To investigate the effect of enhanced fermentation with different species of Aspergillus on the flavor of Douchi, the quality differences between enhanced fermentation by A. flavus 7214, A. flavus 7622 or A. oryzae and natural fermentation were compared. The results showed that enhanced fermentation with A. flavus promoted an increase in the total amount of free amino acids and organic acids, particularly glutamic and acetic acids, in Douchi. A total of nine classes of compounds including esters (20), acids (11), aldehydes (12), ketones (9), alcohols (7), pyrazines (8), alkanes (10), phenols (7) and other compounds (8) were identified by gas chromatography-mass spectrometry (GC-MS). While higher levels of pyrazine and phenolic compounds were observed in naturally fermented Douchi, enhanced fermented Douchi had more acids and aldehydes. In addition, some new volatile components, such as methyl phenyl acetate and ethyl linoleate, were formed by enhanced fermentation. Electronic nose analysis revealed that the flavor of A. flavus 7622 fermented Douchi was closest to that of naturally fermented Douchi, while the largest flavor differences were found between A. flavus 7214 fermented and naturally fermented Douchi. Furthermore, A. flavus 7214-fermented Douchi had better overall flavor in sensory evaluation, mainly in terms of aroma, sourness, sweetness, and umami. To summarize, enhanced fermentation promoted the release of free amino acids, organic acids, and volatile compounds, thereby having a significant impact on sweetness, umami, and aroma. These results could provide theoretical guidance for the industrial production of Liuyang Douchi.

In order to investigate the metabolic differences among Liuyang Douchi fermented by different starter cultures, the contents of total acid and amino nitrogen in Douchi fermented naturally and by using Aspergillus flavus 7214 (AF 7214), A. flavus 7622 (AF 7622), their mixture (AF 7214 + AF 7622) or A. orzyae were determined, and the difference in metabolites among these fermentation strategies was explored by liquid chromatography-mass spectrometry (LC-MS). The results showed that among the five Douchi samples, Douchi fermented by AF 7214 had the highest contents of total acid (3.52%) and amino nitrogen (1.47 g/100 g). The results of partial least squares discriminant analysis (PLS-DA) indicated that significant differences in metabolites were observed among the Douchi samples, and the composition of metabolites in Douchi fermented by AF 7622 showed the smallest difference from that in naturally fermented Douchi. Further analysis revealed that the differential metabolites between Douchi fermented naturally and by using starter cultures were mainly involved in amino acid metabolic pathways, especially arginine biosynthesis. A total of 62 key differential metabolites among the five samples were identified using variable importance in the projection (VIP) greater than 1.5 and P < 0.05 as criteria, including 26 amino acids and their derivatives such as L-lysine, L-serine and 2-methylserine, which indicated that enhanced fermentation showed the most notable influence on the metabolism of amino acids. This study has provided a new understanding of the formation of metabolites during the enhanced fermentation of Liuyang Douchi.

In order to investigate the effect of inoculated fermentation with different Aspergillus strains on the umami taste of Liuyang Douchi, the differences in the umami taste and protein degradation of Liuyang Douchi fermented naturally (NF) or by four dominant Aspergillus flavus strains isolated from Douchi (A. flavus 7214, A. flavus 7622, A. flavus 6112, and A. flavus 5322) alone and their mixture in an equal proportion (M4) were compared. The results showed that the major protease produced by each of the four strains was serine proteases, and A. flavus 5322 had the strongest protease activity, with significantly higher activity of acidic protease than the other three strains. The contents of amino acid nitrogen and water-soluble protein in Aspergillus fermented Douchi were higher than those in NF Douchi after the end of pile-fermentation stage. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the degradation trends of six proteins in Douchi were consistent, all of which exhibited rapid degradation during the pile-fermentation process. Additionally, the results of sensory evaluation and electronic tongue analysis showed that Douchifermented by A. flavus 5322 showed the strongest umami taste, which significantly increased when compared with NF. Using ultra-high performance liquid chromatography-electrospray/quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q-TOF-MS/MS) combined with correlation and peptidomics analysis, a total of 43 potential umami peptides were identified. The total peak area of potential umami peptides in Douchi fermented by A. flavus 6112 was the highest, followed by A. flavus 5322 with little difference, significantly greater than that in NF Douchi. In summary, inoculated fermentation, especially with A. flavus 5322, could promote protein degradation and the release of potential umami peptides, thereby enhancing the umami taste of Liuyang Douchi. This study will provide theoretical guidance for improving the umami taste of Liuyang Douchi and development of industrial production strains.

In order to investigate changes in the quality, flavor and major aroma components of pickled chilies in ceramic jars during fermentation, this study used headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS) to analyze the volatile substances of pickled chilies in ceramic jars from a certain producer after storage for 1–5 years and determined their physicochemical indexes and total bacterial counts. The results showed that the levels of soluble salt-free solids, total acids, total esters, amino nitrogen and total colony count of chopped chili peppers kept changing with storage time, and the quality and aroma kept improving in the first three years, and then the quality declined in the fourth year. A total of 112 volatile compounds were detected, including 22 acids, 12 alcohols, 36 esters, 5 ketones, 9 aldehydes, 5 phenols, 18 hydrocarbons, and 5 other compounds. The volatile composition and content varied with storage time, and 36 volatile compounds with odor activity values (OAVs) greater than 1 were found, which played an important role in the overall aroma of chopped chili peppers. Among them, linalool and methyl salicylate, ethyl salicylate, 4-ethyl-2-methoxyphenol were common to all samples. The samples stored for 1–3 years were superior in terms of organoleptic quality, physicochemical properties and volatile components, while those stored for more than 3 years had a strong aroma but poorer texture.