Screening of a lactobacillus rhamnosus strain and its application in production of air-dried chicken

Abstract

This invention belongs to the field of microbial screening and food fermentation technology, and discloses the screening of a strain of *Lactobacillus rhamnosus* and its application in the production of air-dried chicken. The strain is classified and named *Lactobacillus rhamnosus* strain L8, and is deposited at the China Center for Type Culture Collection (CCTCC) with accession number CCTCC M 2026609 LR01. The strain was isolated from air-dried chicken samples. This invention provides a strain of *Lactobacillus rhamnosus* isolated from traditional air-dried chicken, which has strong salt and nitrite tolerance, can degrade nitrite, inhibit the growth of *Pseudomonas fluorescens*, and has excellent compatibility with the air-dried chicken production system. Furthermore, the strain exhibits low-temperature growth adaptability, textural stability, and food safety, and can be adapted to the entire air-dried chicken production process, resulting in strong controllability of product quality. It also provides multi-formula microbial agents, especially the freeze-dried bacterial powder, which uses a compound protectant formulation, resulting in high strain survival rate and good stability, suitable for industrial storage, transportation, and application needs.

Description

Technical Field

[0001] This invention belongs to the field of microbial screening and food fermentation technology, and in particular relates to the screening of a strain of Lactobacillus rhamnosus and its application in the production of air-dried chicken. Background Technology

[0002] Air-dried meat products are made from fresh meat that has been trimmed, processed, and marinated. This meat is then sun-dried for an extended period in well-ventilated areas to remove some moisture, followed by either cooked or non-cooked processing. Traditional air-dried meat products undergo a complex microbial community evolution during processing, resulting in significant changes in the microbial community structure. During this process, various microorganisms present in the raw meat and environment naturally accumulate, and through competition and adaptation, a relatively stable dominant microbial community system is eventually formed. Lactic acid bacteria, yeasts, and Staphylococcus bacteria during processing help maintain the stability of the physicochemical quality of air-dried meat and promote the enhancement of flavor compounds.

[0003] Currently, there are patent reports that Lactobacillus sakei has been screened from meat products for use in meat product fermentation, and there are also patents that have screened Pediococcus pentosaceus for use in meat product fermentation. However, there are few applied studies on screening dried chicken for use in dried chicken inoculation and fermentation, and there are also very few types of functional strains specifically for the fermentation of dried chicken products, which makes it difficult to support the standardization and large-scale production upgrade of the dried chicken industry.

[0004] Therefore, it is necessary to screen a strain of Lactobacillus rhamnosus and apply it in the production of air-dried chicken to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a screening of Lactobacillus rhamnosus and its application in the production of air-dried chicken to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A strain of Lactobacillus rhamnosus derived from air-dried chicken, classified and named Lactobacillus rhamnosus strain LR01, is deposited at the China Center for Type Culture Collection (CCTCC) with accession number CCTCC NO: M 2026609. It was isolated from an air-dried chicken sample and the deposit date is April 7, 2026.

[0008] This strain is an endogenous microorganism of air-dried chicken, which is naturally adapted to the air-dried chicken production system. It can quickly colonize and form a dominant microbial community during the fermentation process. Compared with exogenous lactic acid bacteria, it is more adaptable to the special process environment of air-dried chicken production and is the core germplasm basis of the entire technology system.

[0009] Preferably, the strain exhibits typical morphological and physiological characteristics of lactic acid bacteria: Gram-positive bacilli arranged in short chains; forming white, round colonies on MRS agar medium with a moist surface, regular edges, and a slightly transparent to opaque state; negative for catalase test; and capable of fermenting glucose to produce acid.

[0010] The above characteristics are the basis for strain classification and identification, and can quickly complete the preliminary identification and purification of strains.

[0011] Preferably, the strain has excellent stress resistance: it can grow normally under sodium chloride conditions with a mass-volume percentage not exceeding 8%, and under sodium nitrite conditions with a concentration not exceeding 150 mg / L.

[0012] This stress tolerance can cover the range of conventional salinity and nitrite addition during the curing stage of air-dried chicken, ensuring that the strain can survive and start growing during the curing stage, avoiding the decline of viable bacteria caused by environmental stress, and ensuring the effectiveness of inoculation and fermentation.

