A composite freeze-drying protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses thereof

Abstract

The present invention belongs to the technology of fermented milk, and particularly relates to a composite freeze-drying protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses thereof. The probiotic is 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 agent can significantly improve the freeze-drying survival rate of Streptococcus thermophilus JM905 and overcome the technical problem that this probiotic is difficult to tolerate the freeze-drying process. The freeze-dried bacterial powder of Streptococcus thermophilus JM905 prepared therefrom can maintain a high viable bacteria count. The fermented milk prepared by using the freeze-dried bacterial powder of Streptococcus thermophilus JM905 has high contents of free amino acids, protein and total nucleotides, and the digestibility is significantly improved, and the content of soluble protein after gastrointestinal digestion is increased.

Description

Technical Field

[0001] The present invention belongs to the technology of fermented milk, and specifically relates to a composite freeze-drying protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses thereof. 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 been gradually applied to the fermentation industry, and Streptococcus thermophilus has always been an excellent fermentation strain indispensable in fermented products due to its good acid-producing ability.

[0003] Different fermentation preparation methods can endow fermented milk with different fermentation characteristics. By appropriately adding the protein content in the fermentation matrix such as whey protein and separated protein to the fermented dairy products, the traditional skimmed fermentation matrix can reach a higher protein content. Freeze-drying technology is an effective means to ensure the microbial activity by reducing the water activity and water content. However, due to the differences in the components and dosages of the freeze-drying protective agents added in the process, the finished freeze-dried bacterial powder has different activities, and thus different characteristics will be presented during the fermentation process. At present, the single formula composition of the freeze-drying protective agent also affects the fermentation characteristics of the strains, resulting in relatively serious homogenization of fermented milk products and lower nutritional components. Summary of the Invention

[0004] In view of the above problems, the purpose of the present invention is to provide a composite freeze-drying protective agent for probiotics, freeze-dried bacterial powder, fermented milk and uses thereof. 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 freeze-drying protective agent for probiotics. The probiotic is Streptococcus thermophilus JM905, and the composite freeze-drying protective agent is composed of the following parts by weight of freeze-drying protective agents: 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. That is, a composite freeze-drying protective agent for Streptococcus thermophilus JM905 is provided.

[0006] Preferably, the composite freeze-drying protective agent is composed of the following parts by weight of freeze-drying protective agents: 3 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of L-sodium glutamate.

[0007] Among them, arabic gum is a polysaccharide polymer with emulsifying and stabilizing properties. During the freeze-drying process, arabic gum can form a protective layer to wrap Streptococcus thermophilus, reduce the damage of the cell membrane, and prevent the rapid evaporation of intracellular water. In addition, it can also adjust the osmotic pressure of the solution to maintain the balance of the internal and external environments of the cells, thereby improving the survival rate of the cells.

[0008] Among them, maltodextrin is a product of hydrolyzed starch, which has the characteristics of low viscosity and easy solubility. It plays an important role in the lyoprotectant, can form a protective film on the cell surface, reduce the formation of ice crystals, and at the same time provide an energy reserve to help the cells maintain their activity during lyophilization. Maltodextrin can also interact with cell membrane proteins to 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 cells and reduce cell damage caused by osmotic pressure changes during the freeze-drying process. It can also react with proteins on the cell membrane to stabilize the membrane structure and protect the cells from oxidative stress. The use of sodium glutamate, especially in synergistic action with other protectants such as maltodextrin and gum arabic, can significantly improve the freeze-drying survival rate of Streptococcus thermophilus.

[0010] Among them, manganese sulfate can regulate the osmotic pressure of the solution, reduce the osmotic pressure difference inside and outside the cells, and thus reduce cell damage during lyophilization. Manganese sulfate can be a supplier of trace elements for microorganisms and can promote the normal metabolism of cells.

[0011] In a second aspect, the present invention provides a freeze-dried powder of Streptococcus thermophilus, which is prepared from Streptococcus thermophilus JM905 and the aforementioned composite lyoprotectant.

