| Probiotic fermented milk is loved by consumers around the world due to its good flavor and health benefits.The addition of suitable prebiotics to fermented milk can promote the growth of probiotics,thereby improving the fermentation characteristics,quality and functional properties of probiotics in fermented milk.In this paper,three strains of probiotics with significant probiotic properties screened in the laboratory were combined and tested to prepare fermented milk.The development of milk provides theoretical guidance.The main findings are as follows:（1）The effect of prebiotic combination on the viable count of probiotic bacteria.The effects of different prebiotic combinations on the viable counts of Lactobacillus fermentum grx 08,Lactobacillus plantarum grx 16,Lactobacillus rhamnosus grx 10 and three probiotics were investigated by quadratic regression orthogonal rotation design.The study found that the prebiotic combination of L.fermentum grx 08 was 0.49%fructooligosaccharides,0.37%galactooligosaccharides,0.62%inulin,0.48%stachyose,0.28%xylo-oligosaccharides,optimized L.fermentum grx 08.The number of viable bacteria was 1.82 × 108 CFU/mL,which was 2.14 times that of the control group.The prebiotic combination of L.plantarum grx 16 is 0.61%fructooligosaccharide,0.33%galactooligosaccharide,0.46%inulin,0.48%stachyose,0.14%xylo-oligosaccharide,and the optimized number of viable bacteria of L.plantarum grx 16 is 1.65 × 108 CFU/mL,2.58 times that of the control group.The prebiotic combination of L.rhamnosus grx 10 is 0.58%fructooligosaccharides,0.44%galactooligosaccharides,0.47%inulin,0.53%stachyose,0.24%xylo-oligosaccharides,and the optimized viable count of L.rhamnosus grx 10 is 6.65 ×108 CFU/mL,1.95 times that of the control group.The prebiotic combination of compound probiotics is 0.56%fructooligosaccharide,0.38%galactooligosaccharide,0.52%inulin,0.49%stachyose,0.22%xylo-oligosaccharide,and the optimized number of viable bacteria is 8.7×108 CFU/mL,1.43 times that of the control group.（2）The effect of prebiotic combination on the quality of fermented milk.The effect of prebiotic combination on the quality of fermented milk was determined by the number of viable bacteria in fermented milk,pH,acidity,water holding capacity,texture,microstructure,sensory evaluation and volatile flavor.The results showed that the addition of compound prebiotics had the greatest increase in the number of viable bacteria in grx 08 in St-Lf（L.fermentum grx 08 + Streptococcus thermophilus grx 02）and in the St-Lp（L.plantarum grx 16 + Streptococcus thermophilus grx 02）group.Significantly,from 0.85,0.64×108 CFU/mL to 1.82,1.40×108 CFU/mL,which were 2.14 and 2.19 times that of the control group.Compound prebiotics had the most significant effect on the water holding capacity of the CP（L.fermentum grx 08+L.plantarum grx 16+L.rhamnosus grx 10+Streptococcus thermophilus grx 02）group（p<0.05）,from 64.83%to 68.47%.The addition of compound prebiotics had the most significant effect on the volatile flavor and sensory properties of fermented milk in the CP group.The content of ketone volatile flavor substances such as ketone,2-heptanone,2-nonanone,etc.reduces the content of volatile flavor substances such as octanoic acid,n-decanoic acid,1,3,5-cycloheptatriene,and improves its content.Smoothness,smell and acidity of fermented milk.Adding compound prebiotics can improve the microstructure of fermented milk to varying degrees,among which the improvement of the microstructure of fermented milk in St-Lf and St-Lr（L.rhamnosus grx 10+Streptococcus thermophilus grx 02）groups is the most obvious,which helps the fermented milk to form a complete and compact structure.（3）Effects of prebiotic combination on functional properties of fermented milk.The fermented milk’s inhibition of pathogenic bacteria,anti-oxidation,adhesion to Caco-2,immune activity,anti-cancer activity,inhibition of α-amylase activity,and inhibition of ACE activity were used as evaluation indicators.The results showed that the addition of compound prebiotics had a broad-spectrum improvement on the ability of fermented milk to inhibit Pseudomonas aeruginosa,Listeria monocytogenes and Bacillus subtilis,among which compound prebiotics inhibited the St-Lr group fermented milk.Escherichia coli,Salmonella,Pseudomonas aeruginosa,and Listeria monocytogenes had the most significant improvements（p<0.05）.The addition of compound prebiotics generally improved the antioxidant capacity of fermented milk.The DPPH clearance rate of fermented milk in CP group was 53.50±1.89%,which was significantly better than that of St-Lp and St-Lr（p<0.05）.There was no significant difference with the St-Lf group（p>0.05）.The ABTS and hydroxyl radical scavenging rates of fermented milk in the grx 10 group increased the most after adding compound prebiotics,from 30.74%and 28.38%to 36.99%and 52.85%,respectively.The total antioxidant capacity of StLf group fermented milk before and after adding compound prebiotics was the strongest.Adding compound prebiotics improved the ability of probiotic fermented milk to adhere to Caco-2 cells and proliferate RAW 264.7 cells.The ability of all groups to add compound prebiotics to inhibit colon cancer Caco-2 cells was generally improved.Among them,the StLf fermented milk group and the CP group fermented milk group had the most obvious improvement in the inhibition of Caco-2 cells.When the concentration reaches 8 mg/mL,the inhibitory effect tends to be stable,and the cell viability is about 30%and 20%,respectively.After adding compound prebiotics,the ability of fermented milk to inhibit α-amylase was improved to varying degrees,and the St-Lf group had the most obvious improvement,and the inhibition rate of α-amylase increased from 34.30%to 43.80%.The inhibitory ability of probiotic fermented milk on ACE after adding compound prebiotics was generally improved（p<0.05）,among which the St-Lf group had the greatest increase,and the inhibitory rate on ACE after adding compound prebiotics was 62.81%,an increase of 63.40%.（4）The effect of prebiotic combination on the stability of fermented milk during storage.The number of viable bacteria,pH,acidity,water holding capacity,color,texture and antioxidant capacity（DPPH,ABTS,hydroxyl radical scavenging capacity,total antioxidant capacity）of fermented milk during storage period（0,7,14,21 d）is the evaluation index.The results showed that the addition of compound prebiotics slowed down the decay of the viable counts of probiotics during the storage period,and the viable counts of the fermented milk were all above 107 CFU/mL at the end of the storage period.The acidity of probiotic fermented milk after adding compound prebiotics was lower than that of the control group,which slowed down the post-acidification of fermented milk.The addition of compound prebiotics improved the water holding capacity of fermented milk samples to varying degrees during storage,among which the water holding capacity of the St-Lf group increased most significantly on the 14th day of storage（p<0.05）.which increased from 72.26%to 77.48%.The addition of compound prebiotics had a significant effect on the L*（brightness）value of fermented milk（p<0.05）,and its L*value decreased to varying degrees,among which the St-Lf group had the most significant decrease,from 85.34 to 82.18.The change of b*value after 21 days of storage showed different trends.The b*value of St-Lfs.St-Lp and St-Lps groups showed an upward trend,and the other groups all showed a decline.Compound prebiotics improved the texture of fermented milk to varying degrees during storage,and the St-Lr and CP groups had more stable textures during storage after adding compound prebiotics and were significantly better than other groups（p<0.05）. |
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