Lactococcus lactis, compound dairy starter and application thereof

Abstract

The application provides a lactococcus lactis, a compound dairy product fermenting agent and application thereof, the lactococcus lactis has the functions of sweetening and flavoring, and is classified and named as lactococcus lactis Lactococcus lactis The strain has significant sweetening and flavoring characteristics, exhibits strong fructo-oligosaccharide (FOS) and acetaldehyde generation capacity in the fermentation process, can provide endogenous sweet taste and aroma flavor substances for fermented milk, and significantly improves the taste and flavor of the fermented milk.The lactococcus lactis C15 is compounded with lactobacillus casei AU9077 and lactobacillus rhamnosus AU9260 to obtain a compound dairy product fermenting agent, which has a significant synergistic effect in releasing fructo-oligosaccharide, continuously provides stable endogenous sweetening effect for fermented milk, improves the quality of dairy products, and meets the needs of consumers for healthy and flavored dairy products.

Description

Technical Field

[0001] This invention belongs to the field of microbial fermentation technology, and specifically relates to a strain of lactic acid lactococcus, a compound dairy fermentation agent and its application. Background Technology

[0002] In the field of food fermentation, especially in the dairy fermentation industry, improving the flavor and texture of products has always been a key research focus and a primary goal. Consumers have increasingly higher demands for the flavor of fermented dairy products such as yogurt, expecting a richer, more intense, and unique sensory experience. This has prompted researchers and companies to continuously explore new fermentation strains and technologies.

[0003] Currently, a wide variety of lactic acid bacteria are commonly used in yogurt fermentation, but they have certain limitations in imparting aroma and sweetness to yogurt. Some traditional lactic acid bacteria fermentation produces relatively simple flavor compounds, only providing basic yogurt flavor and failing to meet consumers' demands for diverse flavors. For example, the common combination of Lactobacillus bulgaricus and Streptococcus thermophilus mainly produces conventional metabolites such as lactic acid. While this gives yogurt a basic sour taste and fermented aroma, it lacks richness and uniqueness in aroma, failing to create a multi-layered flavor experience.

[0004] In terms of sweetening, current technologies mainly rely on adding exogenous sweeteners, such as sucrose, glucose, and high-fructose corn syrup. However, the extensive use of these sweeteners not only increases the product's calorie content but may also pose health risks, such as causing blood sugar fluctuations and obesity, which does not align with current consumer trends towards healthier foods. Research on the natural production of sweet substances through fermentation is still insufficient. The reported fermentation strains with sweetening effects are very limited, and their sweetening capabilities are weak, making it difficult to meet the needs of practical production applications.

[0005] Regarding flavor enhancement, although some lactic acid bacteria can produce aroma compounds such as acetaldehyde and diacetyl, the yield is low and insufficient to fully enhance the aroma of yogurt. Furthermore, the types and proportions of aroma compounds produced by different lactic acid bacteria vary significantly, leading to unstable product aromas and making standardized production difficult. For example, some undesirable flavor compounds produced by lactic acid bacteria during fermentation, such as bitterness and rancidity, can affect the overall flavor quality of yogurt. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a strain of Lactococcus lactis, a compound dairy fermentation agent, and their applications. The Lactococcus lactis C15 strain possesses the ability to endogenously produce high levels of fructooligosaccharides (FOS) and acetaldehyde. When used in yogurt fermentation, it can endogenously increase the sweetness and aroma of the product, reduce the addition of exogenous sugars and flavoring substances, and meet the low-sugar, low-additive requirements of healthy foods.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] The first aspect of the present invention provides a strain of Lactococcus lactis C15 that has both sweetening and aroma-enhancing effects. The strain is classified as Lactococcus lactis and is deposited at the China General Microbiological Culture Collection Center (CGMCC) with accession number CGMCC No. 18950 and accession date of November 13, 2019.

[0009] The second aspect of the present invention provides a compound dairy fermentation agent that has both sweetening and aroma-enhancing effects, the compound dairy fermentation agent comprising Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 as described in the first aspect;

[0010] The Lactobacillus casei AU9077 strain is classified and named Lactobacillus casei. This strain is deposited at the China General Microbiological Culture Collection Center (CGMCC) with accession number CGMCC No. 21663 and deposit date of January 18, 2021.

[0011] The Lactobacillus rhamnosus strain AU9260 is classified and named Lactobacillus rhamnosus. This strain is deposited at the China General Microbiological Culture Collection Center (CGMCC) with accession number CGMCC No. 21662 and deposit date of January 18, 2021.

