The Studies Of The Key Odorants Causing Jiao-aroma And Their Formation Pathways In The Strong-aroma Types Of Baijiu

Strong-aroma?also known as Luzhou-flavored?types of Baijiu?SAB?,which account for approximately 70%of the total national liquor production,are among of the foremost traditional distillates in China.Compared with the other aroma types of Baijiu,fermentation in a unique fermentation container covered with pit mud is the most important process for setting the characteristics of the SAB,as this step endows SAB with its characteristic jiao-aroma.In recent years,with the development of foodomics and sensomics approach,most studies have been focused on the determination of key aroma compound in SAB,investigation of the mechanism of microbial metabolism,and the construction of functional strains.However,until now,no comprehensive study has been performed on the key aroma compound responsible for jiao-aroma characteristics in SAB;more importantly,the metabolism of the aforementioned key aroma compound during the brewing of SAB remain unclear.Therefore,for the scientific design of SAB and improve the stability of its quality.In this thesis,the SAB with strong jiao-aroma was selected as the main research object.Moreover,the objectives of this study were?i?to clarify the potent odorants responsible for the typical jiao-aroma in SAB;?ii?to investigate the relationships between jiao-aroma and the mud-like off-odor;?iii?to highlight the interactions between the roasted and jiao aroma compounds in SAB;?iv?to preliminary elucidate the metabolic pathway of the key compound responsible for the jiao-aroma during the brewing of SAB.The main research contents and results were listed as follows:?1?The aroma profile of the SAB was investigated by means of liquid-liquid extraction?LLE?combined with the gas chromatography-mass spectrometry/olfactometry?GC-MS/O?analysis.Moreover,to further determine the differences in the overall aromas of LZ-2016base SAB and GJ-1573,descriptive profile analyses were perforemed.A total of 68 and 64odor-active regions were detected in the base and commercial SAB by comparative aroma extract dilution analysis,with an FD factor in range of 1-59049;moreover,67 and 64 of these were further identified by a comprehensive comparison of their odor descriptions,their mass spectra obtained in SCAN mode,and their retention indices?RIs?with those of the pure standards on both capillary columns.Among them,64 were common to both SAB samples,but the FD values were considerably different.The most significant differences were found for 3-?methylthio?propanal,3-methylthiopropanol,and 3-?methylthio?-propanoic acid ethyl ester.These roasted potato and onion-like odorants showed high FD factors?27-729?in LZ-2016 but were not sensorially detectable in GJ-1573.?2?For a long time,it was experientially assumed that‘mud'-like odors present in the SAB may have originated from the pit mud since similar odors were present.In this work,volatile compounds in pit mud and SAB samples were isolated by HS-SPME and LLE.An odoriferous zone was detected by GC-MS/O and GC-MS and was attributed to3-methylindole rather than ethyl oleate based on their odor characteristics,MS data,and retention indices.Combined with the detection threshold of 3-methylindole?3-MI?determined by a three-alternative forced-choice?6.09?g/L?procedure,OAVs in the tested base SAB were from 1 to 23.Sensory studies and the prepared heat map highlighted the contribution of this compound to the odor characteristics for the first time.?3?Based on the results mentioned above,the odorants with flavor dilution?FD??9were further quantified by multiple quantitative techniques,including the direct injection combined with gas chromatography-flame ionization detector,the vortex-assisted surfactant-enhanced emulsification liquid-liquid microextraction?VSLLME?combined with GC-MS,and the derivatization method combined with VSLLME-GC-MS,and 37 and 26odorants were shown to be important odorants as their OAVs?1.Among these components,5compounds,hexanoic acid,butyric acid,p-cresol,3-MI,and 3-?methylthio?propanal showed higher OAVs in base than in commercial SAB,and based on aroma recombination and omission analysis,they were confirmed to be the key odorants responsible for the jiao-aroma and roasted odors.The evaluation of the aforementioned five key odorants by the“addition”test showed that the roasted and jiao aromas of base SAB were related,and the odors were mainly attributed to additive or masking effects among the compounds.?4?A poly?allylamine?-modified magnetic graphene oxide?GO@PAA@Fe₃O₄?with a high adsorption capacity for 3-indoleacetic acid?IAA?and 3-indolepropionic acid?IPA?was prepared by a simple and facile method at room temperature.Moreover,according to the results related to the adsorption mechanism,the introduction of PAA and Fe₃O₄ not only altered the polarity of GO but also provided more adsorption sites for IAA and IPA,mainly through the generation of hydrogen bonding sites.More importantly,using 0.5%acidic acetone to activate GO@PAA@Fe₃O₄ significantly enhanced its extraction efficiency for IAA and IPA.The as-prepared materials showed an excellent adsorption ability for IAA and IPA,and the adsorptions of IAA and IPA on GO@PAA@Fe₃O₄ were fitted to the pseudo-second-order kinetic model and Langmuir isotherm,respectively.The adsorption equilibrium of the analytes could be reached within 10.0 min;moreover,the maximum adsorption capacity of GO@PAA@Fe₃O₄ for IAA and IPA obtained from a Langmuir isotherm was 14.0 and 14.3 mg/g,respectively,at 35?,p H 4.0.Owing to its good thermal and mechanical stability,the GO@PAA@Fe₃O₄ showed a long lifetime,and it could be reused for at least 9 cycles without a significant decrease in the adsorption capacity.?5?This is the first time that IAA and IPA have been determined in SAB using the newly developed GO@PAA@Fe₃O₄-based magnetic solid-phase extraction?MSPE?coupled with HPLC-MS/MS technique.To obtain the optimal MSPE conditions for extracting IAA and IPA from the base SAB samples,the major experimental parameters,including the amount of GO@PAA@Fe₃O₄,the p H and ionic strength of the sample solution,and the extraction time,as well as the desorption solvent,temperature and time,were investigated.Under the optimal conditions,IAA and IPA were determined in 16 base SAB for the first time,and their concentrations ranged from 0.64-11.32?g/L and 0.66-18.74?g/L,respectively.Moreover,these findings indicate that the metabolism of 3-MI during base SAB brewing follows two pathways:?a?the conversion of L-tryptophan into 3-MI via IAA,and?b?the direct conversion of L-tryptophan into IPA.