[0013] Preferably, the strain possesses the following core functional activities: nitrite degradation ability with a degradation rate of not less than 40%; and antibacterial activity against Pseudomonas fluorescens with an inhibition zone diameter of not less than 12 mm.

[0014] The strain reduces product residues by degrading nitrites and reduces the risk of spoilage by inhibiting putrefactive bacteria; these two functions work synergistically to improve the food safety level of products.

[0015] Preferably, the strain is process-adaptable: it has the ability to grow at low temperatures of 5~15℃, which is suitable for the low-temperature air-drying process of air-dried chicken; no significant protease and lipase activities were detected under plate testing conditions, which can maintain the stability of product texture and avoid texture deterioration caused by excessive decomposition of meat tissue.

[0016] Preferably, the strain possesses both food safety and traceability: it is negative for amino acid decarboxylase detection, does not produce bioamines, and is highly safe for consumption; its 16S rDNA sequence has a similarity of no less than 99% with the standard sequence of Lactobacillus rhamnosus, making the strain's identity clear and traceable, which facilitates quality control during the production process.

[0017] In a second aspect, the present invention provides a microbial agent for the production of air-dried chicken, containing at least one strain of Lactobacillus rhamnosus cells, fermentation broth or metabolites as described in any one of the following, and the formulation is any one of liquid agent, concentrated bacterial mud or freeze-dried bacterial powder.

[0018] Multi-form microbial agents can be adapted to production scenarios of different scales, solving the pain point that the original slant strains cannot be directly used in industrial production, and providing a product carrier for the large-scale promotion of strains.

[0019] Preferably, when the bacterial agent is a freeze-dried bacterial powder, a compound freeze-drying protectant is used, containing 14% skim milk powder and 2%~10% sucrose, and the freeze-drying survival rate of the strain is not less than 85% under the optimal ratio;

[0020] By combining skim milk powder and sucrose in a synergistic manner, damage to the microorganisms during the freeze-drying process is reduced, ensuring a high number of viable bacteria and storage stability. The wide range of formulations is adaptable to different preparation equipment and production scales, and the process tolerance is stronger.

[0021] Thirdly, the present invention provides a method for screening the above-mentioned Lactobacillus rhamnosus, comprising three steps: sample preparation, separation and purification, and functional screening.

[0022] This method uses traditional air-dried chicken as the isolation source, targets and mines endogenous suitable strains, and locks in the target through multi-level functional screening. The screening process is clear and highly targeted, and can efficiently obtain excellent lactic acid bacteria suitable for air-dried chicken production.

[0023] As a preferred method, the separation and purification were carried out on MRS agar medium and cultured at 30-40℃ for 24-72 hours; the functional screening covered multiple core indicators such as acid production, stress resistance, damage reduction, and antibacterial activity.

[0024] A wide range of culture parameters are available to suit the growth requirements of strains with different performance characteristics, and multi-dimensional screening indicators are used to fully match actual production needs, ensuring that the selected strains can be directly applied to production.

[0025] Fourthly, the present invention provides the application of the above-mentioned Lactobacillus rhamnosus or microbial inoculant in the preparation of fermented air-dried chicken;

[0026] Applying bacterial strains or agents to the production of air-dried chicken can regulate the fermentation microbial community in a targeted manner, accelerate the acid production rate, reduce nitrite residues, inhibit the growth of putrefactive bacteria, improve batch stability, and help the traditional air-dried chicken industry to upgrade to standardization.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] This invention provides a strain of Lactobacillus rhamnosus isolated from traditional air-dried chicken. It has strong salt and nitrite tolerance, can degrade nitrite, inhibit the growth of Pseudomonas fluorescens, and has excellent compatibility with the air-dried chicken production system.

[0029] The strain of this invention has the characteristics of low temperature growth adaptability, textural stability and food safety, and can be adapted to the air-dried chicken production process throughout the entire process, with strong controllability of product quality.

[0030] This invention provides multi-form microbial agents, especially freeze-dried bacterial powder with a compound protectant formulation. Under optimal ratio, the strains have high survival rate and good stability, which is suitable for industrial storage, transportation and application needs.

[0031] This invention provides a targeted screening method with a clear process and reliable results, which can efficiently screen for target functional strains from air-dried chicken.

[0032] The strain of this invention, when applied to the production of air-dried chicken, can promote the generation of volatile flavors, increase the content of esters, effectively improve product quality, and achieve standardized production.

