The Study On Screening Of Probiotic Lactobacillus And Changes In Quality And Flavor During Fermentation In Sichuan Pickled Cowpeas

Sichuan pickles is generally salted with freshly fermented vegetables,of which pickled cowpeas are typical,easy to make,easy to eat,unique flavor,high nutritional value and unique flavor.At present,pickles is mainly divided into two types: traditional natural fermented pickles at home and industrial fermented pickles.Natural fermented pickles is easy to operate,and the microbial flora is mostly Lactobacillus,with a relatively single strain and a slow fermentation rate;industrial fermented pickles is richer in microbial species,with a fast fermentation rate and better sensory quality,but the operation process is more complicated.Different fermentation methods lead to different quality and flavor of pickles,and most of the quality and flavor changes of pickles come from the fermentation of Lactobacillus,so in addition to changing the fermentation method,screening good Lactobacillus can also improve the quality and flavor of pickles.However,there are few studies on the screening of dominant Lactobacillus in pickles and the fermentation of dominant Lactobacillus.In order to investigate the influence of dominant strains and fermentation methods on the flavor and quality of pickles,it is important to screen the dominant strains in the fermentation process and compare the advantages and disadvantages of different fermentation methods to further improve the quality of pickles.In this study,the dominant Lactobacillus with probiotic properties were isolated from industrially produced Sichuan pickled cowpeas as raw materials for strain fermentation.Volatile flavor substances,organic acids,physicochemical indexes,Lactobacillus population,and textural characteristics were used as research indexes to compare the quality and flavor changes during 0～18 days of fermentation for three different fermentation methods: strain fermentation,natural fermentation,and industrial fermentation,and to select the best fermentation method.The main research results are as follows:(1)56 species of typical Lactobacillus were isolated from industrially fermented raw pickled cowpeas.These Lactobacillus showed typical white colonies and obvious calcium soluble circles on MRS+2%CaCO3 medium and were identified as Lactobacillus by Gram staining and peroxidase experiments with good motility under oil microscopy.According to the acid production rate and growth curve of these 56 Lactobacillus,8 Lactobacillus with apparently faster growth and higher acid production rate were selected from them.The physiological and biochemical identification and 16S rDNA strain identification of these eight strains showed that 4 of them were Lactobacillus plantarum and the other 4 were Lactobacillus fermentum.(2)4 probiotic properties were determined for these 8 dominant Lactobacillus strains.The results showed that,among the 8 strains,F38 and F50 had significant growth characteristics,i.e.salt tolerance,temperature tolerance and pH tolerance,F6,F11,F38 and F44 had significant antibacterial ability,F6,F26 and F41 had significant extracellular polysaccharide production ability,and only 3 strains,F6,F38 and F44,produced folic acid.In summary,F6,F38 and F44 were identified as "probiotic Lactobacillus" and made into suspension according to 1:1:1,which was used as raw material bacteria for strain fermentation.(3)The results of quality changes during the fermentation of the three methods showed that the highest sensory evaluation of strain fermentation was 96 points,which was better than 93 points of industrial fermentation and 91 points of natural fermentation;the hardness and crispness of strain fermentation was higher,and the elasticity of industrial fermentation was better;the acid production of industrial fermentation was faster,and strain fermentation degraded more reducing sugars,while natural fermentation was inferior to the other two fermentations in all aspects.In conclusion,the inoculated Lactobacillus strains were able to significantly improve the quality characteristics of the fermented cowpea pickled cowpeas products.(4)The organic acids were determined by high performance liquid chromatography(HPLC)and the volatile components were detected by headspace solid-phase microextraction(HSSPME)coupled with gas chromatography-mass spectrometry(GC-MS)technique in Sichuan pickled cowpeas.The results from the changes of flavor during fermentation in the three methods showed that the industrial fermentation method produced tartaric acid,acetic acid,and lactic acid,which all ranked first,and the content of succinic acid and citric acid in the bacteriophage fermentation method was slightly higher than the other two methods;among the various classes of volatile chemicals,the number and content of esters were the highest,followed by alcohols;the variety of substances in industrial fermentation was richer compared to the other two;the content of typical substances The highest is industrial fermentation,followed by the content of strain fermentation,while natural fermentation has a certain gap in both the content and quantity of volatile substances compared to the other two fermentation methods.It proved that among the three fermentation methods,industrial fermentation produced better flavor substances than the other two,followed by strain fermentation,indicating that the screened lactic acid bacteria had a certain improvement in the flavor of the pickled cowpeas but not as much as industrial fermentation,which still needs to be improved.(5)From the fermentation process of 0～18 d,the number of Lactobacillus reached its peak at9～12 d and began to decline,when the p H value had stabilized and was not changing,and the reducing sugar content had also stabilized,indicating that the main process of fermentation had been completed;while organic acids and volatile flavor substances grew faster at 0～12 d,and the rate became significantly slower at 12～18 d although they were also growing;meanwhile,the sensory score of pickled cowpea was highest at 9 The organoleptic scores of pickled cowpeas were highest at ～12 d and decreased as it went on.The results proved that the quality and flavor of fermented cowpeas reached the best at 9～12 d,and the quality decreased when fermentation continued later.In conclusion,among the three fermentation methods,the quality of strain fermentation was superior,but the flavor matter formed by industrial fermentation was optimal,and both were significantly better than natural fermentation,indicating that the three strains of Lactobacillus inoculated contributed to the improvement of the quality and flavor of the pickled cowpeas,but still had to be improved.In contrast,the best quality of the fermented cowpeas was achieved at 9-12 d.This paper has some reference value for the subsequent investigation of the methods to improve the quality and flavor of pickled cowpeas cowpeas.