includes (1) deadwood, fallen leaves, pruning branches and leaves, seed shells and other tea residues produced during tea cultivation;(2)fannings(accounting for about 10%-30%of total finished tea production), thick stems, old stems and tea ash produced during tea processing;(3)tea residues and tea oil cakes produced during deep processing of tea beverage, instant tea, tea polyphenols and tea oil;(4)tea leaves after drinking of tea. China is the biggest producer and consumer of tea across the world.The cultivation area and production of tea in China in 2012 were 1 513 000 hm2and 1 714 902 t, respectively, accounting for 46.18% and 35.59% of the world’s area harvested and yield of tea. It is estimated that China will produce more than 5 000 000 t of tea waste per pear.Currently,only a small fraction of tea waste has been utilized rationally, and the most is treated as agricultural waste, resulting in both environmental pollution and enormous natural resource waste. Tea waste is rich in cellulose, hemicellulose, lignin,theanine, proteins, polyphenols,polysaccharides, caffeine, pigment,saponin, vitamins, trace elements and other useful components. It has extensive application potential in food processing, agriculture and animal husbandry,health care,environmental protection and other fields. Therefore,understanding the research status and development trend of comprehensive utilization of tea waste plays an important role in researches about and development of comprehensive utilization technology for tea waste in China.

Active Ingredients

Tea waste is rich in tea polyphenols, polysaccharides, theanine, caffeine,pigment,saponin and other biological active ingredients that have a variety of physiological functions and a variety of health effects on humanbodies.These active ingredients in tea waste all can be extracted and turned into treasure, enhancing the added value of the tea industry. In recent years, there have been more and more studies on extraction of active ingredients from tea waste, especially on extraction of tea polyphenols, tea polysaccharides and caffeine from tea waste.

Tal polyphenols

Tea polyphenols are the collective name of phenols and their derivatives in tea. They are mainly composed of catechin, and are the most common deep processing product of tea. In fresh tea leaves, polyphenols content generally ranges from 18% to 36%(dry weight), and catechin content ranges from 12%-24% (dry weight)[2].Tea polyphenols have a series of important pharmacological functions, including oxidation resistance, anticancer, anti-aging, radioresistance,scavenging free radicals, reducing blood glucose, reducing blood fat, reducing blood pressure and suppressing the mutant[3]. Therefore, the extraction and separation technology for tea polyphenols has been studied extensively.At present,the reported extraction methods for tea polyphenols include solvent extraction, ion precipitation, resin adsorption, supercritical fluid extraction, microwave extraction and ultrasonic extraction[4].Among them, solvent extraction and ion precipitation are currently the most widely used extraction methods. But both the extraction methods have disadvantages of low extraction rate and low purity. Supercritical fluid extraction does not use toxic reagents and has high extraction purity. Moreover, it can be used to extract caffeine, so it has high application potential. However,tea polyphenols have low solubility in carbon dioxide fluid, so further research is needed. Considering the technical and cost factors, the combination of the two kinds of extraction methods may be currently a good choice for extracting high-purity tea polyphenols. In the process of refining crude polyphenols, silica gel column chromatography can be used as a good choice,and it is characterized by simple operation, good separation effect and low cost.Yuan et al.[5]purified extracted crude tea polyphenols using silica gel column chromatography with silica gel as adsorbent and ethyl acetate as eluent.In the obtained refined product, the total content of tea polyphenols and catechin accounts for more than 90%; the ester catechins content accounts for more than 75%;the EGCG content is up to 60%; the caffeine content can be reduced to be undetectable.

Tea polysaccharides

Tea polysaccharides refer to a class of acidic polysaccharides or acidic glycoproteins with special biological activity, and they are combined with proteins[6].At present,the reported extraction methods for tea polysaccharides include water extraction, ultrafiltration extraction, enzymatic extraction, microwave-assisted extraction and ultrasonic extraction. Among them, enzymatic extraction method can improve polysaccharides yield and facilitate the removal of pectin, proteins and other substances from obtained crude polysaccharides. Zhou et al.[7]extracted tea polysaccharides from Laoshan rough old tea using pectinase, trypsin and compound enzyme extraction methods. The results showed that the extraction rate of the compound enzyme method was highest(5.17%±0.17%);the tea polysaccharides extracted by the pectinase method had the highest purity with total sugar content up to 95.26% ±4.09%, and the amino acid content was lowest.Tea oil cakes are the main by-product after oil extraction from tea seeds, and they can be used as good material for the extraction of tea polysaccharides. After degreasing the tea seeds, An et al.[8]extracted tea polysaccharides from tea oil cakes.They found that increased extraction temperature, extraction time, solid-toliquid ratio and pH of extract all could improve the extraction rate of tea polysaccharides. Under conditions of extraction temperature of 100 ℃, extraction time of 4.7 h,solid to liquid ratio of 1: 43 and pH value of 11.0, the extraction rate of tea polysaccharides reached 74.14%.

