Composite freeze-drying protective additive for probiotics, freeze-dried bacterial powder, fermented milk and application

Abstract

The invention belongs to a fermented milk technology, and particularly relates to a composite freeze-drying protective agent for probiotics, freeze-dried bacterial powder, fermented milk and application. The probiotics are streptococcus thermophilus JM905, and the composite freeze-drying protective agent is composed of 3-3.5 parts by weight of Arabic gum, 1-2 parts by weight of manganese sulfate, 2-3 parts by weight of maltodextrin and 2-4 parts by weight of L-sodium glutamate. The composite freeze-drying protective additive can significantly improve the freeze-drying survival rate of streptococcus thermophilus JM905, and overcomes the technical problem that probiotics are difficult to tolerate a freeze-drying process. The streptococcus thermophilus JM905 freeze-dried bacterial powder prepared from the streptococcus thermophilus JM905 freeze-dried bacterial powder can keep a relatively high viable count. Fermented milk prepared from the streptococcus thermophilus JM905 freeze-dried bacterial powder is high in free amino acid content, protein content and total nucleotide content, the digestibility is remarkably improved, and the soluble protein content after digestion of gastrointestinal tracts is improved.

Description

Technical Field

[0001] The invention belongs to the fermented milk technology, and specifically relates to a composite freeze-dried protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses. Background Art

[0002] As a new type of special food in the current market and consumer demand, fermented milk not only has a good taste and flavor, but also has multiple nutritional values. With the gradual expansion of market demand, a variety of lactic acid bacteria have gradually been used in the fermentation industry, and thermophilic streptococci have been an indispensable excellent fermentation strain in fermented products due to their good acid production ability.

[0003] Different fermentation preparation methods can give fermented milk different fermentation characteristics. By appropriately adding the protein content in the fermentation matrix, such as whey protein and protein isolate, it can be supplemented into fermented dairy products, so that the traditional defatted fermentation matrix can reach a higher protein content. Freeze-drying technology can be an effective means to ensure the activity of microorganisms by reducing water activity and moisture content, but due to the differences in the composition and dosage of the freeze-dried protective agent added during the process, the finished freeze-dried bacterial powder has different vitality, and thus will show different characteristics after the fermentation process. At present, the single ingredient of the freeze-dried protective agent formula also affects the fermentation characteristics of the strain, resulting in serious homogeneity of fermented milk products and low nutritional content. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a composite freeze-dried protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses. To achieve the above purpose, the technical solution of the present invention is as follows:

[0005] In the first aspect, the present invention provides a composite lyophilization protective agent for probiotics, wherein the probiotic is Streptococcus thermophilus JM905, and the composite lyophilization protective agent is composed of the following lyophilization protective agents in parts by weight: 3 to 3.5 parts by weight of gum arabic, 1 to 2 parts by weight of manganese sulfate, 2 to 3 parts by weight of maltodextrin, and 2 to 4 parts by weight of sodium L-glutamate. That is, a composite lyophilization protective agent for Streptococcus thermophilus JM905 is provided.

[0006] Preferably, the composite lyoprotectant is composed of the following lyoprotectants in parts by weight: 3 parts by weight of gum arabic, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

[0007] Gum arabic is a polysaccharide polymer with excellent emulsification and stabilization properties. During the freeze-drying process, gum arabic can form a protective layer to wrap the thermophilic Streptococcus, reduce damage to the cell membrane, and prevent rapid evaporation of water in the cell. In addition, it can also regulate the osmotic pressure of the solution and maintain the balance of the internal and external environment of the cell, thereby improving the survival rate of the cell.

[0008] Maltodextrin is a product of hydrolyzed starch, which has the characteristics of low viscosity and easy solubility. It plays an important role in freeze-drying protective agents. It can form a protective film on the cell surface, reduce the formation of ice crystals, and provide energy reserves to help cells maintain activity during the freeze-drying process. Maltodextrin can also interact with cell membrane proteins, stabilize the membrane structure, and reduce mechanical damage caused by freeze-drying.

