| Edible mushroom by-products are produced in the cultivation and deep processing of edible mushroom,including deformed mushrooms,mushroom feet,mushroom slices,mushroom cap,and the solid residue after extraction of functional components in mushroom.Studies have shown that edible mushroom by-products are rich in diverse nutrients that are similar to those in normal edible mushrooms.However,edible mushroom by-products are generally difficult to be directly consumed due to its relatively poor appearance and flavor,therefore,they are often used as animal feed or directly discarded as waste.Their nutritional,health,and economic values have been severely underestimated,resulting in a huge waste of resources.It has been reported that fermentation or enzymatic hydrolysis can promote the release of flavoring substances and up-regulation of bioactive components of edible mushrooms,therefore,fermentation or enzymatic hydrolysis is of great research interest for the reutilization of edible mushroom by-products.Douchi,one of China’s traditional fermented products,is obtained by hydrolyzing the macromolecular material of soybeans through abundant enzymes in the Koji-making stage,followed by complex biochemical reactions in the post-fermentation stage to obtain a finished product with full color,aroma,and taste.However,due to the high sodium content of Douchi during the post-fermentation stage,the bioactive ingredients cannot be retained to the greatest extent,and the long-term consumption of high-sodium foods can increase the incidence of many chronic diseases,which greatly limits the development of Douchi.Therefore,in order to fully utilize the nutrients,flavor components,and health value of edible mushroom by-products,this project investigated the optimal process conditions for low-sodium edible mushroom by-product Douchi(EMD)co-fermentation using edible mushroom by-product powder(EMP)and soybean as the fermentation matrix.Subsequently,the UPLC-MS and GC-MS techniques were used to explore the changes of components in the fermentation process of EMD in order to provide theoretical basis for the deep processing of edible mushroom by-products.(1)Selection of fermentation strains and optimization of the Koji-making process.Four commonly used strains,including Mucor racemosus(CICC 40481),Actinomucor elegans(CICC 40252),Mucor Wutungkiao(CICC 3109),and Aspergillus oryzae(CICC 41736),were screened to discover Mucor racemosus(CICC 40481)and Aspergillus oryzae(CICC 41736)with high activities of neutral protease and β-glucosidase.On this basis,the Box-Behnken response surface experiment was used to optimize four factors,including the inoculation amount,proportion,EMP amount,and Koji-making time,taking neutral protease andβ-glucosidase activities as the index.The results showed that the optimal Koji-making process of EMD was as follows: inoculation amount of 6%,strain ratio of 1:1(v/v),EMP amount of21%,and Koji-making time of 63 h.Under these conditions,the activities of neutral protease and β-glucosidase of edible mushroom by-product Douchi Koji(EMDK)reached796.03±15.01 U/g and 1175.40±36.98 U/g,respectively.In addition,under the optimal Koji-making conditions,the contents of amino nitrogen,total phenolics,total flavonoids,and isoflavone glycosides,as well as the antioxidant capacity of EMDK were significantly higher than those of unfermented raw material samples and Douchi Koji without EMP.(2)Optimization of post-fermentation processes.The effects of the ratio of potassium salt substitutes for sodium salt,the amount of mixed wine added,and the post-fermentation time on the amino nitrogen content were mainly investigated,supplemented by the effects of the post-fermentation time on the browning,which was optimized using the Box-Behnken design.The results showed that the optimal post-fermentation process conditions of EMD were as follows: potassium salt substitution ratio of 31.0%,mixed wine addition amount of6.0%,and post-fermentation time of 30 days,under which the amino nitrogen content reached1.9348±0.007 g/100 g.In addition,under optimal post-fermentation conditions,the total phenolics and total acids of EMD accumulated as the post-fermentation time lengthened,but there was no significant difference in the total flavonoid content before and after post-fermentation.(3)Non-volatile metabolomic analysis and flavor changes during fermentation of the EMD.The UPLC-MS technique identified 695 non-volatile metabolites of the EMD,including 113 lipids,110 flavonoids,102 amino acids and derivatives,79 terpenoids,74 phenolic acids,60 organic acids,42 nucleotides and derivatives,33 alkaloids,18 lignans and coumarins,1 tannin,and 63 others.The results of metabolomic analysis showed that Koji-making and the first 5 days of post-fermentation had the greatest impact on non-volatile metabolites,and 366 differential metabolites were identified throughout the fermentation process,of which flavonoids were the largest class of differential metabolites,followed by amino acids and their derivatives.Moreover,the antioxidant capacity of EMD increased significantly with the extension of the fermentation time,which was closely related to the conversion of isoflavone glycosides to aglycones,amino acids and their derivatives,free fatty acids,group A saponins,and phenolic acids.A total of 169 volatiles were identified by the GC-MS technique,and their total content gradually increased with the fermentation time and finally stabilized.It was found that a rich volatile composition endowed the EMD with distinctive flavor,including volatile substances,such as 3-methyl 1-butanol,benzaldehyde,ethyl oleate,ethyl benzoate,maltol,acetic acid,and pyrazines.Therefore,the low-sodium co-fermentation process between edible mushroom by-products and soybeans was established,and the changes of non-volatile metabolites and the correlation between antioxidant activities during fermentation were explored,and the evolution of flavor substances was elucidated.This provides a certain theoretical basis for the deep processing of edible mushroom by-products and the Douchi process of potassium salt replacing part of the sodium salt. |
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