Other active ingredients

The other active ingredients in tea leaves,such as caffeine,theanine,tea pigment and tea saponin, all have a strong role in health care to human bodies, and their processing added values are also very high.Wang et al.[9]extracted caffeine from tea fannings and other tea wastes using magnesium oxide segmented leaching method, and the quality of obtained caffeine finished products met quality standards required by the new Chinese Pharmacopoeia.Icen and Guru[10]used the tea fibers and tea stems produced by Turkish factory as raw materials, and they extracted caffeine by supercritical carbon dioxide extraction method. From tea fibers and tea stems, the highest extraction amounts reached 19.2 and 14.9 mg/g, respectively, which were increased by 65.5%and 61.9% , respectively compared with those by chloroform extraction method. Theanine is a unique free amino acid in tea,and its content in tea ranges from 1.2% to 2.0% (dry weight). Zhang et al.[11]found that theanine could be extracted from waste liquor produced during the extraction of tea polyphenols by surfactants(CTAB and SDS).

Biological Adsorbent

Tea leaves have porous structure and large surface area. They are also rich in tea polyphenols, cellulose,hemicellulose, lignin and other functional substances containing oxygen,nitrogen, phosphorus and sulfur, and can be used as a good raw material for biological adsorbents. However, if tea leaves are directly used as raw material,the cost is high. It is reported that the adsorption effect of tea residue is better than that of tea leaves[12]. Tea residue is the main tea waste that is used as raw material for adsorbent.One of the main components in tea residue is tea dietary fiber, especially water-insoluble dietary fiber, which may be the main active ingredient in tea residue adsorbent[13]. In recent years, the researches on biological adsorbents with tea residue as raw material have aroused widespread attention.There have been many reports on the adsorption of heavy metals,organic dyes, radioactive elements and other pollutants from wastewater by tea residue adsorbent.

Adsorbing heavy metals from waste water

Tea residue can effectively re-move heavy metals from waste water.Zhang et al.[14]used tea residue to adsorb Pb(II)and Cd(II)from waste water, and the adsorption rate reached 72.13%and 93.75%, respectively. Cui et al.[15]prepared tea fiber (one of the main components in tea residue) with fresh tea leaves, and they found the tea fibers showed high adsorption property for Cu(II)with the highest adsorption amount of 16.78 mg/g. Albadarin et al.[16]utilized tea residue and date pits to adsorb Cr(VI) from waste water.They found that at the temperature of 60 ℃, the calculated maximum adsorption capacities of tea residue and date pits for Cr(VI)were 5.768 and 3.199 mmol/g,respectively.During the adsorption, Cr(VI) was first reduced to Cr(III)and then combined with adsorption sites. In addition, tea residue also had good adsorption effects for Ni(II)[17],Mn(II)[18-19],As(III)[20],As(V)[20]and other heavy metal ions.

Adsorbing organic dyes from waste water

Tea residue can effectively remove organic pollutants, such as dyes, from waste water. In 2009, Uddin et al.[21]collected tea residues from local families and made them into adsorbents, which were used to adsorb cationic dye methylene blue from aqueous solutions. The results showed that the maximum adsorption amount was up to 85.16 mg/g.Giahi et al.[22]used tea wastage to adsorb methylene blue, and the maximum adsorption amount reached 210.79 mg/g at temperature of 40 ℃. When the initial concentration of methylene blue was 0.01 mmol/L, the adsorption rate was up to 85%.

Adsorbing radioactive elements from waste water

The nuclear industry has been rapidly development since its birth in the 1990s. Currently, nuclear technology and products have been widely used in various fields, such as industry, agriculture, medicine and so on.However, large amounts of nuclear waste are produced every year. Tea residue can remove radioactive elements from waste water polluted by nuclear material. Eroglu et al.[23-24]found that black tea residue could efficiently adsorb radioisotopes titanium(201Ti) and gallium (67Ga) from waste water. Cesium (137Cs) is a product of nuclear fission. The radioactive contaminants leaked from the Japan’s Fukushima Daiichi Nuclear Power Plant in 2011 included137Cs.Gurung et al.[25]found that the green tea fannings cross linked by concentrated sulfuric acid showed a high selective adsorption ability for Cs (I), and it could be used for adsorb137Cs from waste water. In addition, tea residue can also adsorb precious metals[12], formaldehyde[26], etc. Due to strong adsorption capacity for heavy metals, tea residue can also be used to fix heavy metals in soil, repairing soil contaminated by heavy metals. In recent years, with rapidly increased production of tea beverages, instant tea, tea polyphenols and other deep-processing products of tea, the production of tea residue has been increased year by year. As a biological adsorbent with wide sources and low costs, tea residue has a broad development prospect in pollution control.

