Study On The Characterisation And Regulation Of Flavor Compounds In Unfermented Stinky Tofu

Unfermented stinky tofu is a special Chinese snack,popular for its history,unique flavor and nutritional value.However,the raw material sticky brine of the sticky tofu is mainly produced in open environment,making it easy to be contaminated by undesired microorganisms and resulting in a flavor variation.In this research,the dynamic changes of flavor characteristics of stinky tofu stored with different times were studied to establish the relationship between key aroma compounds and sensory attributes,and determine the best consumption period of products.Then,Gas Chromatography-Mass Spectrometer(GC-MS)analysis combined with highthroughput sequencing was adopted to determine the correlation between the microbial community composition and the content volatile compounds in brine was established.According to the pretest result,the superior aroma-producing bacteria selected from the sticky brine were mixed into the brine solution to make the flavor-enhancing stinky tofu.The aroma-producing microorganisms with best aroma-producing ability were identified.This study provides theoretical guidance and data support for stabilizing and optimizing the flavor quality of semi-fermented stinky tofu in China,and the main conclusions are as follows.:(1)Unfermented stinky tofu with different storage times were studied by Quantity Description Analysis(QDA)sensory evaluation combined with e-nose and Headspacesolid phase micro extraction-Gas Chromatography-Mass Spectrometer(HS-SPMEGC-MS).Total 60 volatile compounds with aroma characteristics were isolated and identified from stinky tofu samples by HS-SPME-GC-MS,and correlation analysis showed that the esters could provide an overripe apple-like odor for the aroma profile of stinky tofu.The of esters content was significantly higher in stinky tofu stored for more than 7 days.Therefore,the content of esters can be used as an indicator to determine the product’s storage time.Compounds such as 1-hexanol,2-pentylfuran,hexanal,decanal,(E)-2-Nonenal and(E,E)-2,4-nonadienal positively correlated with the beany properties of the samples,with higher levels in the early stages of storage.According to the results of sensory evaluation and analysis of volatile compounds,the score of bad flavor attributes in the samples stored for more than 10 days showed an obvious upward trend.The final conclusion was that the optimal consumption period for Unfermented stinky tofu should be less than 7 days from the date of production.(2)The composition of bacterial and fungi communities in semifermented stinky tofu samples from five different regions was analyzed by high-sequencing throughput.In terms of abundance and diversity,CS and SX had the highest species abundance and diversity of all samples,while JCD had the lowest values in both.The microbial community composition of stinky tofu samples from different regions was different,but CS and SX samples overlapped more at the bacterial level,while CS,SX and QM samples were similar at the fungal level.The dominant bacterial genera in the stinky tofu from the different regions mainly included Georgenia,DMER64,Sphaerochaeta,Sporanaerobacter and SAVAGEA,while the dominant fungal genera mainly included Aspergillus,Penicillium,Candida,Coprinopsis,Trichoderma and Wallemia.The results of GC-MS analysis showed that NB had the highest total concentration of aroma compounds(238.012 mg/kg)and high concentration of nitrogen compounds(40.079mg/kg).There were more phenols and aldehydes in SX,more alcohols in CS,and more acids in NB and JCD.The correlation between volatile compounds and microbial community was established by Partial least squares regression(PLS).it was found that Bacillus,Trichococcus and Trichoderma could promote bean flavor in samples Georgenia,SAVAGEA,Clostridium sensu stricto,Trichoderma,Aspergillus,Penicillium and Wallemia may be the aroma-producing microorganisms that contribute to the characteristic odor of Unfermented stinky tofu.(3)A total of 61 strains including 24 strains of Bacillus subtilis,2 strains of Bacillus pakistanensis,7 strains of Bacillus velezensis,3 strains of Bacillus licheniformis,2 strains of Bacillus pumilus,3 strains of Lysinibacillus pakistanensis,2strains of Bacillus altitudinis,1 strain of Bacillus amyloliquefaciens,9 strains of Bacillus Aneurinibacillus,3 strains of Bacillus tequilensis and 5 strains of Aspergillus oryzae were screened and identified from brine samples from different regions in China.Through quantitative descriptive sensory evaluation,nine dominant aroma-producing strains(Bacillus subtilis SJ2-15、Bacillus subtilis SJ2-36、Bacillus velezensis SJ-1-8、Bacillus pumilus YB5、Bacillus licheniformis YB14、Bacillus licheniformis YB24、Aspergillus oryzae QM-3-12、Aspergillus oryzae QM-3-14、Aspergillus oryzae QM-3-23)were selected,which had strong ability to produce aroma,the flavor-enhancing and stinky tofu was analyzed by GC-MS to further verify and quantify its flavorenhancing ability.The results showed that the concentration of phenols(peaked at 51.83mg/kg)and acids(peaked at 52.50 mg/kg)in the samples after inoculation increased significantly,while the species of nitrogen compounds decreased slightly.In general,Bacillus Velezensissj-1-8 was the most effective aroma-producing bacterium.The best aroma-producing fungus was Aspergillus oryzae QM-3-15.The contents of acids and phenols related to the flavor characteristics of stinky tofu had increased significantly in the samples prepared with these two strains.The sensory analysis results indicated that using the optimized mixed bacterial fermentation conditions(3% Bacillus velezensis SJ-1-8,2.67% Bacillus subtilis SJ-2-36,and 1.33% Aspergillus oryzae QM-3-15,with a fermentation time of 5 days)produced stinky tofu exhibited an improvement in the flavor attributes of bean flavor,freshness,and saltiness,while bitterness and grassy flavor were reduced.Furthermore,the overall likability of the stinky tofu was enhanced compared to the original sample in terms of various sensory attributes.