Isolation And Identification Of Flavor Producing Strains Of Stinky Tofu And Research On Its Flavor Formation Mechanism

This thesis regards stinky tofu (strong-smelly fermented bean curd)---the traditional snack food in Wuhan as the research object. The preliminary investigation on the changes of chemical compositions during processing was done first. Then, three kinds of common extraction methods were used respectively to extract the volatile components from stinky tofu which were analysis with GC-MS. After that, isolate and select the special strains with flavor producing ability from the fermented stinky brine and to identify the strains with physiological and biochemical method and molecular biology. Finally, utilize the isolated strains to simulate the fermented brine for soaking fermentation production of stinky tofu and the investigaion on the flavor formation mechanism through the microbial point of view by sensory evaluation, physical analysis and comparing the volatile components before and after leaching. The main research content and the result are as follows:1. Preliminary investigation on chemical compositionsThe samples of different stages of soaking were selected for analysis. The results showed that:After fermented brine immersion, the water content in stinky tofu increased a bit, while protein and the crude fat decreased due to partially hydrolyzed, the total acid and free amino nitrogen increased rapidly during processing, by contrast, the the increasing rate of fatty acid was not significant, showing a trend of slow growth.2 Isolation and identification of bacteria from stinky tofuTwelve strains were isolated from the fermented brine and stinky tofu and four strains were screened out with special flavor producing ability and were identified. Based on the physiological and biochemical identification and by the means of molecular biology methods, these four strains were determined:Lactobacillus fermentum, Bacillus amyloliquefaciens, Lactobacillus plantarum, Bacillus subtilis, all their homology detection have achieved more than 99%.3 Extraction and analysis of volatileUtilize three kinds of common extraction methods:simultaneous distillation extraction (SDE), solvent extraction (SE) and solid phase micro extraction (SPME) to detecte volatile components in the stinky tofu. Totally 110 species of compounds were detected including 13 kinds of acids,13 aldehydes,24 hydrocarbons,10 ketones,16 alcohols,16 esters and 18 other compounds.Finally,21 kinds of compounds were identified to be the main flavor contributors of stinky tofu, and there four compounds 1H-2-benzopyran-1-one 3,4-dihydro-8-hydroxy-3-methyl, disulfide dimethyl, dimethyl trisulfide,3-methylindole, are the characteristic flavor contributors.4 Flavor producing strain and fermentation mechanismThe simulation of the production processes with a single strain isolated from fermented brine was done to prepare the experimental samples. Sensory evaluation, texture analysis and volatile components analysis had showed that:Bacillus subtilis and Lactobacillus plantarum are the predominant strains for producing the chief flavor of stinky tofu, Lactobacillus fermentum generated a relatively little contributions on the flavor of stinky tofu, and Bacillus amyloliquefacien is not the main flavor producing strains of advantage.