[0012] In a third aspect, the present invention provides a method for preparing the aforementioned freeze-dried powder of Streptococcus thermophilus, which includes the following steps:

[0013] Activate Streptococcus thermophilus JM905 and culture it at 37 °C using M17 broth medium; after fermentation, centrifuge to collect the cells of Streptococcus thermophilus JM905, add the composite lyoprotectant and homogenize to obtain a mixture of Streptococcus thermophilus cells and the lyoprotectant. After pre-freezing at -80 °C for 48 h, put it into a freeze-dryer and freeze-dry for 48 h to obtain the freeze-dried powder of Streptococcus thermophilus.

[0014] Preferably, the weight ratio of the cells of Streptococcus thermophilus JM905 to the composite lyoprotectant is (0.5 - 1.5):10. More preferably, the weight ratio of the cells of Streptococcus thermophilus JM905 to the composite lyoprotectant is 1:10.

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

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

[0017] Take the aforementioned freeze-dried powder of Streptococcus thermophilus and inoculate it into a 10% - 15% skim milk medium, then inoculate it into the fermented milk. After homogenization, culture it at 42°C until the pH value reaches 4.5. As is well known in the art, the concentration of the skim milk medium is usually 10% - 15%, for example, the skim milk content is 12 g / 100 mL - 15 g / 100 mL.

[0018] Sixth aspect, the present invention provides the application of the aforementioned freeze-dried powder of Streptococcus thermophilus or the freeze-dried powder of Streptococcus thermophilus 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] Research has found that Streptococcus thermophilus JM905, as a thermophilic bacterium, has poor low-temperature tolerance. However, freeze-dried bacterial powder is the best storage method for probiotics, which is conducive to reducing the adverse effects of the environment on the bacterial cells. In the screening of cryoprotectants for Streptococcus thermophilus JM905, the inventors obtained a composite cryoprotectant 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 viable cell count. The fermented milk prepared with the freeze-dried bacterial powder of Streptococcus thermophilus JM905 has a high content of free amino acids, protein, and total nucleotides, and the digestibility is significantly improved, and the content of soluble protein after gastrointestinal digestion is increased. Description of the drawings

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

[0022] Figure 2 It is a diagram of the protein content of the fermented milk of the present invention;

[0023] Figure 3 It is a diagram of the total nucleotide content of the fermented milk of the present invention;

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

[0025] In the figure, *** indicates p < 0.001 when comparing the two groups. Detailed implementation manners

[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 ratios (g / L) of M17 agar medium are as follows: peptone from soybeans 5.0 g / L; peptone 2.5 g / L; peptone from casein 2.5 g / L; yeast extract powder 2.5 g / L; beef extract powder 5.0 g / L; lactose 5.0 g / L; sodium ascorbate 0.5 g / L; β-glycerophosphate disodium 19.0 g / L; magnesium sulfate 0.25 g / L; agar 12.75 g / L; pH value 7.2 ± 0.2 at 25 °C, and the solvent is water.

[0032] The components and ratios (g / L) of M17 broth medium are as follows: peptone from soybeans 5.0 g / L; peptone 2.5 g / L; peptone from casein 2.5 g / L; yeast extract powder 2.5 g / L; beef extract powder 5.0 g / L; lactose 5.0 g / L; sodium ascorbate 0.5 g / L; β-glycerophosphate disodium 19.0 g / L; magnesium sulfate 0.25 g / L; pH value 7.2 ± 0.2 at 25 °C, and the solvent is water.

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

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

[0035] Take the activated Streptococcus thermophilus JM905 and inoculate it into M17 broth medium at an inoculation amount of 5%. After expanding the culture at 37 °C for 24 h, perform 6 L high-density fermentation at an inoculation amount of 5% and ferment for 24 h to obtain the fermentation broth. Centrifuge the obtained fermentation broth, collect the Streptococcus thermophilus cells, measure the cell mass, and add the freeze-drying protectant according to the set mass ratio for homogenization to obtain a mixture of Streptococcus thermophilus cells and the freeze-drying protectant. After pre-freezing at -80 °C for 48 h, put it into a freeze-dryer and freeze-dry for 48 h to obtain the Streptococcus thermophilus JM905 freeze-dried bacterial powder. Freeze-drying conditions: cold trap temperature -73 °C, cold trap vacuum 0.1 Pa.

[0036] 1.2 Investigation of the effects of different freeze-drying protectants

[0037] To investigate the protective effects of different freeze-drying protectants on Streptococcus thermophilus JM905 during freeze-drying, add freeze-drying protectants with different ratios according to different mass ratios, and use the above freeze-drying process of Streptococcus thermophilus JM905 freeze-dried bacterial powder to prepare different samples. After preparing different Streptococcus thermophilus JM905 freeze-dried bacterial powders, perform viable count according to GB4789.35-2016 and measure the viable bacteria count in the samples.