[0033] To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the overall process of the present invention;

[0035] Figure 2 This is a schematic diagram illustrating the acid production of the lactic acid bacteria of the present invention in MRS liquid culture medium;

[0036] Figure 3 This is a schematic diagram illustrating the growth capacity of the lactic acid bacteria of the present invention under different NaCl concentrations.

[0037] Figure 4 This is a schematic diagram illustrating the growth capacity of the lactic acid bacteria of the present invention under different sodium nitrite concentrations.

[0038] Figure 5 This is a schematic diagram illustrating the nitrite degradation ability of the lactic acid bacteria of the present invention.

[0039] Figure 6 This is a schematic diagram illustrating the growth capacity of the lactic acid bacteria of the present invention under different temperature conditions;

[0040] Figure 7 This is a schematic diagram showing the effect of the cryoprotectant of the present invention on the freeze-dried survival rate of LR01. A: Skim milk powder is the freeze-dried protectant; B: Compound freeze-dried protectant (skim milk powder + sucrose).

[0041] Figure 8 This is a morphological diagram of the lactic acid bacteria strain LR01 of the present invention;

[0042] Figure 9 This is a schematic diagram of the phylogenetic tree of the 16S rDNA gene sequence of the lactic acid bacteria LR01 of this invention. Detailed Implementation

[0043] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples. Those skilled in the art can make similar extensions without departing from the spirit of the present invention; therefore, the present invention is not limited to the specific embodiments disclosed below.

[0044] Example 1: Isolation, purification, and morphological and physiological identification of Lactobacillus rhamnosus

[0045] I. Sample Source and Preparation: Traditional air-dried chicken samples were collected from Jingmen, Hubei, China. Under aseptic conditions, 5.0g of air-dried chicken sample was weighed and added to a sterile conical flask containing 45mL of 0.85% (w / v) sterile physiological saline. The mixture was shaken at 180rpm for 5min to prepare a homogenate.

[0046] II. Isolation and Purification: Take 1 mL of homogenate and perform a tenfold serial dilution. Select 200 μL of the bacterial solution with an appropriate dilution and spread it on an MRS agar plate. Incubate at 36°C upside down for 48 h. After the culture is completed, pick a single colony of lactic acid bacteria with typical morphology and repeatedly streak it on an MRS agar plate for purification 3 times until a pure culture strain is obtained. A total of 10 lactic acid bacteria strains were isolated. The strain numbered LR01 was selected as the target strain for subsequent identification.

[0047] III. Morphological observation: The purified LR01 strain was inoculated onto MRS agar plates and cultured at 36℃ for 48 hours. The colony morphology was then observed: the colonies were white and round, with a moist surface, regular edges, and were translucent to opaque. After Gram staining and observation under an oil microscope, the strain was found to be a Gram-positive bacillus arranged in short chains.

[0048] IV. Physiological and Biochemical Identification: Catalase Test: Take a clean glass slide, pick up the LR01 strain cells with an inoculation loop, add 3% H2O21-2 drops. If no bubbles are produced, it is determined to be catalase negative.

[0049] Glucose fermentation test: The LR01 strain was inoculated into glucose fermentation medium vials, with a blank control set up, and cultured at 36℃ for 24h; the results showed that the medium changed from purple to yellow, and there were no bubbles in the Durham tubes, indicating that it could ferment glucose to produce acid but not gas.

[0050] This example verifies that strain LR01 was isolated from a dried chicken sample.

[0051] Example 2: Determination of stress resistance of Lactobacillus rhamnosus

[0052] I. Salt tolerance test: MRS liquid culture medium containing 0%, 2%, 4%, 6% and 8% (w / v) sodium chloride was prepared. The activated LR01 strain was inoculated at 16.67% and cultured at 36℃ for 24 h. The absorbance was measured at 600 nm wavelength and three replicates were set up.

[0053] The results showed that strain LR01 could grow in the range of 0-8% sodium chloride concentration, and maintained good growth even at 8% sodium chloride concentration, demonstrating excellent salt tolerance.

[0054] II. Nitrite tolerance test: MRS liquid culture medium containing 0 mg / L, 50 mg / L, 100 mg / L and 150 mg / L sodium nitrite was prepared. LR01 strain was inoculated at an inoculum of 16.67% and cultured at 36℃ for 24 h. The absorbance at 600 nm was then measured.

