| The traditional bacterial fermented Douchi,which is popular in southwest China,has different edible ways in different producing areas,which makes the treatment methods after fermentation different.At present,the research on bacterial fermented Douchi mainly focuses on the nutrition,flavor and microbial characteristics of bacterial fermented Douchi.Dry fermented Douchi is a popular fermented Douchi product in southwest China because it can be stored and eaten for a long time.There are few reports on the nutrition and flavor changes of dried fermented Douchi in air drying or drying storage.Considering the changes of flavor and possible nutritional quality and safety of dry fermented Douchi after long-term storage,this paper takes artificial inoculation and natural fermentation of bacterial fermented Douchi as the research object,based on the water content range of dry fermented Douchi commonly found in the market.To study the changes of nutritional components,functional components,edible safety indexes,sensory and taste quality of dried fermented Douchi during storage under different water contents and packaging methods,and further analyze the volatile flavor characteristics and quality change mechanism during storage,so as to provide reference for rational consumption and commercialization of dried fermented Douchi in the future.The main research contents and results are as follows:(1)The changes of nutritional quality and food safety of bacterial fermented Douchi in storage were detected and analyzed.The results showed that the moisture content of fermented Douchi samples in vacuum packaging did not change significantly,while the traditional preserved samples showed a downward trend.Both artificial and natural inoculation samples had decreased fat content,but the amount of artificial inoculation was lower than that of natural inoculation,and the degradation of traditional preserved fat was higher than that of vacuum preservation.The change of protein content in natural fermentation of N1 showed a gradual decrease trend,and the other treated samples had small fluctuations.The ash content changes slightly in the early stage of storage and tends to be stable in the later stage.From the degradation of fat and protein,the suitable storage time of Douchi is about 60 days.The analysis of functional components showed that the total polyphenols of artificially inoculated fermented Douchi decreased first and then increased,while the natural fermented samples showed a slow increase trend.The total polyphenols in traditional preservation treatment was higher than that in vacuum preservation treatment.Free phenol and bound phenol showed an increasing trend during the whole storage period,but the change law between different fermentation treatments was not obvious.The high value of total isoflavones and free aglycogenin by artificial inoculation appeared on 90 days and 60-150 days of storage,respectively.The optimal storage time for active free aglycogenin was 60 days.The high value of naturally fermented total isoflavones and the main free aglyphs genistein appears after60 days of storage(high Np value is 120-150 days)and about 60-120 days of storage,respectively.The suitable storage time is 90 days of high active free aglyphs.The suitable storage time of artificially inoculated Douchi was about 90 days,and the suitable storage time of naturally fermented Douchi was about 60 days.The variation of saponins content in all samples gradually decreased,the decrease of artificial inoculation was lower than that of natural inoculation,and the decrease of traditional preservation was lower than that of vacuum preservation.On the whole,the preservation of saponins should not exceed 60 days.In the change of polysaccharide content,the content of natural inoculated Douchi was higher than that of artificial inoculation at the early stage of storage,but was close to that at the late stage of storage.Oligosaccharide content change of artificial inoculation lobster sauce down before they rise,the rising trend natural fermented Douchi,storage end content were increased,vacuum preservation rise higher than traditional preservation.The analysis of food safety indexes showed that the variation of TVB-N value between artificial inoculation and natural inoculation was significantly different.In general,the TVB-N value of natural fermentation was lower than that of artificial inoculation.The optimal preservation time of artificial inoculation was 60-90 days,while the optimal preservation time of natural inoculation was about 60 days.The TBA values of the two inoculation methods showed an increasing trend.Overall,the TBA preserved by natural fermentation was lower than that by artificial inoculation.The preservation by artificial inoculation should not exceed 60 days,and the preservation by natural inoculation was stable.Both peroxide value and acid value increased with the extension of storage time,and did not exceed the limited value of POV value in T/GZSX 014-2018 standard of Douchi(≤0.25 g/100g)during the whole period.However,from the general trend of POV changes,R3 and N3 were not suitable for long-term storage,which should not exceed 60 days,and other treatments were stable.The limit value of acid value(≤5 mg/g)of fermented Douchi exceeded the limit value after 60 days of artificial inoculation,and N3 and N2 exceeded the limit value after 90 days of natural inoculation and 120 days of natural inoculation.Changes of nitrite content in both inoculation methods gradually decreased to 60 days to reach the peak value,and then gradually increased to 120 days after storage,except natural fermentation N1 and N3 treatment,other samples have exceeded the safety standard of nitrite stipulated in GB2760-2014(≤30 mg/kg).Microbiological testing shows that the process did not check out the mold,yeast and lactic acid bacteria,artificial inoculation with Douchi colony than natural fermented Douchi,total number of bacillus upward trend in the same as the total number of colonies,traditional preservation faster than vacuum preservation colonies to breed.From the food safety indicators comprehensive consideration,artificial inoculation fermentation Douchi safety storage time should not be more than 60 days,natural inoculation fermented Douchi water content lower than 40% when storage should not be more than 120 days,moisture content more than 40% when the storage time should not be more than 90 days.