Formation Pathway Of Key Aroma-active Compounds Generated By Maillard Reaction During Roasting Of Wuyi Rock Tea

People have been drawn to Wuyi rock tea（WRT）,a sub-category of oolong tea in China,for thousands of years because of its exceptional and premium aroma.The roasting process has a significant impact on the production of WRT aromas and can mold various sorts of WRT aromas.The Maillard reaction is an important step in the roasting process.The key aroma-active compounds in WRT,however,are still unknown.The role of important Maillard reaction substances on the aroma of WRT is uncertain.Furthermore,the formation mechanism of Maillard reaction aroma compounds during roasting is unknown.Therefore,this subject systematically explored the changes in aroma during roasting,the impacts of the Maillard reaction on the aroma formation of WRT,and the formation mechanism of key aroma-active substances relating to the Maillard reaction.The following are the main research outcomes for this subject:（1）The influence of different roasting temperatures on WRT aroma formation was studied.SPME combined with GC-O-MS and GC×GC-O-MS were used to identify the aroma compounds of WRT with three different roasting temperatures（light fire,LF,120℃;moderate fire,MF,130℃;high fire,HF,140℃）.The significant difference between MF/HF and LF was found to be due to the nitrogen-containing compounds,which typically present roast aroma,according to sensory evaluation and e-nose analysis.MF received the highest overall score in the professional sensory evaluation.In total,97 aroma-active compounds were identified in three samples,52 of which were r-OAV>1.Three samples have their own characters.LF samples contained more alcohols,aliphatic aldehydes,and ketones,which contribute to the floral and fruity aroma,such as（E,E）-2,4-hexadienal,octanal,hexanal,（E,Z）-2,6-nonadienal,（E）-β-Ionone,（E）-3-octene-2-one,etc.Strecker aldehydes are important in MF,including 2-methylpropanal,2-methylbutanal,and 3-methylbutanal.At the same time,the aroma components with baking aroma are generated at an appropriate temperature without affecting the aroma components of flowers and fruits.In HF,the roasted and burnt aroma was strong,which weakened the floral and fruity aroma,because of the high content of heterocyclic compounds produced by the Maillard reaction,such as 5-methylfurfural,1-furylpyrrole,2,5-dimethyl-3-ethylpyrazine,6-methyl-2-ethylpyrazine,5-methyl-2-ethyl pyrazine and 1-methylpyrrole-2-formaldehyde.（2）The key aroma-active compounds of Rougui WRT（RR）and Rougui WRT with moderate roasting（RM）were identified.The aroma extracting methods of DHS（Dynamic headspace sampling）and SAFE（Solvent assisted flavor evaporation）combined with GC-O-TOF MS,GC×GC-O-MS,and GC-Qq Q were utilized to characterize the key aroma-active compounds in RR and RM.Totally,41 aroma active compounds with FD>8 were obtained by AEDA,of which 30 key aroma-active compounds with OAV>1 were characterized through external standard quantitative analysis.Finally,26 key aroma-active compounds in RR and RM were identified by aroma recombination and omissions experiment.（E）-β-Ionone,2-/3-methylbutanal,hexanal,phenylacetaldehyde,linalool,and（Z）-3-hexene-1-ol are the most important key aroma substances for the overall aroma of RR and RM.The roasted flavor was contributed by 2-/3-methylbutanal,furanone,3-methylthiopropanal,β-myrcene,furfural,and 5-/6-methyl-2-ethylpyrazine;hexanal,（Z）-4-Heptenal,（E）-2-Heptenal,geraniol,pentanal,β-ionone,（E）-nerolidol and benzyl alcohol were associated with the aroma attributes of floral,fruity and grass/fresh leaf.（3）The free amino acid,sugars,polyphenol,and the aroma changes in different WRT cultivars（Shuixian,Rougui,and Dahongpao）with different roasting temperatures were investigated.It was found in my study that theanine and glutamic acid which were showing an umami taste,were the main amino acids in all samples.They gradually decreased after roasting and with the increase of roasting temperature.Sucrose showed a downward trend.EGCG and EGC were the main polyphenols that declined when raised roasting temperature.Sensory evaluation and e-nose analysis showed that the aroma profiles of the three cultivars tend to be similar when roasting at the highest roasting temperature.A total of 85 aroma-active compounds were identified by SPME combined with GC-O-MS and GC×GC-O-MS,45 of which with r-OAV>1.Eighteen aroma-active compounds were regarded as the characteristic components in WRT due to it could be identified in all samples,containing 1-octene-3-one,2-acetylpyrroline,3-methylthiopropanal,geraniol,3-methylbutanal,β-myrcene,linalool,hexanal,（E）-β-ionone,（E）-nerolidol,indoleβ-cyclocitral,nonanal,（E,E）-2,4-heptadienal,6-methyl-5-heptene-2-one,2-pentylfuran,hexyl 2-methylbutyrate,and benzaldehyde.They might be related to the“Yan flavor”of the WRT.The enhancement of roasted aroma attribute in each sample was closely related to the increased contents of 1-ethyl-1H-pyrrole,5-methyl-2-ethylpyrazine,6-methyl-2-ethylpyrazine,3-methyl-2-ethylpyrazine,furfural and 5-methylfurfural that had a great correlation with theanine and glucose according to correlation analysis.（4）The formation pathway of key aroma compounds in WRT were explored.The aroma compounds in three Maillard reaction systems（glutamic acid-glucose,theanine-glucose,and theanine-glucose-EGCG）were identified and compared by SPME combined with GC-O-MS and GC×GC-O-MS.The reactants were reacted at a 1:1 mole ratio.The time and temperature of reaction were the same as the MF’s.The results showed that the glutamic acid-glucose system could produce more pyrazine compounds than the theanine-glucose system,while theanine-glucose was more conducive to the formation of pyrroles.In addition,furfural mainly comes from the theanine-glucose system,and EGCG inhibits the Maillard reaction.Theanine is not easy to undergo Maillard reaction,thus accumulating furfural,dicarbonyl compounds,and other Maillard reaction intermediates,which is conducive to the long-term roasting process of Wuyi rock tea.Using carbon module labeling（CAMOLA）technique investigated the sources of C-skeleton of key aroma compounds.¹³C₆-Glucose and D₅-Theanine were used to semi-label and fully label the theanine-glucose and theanine-glucose-EGCG Maillard systems.It was found that the C-skeleton of furfural and 5-hydroxymethylfurfural was mainly provided by glucose,and the part of glutamate of theanine could also participate in the reaction.The C-skeleton of pyrrole compounds is provided by glucose,the N-atom on the pyrrole ring was provided by ethylamine part of theanine.EGCG can promote the Strecker degradation pathway to form furfural and affect the formation of pyrrole compounds by changing the cleavage unit of glucose.In addition,EGCG itself can also form pyrrole compounds.（5）The above-mentioned pathways were verified and applied by theanine-glucose Maillard reaction system using Wuyi Rock tea infusion as matrix.Three Maillard reaction application models（RR,RR-roast,RR+aa,and RR+glu）were established.The aroma of different reaction models was analyzed and identified by SPME combined with GC-O-MS.The results showed that glucose had a great effect on the formation of furfural,which could also be influenced by theanine.Furfural/glucose and tea pyrrole,1-ethyl-1H-pyrrole/theanine could be used as two groups of indicators to monitor the degree of roasting to some extend...