Analysis Of The Diversity Of Bacterial Flora In Sour Bamboo Shoots And The Screening,Characterization And Application Of Lactic Acid Bacteria

Sour bamboo shoot is a traditional fermented vegetable from the Guangxi region of China.It extends the shelf life of fresh bamboo shoots and,because of its unique flavour of fresh,sour and smelly,has entered the national public eye in recent years,along with river snails rice noodle.At present,the traditional natural fermentation method is still used in the production of sour bamboo shoots,which has a complex microbial flora,inadequate utilization of nutrients,and difficulty in controlling nitrite content,resulting in uneven quality of sour bamboo shoots products and difficulty in ensuring food safety.Therefore,it is important to explore the composition of the natural fermented asparagus flora,and to screen the excellent lactic acid bacteria for research and application to improve the nutritional value and food safety of asparagus,which is of great practical significance for the standardization and industrialization of asparagus production.In this thesis,the diversity of microorganisms in naturally fermented sour bamboo shoots produced by different manufacturers was studied,and lactic acid bacteria strains with good nitrite degradation ability,high γ-aminobutyric acid(GABA)production and good tolerance were screened out from them.The quality characteristics of natural and pure fermented asparagus were compared.The details of the study and the experimental results are as follows:(1)Analysis of microbial diversity in naturally fermented sour bamboo shoots:The composition of the bacterial flora of naturally fermented sour bamboo shoots produced by different manufacturers was determined by high-throughput sequencing,and the results showed that,at the phylum level,the main dominant phyla in the three sour bamboo shoots were the thick-walled phylum and the cyanobacterial phylum.At the genus level,the main dominant bacterial genus in both SS1 group(1st family sour bamboo shoot)and SS2 group(2nd family sour bamboo shoot)was Lactobacillus sp.SS11 and SS12 accounted for 88.12% and 65.90% of the sample abundance,respectively,while SS21 and SS22 accounted for 41.78% and 41.97%,respectively;SS3 group(3rd family sour bamboo shoot)accounted for a lower percentage of Lactobacillus sp.with 16.83% and20.40%,respectively,but it was still the first dominant genus among the culturable bacterial genera.(2)Screening of lactic acid bacteria with specific functional properties:Based on high-throughput sequencing results,strains from three naturally fermented sour bamboo shoots were screened with MRS(De Man,Rogosa and Sharpe)and MC(Modifiedivchalmers)selective media,and 40 lactic acid bacteria strains were initially identified by Gram staining and peroxidase tests,and identified as five strains by 16 S r RNA sequencing,namely Lactobacillus zymae,Lactobacillus spicheri,Pediococcus acidilactici,Lactobacillus plantarum,Levilactobacillus brevis.The functional characteristics of the screened lactic acid bacteria were studied,and the results showed that all 40 strains wereγ-hemolytic,and strain R9 had the best ability to degrade nitrite,and the degradation rate could reach 99.92%,respectively;strain R22 produced the highest GABA content of71.77 μg/m L.tolerance studies on strains L29,R9,R19,R22,and R25 showed that strains R9,R19 and R25 had strong tolerance to Na Cl concentration ≤ 7%,bile salt concentration≤ 0.7%,p H 3.5 and p H 4.Therefore,Pediococcus acidilactici R9,Lactobacillus plantarum R19 and R25 were selected as the subsequent pure fermentation experimental strains of sour bamboo shoot.(3)Optimization of conditions for the fermentation of lactic acid bacteria in pure strains of kefir:The screened lactic acid bacteria strains were applied to the pure strain fermentation of kefir,and the GABA content,nitrite residue,p H and sensory evaluation were used as indicators to first determine the best strain ratio for pure fermentation,followed by a single-factor test on the inoculum amount and fermentation temperature,and a two-factor completely randomized test to optimize the results,which showed that the best strain ratio was(Pediococcus acidilactici R9: Lactobacillus plantarum R25 = 1:1),the best fermentation conditions were 4% inoculum and 31℃ fermentation temperature,and the obtained fermented sour bamboo shoot had GABA content of 817.44μg/m L,nitrite residue of 0.43 mg/kg,p H value of 3.41,and sensory score of 82.13.(4)Verification of optimal fermentation conditions and quality evaluation:Using the GABA content as an indicator,two replicate experiments were conducted using optimal fermentation conditions,while the qualitative characteristics and taste of pure fermented sour bamboo shoots and natural fermented sour bamboo shoots under optimal conditions were compared and analyzed,and the results showed that:The reproducibility of GABA content in pure fermented sour bamboo shoots under this condition was good,which initially indicated that the optimal fermentation conditions were stable and reliable.In terms of textural characteristics,the hardness and crispness of pure-bred fermented sour bamboo shoots were better than those of naturally fermented sour bamboo shoots,and the taste may be more popular among the public.Taste analysis by electronic tongue revealed that pure fermented sour bamboo shoots had significantly higher acidity than naturally fermented sour bamboo shoots,and were lower than naturally fermented sour bamboo shoots in bitterness,astringency and corresponding bitter aftertaste and astringent aftertaste;in freshness,richness and saltiness,pure fermented sour bamboo shoots were slightly lower than naturally fermented sour bamboo shoots.