(2)the type of bacteria Douchi sensory and flavor quality in storage change detection analysis,show that artificial and natural inoculated fermentation lobster sauce with the extension of storage time L* value fluctuation change,but the traditional preservation processing after 60 days will rise rapidly,a* value with the extension of storage time gradually decreases and respectively in the 120 days and60-120 days in the valley value after the rebound,The value of b* fluctuated slightly,but the increase of traditional preservation method was higher.In terms of color difference,the color change of artificial inoculation treatment was accelerated after 90 days of storage,while the color difference of natural inoculation treatment remained stable,and the degree of color change could be reduced by traditional preservation.The grain values of artificially inoculated and naturally inoculated fermented Douchi were stored for 60-90 days and 60 days,respectively,and there was no significant difference between vacuum preservation and traditional preservation.5-HMF increased gradually with the extension of storage time,and began to increase rapidly after 90 and 60 days of storage by artificial inoculation and natural inoculation,respectively,and the increase range of traditional preservation was higher than that of vacuum preservation.The analysis of texture characteristics showed that the hardness of traditional preserved Douchi was much higher than that of vacuum preserved Douchi,and the hardness of vacuum preserved Douchi had little change during storage.The elasticity and cohesiveness are different among the samples.Chewability is related to hardness.The greater the hardness,the higher the chewability of the sample.Traditional chewability of preserved Douchi is much higher than that of vacuum preserved Douchi,which is related to the water loss of traditional preserved Douchi.Taste quality analysis showed that soluble total sugars,reducing sugars,free amino acids and total acids of all treatments maintained a stable rising trend during storage,in which N3 and R3 with high water content increased significantly.In terms of total acid content,the contents of artificial inoculation of R1,R2 and R3 and natural inoculation of N2 and N3 exceeded the limit of total acid content in T/GZSX014-2018 standard(≤2.5 g/100g)after 60 days.Overall,the preservation of artificial inoculation and fermentation should not exceed 90 days,and the traditional preservation method was better.The sensory evaluation showed that the evaluators preferred the samples stored in the early and middle stages.The traditional storage time after natural fermentation should be within 60 days,the vacuum storage time should not exceed 30 days,and the samples were more acceptable when the water content was 30%～40%.(3)The preliminary study on the mechanism of changes of volatile flavor substances and edible quality during storage of Douchi showed the following results:(1)A total of 138 volatile flavor substances were detected,and hydrocarbon,heterocycle and alcohol were the main volatile flavor substances,accounting for62.7%～69.7% of the total volatile flavor substances.The esters were higher at 60 days of storage,and the aldehydes were higher at 120 days of storage.The isovaleric acid with sweat odor and indoles with foot odor in traditional fermentation and preservation treatment Np gradually increased during storage,which was consistent with the special odor of traditional fermented Douchi in storage.Analysis of flavor characteristics and contribution value showed that 2,3,5-trimethylpyrazine and 2,5-dimethylpyrazine were the highest OCV,and increased with storage time.Other factors affecting OCV were D-limonene,α-pinene and laurene at 120 days of storage.From the changes of volatile flavor substances,short-term storage(0-60 days)can maintain good flavor,but with the extension of storage time,the traditional storage Np retention ability at 120 days is lower than other treatments.(2)Analysis of the change mechanism of edible quality in fermented soybean showed that non-protein nitrogen,soluble peptide and ammoniaceptide nitrogen of protein degradation products increased gradually with the extension of storage time.The degradation products of artificially inoculated proteins were higher than those of traditional stored proteins.The content analysis of free amino acids showed that the content of free amino acids in other treatments was relatively higher after 60 days of storage except R2.The content of free amino acids in artificial inoculation was higher than that in natural inoculation was glutamic acid,alanine,methionine and phenylalanine,while the content of free amino acids in natural inoculation was higher than that in normal inoculation.In the process of storage,glutamic acid can be preserved better by vacuum treatment than by traditional storage treatment.The carbonyl group of protein oxidation products increased and the active sulfhydryl group decreased,indicating that protein was oxidized,and the effect of vacuum packaging on inhibiting Douchi protein was better than that of traditional storage.The in vitro digestibility of each treatment was stable in the early stage of storage,but decreased in the later stage,and protein oxidation could change the in vitro digestibility.The analysis of fat changes showed that the total content of free fatty acids was higher at 60 days of storage,and gradually decreased when storage continued.The total amount of free fatty acids in vacuum storage was higher than that in traditional storage.Further analysis of the composition of free fatty acids revealed that 12 kinds of fatty acids were detected,and monounsaturated fatty acids(mainly oleic acid)were higher in each treatment,followed by saturated fatty acids.The polyunsaturated fatty acids were relatively low,which may be related to the degradation of unstable fatty acids in fermented and stored Douchi.Correlation analysis showed a significant negative correlation between non-protein nitrogen and protein(P<0.05),non-protein nitrogen was significantly positively correlated with amino nitrogen and soluble peptide(P<0.01),were significantly positively correlated with free amino acids(P<0.05),carbonyl group was negatively correlated with active sulfhydryl group(P<0.05),in vitro digestibility and TVB-N were not highly correlated with other factors,and the influencing factors were more complex.Fat was negatively correlated with TBA,POV,acid value,aldehydes and alcohols,and POV was significantly negatively correlated with fat(P<0.05),free fatty acids were negatively correlated with aldehydes(P<0.05).On the whole,the oxidative degradation of protein and fat not only affected the changes of flavor characteristics of Douchi in storage,but also was related to food safety. |
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