[0009] Among them, sodium L-glutamate, as an amino acid salt, can regulate the osmotic pressure inside and outside the cell and reduce the cell damage caused by the change of osmotic pressure during freeze-drying. It can also react with the protein on the cell membrane, stabilize the membrane structure, and protect the cells from oxidative stress. The use of sodium glutamate, especially in synergy with other protective agents such as maltodextrin and gum arabic, can significantly improve the freeze-dried survival rate of Streptococcus thermophilus.

[0010] Manganese sulfate can regulate the osmotic pressure of the solution and reduce the difference in osmotic pressure between the inside and outside of the cell, thereby reducing cell damage during the freeze-drying process. Manganese sulfate can be used as a trace element supplier for microorganisms and promote the normal metabolism of cells.

[0011] In a second aspect, the present invention provides a thermophilic streptococcus lyophilized powder, which is prepared by using thermophilic streptococcus JM905 and the aforementioned composite lyophilization protective agent.

[0012] In a third aspect, the present invention provides a method for preparing the aforementioned thermophilic Streptococcus lyophilized powder, comprising the following steps:

[0013] The thermophilic Streptococcus JM905 was activated and cultured in M17 broth medium at 37°C. After fermentation, the thermophilic Streptococcus JM905 cells were collected by centrifugation, and a composite lyophilization protective agent was added for homogenization to obtain a mixture of thermophilic Streptococcus cells and lyophilization protective agent, which was pre-frozen at -80°C for 48 hours and then freeze-dried in a freeze dryer for 48 hours to obtain thermophilic Streptococcus lyophilized powder.

[0014] Preferably, the weight ratio of the thermophilic Streptococcus JM905 cells to the composite lyophilization protective agent is (0.5-1.5): 10. Further preferably, the weight ratio of the thermophilic Streptococcus JM905 cells to the composite lyophilization protective agent is 1:10.

[0015] In a fourth aspect, the present invention provides a fermented milk prepared by using the aforementioned thermophilic streptococcus freeze-dried powder. That is, the fermented milk contains thermophilic streptococcus JM905, gum arabic, manganese sulfate, maltodextrin and sodium L-glutamate, wherein the weight ratio of the gum arabic, manganese sulfate, maltodextrin and sodium L-glutamate is: (3-3.5): (1-2): (2-3): (2-4).

[0016] In a fifth aspect, the present invention provides a method for preparing the aforementioned fermented milk, comprising the following steps:

[0017] The aforementioned thermophilic Streptococcus lyophilized powder is inoculated into 10% to 15% skim milk medium, and then inoculated into fermented milk, homogenized, and cultured at 42°C until the pH value reaches 4.5. It is well known in the art that the concentration of the skim milk medium is usually 10% to 15%, for example, the skim milk content is 12 g / 100 mL to 15 g / 100 mL.

[0018] In a sixth aspect, the present invention provides a use of the aforementioned thermophilic streptococcus lyophilized powder or the thermophilic streptococcus lyophilized powder obtained by the aforementioned preparation method or the aforementioned fermented milk or the fermented milk obtained by the aforementioned preparation method in the preparation of food or health food.

[0019] Beneficial effects:

[0020] Studies have found that as a thermophilic bacterium, Streptococcus thermophilus JM905 has poor tolerance to low temperatures. However, freeze-dried bacterial powder is the best storage method for probiotics, which helps reduce the adverse effects of the environment on the bacteria. In the screening of freeze-dried protective agents for Streptococcus thermophilus JM905, the inventors obtained a composite freeze-dried protective agent that can significantly improve the freeze-dried survival rate of Streptococcus thermophilus JM905. The freeze-dried bacterial powder of Streptococcus thermophilus JM905 prepared therefrom can maintain a high number of viable bacteria. The fermented milk prepared using the freeze-dried bacterial powder of Streptococcus thermophilus JM905 has a high free amino acid content, protein content, and total nucleotide content, and the digestibility is significantly improved, and the soluble protein content after gastrointestinal digestion is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Figure 1 is a picture of colonies of different morphologies in a culture dish of the present invention-7 gradient;

[0022] Figure 2 This is a graph showing the protein content of the fermented milk of the present invention;

