Study On Preservation Mechanism Of Green Tea

Free radical theory as new ageing mechamism of tea is proposed in this paper.The advance in study on tea ageing mechamism and preservation technology are reviewed.The changes of main components in the ageing process of tea and its effects on sensory quality,environmental factors and preservation technology focusing on these factors are introduced. The properties,detection method and progress in research of tea radical are discussed in detail.In this experiment,tea samples exhibit a high,symmetric ESR (electron spin resonance)singlet centered at g = 2.002±0.002 with peak-to-peak line widths 0.98mT,superimposed on the other broad signal with peak-to-peak line widths 55 mT.The broad signal is a characteristic of manganese(Ⅱ)complex,while the sharp signal is related to a stable organic radical,similar to the semiquinone radicals signal which origin from tobacco tar and other natural plant leaves.And the signal intensity is affected by temperature,humidity,irradiation and mechanical effect.All the samples were kept in constant temperature and humidity cabinet at 40℃,and 75%relative humidity to accelerate ageing.For further study on radical generation and variation mechamism,tea polyphenol extraction,tea residue and the tea residue added certain amount of tea polyphenol were used as experiment material to carry out comparative test.The radical content of all tea residues show increasing tendency after high temperature storage for12 days,but the radicals concentration of tea polyphenol extraction show a extremely low level,and exhibit a decreasing tendency throughout the whole process.This indicate that the radicals origin from the high polymers of tea residue rather than tea polyphenols.Meanwhile,we find the chemical property of tea polyphenol extraction which has little radicals is very stable,and hardly can be oxidized.However,the tea polyphenols added to tea residue are not stable and easily oxidized,moreover,there is apositive relation between the retention rate of tea polyphenols and final radicals content of tea residue. It is clear that the radicals origining from high polymers rather than external oxygen are the real motivation for tea polyphenol oxidation. During the high-temperature storage,tea polyphenols scavenge free radicals and oxidize themselves at same time.The impact of environmental factors on radicals cotent in tea residues was discussed,positive effect of oxygen,moisture and high temperature on radicals concentration was revealed.Furthermore,the radicals concentrate of tea powder increase obviously after mechanical grinding,which indicats the polymer chains are broken during the grinding process,and H-deprivers of the polymer fracture terminals can increase radicals content directly.In addition,effects of various cold storage temperature and temperature difference in and out cold storage on radicals content and tea quality were also tested and analysised.In our experiment,tea processing technology is upgraded according to the radical theory.Decreasing cells destruction ratio and annealing procedures to prolong the drying and heating procedure are available to enhance preservation properties of Longjing-tea.