[0038] Table 1 Viable bacteria count of Streptococcus thermophilus JM905 freeze-dried bacterial powder with single freeze-drying protectant

[0039]

[0040] Table 2 Viable cell count and survival rate of freeze-dried Streptococcus thermophilus JM905 powder with composite cryoprotectants

[0041]

[0042]

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

[0044] As can be seen from Table 2, the freeze-drying protection effects of the two-component and three-component cryoprotectants on Streptococcus thermophilus JM905 are only slightly improved compared with the single soy protein (Group 4 in Table 1), and the improvements in the A, B, E, F, and H groups are 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 cryoprotectant on Streptococcus thermophilus JM905 is improved to a greater extent compared with the single-component, two-component, and three-component cryoprotectants. However, the survival rate improvement in Group I is small, with a survival rate of less than 37%; among them, the four-component cryoprotectant in Group K has the best freeze-drying protection effect on Streptococcus thermophilus JM905, with a survival rate reaching 67.4%.

[0046] As can be seen from Table 1, the single cryoprotectant has a poor freeze-drying protection effect on Streptococcus thermophilus JM905, and sodium acetate has the worst protection effect, while the protection effects of other single cryoprotectants have small differences. As can be seen from Table 2, the combined action of the four cryoprotectants on the freeze-drying protection of Streptococcus thermophilus JM905 is higher than that of the combination of two and three cryoprotectants.

[0047] Although only the ratio of Streptococcus thermophilus cells to cryoprotectant of 1:10 was investigated in this example, based on the basic principle and matrix of freeze-drying protection, it can be reasonably predicted in this field that within the range of Streptococcus thermophilus cells to cryoprotectant of 0.5:10 to 1.5:10, the freeze-drying effect changes little.

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

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

[0050] Prepare the freeze-dried Streptococcus thermophilus JM905 powder according to the formula and preparation method of Group J and Group K in Example 1. After measuring the viable cell count of the two groups of freeze-dried powder, respectively, according to the viable cell count of 10 7Inoculate into the sterilized modified skim milk powder medium at a CFU / g inoculation amount. Ensure that the temperature of the medium drops below 40 °C during inoculation, and inoculate into the fermented milk at a viable count of 10 7 CFU / g. After thorough homogenization, place it in an incubator at 42 °C for cultivation, and monitor the pH value of the fermented milk during the cultivation process. When the pH value reaches 4.5, it indicates that the fermentation of the fermented milk is complete. Fermented milks prepared from the freeze-dried powder of Streptococcus thermophilus JM905 in Group J and Group K are obtained respectively, and then the quality of the two fermented milks is investigated.

[0051] Among them, the preparation method of the modified skim milk medium is as follows: Heat an appropriate amount of milk to boiling for 20 - 30 min, cool overnight, and the fat will float to the surface. 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% separated protein (food grade) to the obtained skim milk according to the mass percentage. Add the whey protein and separated protein to the skim milk at 25 °C - 30 °C, and homogenize with a food-grade homogenizer. The homogenization pressure is 20 - 22 Mpa. After homogenization for 60 s, filter the above-mentioned homogenized product with a pre-filter formed by hydrophobic resin with a pore size of 0.08 - 0.20 μm to obtain a separated and purified protein solution (the protein solution is skim milk + 2% whey protein and 1.75% separated protein). Place the protein solution in test tubes and Erlenmeyer flasks, seal them, and place them in an autoclave for steam sterilization at 95 °C for 15 min to obtain the modified skim milk medium. The content of skim milk in this skim milk medium is 12% (g / 100 mL), the content of whey protein is 2% (g / 100 mL), and the content of separated protein is 1.75% (g / 100 mL). Since different fat content skim milk can be used, the skim milk content of this skim milk medium is allowed to be 10% - 15% (g / 100 mL).