[0023] Figure 3 This is a graph showing the total nucleotide content of the fermented milk of the present invention;

[0024] Figure 4 This is a graph showing the soluble protein content of the fermented milk of the present invention after simulated gastrointestinal digestion;

[0025] *** in the figure indicates p<0.001 compared between the two groups. DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with the accompanying drawings and embodiments. The Latin name of the thermophilic streptococcus JM905 in the embodiment is: Streptococcus thermophilus JM905; isolated from traditional fermented dairy products in Inner Mongolia, see the document "Streptococcus thermophilus JM905-Strain Carbon Source Utilization and Its Fermented Milk Metabolic Profile at Different Fermentation Stages" (Foods, 2023, 12 (19): 3690). The thermophilic streptococcus JM905 has been preserved, and the preservation unit is the Guangdong Provincial Microbiological Strain Collection Center; the preservation address is 5th Floor, Building 59, No. 100 Xianlie Middle Road, Guangzhou; the preservation date is October 30, 2024; the preservation number is GDMCC No: 65372; the classification name is Streptococcus thermophilus, that is, thermophilic streptococcus.

[0027] The following examples screened the formula of freeze-dried powder of Streptococcus thermophilus JM905 and the quality of fermented milk prepared with the freeze-dried powder.

[0028] Isolation and identification of thermophilic Streptococcus JM905:

[0029] Inner Mongolia traditional fermented dairy products (cheese) were stored at 4°C by laboratory staff and transported to the laboratory at low temperature. Take 10g of dairy products and put them into 90mL of sterile saline, homogenize them through a homogenizer at 200Pa, and after thorough mixing, take 1mL as a dilution sample in a sterile environment. Take the dilution sample and add 9mL of sterile saline to dilute it 10 times. Repeat the 10-fold dilution for a total of 7 times to obtain a -7 gradient dilution sample, and select the -7 gradient dilution sample as the colony counting culture plate gradient. Take the corresponding 1mL of dilution solution and add it to different culture dishes, pour 20mL of M17 agar medium at a temperature of 55°C, and fully mix the dilution solution and the culture medium. After it solidifies, culture it at 37°C for 48h.

[0030] Figure 1 The left and right pictures are parallel photos of the culture of the -7 gradient dilution sample colony counting culture dish. The colonies of different morphologies were picked from the culture dish, cultured in M17 broth for 24 hours, centrifuged to obtain the bacterial sludge, and DNA was extracted using a bacterial DNA kit. After sequencing and splicing and base database retrieval, a strain of thermophilic Streptococcus was confirmed, named thermophilic Streptococcus JM905, and preserved.

[0031] The components and proportions of M17 agar culture medium (g\L) are: soy peptone 5.0g\L; peptone 2.5g\L; casein peptone 2.5g\L; yeast extract powder 2.5g\L; beef extract powder 5.0g\L; lactose 5.0g\L; sodium ascorbate 0.5g\L; sodium β-glycerophosphate 19.0g\L; magnesium sulfate 0.25g\L; agar 12.75g\L; pH value is 7.2±0.2 at 25℃, and the solvent is water.

[0032] The components and proportions (g\L) of M17 broth culture medium are: 5.0g\L soytone; 2.5g\L peptone; 2.5g\L casein peptone; 2.5g\L yeast extract powder; 5.0g\L beef extract powder; 5.0g\L lactose; 0.5g\L sodium ascorbate; 19.0g\L sodium β-glycerophosphate; 0.25g\L magnesium sulfate; pH value is 7.2±0.2 at 25℃, and the solvent is water.

