| Fermented milk is the main food source for people to use probiotics.Because of its unique texture and special fermented aroma,it is popular among consumers.By fermentation,milk protein,carbohydrates and lipids are broken down into smaller nutrients,and generate a variety of flavor ingredients to make products richer flavor,more easily absorbed and utilized.Among them,fermented milk contains a large amount of active lactic acid bacteria,which is of great significance for regulating human intestinal health,slowing down aging,reducing fat,and preventing cancer.In order to develop more characteristics of lactic acid bacteria fermentation products to meet market demand for different types of dairy products,fermented milk research on the structure and properties of the gel has a positive meaning.At present,the domestic research on the structure of fermented milk mainly focuses on the use of rennet and thickener,the addition of whey protein,and the effect of homogenization and heat treatment on the stability of fermented milk.However,there are few studies on the relationship between the different traits of fermented milk and the rheological properties of gel and the gastrointestinal digestibility of fermented milk with different structures.In this paper,lactic acid bacteria with good fermentation properties in milk were screened from Xinjiang's characteristic fermented milk products,namely yogurt and milk custard,and their gel-forming ability was analyzed.Based on the different ropy properties of fermented milk,the physical and chemical changes of fermented milk were measured,and the differences in structure and rheological properties such as water holding capacity and particle size were studied;Furthermore,the in vitro simulated digestion model was used to analyze the protein degradation,small peptides and amino acid delivery of different lactic acid bacteria fermented milk during the digestion process,so as to provide a certain theoretical basis for the research on fermented milk structure and nutrient digestion.The research contents and results are as follows:(1)Using the culture medium for the isolation and purification of lactic acid bacteria,6 strains of bacteria were selected from Xinjiang traditional fermented dairy products(yogurt and milk custard),and performed morphological observation and Molecular biology identification,together with 1 strain of bacteria not identified in the laboratory.The six lactic acid bacteria were identified as L.helveticus S-NA12,L.paracasei subsp.tolerans S-NA,L.paracasei subsp.tolerans S-NB,and L.paracasei subsp.tolerans S-NA5,L.paracasei subsp.tolerans S-NB8 and L.delbrueckii subsp.bulgaricus GD-Al,and a strain from the laboratory is L.fermentum Z-15.(2)The lactic acid bacteria and the four identified strains were fermented in milk,and by comparing the results of gel-forming time and drawing experiments,it was found that the fermented milk prepared by L.pentosus R-17,L.helveticus MB 2-1,L.helveticus R-5,L.helveticus S-NA12,L.paracasei S-NA,L Paracasei S-NB strains had obvious ropy properties,while the fermented milk produced by L.fermentum Z-15,L.bulgaricus LB,L.bulgaricus GD-Al,L.paracasei S-NA5,L.paracasei Fermented milk prepared from 5 strains of lactic acid bacteria such as S-NB 8 did not have the property of producing viscosity.The pH and acidity results of fermented milk showed that at the end of gel,the pH of ropy strains such as L.pentosus R-17,L.helveticus MB 2-1,L.helveticus R-5,L.helveticus S-NA12,L.paracasei S-NA and L paracasei S-NB were all lower than 4.6,with higher acidity;For non-ropy fermented milk prepared from L.fermentum Z-15,L.bulgaricus LB,L.bulgaricus GD-A1,L.paracasei S-NA5,L.paracasei S-NB8,they had a long time for gel,a pH higher than 4.6 at the time of curdling,and a lower acidity value in total,indicating that there was a significant difference in the ability of fermented milk to produce acid with different viscose-producing properties(P<0.05),and the acid and metabolism of viscous lactic acid bacteria in milk was faster.(3)By measuring the water holding capacity of fermented milk,it was found that the water holding capacity of ropy fermented milk was more than 15%higher than that of non-viscous fermented milk.Further analysis of the particle size distribution and zeta potential of fermented milk from different lactic acid bacteria,the results showed that the zeta potential of ropy fermented milk was between 10 and 20 mV,indicating that the fermented milk is positively charged and the system is relatively stable;The particle size distribution showed that most of the fermented milk had a single peak at 1?100?m,and a few of them appeared skewed,indicating that the fermented milk of 11 strains of lactic acid bacteria was evenly distributed.Fermented milk from S-NA5 and Z-15 have independent small peaks at 0.1?0.5 ?m,which indicates that their gel condition is poor,which is related to pH and fermentation acidity.Combining the particle size parameters D[4,3]and D[3,2]for comparison,it is found that the particle shape of ropy fermented milk is more regular than that of non-viscous fermented milk.(4)Based on the ropy and non-ropy properties,the apparent viscosity of fermented milk was measured using a rheometer.The results showed that all fermented milks were frequency-dependent and decreased with increasing shear rate;Through frequency scanning experiments to further analyze the viscoelasticity of fermented milk,it was concluded that fermented milk samples all had a certain viscoelasticity.The G' and G" values increased with the increase of the scanning frequency,showing the same frequency dependence.And.their elastic modulus G' is higher,showing a solid-like characteristic;The results of fermented milk shear scan indicated that all fermented milks tested had a certain thixotropy.By comparison,ropy fermented milk had a higher apparent viscosity,but also had a lower modulus(G',G")and a large thixotropic ring area.These results may be related to the way that EPS is integrated into the gel network during lactic acid bacteria fermentation.(5)By extracting EPS from 11 strains of lactic acid bacteria,measuring the primary structure of EPS,and analyzing its zeta potential and rheology,the correlation between EPS and the ropiness and gelling of fermented milk was explored.Ultraviolet spectrum showed that none of the 11 kinds of crude EPS had nucleic acid and protein.The characteristic absorption peaks of polysaccharides O-H,-CH3 or-CH2 and C-O were found in ir spectra.According to the yield analysis,the average extraction rate of crude EPS from ropy fermented milk was higher than that from non-ropy fermented milk,and the molecular weight distribution of crude EPS from viscous fermented milk was concentrated in the range of 2.44×108?2.30×109 Da,with a larger molecular weight.The results of monosaccharide composition showed that EPSs from ropy fermented milk was mainly composed of galactose and glucose,while the non-ropy fermented milk EPS contained galactose and glucose,and partly contained rhamnose and mannose with higher molar mass.Zeta potential results indicated that EPSs were all negatively charged and the system were stable.By analyzing and comparing the apparent viscosity of EPS,it was found that the shear viscosity of 11 EPSs showed a downward trend with the increase of shear rate;compared with non-ropy fermented milk,the apparent viscosity value of EPS of ropy fermented milk was higher.(6)The in vitro simulated gastrointestinal digestion model was used to study the protein degradation and digestion of different LAB fermented milk during digestion.From the change of soluble protein content and SDS-PAGE electrophoresis,the degradation of protein was related to its gel state,and the curd structure could resist further hydrolysis and digestion of protein and its hydrolysate.During gastric digestion,?-casein was firstly degraded,followed by ?-casein and as-casein.There were differences in the utilization rate of ?s-casein and ?-casein by viscous fermented milk.In the intestinal digestion stage,non-ropy fermented milk showed rapid enzymatic hydrolysis of whey protein at the beginning of digestion due to whey excretion.The analysis of small peptides and amino acids showed that during the digestive phase of the stomach,the release of small peptides and the generation of amino acids were limited,which may be related to the re-aggregation of proteins in an acidic environment;While during intestinal digestion,pancreatic enzymes showed specific enzymatic hydrolysis of proteins and small peptides,and the lactic acid bacteria's ability to release small peptides and amino acids was significantly enhanced. |
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