| There are many problems during the process and storage of tea beverage, among which the formation of sediments in tea beverage still cannot be well solved because the formation is complicated. The formation mechanism of sediments should be further studied and the available control method is expected to establish. China is one of the major countries which produce and consume green tea beverage products. The formation of cream and sediments during the process, brewing or storage of instant green tea powder, green tea concentrate and green tea beverage, greatly influences the exterior quality and flavor of these products. In the present study, we investigated the formation and chemical components of green tea sediments in green tea infusion and concentrate, the formation mechanism of irreversible tea sediment, key factors and control basis of green tea sediment formation. The results can help to reveal the formation mechanism of green tea sediment, and provide the theoretical principle to improve the quality of green tea beverage. The main results were listed as follows:1. Green tea infusion quickly appeared turbid when it was stored at low temperature. Sediment formation and its amount of green tea infusion were influenced by the green tea materials. Most of the chemical components in green tea infusion were found to participate in green tea sediment formation, which include polyphenols, catechins, carbohydrates, pectin, polysaccharides, protein, caffeine, flavones, and 7 metal ions. And polyphenols, total sugar and caffeine were the major components of tea sediments. Gallated catechins and caffeine were one of the principle chemical compositions of tea sediments, and the amount of sediments can be forecasted by the contents of caffeine and gallated catechins in green tea infusion.2. With the increase of the solid concentration (5-60 Brix), the sediment amount of green tea concentrate increased first and then decreased. The sediment amount increased gradually when the solid concentration increased from 5 to 40 Brix, but the amount decreased greatly when the solid concentration reached 50 and 60 Brix. The large decrease of the amount of the sediment may be due to the sharp increase of the viscosity of the tea concentrate, which helped to improve the stability of tea concentrate. There was demixing phenomena of the tea sediment when the solid concentration reached 30 Brix, most part of the sediment (upper) was black while some part of the sediment was white (lower). The sediment was dissolved with 60℃ water for 20 min, and the dissolved part was denoted as reversible tea sediment (RTS), the non-dissolved part was called irreversible tea sediment (IRS). Most of the black sediment was the RTS, while most the white sediment was IRS.3. The sediment amount of green tea concentrate increased with the storage time. RTS was mainly formed in the first 10 days, and IRS was mainly formed between 20 and 40 days of storage. The RTS mainly include polyphenols, flavones, caffeine, protein and carbohydrates, while IRS contained large amount of metal. The total mineral content in the IRS (16.5%) was much higher than that in the RTS. The Ca, Mg, Mn and Ga contents in IRS were over 1.0%(w/w) each.4. The amount of IRS in the green tea infusion from different cultivars was significantly different. The formation of IRS was influenced by the chemical constituents of tea infusion. By dissolving in HCl solution, IRS was found to contain large amount of oxalic acid ions, and oxalate was the main ingredient of IRS, in which the content of calcium oxalate was the highest, including calcium oxalate monohydrate and calcium oxalate dehydrate. However, there was also 25%(w/w) of IRS cannot be dissolved in HCl solution, and its chemical components and formation mechanism need to be further analyzed. Meanwhile, ellagic acid was found to combine with protein and form IRS in green tea infusion, which may be one of the ways for IRS formation.5. The formation of green tea infusion was found to be significantly influenced by the extraction temperature, extraction time and the leaves/water ration. With the extraction temperature increased, the solid concentration and sediment amount significantly increased, especially when the extraction temperature increased from 40 to 50℃, the amount of tea sediment increased greatly. And with the extraction time increased, the solid concentration and amount of sediment also significantly increased. However, the extraction time increased to an extent, then the solid concentration and amount of sediment was slightly influenced by the extraction time. With leaves/water ratio increased, the solid concentration and amount of sediment was also found to increase, and lower leaves/water ratio helped to inhibite the formation of tea sediment.6. Adding Ca2+ could increase the formation of RTS (8%) and IRS (92%), while adding chelating ions of Na2EDTA significantly decreased the amount of RTS by 14.6%, but not the amount of IRS. It was because that, under acid conditions, Ca2+ combined with oxalic ions to form indissoluble oxalate despite the existence of the chelating ions. Decaffeination largely inhibited the formation of RTS (73%) and IRS (60%), even in the presence of Ca2+. The amount of sediment could be reduced by removing polyphenols using polyvinyl-polypyrrolidone, which also inhibited other chemicals to participate in sediment formation.7. It was found that the amount of tea sediments significantly decreased with the addition of simple sugars. And the rates of polyphenols/caffeine in tea sediments sharply decreased; while the total sugars increased greatly. It was found that fructose and sucrose showed better effects in reducing tea sediment and improving the concentration of polyphenols and caffeine in tea concentrate than maltose and glucose, indicating that fructosyl was a highly effective functional group. The difference in sediment formation between adding fructose and adding glucose may be due to the interaction energies difference between typical sugars and polyphenols and caffeine. |
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