[0033] Example 1 Preparation process of freeze-dried powder of Streptococcus thermophilus JM905 and investigation of freeze-drying protective agent

[0034] 1.1 Preparation of freeze-dried powder of Streptococcus thermophilus JM905

[0035] The thermophilic Streptococcus JM905 was activated and inoculated into M17 broth medium at a 5% inoculation amount. After 24 hours of expansion culture at 37°C, 6L high-density fermentation was carried out at a 5% inoculation amount for 24 hours to obtain a fermentation broth. The obtained fermentation broth was centrifuged, the thermophilic Streptococcus cells were collected, the cell mass was determined, and a freeze-drying protective agent was added according to the mass ratio set for investigation and homogenization to obtain a mixture of thermophilic Streptococcus cells and freeze-drying protective agent. After pre-freezing at -80°C for 48 hours, it was placed in a freeze dryer and freeze-dried for 48 hours to obtain freeze-dried Streptococcus thermophilus JM905 powder. Freeze-drying conditions: cold trap temperature -73°C, cold trap vacuum 0.1Pa.

[0036] 1.2 Effects of different lyophilization protective agents

[0037] In order to investigate the protective effect of different lyoprotectants on Streptococcus thermophilus JM905 during the lyophilization process, lyoprotectants of different proportions were added according to different mass ratios, and different samples were prepared using the lyophilization process of the above-mentioned lyophilized powder of Streptococcus thermophilus JM905. After preparing different lyophilized powders of Streptococcus thermophilus JM905, the live bacteria count was performed according to GB4789.35-2016 to determine the number of live bacteria in the samples.

[0038] Table 1 Live bacteria count of thermophilic Streptococcus JM905 freeze-dried powder with single freeze-drying protectant

[0039]

[0040] Table 2 Live bacterial count and survival rate of thermophilic Streptococcus JM905 freeze-dried powder with composite freeze-drying protective agent

[0041]

[0042]

[0043] It can be seen from Table 1 that the freeze-drying protection effect of a single freeze-drying protective agent on thermophilic Streptococcus JM905 is relatively poor. Among them, sorbitol, mannitol, skim milk powder, soy protein, maltodextrin, and gum arabic have relatively good freeze-drying protection effects, sodium acetate has the worst protection effect, and manganese sulfate and sodium L-glutamate are only slightly better than sodium acetate.

[0044] It can be seen from Table 2 that the freeze-drying protection effect of two-component and three-component freeze-drying protectants on Streptococcus thermophilus JM905 is slightly improved compared with that of single soybean protein (Group 4 in Table 1), only Group A, Group B, Group E, Group F, and Group H, but the improvement of protection effect is not significant. It can be seen that Streptococcus thermophilus JM905 has poor tolerance to the low temperature environment of the freeze-drying process.

[0045] The freeze-drying protection effect of the four-component freeze-dried protective agent on thermophilic Streptococcus JM905 was significantly improved compared with the freeze-drying protection effect of the single-component, two-component, and three-component freeze-dried protective agents on thermophilic Streptococcus JM905. However, the survival rate of group I was slightly improved, with a survival rate of less than 37%; the freeze-drying protection effect of the four-component freeze-dried protective agent in group K on thermophilic Streptococcus JM905 was the best, with a survival rate of 67.4%.

[0046] It can be seen from Table 1 that the freeze-drying protection effect of a single freeze-drying protectant on thermophilic Streptococcus JM905 is poor, among which sodium acetate has the worst protection effect, and the protection effects of other single freeze-drying protectants are relatively small. It can be seen from Table 2 that the freeze-drying protection effect of the combination of four freeze-drying protectants on thermophilic Streptococcus JM905 is higher than that of the combination of two or three freeze-drying protectants.

[0047] Although this example only examines a ratio of thermophilic Streptococcus cells: lyoprotectant of 1:10, based on the basic principles and matrix of lyoprotection, it can be reasonably predicted in the art that the lyoprotection effect of thermophilic Streptococcus cells: lyoprotectant will not change much when the ratio is within the range of 0.5:10 to 1.5:10.