[0052] 2.2 Investigation of the quality of fermented milk prepared from the freeze-dried powder of Streptococcus thermophilus JM905

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

[0054] Accurately weigh 1.0 g of the sample into a 250 mL conical flask, add 200 mL of boiling water, heat in a water bath at 85 °C, and mix well every 5 minutes. After extraction for 10 minutes, take it out and filter while it is hot. After the filtrate is cooled to room temperature, make up the volume to 250 mL with water, mix well, take an appropriate amount of the sample solution, and filter it through a 0.25 μm aqueous filter membrane. Pipette 10 μL of the prepared sample solution into a clean glass inner cannula, add 70 μL of the buffer solution for derivatization (glycine-hydrochloric acid buffer solution (0.05 mol / L)), and then add 20 μL of the derivatization reagent (phthalic acid-hydrochloric acid buffer solution (0.05 mol / L)), vortex mix for 10 s, seal it with a lid in a sample vial, let it stand at room temperature for 10 minutes, and determine the amino acid content and composition using an automatic amino acid analyzer.

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

[0056] Determine the protein content in the sample using the national standard GB 5009.5-2016 Determination of protein in foods.

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

[0058] Standard substances: Guanosine monophosphate (GMP), purity 98.8%; Adenosine monophosphate (AMP), purity 92.4%; Inosine monophosphate (IMP), purity 100.0%; Cytidine monophosphate (CMP), purity 99.3%; Uridine monophosphate (UMP), purity 99.7%.

[0059] Samples: Two kinds of fermented milk prepared with the freeze-dried bacterial powder of Streptococcus thermophilus JM905 in group J and group K respectively.

[0060] Weigh appropriate amounts of CMP, UMP, AMP, GMP and IMP standard substances accurately to 0.1 mg, dissolve them with water, and prepare standard stock solutions with concentrations of 1 mg·mL -1 respectively; The chromatographic conditions are as follows: The chromatographic column is Acquity UPLCHSS T3, the mobile phases are potassium dihydrogen phosphate (pH = 3.40) at 0.4 mol / L and ammonium formate (pH = 3.80) at 0.04 mol / L, the elution ratio is 8:2, the injection volume is 3 μL, the column temperature is 30 °C, the flow rate is 0.5 mL / min, the detection wavelength is 254 nm. Pipette certain volumes of each standard stock solution respectively, dilute them with water to prepare mixed standard working solutions with different concentrations. Inject 8 μL of the standard solutions with different concentrations into the column according to the above chromatographic conditions respectively. Taking the mass concentration as the abscissa and the peak area as the ordinate, draw a standard curve, obtain the regression equation and correlation coefficient through regression calculation, add up the final contents of the 5 nucleotides, which is the total amount of nucleotides. Determine the samples under the same detection conditions and calculate the total amount of nucleotides in the samples.

[0061] (4) The digestion characteristics of the samples were experimentally determined through an in vitro gastrointestinal digestion model.

[0062] Two kinds of fermented milks prepared with the freeze-dried Streptococcus thermophilus JM905 powder of group J and group K were simulated for gastrointestinal digestion.

[0063] Salivary amylase solution: Diluted to the required 0.5% concentration with potassium phosphate buffer solution at pH 7.0 as the solvent.

[0064] Pepsin solution: Dissolved pepsin powder with citric acid-disodium hydrogen phosphate buffer solution at pH 2.2 as the solvent, with a concentration of 0.35%.

[0065] Trypsin solution: Dissolved trypsin powder with PBS buffer solution at pH 7.3 as the solvent, with a concentration of 0.1%.

[0066] The entire digestion process was completed in a thermostatic oscillator (37 °C). Fermented milk: Salivary amylase solution: Pepsin solution: Trypsin solution (mass ratio) = 2.0:2.0:1.0:1.0. Equal-volume sampling and determination were carried out at 0.5, 1, 1.5, 2, 2.5, and 3 h respectively. The samples at each digestion stage were fixed to 14 mL with deionized water to ensure the same dilution degree of the samples at all digestion stages. The fixed samples were placed in boiling water for a water bath for 5 min to inactivate the enzymes. Then, 2 mL was aspirated and placed in a centrifuge tube for centrifugation (11000 r / min, 5 min). After centrifugation, the supernatant was aspirated to determine the protein components to detect the digestion characteristics of the fermented milk.

