## 2 Results and Discussion
### Volatile Compounds of FST  
From Table 2, it could be seen that total of 39 volatile compounds are identified in FST samples, including nine esters, seven alcohols, five alkenes, four sulfides, three heterocycles, three carboxylic acids, three ketones, two aldehydes, one phenol, one amine and one ether (eucalypt). The predominant volatile compound in FST is indole, followed by dimethyl trisulfide, phenol, dimethyl disulfide and dimethyl tetrasulfide. The number of ester compounds is the largest among the detected compounds. The esters identified are acetate, propanoate and butanoate. They are formed by the esterification reaction of organic acids with alcohols under catalysis of enzymes, which were produced by molds. The three kinds of organic acids and most of alcohols forming these esters are also identified in the FST sample. These ester compounds can impart FST with fruity notes and make the odor of FST lifting and diffusive. Ethyl acetate has pleasant ethereal-fruity like aroma; 3-methyl-1-butyl acetate has sweet, banana, fruity with a ripe ester nuance; hexyl acetate has green, fruity, sweet, fatty odor; ethyl propanoate has fruity, rum, fermented and pineapple aroma; ethyl butanoate has sweet, fruity and tutti frutti odor [39]. Carboxen/PDMS and PDMS/DVB are more suitable for the extraction of ester compounds. When they were used, nine esters were all identified. Alcohol compounds are also isolated from FST, and their formations may be due to the fermentation of carbohydrates from soybean during the ripening step. 1-Butanol is the main linear aliphatic alcohol. The detected alkenes, mainly including limonene, copaene, \(\alpha\)-caryophyllene, aromadendrene and \(\alpha\)-panasinsen, may be from materials used in the manufacture process of stinky tofu, like pepper, which contains these alkene compounds [40]. Carboxen/PDMS and PDMS/DVB are also suitable for the extraction of alcohol compounds  
Phenol is the only phenol compound identified in FST. Maybe it comes from the decomposition of tyrosine. The reason is that structural formula of tyrosine contains the structure of phenol, and the content of tyrosine fluctuates and even sometimes cannot be detected during the ripening process [41]. Phenol is not only a flavor compound but also a kind of bactericide. As a flavor compound, it has phenolic, plastic and rubber odor [39]; as a kind of bactericide, it can extend the shelf life of stinky tofu. The detected organic acids are acetic acid, propanoic acid and butanoic acid. They are considered from the hydrolysis of soybean lipids [3] and from the action of deaminase of amino acids. In the course of deaminase, ammonia compounds are also formed. The isolation of dimethylamine in the experiments can prove this statement. PDMS was more suitable for the extraction of carboxylic acids. When it was used, three carboxylic acids were all identified in FST 1 and FST 2. Aldehydes and ketones might be formed by beta-oxidation of fatty acids, which generated a few of important flavor compounds [11]. Two aromatic aldehydes, two methylketones and a cyclopentenone are observed in FST. Benzaldehyde is described as almond-like aroma and can give FST a nutty aroma, benzeneacetaldehyde is described as floral, sweet, sauce and soy sauce odor and can impart FST savory odor, 2-pentanone and 2-heptanone have sweet, fruity and ethereal aroma and make FST have sweet nuance, and 2-methyl-2-cyclopenten-1-one gives FST wood smoke notes. Among the four fiber used, Carboxen/PDMS is the most suitable for the extraction of these compounds  
Furans and their derivatives are considered derived from Maillard reactions [11]. 2-Pentyfuran and 2-pentylthiophene are detected in FST. Furan derivatives possess caramel, sweet, roasted, burnt and sugar notes. Four sulfides, including dimethyl disulfide, dimethyl trisulfide, dimethyl tetrasulfide and methyl (methylthio) methyl disulfide, are isolated from FST. They arise from the degradation of amino acids containing sulfur.