Effect And Mechanism Of Theanine On Tea Roasted Aroma Formation

Tea aroma is the soul of tea(Camellia sinensis),which is related to tea flavor quality,affecting consumer's preference and selectivity.Most of tea aromas are generated from odorless precursors in a series of physical and biochemical reactions during tea manufacturing processes.Theanine,the most abundant non-protein amino acid in tea,which have important health benefits,is widely believed to be associated with the umami taste,however,its contribution to the formation of tea aroma and the related aroma formation mechanism is still unknown.In this study,Shuixian oolong tea(Rock tea)and Large-leaf yellow tea(LYT)were chosen as the test materials to conduct the research.The volatile compositions were detected and aroma properties were studied during tea manufacturing processes,coupled with the determination of total sugar and theanine content,to investigate and confirm the effect of theanine on tea aroma formation,and its related mechanism was preliminarily proposed.The main results are as follows:1)The volatile composition of Shuixian oolong tea(undergoing full fire processing,FFOT)was determined by different extraction methods,steam distillation(SD),solid phase micro-extraction(SPME)and headspace(HS)were applied for analysis of tea body notes,tea infusion and dry tea aroma,respectively.Meanwhile,the SD was used for extraction the volatiles during Shuixian oolong tea manufacturing processes.Heterocyclic compounds,aromatic compounds,terpenes and alcohols were the main volatiles of FFOT,and with the movement of processing,there were numerous newly generated heterocyclic compounds with pyrazine and pyrrole structures,which were of nutty or burnt odor,and the total sugar and theanine content decreased significantly after full fire processing compared to 2nd roasted(SR),it was speculated that carbonyl-amine reaction might lead to the sharp decrease in concentration.And the new formation of nitrogen-containing volatiles and decreasing of theanine content suggested that theanine probably involved in the volatile formation during manufacturing processes.2)The comprehensive and multi-technology of sensory evaluation,electronic nose(E-nose)and GC-O technology was used to investigate the aroma properties during Shuixian oolong tea manufacturing processes.The results showed that caramel-like,nutty and floral flavor were the main aroma note of FFOT,and the nitrogen oxides,aromatic compounds and compounds containing methyl-or ethyl-structure were the material basis for the difference of aroma properties during tea manufacturing processes.At the same time,the GC-O description results revealed that most of the nitrogen and oxygen-containing compounds were of roasted,nutty or burnt flavor,and had strong or highest intensity,these may be the aroma-producing component of FFOT.The deconstruction of aroma properties of Shuixian oolong tea by three methods was consistent with the results of volatile analysis,and the formation of these volatiles is related to thermal reaction in previous study,so it further implied the possibility of theanine participating in thermal reaction to form volatiles during tea manufacturing processes.3)The model thermal reaction(MTR)with D-glucose and L-theanine was conducted and confirmed that theanine may generate volatiles with glucose in a Strecker degradation way,and methylpyrazine and 2,5-dimethylpyrazine,two newly formed compounds in FFOT,together with other volatiles were successfully detected in MTR.And the model tea roasted experiment further verified that L-theanine contributed to the formation of 2,5-dimethylpyrazine in oolong tea by adding additional L-theanine to 2nd roasted(SR)in a same way of processing from SR to FFOT.4)The model thermal reaction with heating L-theanine alone in 120? or higher temperature(130?)confirmed that theanine can be an aroma precursor of tea volatiles,contribution to the formation of N-ethylformamide,N-ethylacetamide and N-ethylsuccinimide in tea.2,5-Dimethylpyrazine and amide compounds,which were formed from theanine as an aroma precursor molecules,were verified in the aroma of L YT with the similar roasting process,and further confirmed the contribution of theanine to the tea aroma formation.The formation mechanism of these volatiles,which were formed from precursor molecules that are produced from theanine by a thermal reaction were proposed.The exploring of contribution of theanine to tea roasted aroma formation and its mechanism provide important theoretical basis for regulating tea aroma quality and the optimization of tea processing technology,provide a new insight of L-theanine on tea aroma beyond umami taste,and expand the interpretation scope of tea aroma formation mechanism.At the same time,the deconstruction of aroma properties provides technical support for tea processing,tea perfume making,and would assist tea manufacturers to either improve the aroma of tea or avoid the formation of undesired odors.