[0048] Example 2 Preparation and quality assessment of fermented milk using freeze-dried powder of Streptococcus thermophilus JM905

[0049] 2.1 Preparation of fermented milk with freeze-dried powder of Streptococcus thermophilus JM905:

[0050] The thermophilic Streptococcus JM905 freeze-dried powder was prepared according to the formula and preparation method of Group J and Group K in Example 1. After the viable bacterial counts of the two groups of freeze-dried powders were determined, the viable bacterial counts were 10 7CFU / g inoculum was inoculated into the sterilized modified skim milk powder medium. During inoculation, the temperature of the medium was ensured to drop below 40°C. 7 The number of CFU / g live bacteria was inoculated into the fermented milk. After sufficient homogenization, it was placed in a 42°C incubator for culture. The pH value of the fermented milk was monitored during the culture process. When the pH value reached 4.5, the fermented milk was completely fermented. Fermented milks prepared with freeze-dried powder of thermophilic Streptococcus JM905 of group J and group K were obtained respectively, and then the quality of the two fermented milks was investigated.

[0051] Among them, the preparation method of the improved skim milk culture medium is as follows: heat and boil an appropriate amount of milk for 20 to 30 minutes, cool it overnight, and the fat will float. Remove the upper layer of milk fat to ensure that the fat content is less than 0.75% to obtain skim milk. Add 2% whey protein and 1.75% protein isolate (food grade) to the obtained skim milk according to the mass percentage. Whey protein and protein isolate are added to skim milk at 25°C to 30°C, and homogenized by a food-grade homogenizer with a homogenization pressure of 20 to 22Mpa. After homogenization for 60s, the above homogenized product is filtered using a pre-filter with a pore size of 0.08 to 0.20μm and formed by a hydrophobic resin to obtain a separated and purified protein solution (the protein solution is skim milk + 2% whey protein and 1.75% protein isolate). The protein solution is placed in a test tube and a triangular flask, sealed and placed in a sterilizing pot for steam sterilization at 95°C for 15 minutes to obtain a modified skim milk culture medium. The skim milk culture medium has a skim milk content of 12% (g / 100mL), a whey protein content of 2% (g / 100mL), and an isolate protein content of 1.75% (g / 100mL). Since skim milk can be skim milk with different fat contents, the skim milk content of the skim milk culture medium is allowed to be 10% to 15% (g / 100mL).

[0052] 2.2 Quality evaluation of fermented milk with freeze-dried powder of Streptococcus thermophilus JM905

[0053] (1) Determination of free amino acids in fermented milk

[0054] Accurately weigh 1.0g of the sample into a 250mL conical flask, add 200mL of boiling water to brew, heat in a water bath at 85℃, and mix every 5min. Take out after 10min of extraction and filter while hot. After the filtrate cools to room temperature, dilute to 250mL with water, mix well, take an appropriate amount of sample solution, and filter through a 0.25μm aqueous filter membrane. Pipette 10μL of the prepared sample solution into a clean glass insert, add 70μL of the derivatization buffer solution (glycine-hydrochloric acid buffer (0.05mol / L)), then add 20μL of the derivatization reagent (phthalic acid-hydrochloric acid buffer (0.05mol / L)), vortex mix for 10s, cover and seal in the injection bottle, let stand at room temperature for 10min, and use a fully automatic amino acid analyzer to determine the amino acid content and composition.

[0055] (2) Determination of protein content in fermented milk

[0056] The protein content in the sample was determined using the national standard GB 5009.5-2016 for the determination of protein in food.

[0057] (3) Determination of nucleotides in fermented milk

[0058] Standards: guanine nucleotide (GMP), purity 98.8%; adenine nucleotide (AMP), purity 92.4%; inosine nucleotide (IMP), purity 100.0%; cytosine nucleotide (CMP), purity 99.3%; uracil nucleotide (UMP), purity 99.7%.

[0059] Samples: Two types of fermented milk prepared using freeze-dried bacterial powder of group J and group K Streptococcus thermophilus JM905, respectively.

[0060] Weigh appropriate amounts of CMP, UMP, AMP, GMP and IMP standards to an accuracy of 0.1 mg, dissolve in water and prepare concentrations of 1 mg mL -1 The standard stock solution of ; the chromatographic conditions are: the chromatographic column is Acquity UPLCHSS T3, the mobile phase is 0.4mol / L potassium dihydrogen phosphate (pH=3.40) and 0.04mol / L ammonium formate (pH=3.80), the elution ratio is 8:2, the injection volume is 3μL, the column temperature is 30℃, the flow rate is 0.5mL / min, the detection wavelength is 254nm, and a certain volume of each standard stock solution is drawn and diluted with water to form a mixed standard working solution of different concentrations. 8μL of standard solutions of different concentrations were injected according to the above chromatographic conditions, and the standard curve was drawn with mass concentration as the horizontal axis and peak area as the vertical axis. The regression equation and correlation coefficient were obtained by regression calculation, and the final 5 nucleotide contents were added together to obtain the total amount of nucleotides. The samples were measured under the same detection conditions to calculate the total amount of nucleotides in the samples.

