Studies On The Volatile Flavor Compounds Of Aspergillus Oryzae Fermented Douchi And Its Changes During The Processing

Flavor is one of the most important sensory properties of douchi product. However, little reports on the volatile compounds of Chinese douchi were present, especially little reports was on the flavor of Aspergillus-type douchi. Encompassing the central task of characteristic aroma component of Aspergillus-type douchi, the douchi fermented by Aspergillus oryzae A3042 were the object of this Paper. The volatile compounds and aroma active components of Aspergillus-type douchi were identified by SDE, SPME, GC-MS and GC-O. During the fermentation process of Aspergillus oryzae douchi, the changes of microbe, physiochemical index, protein degradation, color and hardness were studied systematically. Comparison of the volatile flavor components in the Aspergillus oryzae douchi from different fermentation process was investigated. The results could be benefit to evaluate aroma quality of Aspergillu-type douchi and provide important information about Aspergillu-type douchi processing and products quality improving. The contents and result conclusions are made as follows:1 Process optimization of douchi fermented by Aspergillus oryzaeThe growth temperature and pH of Aspergillus oryzae were determined. On the basis of these result, the fermentation conditions were optimized. The optimal conditions of soybean pre-treatment are soaking for 220min under 30℃, steaming for 40min under 0.1 Mpa. With neutral protease enzyme activity of Aspergillus oryzae as an indicator, the primary fermentation process was optimized. The conditions was 0.4%for inoculum size, 32℃for incubation temperature,44 h for culture time, thickness of 2 cm, and interval 9h for turn over. With free amino acid content as the index, post-fermentation process was optimized, fermentation time of 10d, temperature of 35℃and 14%salt content.2 Analysis for volatile flavor composition in traditional Aspergillu-type douchiIn SDE method, dichloromethane was the better extractant for Liuyang douchi. In SPME method,50/30μm DVB/CAR/PDMS fiber had the best effect for douche.Volatile components of Liuyang douchi extracted by HP-SPME and SDE, and analyzed by GC-MS. Total 78 compounds were detected, including 18 esters,14 hydrocarbons,13 pyrazines,9 aldehydes,6 phenols,5 acids,3 ketones,2 alcohols,2 sulfur compounds and 6 other compounds. HP-SPME and SDE can complement each other for douchi volatile flavor analysis. Volatile components of three kinds of douchi were compared. The number of volatile components in Liuyang douchi,Yongchuan douchi and bulk douchi were 56,50 and 42, respectively. There are large differences in species of the volatile components for three kinds of douchi. The aroma-impact compounds of the SPME extract were then determined using GC-MS and GC-O. Compounds with the greatest aroma intensities include:2-methyl-butanal, ethyl 2-methylbutyrate, isoamyl acetate, 2,6-dimethylpyrazine, 1-octen-3-ol,2-pentyl furan, benzeneacetaldehyde, phenylethyl alcohol, phenethyl acetate and phenethyl butyrate.3 Analysising of volatile flavor composition in the douchi fermented by Aspergillus oryzaeIn SDE method, methylene chloride extraction effect is better. The optimal extraction conditions of SPME method were sample quality on 3g, balance time on 30min, extraction time on 30min and extraction temperature on 60℃.19 and 15 volatile compounds were detected in soybean douchi and black bean douchi, respectively, using SPME. Only 6 components were found in both of two douchi.26 and 31 volatile compounds were detected in soybean douchi and black bean douchi, respectively, using SDE. And 19 components were found in both of two douchi. There are significant differences in volatile composition of soybean douchi and black beans douchi both qualitatively and quantitatively. Using headspace solid phase microextraction and simultaneous distillation extraction method combined with GC-O,17 aroma active ingredients were identified. Compounds with the greatest aroma intensities include: 3-(methylthio)-propanal, benzaldehyde, 1-octen-3-ol, nonanal, benzeneacetaldehyde, tetramethylpyrazine, decanal,2-methoxy-4-vinylphenol. These compounds might have an impact on the overall aroma of douchi fermented by Aspergillus oryzae4 The changes of main physiochemical index and protein degradation during fermentation of Aspergillus oryzae douchiIn fermentation process, Aspergillus and Bacteria played the main role, no yeast detected. In the fermentation process, water content and pH value decreased; protein, fat and total sugar content reduced, while the amino nitrogen, free fatty acid and reducing sugar content was increased significantly. After cooking and adding salt, hardness and color of soybean changed significantly. So cooking and fermenting with salt are the key technics to the color and texture of douchi. During fermentation, neutral protease and alkaline protease were the main protease. soy protein degraded mainly in the stage of the early primary fermentation and the late post-fermentation.In three stages ofsteaming bean, primary fermentation and post-fermentation, each amino acid content reached the highest in primary fermentation. According to the level of content, the FAA spectrum was Leu, Asp, Glu, Arg and Lys. According to the taste characteristics of amino acids, they were divided into umami, sweet, bitter and tasteless. In the processing stage, bitter amino acids had the highest content. The content of all bitter and umami amino acids were higher than their threshold, where the content of Glu was the highest.5 The effect of volatile flavor compounds of douchi on fermentation processesAroma active components nonanal, hexanal, acetophenone,1-octene-3-ol,2-pentyl furan were the characteristic flavor of cooked soybean, not generated in the fermentation. 3-methyl-butanal,2-methyl-butanal and benzaldehyde were mainly produced in the post-fermentation process; 3-methylbutanoic acid was detected only in the drying process; the contents of 3-octanone and phenylethyl alcohol are increased in primary fermentation, and was the maximum at the end of this stage. Extending post-fermentation time benfited to formation of 2-methylButanal, Benzaldehyde, trimethylPyrazine, Benzeneacetaldehyde, tetramethylPyrazine, ethyl benzoate, Benzeneacetic acid ethyl ester, and made the cooking bean flavor compounds decreasing. Low temperature fermentation was suitable for aldehydes generating. And high temperature fermentation can produce thermal degradation or or thermal reaction products. Too low or too high salt content will inhibit the formation of volatile flavor compounds. The drying process can make partial flavor active substances lost, And the drying temperature was higher, the content of volatile flavor compounds was lower.