| Broad bean mash is one of the important ingredients of Pixian Douban.It is made from the koji of broad bean,which is mixed with a certain proportion of salt water,and then fermented naturally under high salt conditions.Salt content is one of the main factors affecting the fermentation process and product quality of Broad bean mash.In this study,low-salt Broad bean mash(LSB)with salt content of 6% and 9% and high-salt Broad bean mash(HSB)with salt content of 12% and 15% were used as research objects to study the effects of different salt contents on quality indexes such as microbial index,physicochemical index,organic acids,amino acids,volatile flavor compounds and biological amines in the fermentation process of Broad bean mash.Furthermore,the microbial community structure in the fermentation of Broad bean mash with different salt content was studied.In order to further improve the fermentation mode of Broad bean mash,two new brewing processes of "low salt first followed by high salt,low temperature first followed by high temperature" were established.The main research conclusions are as follows:(1)Under the condition of different salt content,the total number of mould in the fermentation process of Broad bean mash decreases continuously,and the lower the salt content,the faster the decrease;The total number of bacteria changes in a consistent pattern,it first decreases and then slowly increases to a level off.Lactic acid bacteria are only detected in the early stage of LSB fermentation,which may be the reason that the total number of bacteria in LSB in the early stage of fermentation is significantly higher than that of HSB.In addition,salt content has a significant effect on the content of total acid,amino acid nitrogen and reducing sugar in broad bean mash.The lower the salt content is,the higher the content of total acid and amino acid nitrogen is,and the lower the content of reducing sugar is.However,the salt content has no significant effect on the crude protein content of Broad bean mash.The results of sensory evaluation showed that the LSB samples tasted too sour,which led to a lower sensory score,indicating that the low salt content was not conducive to the sensory quality of Broad bean mash.Therefore,this study believes that Broad bean mash fermentation should not blindly pursue low salinization,and the salt content should be controlled in the appropriate range(?12%),so that the Broad bean mash are of the best quality.(2)The results of organic acid analysis showed that the lower the salt content is,the higher the organic acid content is;At the end of LSB fermentation,the content of organic acid in the sample is much higher than that of HSB.Lactic acid and acetic acid are the main organic acids in LSB sample;While the main organic acids in HSB sample are oxalic acid and citric acid.The results of free amino acid analysis showed that the content of free amino acid increases first and then decreases slightly in the process of Broad bean mashfermentation under different salinity conditions,and the lower the salt content is,the higher the content of free amino acid is.At the end of fermentation,the contents of free amino acids in 6%,9%,12% and 15% salt-containing Broad bean mash are 63.74,58.00,49.42 and 37.93mg/g,respectively.Through SPME-GC-MS analysis of volatile flavor compounds in Broad bean mash samples fermented for 60 days,it is found that the volatile flavor compounds in HSB samples are more diverse and higher in content,especially the contents of esters and alcohols are significantly higher than those in LSB samples;While the content of acid compounds in LSB sample is higher.PCA analysis show that there are significant differences in volatile flavor substances of Broad bean mash with different salt contents.(3)The results of biogenic amines analysis show that the total amount of biogenic amines in 6% salt content Broad bean mash ample is up to 1558.03 mg/kg,far exceeding the recommended limit value of 1000 mg/kg,which indicates that the fermented Broad bean mash product with 6% salt content has a great potential safety hazard;While the total amount of biogenic amines in 9%,12% and 15% salt content Broad bean mash samples is low,and the product is safe.Therefore,the salt content in the fermentation process of Broad bean mash should be controlled above 9% to ensure the product safety.(4)The results of microbial community structure analysis show that under different salinity conditions,the difference of fungal community structure is small,while the difference of bacterial community structure is significant.The results of fungus at the phylum level analysis showed that Ascomycota was the dominant phylum in broad bean mash with different salt concentration,and its relative abundance was more than 98.7%.The results of fungus at the genus level analysis showed that Aspergillus was the main species in broad bean mash fermentation process under different salt content.However,the relative abundance of Millerozyma in the late stage of LSB fermentation is relatively high,which makes it one of the dominant fungi.The results of bacterial phylum analysis showed that in 0 d broad bean mash samples,the main phylum were Firmicutes(73.3~75.3%),Cyanobacteria(7.1~10.1%),Proteobacteria(8.0~13.3%)and Actinobacteria(6.2~8.2%).The relative abundance of Firmicutes increased in the process of Broad bean mash fermentation,especially in LSB fermentation.The results of bacterial genus analysis showed that in 0 d broad bean mash samples,Staphylococus(22.4~29.5%),Bacillus(15.6~27.2%)and Weissella(9.2~10.2%)were the dominant bacteria.Lactobacillus and Weissella were the dominant bacteria during LSB fermentation,especially Lactobacillus;While the dominant bacteria in HSB fermentation are Staphyloccus and Bacillus.In addition,the relative abundance of Tetragenoccus increased gradually in the later stage of Broad bean mash fermentation with12% salt concentration,which becomes one of the dominant bacteria.The correlation analysis of microorganism and quality index show that the microbial community structure is closelyrelated to the quality index during the fermentation of Broad bean mash,and the difference of microbial community structure is one of the main factors leading to the significant difference of product quality under different salinity conditions.(5)To improve the fermentation mode of Broad bean mash,two different “low salt first followed by high salt,low temperature first followed by high temperature” staged fermentation modes were established.In addition,the traditional high temperature fermentation is used as control group,so as to compare its fermentation property.The results show that under the two kinds of staged-fermentation modes,the Broad bean mash can be fermented normally,and the product quality is in line with the relevant standards.In addition,compared with the control group,the staged-fermentation energy further shortens the fermentation time and improved the product quality.According to the comprehensive product quality and fermentation efficiency,this study recommends a better staged-fermentation mode,that is,the salt content in the early stage is 6%,and the fermentation is conducted at 12 ? for12 days.The medium stage salt content is 6% and fermented at 37 ? for 4 days.The later stage salt content is 15% and fermented at 37 ? for 14 days. |
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