Improvement Mechanism Of The Quality And Flavor In Fermented Tilapia Surimi By Lactic Acid Bacteria

Tilapia(Oreochromis niloticus)is one of the major freshwater aquaculture species in southern China.Currently,tilapia are mainly processed in primary processing,so the product form is relatively simple and the value added is low.The use of biological fermentation technology can effectively improve the easy desorption of the gel as well as the taste and flavor of fresh water fish surimi,contributing to the high value utilization of freshwater fish.In this paper,tilapia surimi was taken as the research object.In order to clarify the types of functional microorganisms that affected the quality and flavor formation of tilapia surimi,the effect of the microbial community structure on the quality and flavor formation during natural fermentation was studied.The selective medium was then used to directionally separate the corresponding microbial strains,whose fermentation,as well as biogenic amine production and removal characteristics were further investigated to screen strains with good fermentation and biogenic amine removal abilities.Therefore,the study established a new technology of green fermentation of tilapia surimi based on microbial orientation control,and analyzed the influence of the starter on the quality,flavor,biogenic amine,and microbial community of fermented tilapia surimi,thus revealing the mechanism of improving the quality and flavor by artificial fermentation.The changes of the texture,gel characteristics,whiteness,p H,free amino acid content,biogenic amine content,volatile compound content,microbial community of tilapia surimi during the natural fermentation process were systematically studied,and the mechanism of microbial community structure on the quality and flavor of naturally fermented tilapia were analyzed in this paper.The results showed that the main physical characteristics(gel strength,whiteness,and hardness),flavor free amino acids(glycine,alanine,and glutamic acid)and featured volatile compounds(hexanal,heptanal,octanal,benzaldehyde,(E)-2-octenal,4-ethylbenzaldehyde,(E)-2-heptenal,(E,E)-2,4-decadienal,1-octene-3-ol,2-pentylfuran,and 2-ethyl-furan)were significantly enhanced after natural fermentation,while p H,adhesiveness,springiness,chewiness,and cohesiveness decreased significantly at the beginning of fermentation(0-18 h).There was little difference in the microbial community of the samples in the late fermentation period(18-30 h),which showed obvious differences with that in the raw surimi.Firmicutes and Proteobacteria were the dominant phylums throughout the fermentation process.Microorganisms such as Lactococcus,Pediococcus,Enterococcus,and Lactobacillus could adapt to the tilapia surimi fermentation environment,and their contents increases significantly during the fermentation process.Among them,lactococcus was most abundant,followed by Pediococcus.Based on the results of correlation analysis,a metabolic network model of key microorganisms was constructed.The results showed that lactic acid bacteria such as Lactococcus,Pediococcus,and Enterococcus played an important role in the formation of the physicochemical properties and the volatile flavor compounds in naturally fermented tilapia surimi.The MRS ager medium was used to select the functional lactic acid bacteria which affected the quality and flavor formation of the naturally fermented tilapia surimi,and their growth,fermentation as well as biogenic amine production and removal characteristics were clarified.Phylogenetic tree was constructed based on the 16 S r RNA gene sequences,and five different Pediococcus pentosaceus strains were further identified.Except for P.pentosaceus 30-3,all strains showed similar growth characteristics,that is,they were in a stagnant phase within 3 h,entered logarithmic growth phase at 3 h,and reached equilibrium value after 15 h.These 5 strains of P.pentosaceus all showed good acid production ability and salt tolerance.Low concentrations of salt(0.5%-2%)showed a certain effect on their acid production ability.These P.pentosaceus strains did not produce biogenic amines during the culture process,and showed high biogenic amine removal ability.Because P.pentosaceus 30-7 and P.pentosaceus 30-15 have relatively good growth,acid production,salt tolerance and biogenic amine removal abilities,they were selected as the starters for tilapia surimi for follow-up research.P.pentosaceus 30-7 and P.pentosaceus 30-15 were used as starters for artificial fermentation to analyze their influence on the physicochemical characteristics,volatile compounds substances,biologic amines,free amino acids,and microbial community of fermented tilapia surimi.The results showed that the addition of P.pentosaceus 30-7could significantly improve the gel strength,hardness,springiness,and cohesiveness of fermented surimi,especially the gel strength of tilapia surimi reaching 758.31 g·cm within 30 h.P.pentosaceus 30-15 significantly increased the contents of free amino acid with umami(glutamic acid and aspartic acid)and sweetness(glycine,alanine,proline,threonine,and lysine)flavor.At the same time,both starters significantly inhibited the formation of biogenic amines in fermented tilapia surimi,of which P.pentosaceus 30-15 had more biogenic amine inhibitory ability than P.pentosaceus 30-7.For volatile compounds,the addition of P.pentosaceus strains could increase the fermentation rate of surimi.Moreover,P.pentosaceus 30-15 could accelerate the generation of volatile flavor compounds better than P.pentosaceus 30-7.After adding starter stains,the dominant genus was significantly changed to Pediococcus,and the abundance of other main microbial genera in the artificial fermentation group was lower than that in the natural fermentation group.The metabolic network model confirmed that the improvement of tilapia surimi physical properties and free amino acids was mainly caused by the metabolism of P.pentosaceus.In addition to their own biogenic amine removal ability,P.pentosaceus strains also inhibited the growth of Enterobacter,Citrobacter and Streptococcus,contributing to the biogenic amine reduction during fermentation.Pediococcus and Lactococcus were the two genus that promoted the most volatile flavors,especially in the P.pentosaceus 30-15 fermentation group,and had significant(r>0.9)correlations with most characteristic flavors.PCA principal component analysis showed that Pediococcus had a higher correlation with volatile compounds such as organic acids,nitrogenous compounds,aromatic compounds,esters,and alcohols.In this paper,the starter strains suitable for tilapia surimi fermentation were selected.A new technology of tilapia surimi fermentation based on microbial targeted control was established,and the mechanisms of microbial metabolism regulation on the fermentation quality and flavor of tilapia surimi were explained.In summary,artificial fermentation could speed up the fermentation speed,improve the quality and flavor of fermented tilapia surimi,and contribute to the safety and stability of the product.The results of the research provide important technical supports for directionally improving the quality,flavor,and safety of fermented surimi.