| With the increasing awareness of health,fermented milk is becoming more and more popular among consumers as a kind of fermented products with excellent flavor and health benefits,which makes the demand for fermenters in the dairy industry increasing.Traditional fermented foods have unique flavor and production process,and are rich in a large number of probiotic resources,so it is important to screen out the basic fermentation strains with excellent performance to enrich the fermentation strain resource base and develop fermenters with independent intellectual property rights.In this paper,we screened edible Lactic acid bacteria(LAB)from traditional fermented foods from Xinjiang to meet the current development needs of fermented milk fermenters,and combined the growth and fermentation characteristics,enzyme production characteristics,exopolysaccharide production capacity and probiotic characteristics of different edible LAB to screen out LAB with excellent fermentation performance or probiotic effects,and provide options for exploring superior strains for fermented milk.Meanwhile,the fermentation ability of different LAB in milk system was studied,combined with the storage stability,textural and rheological characteristics,volatile flavor substances and sensory evaluation analysis of fermented milk,to provide theoretical basis for the application of strains in fermented milk.The specific results of this research are as follows:(1)A total of 18 strains of edible LAB were obtained from 21 traditional fermented products collected from Xinjiang,including 14 strains of Lactobacillus,2 strains of Pediococcus,1 strain of Lactococcus lactis and 1 strain of L.mesenteroides.The analysis of the growth curve,acid production characteristics,enzyme production characteristics and exopolysaccharide production ability of the strains revealed that all strains entered the logarithmic growth phase after 4 h and entered the stable phase after 12 h.JK-L3,JK-L8,JK-L13 and JK-L19 showed better acid production capacity.JK-L19 showed the highest protease and SOD enzyme activities with 3.273±0.048 U/m L and 181.958±0.958 U/m L,respectively.JK-L8 showed the highest β-galactosidase activity(4.925±0.091 U/m L),followed by JK-L7 at 3.510±0.079 U/m L.JK-L3 showed the highest exopolysaccharide production capacity with 189.00±0.19 mg /L,with good potential for viscosity production.The strains JK-L3,JK-L7,JK-L8,JK-L13 and JK-L19 showed good growth and fermentation characteristics as well as good metabolic enzyme production and exopolysaccharide production characteristics.(2)The acid,bile salt and gastrointestinal resistance of 18 edible LAB were studied,the antioxidant and antibacterial activities of different components of LAB were determined,and the autoaggregation,hydrophobic ability and drug resistance of different strains were analyzed and evaluated.The results showed that most of the strains could colonize the intestine through the gastrointestinal environment.Among them,JK-L8 showed the highest survival rate of 83.49%±0.43% at 0.3% bile salt concentration and the viable count was still higher than 7 log CFU/m L after treatment with artificial gastrointestinal fluid;JK-L17 had the highest autoaggregation rate(59.54%±0.23%)and JK-L19 showed the highest hydrophobic capacity in xylene(58.58%±0.95%),with better adhesion properties.The antioxidant activity of the fermentation supernatant(FS)of all strains was significantly higher than that of the intact cells(IC),with the DPPH radical scavenging,reducing power and hydroxyl radical scavenging rates of the supernatants ranging from 74.18% to 90.19%,15.56 to 166.50 μmol/L and 15.98% to 92.90%,respectively,with JK-L19 showing the strongest DPPH radical scavenging ability,JK-L16 showed the highest reducing ability,while JK-L17 showed the strongest hydroxyl radical scavenging ability.In addition,the 18 edible LAB fermentation supernatants showed stronger inhibition of Gram-positive bacteria(Staphylococcus aureus and Listeria monocytogenes),with strains JK-L7 and JK-L16 showing high inhibition of Staphylococcus aureus,as well as high inhibition of Escherichia coli,Listeria monocytogenes and Salmonella typhimurium,with broad-spectrum inhibition.The cells of JK-L16 and JK-L19 were also able to inhibit the growth and multiplication of four pathogenic bacteria,showing good antibacterial ability.All 18 strains of LAB had different degrees of susceptibility to the eight antibiotics commonly used in clinical practice,among which JK-L13,JK-L17 and JK-L19 are more than 75% susceptible and no resistance,with no potential safety risk to human health.(3)Seven edible LAB were selected for fermentation based on the results of the analysis of fermentation and probiotic characteristics.The curd and acid production characteristics,storage stability,textural and rheological characteristics,volatile flavor substances and sensory scoring of the fermented milk were studied using commercial strain SP as a control.It was found that the acidity of the different LAB fermented milk samples,except JK-L19,ranged from 103 °T to 118 °T at 6 h of fermentation,which was slightly higher than that of the commercial strain SP fermented milk(84.4±0.6 °T).The changes in p H,acidity and water retention of the different edible LAB fermented milk samples showed consistency during storage,with varying degrees of decrease as storage time increased.At 6 h of fermentation,the storage modulus(G’)of the different lactic acid bacteria fermented milk was higher than that of SP fermented milk,and the hardness and adhesion were in the range of 96-119 g and124-171 g.sec,respectively,which were suitable for fermented milk production.JK-L3 fermented milk had the highest content of exopolysaccharide(392.14±4.28 mg/L)and JK-L16,JK-L17 and JK-L19 fermented milks had higher antioxidant activity and were consistent with their results in the MRS system.A total of 50 different volatile flavour compounds were detected in the fermented milk,mainly acids,ketones and aldehydes,of which hexanoic acid,octanoic acid and 2,3-butanedione were more abundant.JK-L19 had a poor renneting capacity,with an acidity of 28.7±0.99 °T at 6 h of fermentation and a maximum renneting time of19.03±0.15 h.However,it had a high content of protease and could produce good aroma substances such as acetaldehyde,2,3-butanedione and ethyl acetate,and could be considered as a strain resource for mixed fermenters.JK-L17 fermented milk was richer in flavour matter,had the highest sensory score and insignificant changes in post-acidification and water holding capacity during the storage period of 1-7 d.It can be used as a strain resource for weak post-acidification starter culture that can contribute to the storage of fermented milk. |
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