[0012] Furthermore, the ratio of live bacteria content of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 in the dairy fermentation agent is 2:1~2:1~2.

[0013] A third aspect of this invention provides a method for preparing the compound dairy starter described in the second aspect, comprising the following steps:

[0014] (1) Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 were activated and cultured in liquid medium, the bacterial cells were collected by centrifugation, resuspended in reconstituted skim milk, the effective live bacteria concentration was adjusted and mixed with a protectant and freeze-dried to obtain three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 respectively;

[0015] (2) The three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 are mixed in proportion to obtain the compound dairy fermentation agent.

[0016] Furthermore, the protective agent includes mannitol, trehalose, reconstituted skim milk, dimethyl sulfoxide, and polyvinylpyrrolidone.

[0017] The fourth aspect of this invention provides the application of Lactococcus lactis C15 as described in the first aspect, or the compound dairy starter as described in the second aspect, or the compound dairy starter prepared by the method described in the third aspect, in the preparation of starter or fermented dairy products.

[0018] The fifth aspect of the present invention provides an endogenous sweetening and flavoring fermented dairy product, wherein the raw materials for preparing the fermented dairy product include Lactococcus lactis C15 as described in the first aspect, or the compound dairy starter as described in the second aspect, or the compound dairy starter prepared by the method described in the third aspect.

[0019] Furthermore, the dairy products include yogurt, cheese, milk wine, and dairy beverages.

[0020] Information on strain preservation:

[0021] Lactococcus lactis C15, classified as Lactococcus lactis, is deposited at the China General Microbiological Culture Collection Center (CGMCC), located at No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, 100101, China, with accession number CGMCC No. 18950 and deposit date of November 13, 2019.

[0022] Lactobacillus casei AU9077, classified as Lactobacillus casei, is deposited at the China General Microbiological Culture Collection Center (CGMCC), located at No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, 100101, China, with accession number CGMCC No. 21663 and deposit date of January 18, 2021.

[0023] Lactobacillus rhamnosus AU9260, classified as Lactobacillus rhamnosus, is deposited at the China General Microbiological Culture Collection Center (CGMCC), located at No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, 100101, China, with accession number CGMCC No. 21662 and deposit date of January 18, 2021.

[0024] The advantages of this invention compared to the prior art are as follows:

[0025] 1. This invention screened and obtained a strain of *Lactococcus lactis* C15 with unique metabolic characteristics. This strain possesses the dual function of efficiently synthesizing fructooligosaccharides (FOS) and acetaldehyde. Experimental results showed that, under optimized fermentation conditions, *Lactococcus lactis* C15 exhibited superior FOS synthesis capacity, with a yield reaching 57.7 g / L, significantly higher than conventional lactic acid bacteria strains. Simultaneously, this strain can also simultaneously produce the characteristic flavor compound acetaldehyde, with a maximum yield of 16.2 mg / L. This provides a unique technical advantage for dairy product fermentation, making it of significant application value in the development of functional dairy products.

[0026] 2. This invention scientifically combines Lactococcus lactis C15 with Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 to develop a compound dairy starter. Experimental verification shows that this compound starter has a significant synergistic effect in releasing fructooligosaccharides, continuously providing a stable endogenous sweetening effect for fermented milk, improving the quality of dairy products while meeting consumers' demands for healthy and flavorful dairy products. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0028] Figure 1 Microscopic image of Lactococcus lactis cells after Gram staining with C15;

[0029] Figure 2 The results of FOS yield at different fermentation times for Lactococcus lactis C15 are shown. Detailed Implementation

[0030] The embodiments described are provided to better illustrate the present invention, but are not intended to limit the scope of the invention to the embodiments described. Therefore, non-essential improvements and adjustments made to the embodiments by those skilled in the art based on the above description are still within the scope of protection of the present invention.

[0031] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0032] The present invention will be described in detail below through embodiments. It should be understood that the following embodiments are only used to exemplify and further explain and illustrate the content of the present invention, and are not intended to limit the present invention.

[0033] In the following examples, Lactococcus lactis C15, Lactobacillus casei AU9077, Lactobacillus rhamnosus AU9260, and Lactobacillus plantarum W-4 (accession number CGMCC No. 18260, accession date: July 22, 2019) are all strains preserved by Hunan Agricultural University.