Activated Carbon

Activated carbon is the most commonly used adsorbent material for its convenience, environmentally friendly and low cost.Activated carbon is widely used in water purification,gas filtration and other environmental protection fields. Agricultural and forestry solid wastes, such as tea waste, are rich in carbon, so they can be used as good material for activated carbon. In recent years, there have been more and more researches on application of tea residue in activated carbon preparation. It has been reported that tea waste can be used to absorb chemical dyes[27-28]and heavy metals[29], purify biodiesel[30]and developed as electrode material[30].

Activated carbon preparation generally comprises two main steps:(1) carbonizing raw material under an inert gas atmosphere; (2) activating carbonized products by physical and chemical means. The commonly used chemical activating agents include ZnCl2, H3PO4, KOH, K2CO3, etc. The selection of appropriate activating agent and appropriate use amount is essential for preparation of high quality activated carbon. Gundogdu et al.[32]used the waste produced during the preparation of black tea as raw material,and they prepared 3 kinds of activated carbon using ZnCl2as the chemical activating agent. The results showed that the porosity and surface area of activated carbon were both increased with the increase in ratio between ZnCl2and waste.When the ratio between ZnCl2and waste was 2, the porosity and surface area of activated carbon were 77.7% and 1 141 m2/g,respectively.Gurten et al.[33]used black tea waste as raw material and K2CO3as activating agent for preparing activated carbon, and the surface area of prepared activated carbon was up to 1 772 m2/g.

Bio-organic Fertilizer

Plant polyphenols (tannins) has function of improving soil[34].Since tea waste is rich in tea polyphenols, it can be used as a good raw material for soil conditioner. Mangwandi et al.[35]mixed tea waste produced by cafeteria and limestone powder according to the certain proportion, and they successfully prepared a good soil conditioner.Xia et al.[36]found that the ecological characteristics of soil in tea garden were significantly improved after applying tea residue organic fertilizer into soil. Compared with commercial organic fertilizers, rape-seed cakes and urea for tea gardens,the application of tea residue fertilizer can significantly increase soil organic matter content,reduce soil urease activity and increase soil microorganisms. Many studies have confirmed that tea waste is also a high-quality raw material for compost. Pant et al.[37]used tea residue as raw material, and with the same production technology, the composite index of produced compost was superior to that of common compost produced from chicken manure or green manure.

Many studies have shown that the organic fertilizer produced from tea waste (containing tea polyphenols)can promote crop production, overcome cropping obstacles and mitigate pests and diseases.Sun et al.[38]investigated the effects of different application amounts and different ratios between tea residue organic and inorganic fertilizers on soil in tomato greenhouse. The results showed thatwithin a certain range,the tomato yield was increased with the increased application amount of tea residue fertilizer;the increased application of tea residue fertilizer could reduce the occurrence of diseases in tomato plants;when the application amount of tea residue was 1 875 kg/hm2, the incidence of diseases was lowest.Zhou et al.[39]found that the application of tea residue organic fertilizer could significantly promote the growth of pakchoi and maize, and the fertilizer efficiency was long lasting. The similar effects of tea residue fertilizer have been repeatedly demonstrated on tea, citrus and grape[40-42].

Animal Feed

With the proliferation of the world’s population, food shortage will become more and more serious.Therefore,development and utilization of agricultural products as animal feed has attracted widespread attention.The tea polyphenols, tea proteins,theanine and other ingredients in tea have a significantly role in improving animal slaughter performance, reducing body fat deposition, improving muscle quality, improving animal immunity, etc. In recent years, there have been many reports on developing these active ingredients as feed additives[43-45].

Tea polyphenols in tea waste have excellent oxidation resistance.Developing tea waste as feed or feed additives can reduce loss of nutrients in feed and improve feed efficiency.Kondo et al.[46]found that the addition of green tea residue into oat silage contributed to protein maintainace during storage and protein digestion and absorption by goats.

Tea proteins in tea waste can improve the quality of meat.Zhou et al.[47]extracted proteins from tea residue by alkali-extraction and acid-precipitation method. The hydrolysates of proteins from tea residue can also be added to the basal diet of pigs as feed additives.The studies have shown that hydrolysates of proteins from tea residue can improve pig performance and immunity and increase digestive enzyme activities in pancreas and duodenum.But industrialized extraction of tea proteins from tea residue requires high production costs. To reduce costs, Liu and Tu[48]fermented tea residue by Trichoderma, Aspergillus and combination of beneficial microorganisms.The results showed that the crude protein content in fermented feed reached 26% -29% , which was increased by 20%-30% compared with that of control, and the crude protein content in formula feed met the requirements by piglets.