[0061] (4) Experimental determination of the digestive characteristics of samples using an in vitro gastrointestinal digestion model

[0062] Gastrointestinal digestion simulation was carried out on two fermented milks prepared with freeze-dried bacterial powder of group J and group K Streptococcus thermophilus JM905.

[0063] Salivary amylase solution: Dilute to the required concentration of 0.5% using pH 7.0 potassium phosphate buffer.

[0064] Pepsin solution: Dissolve pepsin powder in pH 2.2 citric acid-disodium hydrogen phosphate buffer to a concentration of 0.35%.

[0065] Trypsin solution: Dissolve trypsin powder in PBS buffer (pH 7.3) to a concentration of 0.1%.

[0066] The entire digestion process was completed in a constant temperature oscillator (37°C), fermented milk: salivary amylase solution: pepsin solution: trypsin solution (mass ratio) = 2.0: 2.0: 1.0: 1.0, and equal volume sampling was performed at 0.5, 1, 1.5, 2, 2.5, and 3 hours. The samples at each digestion stage were fixed to 14 mL with deionized water to ensure that the dilution of the samples at all digestion stages was consistent, and the fixed-volume samples were placed in a boiling water bath for 5 minutes to inactivate the enzyme. Then 2 mL was drawn and placed in a centrifuge tube for centrifugation (11000r / min, 5 minutes). After the end, the supernatant was drawn to determine the protein component to detect the digestion characteristics of the fermented milk.

[0067] The method for determining the protein in the supernatant is determined by the combustion method in accordance with GB 5009.5-2016 "National Food Safety Standard Determination of Protein in Food". Take the sample onto diatomaceous earth so that the weight of the sample and diatomaceous earth is 0.1-1.0g (accurate to 0.0001g), wrap it with tin foil and place it on the sample plate. After the sample enters the combustion reactor (900℃-1200℃), it is fully burned in high-purity oxygen (>99.99%). The product in the combustion furnace is transported to the reduction furnace (800℃) by the carrier gas carbon dioxide or nitrogen, and its content is detected after reduction to generate nitrogen.

[0068] 2.3 Results

[0069] The results of the free amino acid content of fermented milk are shown in Table 3. The free amino acid content of fermented milk prepared by bacterial powder of group K is higher than that of fermented milk prepared by bacterial powder of group J. In particular, compared with the fermented milk prepared by bacterial powder of group J, the free glycine, phenylalanine and histidine contents in the fermented milk prepared by bacterial powder of group K are increased by about 2 to 4 times.

[0070] Table 3 Free amino acid content in fermented milk (g / 100g)

[0071]

[0072] The data in Table 3 are the average values ​​of 3 parallel measurements.

[0073] Figure 2 This is the result of measuring the protein content of the fermented milk of the present invention. The protein content of the fermented milk prepared with the bacterial powder of group K is significantly higher than that of the fermented milk prepared with the bacterial powder of group J.

[0074] Figure 3 The results of total nucleotide content determination in fermented milk showed that the total nucleotide content in fermented milk prepared with bacterial powder of group K was significantly higher than that in fermented milk prepared with bacterial powder of group J.

[0075] Figure 4 The results of the digestion characteristics of fermented milk showed that the content of soluble protein in the fermented milk prepared with group K bacterial powder was significantly higher than that in the fermented milk prepared with group J bacterial powder after digestion with simulated digestive fluid.

[0076] Based on the above results, it can be seen that under the same live bacteria inoculation amount, the fermented milk prepared with group K bacterial powder was significantly higher than that with group J bacterial powder in terms of free amino acid content, protein content, total nucleotide content and digestibility.