[0067] The determination method of the supernatant protein was carried out by the combustion method with reference to GB 5009.5-2016 "National Food Safety Standard Determination of Protein in Foods". The sample was taken onto diatomaceous earth so that the weight of the sample and diatomaceous earth was 0.1 - 1.0 g (accurate to 0.0001 g), wrapped with tin foil and placed on the sample tray. After the sample entered the combustion reaction furnace (900 °C - 1200 °C), it was fully combusted in high-purity oxygen (>99.99%). The products in the combustion furnace were transported by the carrier gas carbon dioxide or nitrogen to the reduction furnace (800 °C), and after reduction to generate nitrogen, its content was detected.

[0068] 2.3 Results

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

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

[0071]

[0072] The data in Table 3 are the averages of 3 parallel determinations.

[0073] Figure 2 These are the determination results of the protein content in the fermented milk of the present invention. The protein content in the fermented milk prepared with the K group of bacterial powder is significantly higher than that in the fermented milk prepared with the J group of bacterial powder.

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

[0075] Figure 4 These are the determination results of the digestion characteristics of the fermented milk. After the fermented milk prepared with the K group of bacterial powder is digested with the simulated digestive juice, the content of soluble protein is significantly higher than that in the fermented milk prepared with the J group of bacterial powder.

[0076] Based on the above results, it can be seen that under the same viable bacteria inoculation amount, the fermented milk prepared with the K group of bacterial powder is significantly higher than the J group of 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 lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3-3.5 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 2-3 parts by weight of maltodextrin, and 3-4 parts by weight of sodium L-glutamate.

2. The composite freeze-drying protective agent for probiotics according to claim 1, wherein The composite lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

3. A freeze-dried powder of Streptococcus thermophilus, characterized in that, The freeze-dried powder of Streptococcus thermophilus is prepared from Streptococcus thermophilus JM905 and a composite lyophilization protectant, and the composite lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3-3.5 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 2-3 parts by weight of maltodextrin, and 3-4 parts by weight of sodium L-glutamate.

4. The freeze-dried powder of Streptococcus thermophilus according to claim 3, characterized in that, The composite lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

5. The preparation method of a freeze-dried powder of Streptococcus thermophilus according to claim 3 or 4, characterized in that, It includes the following steps: Take Streptococcus thermophilus JM905 for activation and culture it at 37 °C using M17 broth medium; after fermentation, centrifuge to collect the cells of Streptococcus thermophilus JM905, add the composite lyophilization protectant and homogenize to obtain a mixture of Streptococcus thermophilus cells and the lyophilization protectant. After pre-freezing at -80 °C for 48 h, put it into a freeze dryer and freeze-dry for 48 h to obtain the freeze-dried powder of Streptococcus thermophilus.

6. The preparation method of a Streptococcus thermophilus freeze-dried powder according to claim 5, characterized in that, The weight part ratio of the cells of Streptococcus thermophilus JM905 to the composite lyophilization protectant is (0.5-1.5):

10.

7. A fermented milk, characterized in that, The fermented milk is prepared by fermenting with the freeze-dried powder of Streptococcus thermophilus JM905, and the freeze-dried powder of Streptococcus thermophilus JM905 is protected by a composite lyophilization protectant. The composite lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3-3.5 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 2-3 parts by weight of maltodextrin, and 3-4 parts by weight of sodium L-glutamate.

8. A fermented milk according to claim 7, characterized in that, The composite lyophilization protectant consists of the following lyophilization protectants in parts by weight: 3 parts by weight of arabic gum, 1 part by weight of manganese sulfate, 3 parts by weight of maltodextrin, and 3 parts by weight of sodium L-glutamate.

9. A fermented milk according to claim 7, characterized in that In the freeze-dried powder of Streptococcus thermophilus JM905, the weight part ratio of Streptococcus thermophilus JM905 to the composite lyophilization protectant is (0.5-1.5):

10.

10. A fermented milk according to claim 9, characterized in that, In the freeze-dried powder of Streptococcus thermophilus JM905, the weight part ratio of Streptococcus thermophilus JM905 to the composite lyophilization protectant is 1:

10.

11. A method for preparing a fermented milk according to any one of claims 7 to 10, characterized in that, It includes the following steps: Inoculate the freeze-dried powder of Streptococcus thermophilus JM905 into 10%-15% skim milk medium, then inoculate it into fermented milk, homogenize, and culture at 42 °C until the pH value reaches 4.

5.

12. Use of a freeze-dried powder of Streptococcus thermophilus 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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