Study On Theanine In Green Tea,Pu-erh Tea And Oolong Tea

L-Theanine is a unique non-protein amino acids in tea which usually constitutes 1%～2% of dry weight of tea leaves. It takes a great proportion among all the free amino acids and contributes a lot to the taste of tea infusion. In recent years, research mostly focus on L-theanine's physiological function, biochemistry, pharmacology, and health functions and so on.. L-theanine is safe and non-toxic and has a lot of physiological functions, which surely will have a prospect future. Researches showed that there exists a small amount of D-theanine in the dry tea besides L-theanine. Both enantiomers of theanine have a similar sweet taste. Though no correlation was found between the absolute concentration of theanine in tea and its enantiomeric composition, the variation of theanine and its enantiomer ratios in different types and grades of tea from the same location is very different, so analysis of enantiomers may be used as an indicator for grades. Therefore, it is necessary to study the changes of theanine in different types of tea. This is the first study on the changes of theanine in green tea, pu-erh tea and oolong tea. The main results are as follows:1. Using accelerated shelf life method to study the changes of theanine in green tea under different storage conditions. Experimental factors and levels are as follows:water content of tea:3.98%,5.03%,6.10%, oxygen partial pressure:2%,10%,20%,then placed the samples in 5℃, 15℃,25℃,35℃for a period of 6 months of storage by sampling every 60 days. Using high performance liquid chromatography coupled with chiral derivatization to determine the content of L-theanine and D-theanine in green tea. The results showed that:among the four factors, water content, oxygen partial pressure, storage temperature and storage time there was no significant diference on L-theanine from oxygen partial pressure, other factors had very different. During storage, in the mid time that was 120d, the content of L-theanine increased rapidly, and then decreased. After 180 days, D-theanine were detected in some samples from 1.3mg/100g to 2.1mg/100g which take up 0.16% to 0.11% of the total theanine. The results suggest that theanine isomerization would be generated after high temperature storage for a long time.2.Research on theanine of riped pu-erh tea and raw pu-erh tea from five different area showed that:L-theanine content of 15 samples of riped pu-erh tea was from 20mg/100g to 104mg/100g,and there was no obvious trend.D-theanine didn't exist in all riped pu-erh tea samples.The content of L-theanine ranged from 628mg/100g to 2131mg/100g in raw pu-erh tea. and there was no obvious trend too.The content of L-theanine of raw pu-erh tea were much higher than riped pu-erh tea. There was only one sample H2 had D-theanine with the content of 4.3mg/100g.3. Research on theanine of seven oolong tea with different various and processing showed that: Different from pu-erh tea and green tea, both L-theanine and D-theanine were detected in all the seven oolong tea.The content of L-theanine was from 9mg/100g-371mg/100g,while D-theanine was from 6mg/100g～18mg/100g,and the ratio of D-theanine and total theanine was from 1.94% to 66.16%, Oolong tea with honey aroma had the highest of 66.16%.The percentage of D-theanine of Oolong tea with heavy flavored was much higher than oolong tea with fragrant. This indicated that L-theanine isomerization may occurre in the thermal processing at a long period. Oolong tea with heavy flavored had a large amount of D-theanine.4.The paper studied theanine and enantiomer of pu-erh tea,oolong tea and green tea in storage. The results showed that:Theanine racemized during long time storage in green tea and processing in oolong tea. Thus, theanine and its enantiomer could be indicators for green tea in storage and quality of oolong tea. And this will provide experiment data for controlling quality of tea.