[0055] The results showed that strain LR01 could grow in the range of sodium nitrite concentration from 0 to 150 mg / L, and maintained strong growth ability at a sodium nitrite concentration of 150 mg / L, demonstrating excellent tolerance to nitrite stress.

[0056] This example verifies that the strain can grow normally under conditions of no more than 8% sodium chloride and no more than 150 mg / L sodium nitrite.

[0057] Example 3: Functional activity determination of Lactobacillus rhamnosus

[0058] I. Determination of nitrite degradation capacity: MRS liquid culture medium containing 100 mg / L sodium nitrite was prepared, and LR01 bacterial culture was inoculated at an inoculum of 16.67% and cultured at 36℃ for 24 h; the residual amount of nitrite was determined by the naphthylethylenediamine hydrochloride colorimetric method in GB 5009.33-2016 "National Food Safety Standard - Determination of Nitrite and Nitrate in Food", and the degradation rate was calculated.

[0059] Determination steps: Centrifuge the fermentation broth at 8000 rpm for 1 min, take 2.5 mL of supernatant, add 0.5 mL of p-aminobenzenesulfonic acid solution (4 g / L), vortex mix, and let stand in the dark for 5 min; add 0.25 mL of naphthylethylenediamine hydrochloride solution (2 g / L) and 1.75 mL of ultrapure water, vortex mix, and let stand in the dark for 10 min; measure the absorbance at a wavelength of 538 nm and calculate the degradation rate.

[0060] The results showed that the degradation rate of nitrite by strain LR01 was 43.76%, which is not less than 40%, indicating excellent degradation performance.

[0061] II. Antibacterial Activity Assay: The antibacterial activity against *Pseudomonas fluorescens* was determined using the agar well diffusion method; activated *Pseudomonas fluorescens* bacterial suspension (10... 6 The bacterial suspension (CFU / mL) was evenly spread on a nutrient agar plate, and 100 μL LRR01 bacterial suspension was added after punching holes. After diffusion at 4℃ for 6 h, the plate was incubated at 36℃ for 48 h, and the diameter of the inhibition zone was measured.

[0062] The results showed that the inhibition zone diameter of strain LR01 against Pseudomonas fluorescens was 12.33 mm, which is not less than 12 mm, indicating a significant antibacterial effect.

[0063] This example verifies that the nitrite degradation rate of the strain is not less than 40%, and the diameter of the inhibition zone against Pseudomonas fluorescens is not less than 12 mm.

[0064] Example 4: Evaluation of the process adaptability and safety of Lactobacillus rhamnosus

[0065] I. Determination of low-temperature growth ability: The LR01 strain was inoculated into MRS liquid medium at an inoculum of 16.67% and cultured at 5℃, 15℃, 25℃ and 35℃ for 24h respectively. The absorbance at 600nm was measured.

[0066] The results showed that the LR01 strain still had growth activity at 5℃ and could grow in the low temperature range of 5~15℃, making it suitable for the low temperature air-drying process of air-dried chicken.

[0067] II. Enzyme activity assay: Protease activity: LR01 strain was inoculated on MRS solid medium supplemented with 5% skim milk powder and cultured at 36℃ for 48 h. The clear zone around the colony was observed. The results showed that there was no obvious clear zone around the colony, and no significant protease activity was detected under the plate test conditions.

[0068] Lipase activity: LR01 strain was inoculated on MRS solid medium supplemented with glycerol tribanilate and cultured at 36℃ for 48 h. The clear zone around the colonies was observed. The results showed that there was no obvious clear zone around the colonies, and no significant lipase activity was detected under the plate detection conditions.

[0069] III. Amino acid decarboxylase activity assay: Under aseptic conditions, 1 mL of LR01 bacterial culture was inoculated into the amino acid decarboxylase test tube and the control tube, covered with sterile liquid paraffin to isolate them from air, and incubated at 36°C for 48 h; the results showed that the control tube and the test tube were both yellow, indicating that the amino acid decarboxylase was negative.

[0070] IV. Molecular biological identification: Genomic DNA of strain LR01 was extracted and PCR amplified using universal 16S rDNA primers. The amplified products were sequenced and the sequences were compared. The results showed that the 16S rDNA sequence of strain LR01 had 100% similarity to the standard sequence of Lactobacillus rhamnosus, and was not less than 99%. It was identified as Lactobacillus rhamnosus.