Nutrients for Edible Fungi Cultivation

Tea waste is rich in cellulose,hemicellulose, lignin, amino acids, vitamins, minerals and other nutrients,and it can be used as good matrix for edible fungi cultivation.The application of tea waste in edible fungi culture not only avoids the waste of tea residue resources but also opens up a new avenue for matrix development for edible fungi cultivation. The used tea waste matrix still contains a lot of nutrients,and can be further used as good raw material for biological fertilizer. There were many related reports in the 1990s, and the technology has been currently mature.Ying et al.[49]used tea seed shells as raw material to cultivate needle mushroom. Lin et al.[50]found that it was feasible to use tea fannings and tea powder, as replacements for sawdust and bran, to cultivate Auricularia polytricha and Auricularia auricula-judae. Tea fannings and tea powder were also proved to be feasible in cultivation of Pleurotus sajor-caju and Pleurotus ostreatus[51]and agrocybechg fruit-body[52]. However, the fungi yield is not high if matrix is only composed of tea waste. Gulser and Peksen[53]found that the yield of Agaricus bisporus cultivated in the matrix composed of tea waste and peat (v∶v,1 ∶1) was higher than those cultivated in the matrixes composed only of tea waste or peat. Therefore, in order to improve the yield and quality of edible fungi,the studies on and industrial application of mixed matrixes comprising tea waste and other components should be strengthened.

Other Aspects

Tea waste is also often used as raw materials for bio-energy, industry,building and environmentally friendly furniture. Agricultural and forestry solid waste is an important renewable fixed carbon resource, and it can be used as raw material for bio-energy.Uzun et al.[54]degraded tea waste into bio-oil and bio-carbon that can be used as bio-fuels by fast pyrolysis, and the highest yields of these two kinds of products reached 30.4% (pyrolysis temperature of 500 ℃)and 43.3%(pyrolysis temperature of 400 ℃), respectively. Tea waste is also a good raw material for wood board. Yalinkilic et al.[55]added a small amount of urea formaldehyde in tea waste, which was used for preparing wood board. Since the prepared wood board was rich in tea polyphenols, it had anti-termite,antiseptic and antibacterial effects.Tea waste, as lignocellulosic material,is also important filler material for polymer. Mattos et al.[56]successfully developed a kind of polypropylene composite with good industrial use by mixing tea waste, eucalyptus particles and polypropylene according to certain proportions. In addition, tea stems are rich in tannic acid and other nutrients,and they can be developed as solid fermentation medium for producing tannase[57]. Wu et al.[58]preliminarily studied the fermentation of tea stems by Aspergillus niger JMU-TS528 for producing tannase. The study results showed that the tannase yield by raw tea waste fermentation was 3.2 times higher than that by cooked tea waste fermentation; sucrose and ammonium chloride are the optimum carbon and nitrogen sources for tannase production;when the addition amounts of sucrose and ammonium chloride were all 7% (m/m), the yield of tannase was highest (23.6 U/g). In addition, tea waste can also be used as good carrier for state fermentation[59]and raw material for building-using clay bricks. It has been reported that the compressive strength and insulation property of clay bricks are all significantly improved when their raw material is added with a small amount of tea waste[60].

Prospects

As an agricultural resource with widespread resources and low costs,the comprehensive utilization of tea waste in line with the view of sustainable development. The comprehensive utilization of tea waste can extendthe industrial chain and significantly improve the economic and social benefits of tea industry in China. But currently, the utilization extent of tea waste is very low in China, and its importance in energy conservation and industrial effectiveness improvement has not been well understood. In addition, the related mechanisms of some utilization technologies for tea waste still lack a complete and clear understanding, so they cannot be promoted in industrial production. Therefore, the mechanisms and industrial application prospects of utilization technologies for tea waste will become hot and key points and difficulties of future researches.

Currently, there have been many reports on application of tea waste.But there are rare reports on comprehensive utilization of tea waste using a variety of techniques.To enhance the utilization efficiency of tea waste and reduce production costs, it is necessary to use a variety of techniques to comprehensively utilize tea waste.Combining the feasibility and costs,the following scheme can be used as reference: (1) tea waste can be first used as raw material for extracting active ingredients,cultivating edible fungi and producing animal feed;(2)the produced tea residue, waste matrix and animal manure can be further mixed together according to certain proportions and fermented to produce organic fertilizer. In addition, the comprehensive utilization of tea waste, as a crossover study area, should extensively integrate high technologies of other disciplines.It should not only further improve the existing technologies but also further broaden the application scope,providing technical support for strengthening and expanding the comprehensive utilization industry of tea waste.

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