Claims

1. A composite freeze-drying protective agent for probiotics, characterized in that: The probiotic is Streptococcus thermophilus JM905, and the composite freeze-drying protective agent is composed of the following freeze-drying protective agents in parts by weight: 3-3.5 parts by weight of gum arabic, 1-2 parts by weight of manganese sulfate, 2-3 parts by weight of maltodextrin, and 2-4 parts by weight of sodium L-glutamate.

2. The composite freeze-drying protective agent for probiotics according to claim 1, characterized in that: The composite freeze-drying protective agent is composed of the following freeze-drying protective agents in parts by weight: 3 parts by weight of gum arabic, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

3. A thermophilic streptococcus freeze-dried powder, characterized in that: The thermophilic streptococcus freeze-dried powder is prepared from thermophilic streptococcus JM905 and a composite freeze-dried protective agent, wherein the composite freeze-dried protective agent is composed of the following freeze-dried protective agents in parts by weight: 3 to 3.5 parts by weight of gum arabic, 1 to 2 parts by weight of manganese sulfate, 2 to 3 parts by weight of maltodextrin, and 2 to 4 parts by weight of sodium L-glutamate.

4. A thermophilic Streptococcus lyophilized powder according to claim 3, characterized in that: The composite freeze-drying protective agent is composed of the following freeze-drying protective agents in parts by weight: 3 parts by weight of gum arabic, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

5. The method for preparing a thermophilic Streptococcus lyophilized powder according to claim 3 or 4, characterized in that: The steps include: The thermophilic Streptococcus JM905 was activated and cultured in M17 broth medium at 37°C. After fermentation, the thermophilic Streptococcus JM905 cells were collected by centrifugation, and a composite lyophilization protective agent was added for homogenization to obtain a mixture of thermophilic Streptococcus cells and lyophilization protective agent, which was pre-frozen at -80°C for 48 hours and then freeze-dried in a freeze dryer for 48 hours to obtain thermophilic Streptococcus lyophilized powder.

6. The method for preparing a thermophilic Streptococcus lyophilized powder according to claim 5, characterized in that: The weight ratio of the thermophilic Streptococcus JM905 bacteria to the composite freeze-drying protective agent is (0.5-1.5):

10.

7. A fermented milk, characterized in that: The fermented milk is fermented by freeze-dried bacterial powder of Streptococcus thermophilus JM905, and the freeze-dried bacterial powder of Streptococcus thermophilus JM905 is freeze-dried and protected by a composite freeze-dried protective agent, and the composite freeze-dried protective agent is composed of the following freeze-dried protective agents in parts by weight: 3 to 3.5 parts by weight of gum arabic, 1 to 2 parts by weight of manganese sulfate, 2 to 3 parts by weight of maltodextrin, and 2 to 4 parts by weight of sodium L-glutamate.

8. The fermented milk according to claim 7, characterized in that: The composite freeze-drying protective agent is composed of the following freeze-drying protective agents in parts by weight: 3 parts by weight of gum arabic, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

9. The fermented milk according to claim 7, characterized in that: In the freeze-dried bacterial powder of Streptococcus thermophilus JM905, the weight ratio of the Streptococcus thermophilus JM905 to the composite freeze-dried protective agent is (0.5-1.5):

10.

10. The fermented milk according to claim 9, characterized in that: In the freeze-dried bacterial powder of the thermophilic Streptococcus JM905, the weight ratio of the thermophilic Streptococcus JM905 to the composite freeze-dried protective agent is 1:

10.

11. The method for preparing fermented milk according to any one of claims 7 to 10, characterized in that: The steps include: The freeze-dried powder of Streptococcus thermophilus JM905 was inoculated into 10%~15% skim milk culture medium, and then inoculated into fermented milk. After homogenization, it was cultured at 42°C until the pH value reached 4.

5.

12. Use of a thermophilic Streptococcus lyophilized powder according to claim 3 or 4 or a fermented milk according to any one of claims 7 to 10 in the preparation of food or health products.

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