[0071] Example 5: Preparation of microbial inoculants

[0072] I. Preparation of liquid inoculum: After activating the LR01 strain by streaking, it was inoculated into MRS liquid seed medium and cultured at 36℃ for 18h to obtain seed liquid; then it was transferred to fermentation medium at a 3% inoculation rate and cultured at 36℃ for 18h. After fermentation, it became liquid inoculum, which could be directly used for small-scale production inoculation.

[0073] II. Preparation of concentrated bacterial sludge: Centrifuge the above fermentation broth at 4℃ and 8000g for 15min, discard the supernatant and collect the bacterial precipitate; wash twice with 0.1mol / L sterile PBS buffer, centrifuge again to obtain concentrated bacterial sludge, which can be used for activation in medium-sized production sites.

[0074] III. Preparation of freeze-dried bacterial powder: The concentrated bacterial mud and freeze-drying protectant are thoroughly mixed at a mass ratio of 1:2, packaged, pre-frozen at -80℃ for 12 hours, and then vacuum freeze-dried for 24 hours to obtain freeze-dried bacterial powder, which is suitable for long-term storage and long-distance transportation.

[0075] This embodiment verifies that the microbial agent includes forms such as microbial cells and fermentation broth, covering three dosage forms: liquid microbial agent, concentrated microbial mud, and freeze-dried microbial powder.

[0076] Example 6: Optimization of the compound protective agent for freeze-dried bacterial powder

[0077] The freeze-drying survival rate was used as an evaluation index to optimize the formulation of the freeze-drying protectant.

[0078] I. Optimization of skim milk powder concentration: Skim milk powder solutions of 2%, 6%, 10%, 14%, and 18% (mass-volume percentage) were prepared, sterilized at 115℃ for 20 min, and then used as a single protectant. After mixing with bacterial sludge, the solutions were freeze-dried, and the freeze-dried survival rate of strain LR01 was determined. The results showed that the freeze-dried survival rate was highest when the skim milk powder concentration was 14%.

[0079] II. Optimization of composite protectant: Based on 14% skim milk powder, 2%, 4%, 6%, 8%, and 10% sucrose were added to form composite protectants, and the freeze-drying survival rate was also measured. The results showed that when the composite protectant contained 14% skim milk powder + 8% sucrose, the freeze-drying survival rate of strain LR01 was the highest, at 85.76%.

[0080] Within the ratio range of 14% skim milk powder to 2%~10% sucrose, the strain can maintain a high freeze-drying survival rate, with the survival rate reaching over 85% under the optimal ratio, meeting the requirements of industrial production.

[0081] Example 7: Validation of the targeted screening method for Lactobacillus rhamnosus

[0082] Using traditional air-dried chicken from Jingmen, Hubei Province as the sample source, targeted screening was carried out according to a three-level process of "sample preparation - separation and purification - functional screening".

[0083] S1. Sample preparation: Under aseptic conditions, take air-dried chicken samples, mix them with sterile physiological saline and shake to prepare a homogenate.

[0084] S2. Separation and purification: The homogenate was serially diluted tenfold and spread on MRS agar medium. It was incubated at 36℃ for 48h. Single colonies were picked and repeatedly streaked for purification to obtain 10 pure culture lactic acid bacteria.

[0085] S3. Functional screening: The acid production capacity, salt tolerance, nitrite tolerance, nitrite degradation capacity, and antibacterial activity were measured in sequence. Finally, Lactobacillus rhamnosus LR01 with the best comprehensive performance was selected.

[0086] In actual screening, the culture temperature can be adjusted in the range of 30~40℃ and the culture time can be adjusted in the range of 24~72h, and the target strain can be obtained in both cases. At least one functional screening index can be selected for initial screening as needed, and then the indexes can be gradually added for secondary screening.

[0087] Example 8: Application of Lactobacillus rhamnosus in air-dried chicken production

[0088] This embodiment is an exemplary application scheme based on the in vitro performance of the strain, used to illustrate the specific use of the strain in the production of air-dried chicken.

[0089] I. Raw material processing: Select fresh and healthy broiler chickens, clean and cut them into pieces, and drain them for later use.

[0090] 2. Pickling and Inoculation: Prepare the pickling solution according to the formula, which contains salt, sodium nitrite, spices and other ingredients; after activating the freeze-dried Lactobacillus rhamnosus powder with sterile physiological saline, add it to the pickling solution at an appropriate inoculation amount and stir evenly.

[0091] 3. Marinating and Fermentation: Soak the chicken in the marinade and marinate at 4℃ for 24 hours to allow the salt and bacteria to penetrate evenly; after marinating, remove the chicken and place it in an environment of 20℃ and 75% relative humidity for 48 hours to ferment.

[0092] IV. Air-drying and maturation: After fermentation, transfer the chicken to an air-drying room at 15℃ and 65% relative humidity for 7-10 days until the moisture content reaches the standard, and obtain the fermented and air-dried chicken product.

[0093] Based on the strain's proven salt tolerance, nitrite tolerance, nitrite degradation, inhibition of spoilage bacteria, and low-temperature growth, it is expected that when applied to air-dried chicken production, this strain can reduce nitrite residues in the product, inhibit the proliferation of spoilage bacteria such as Pseudomonas fluorescens, and improve the batch quality stability of the product.

[0094] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A strain of Lactobacillus rhamnosus, characterized in that, The strain was classified and named Lactaseibacillus rhamnosus strain LR01, and is deposited at the China Center for Type Culture Collection (CCTCC) with accession number CCTCC NO: M 2026609; the strain was isolated from a dried chicken sample.

2. The *Lactobacillus rhamnosus* strain according to claim 1, characterized in that, The strain is a Gram-positive bacillus arranged in short chains; it forms white, round colonies on MRS agar medium with a moist surface, regular edges, and a slightly transparent to opaque state; it is negative for catalase and can ferment glucose to produce acid.

3. The *Lactobacillus rhamnosus* strain according to claim 1, characterized in that, The strain can grow normally under conditions where the sodium chloride concentration is no higher than 8% by volume, and under conditions where the sodium nitrite concentration is no higher than 150 mg / L.

4. The *Lactobacillus rhamnosus* strain according to claim 1, characterized in that, The strain has the ability to degrade nitrite, with a nitrite degradation rate of not less than 40%; it also has antibacterial activity against Pseudomonas fluorescens, with an inhibition zone diameter of not less than 12 mm.

5. The *Lactobacillus rhamnosus* strain according to claim 1, characterized in that, The strain exhibits growth capacity at low temperatures of 5–15°C; no significant protease or lipase activity was detected under plate testing conditions.

6. The *Lactobacillus rhamnosus* strain according to claim 1, characterized in that, The amino acid decarboxylase activity of the strain was negative; the 16S rDNA gene sequence of the strain had a similarity of not less than 99% with the standard sequence of Lactobacillus rhamnosus.

7. A microbial inoculant for the production of air-dried chicken, characterized in that, The agent contains at least one strain of *Lactobacillus rhamnosus* cells, fermentation broth, or metabolites as described in any one of claims 1 to 6; the agent is in the form of any one of liquid agent, concentrated bacterial sludge, or freeze-dried bacterial powder.

8. The microbial agent for air-dried chicken production according to claim 7, characterized in that, When the bacterial agent is a freeze-dried bacterial powder, a compound freeze-drying protectant is used; the compound freeze-drying protectant contains 14% by weight and volume of skim milk powder and 2% to 10% of sucrose, and the freeze-drying survival rate of the strain is not less than 85% under the optimal ratio.

9. A method for screening *Lactobacillus rhamnosus* according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Sample preparation: Take air-dried chicken samples and add sterile physiological saline under aseptic conditions to prepare a homogenate. S2. Separation and purification: The homogenate was serially diluted, spread on MRS agar medium, and incubated at 30-40℃ for 24-72h. Single colonies were picked and purified by streak to obtain pure culture strains. S3. Functional screening: The purified strain is screened for at least one of the following indicators: acid production capacity, salt tolerance, nitrite tolerance, nitrite degradation capacity, and antibacterial activity, to obtain the target Lactobacillus rhamnosus.

10. The use of any one of the strains of Lactobacillus rhamnosus according to claims 1 to 6 or any one of the microbial agents according to claims 7 to 8 in the preparation of fermented air-dried chicken.

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