[0034] The raw milk (fat content 3.66%, protein content 3.14%, total solids content 12.27%) was sourced from Hunan New Hope Nanshan Liquid Emulsion Co., Ltd., and was stored at 4℃ and used within one day.

[0035] Modified MRS liquid medium: 10 g peptone, 5 g yeast extract, 15 g glucose, 15 g sucrose, 10 g beef extract. 2g, diammonium citrate 2g, 0.58 g, 0.25 g of anhydrous sodium acetate, 5 g of Tween 80, 1 mL of cysteine ​​hydrochloride, 0.5 g of distilled water to a final volume of 1 L, adjust pH to 6.5, and sterilize at 121°C for 15 min.

[0036] The detection methods involved in the following embodiments include:

[0037] The determination of fructooligosaccharide content was performed according to the high-performance liquid chromatography (HPLC) method in GB / T 23528.2-2021 "Quality Requirements for Oligosaccharides Part 2: Fructooligosaccharides". Specific conditions included: LC-6A HPLC system (Shimadzu, Japan), and an Agilent Zorbax amino column. The column (5 μm, 4.6 mm × 250 mm × 208 mm, Agilent) was used, with acetonitrile and water (75:25 v / v) as the mobile phase. The injection volume was 10 μl, the column temperature was 35 °C, and the detector temperature was 40 °C. The flow rate was 1.0 ml / min. The content of each sugar component in the product was determined based on the retention time and peak area.

[0038] The method for determining the acetaldehyde content in fermented yogurt is as follows:

[0039] (1) Sample pretreatment: Take the sample to be tested, add an equal volume of 16% TCA solution, mix well, centrifuge at 3500g for 10min, and take the clear supernatant for later use.

[0040] (2) Accurately measure 5.00 ml of 1% concentration The solution was placed in a 250ml Erlenmeyer flask, and 25ml of the supernatant from the treated sample was added and shaken well. After standing at room temperature for 1 hour, 1ml of 1% starch solution was added, and titrated with 0.1mol / L iodine solution until near the endpoint, then titrated with 0.01mol / L iodine solution until the endpoint (a pale blue-purple color that does not fade after 30 seconds) was reached. No further titration was performed. Then, 20ml of 1mol / L iodine solution was added. Mix the solution by shaking for 0.5 min (until the solution turns blue), then add 0.01 mol / L iodine (1 / 2) Titrate the standard solution to the endpoint (pale blue-purple), record the volume of standard iodine solution consumed, and perform a blank test at the same time.

[0041] Perform at least three parallel titrations for each sample and take the average value.

[0042] The formula for calculating acetaldehyde content is:

[0043]

[0044] In the formula, Consumption for blank titration Volume (mL) of standard solution Consumption for sample titration The volume (mL) of the standard solution, C is 1 / 2 Concentration of the standard solution (mol / L).

[0045] Example 1

[0046] A strain was isolated and purified from naturally fermented yogurt, and its morphology was observed under a microscope as follows. Figure 1 As shown, further analysis and identification confirmed that the strain was *Lactococcus lactis*, named *Lactococcus lactis* C15, and sent to a preservation institution for strain depositation. Preservation institution: China General Microbiological Culture Collection Center (CGMCC); Address: No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, 100101, China; Accession number: CGMCC No. 18950; Deposit date: November 13, 2019.

[0047] Culture of Lactococcus lactis C15 and detection of fructooligosaccharides:

[0048] (1) Activation of strains: Inoculate Lactococcus lactis C15 into MRS agar plates and incubate at 37°C for 24 hours;

[0049] (2) Seed culture: The activated strain was inoculated into the modified MRS liquid medium and cultured at 37°C and 180 rpm for 36 h with shaking.

[0050] (3) Shake flask culture: Inoculate the seed culture into the fermentation medium with an inoculum of 2% and culture at 37℃ and 180rpm for 48 h;

[0051] (4) Detection of fructooligosaccharides: The content of FOS in the fermentation broth was detected by HPLC.

[0052] like Figure 2 The results showed that the FOS yield of the strain reached 57.7 g / L after 36 h of fermentation. As shown in the table below, the acetaldehyde content in the fermentation broth of the strain reached 16.2 mg / L after 36 h of fermentation.

[0053]

[0054] This embodiment provides a dairy starter culture using *Lactococcus lactis* C15. The preparation method is as follows: *Lactococcus lactis* C15 is activated on MRS agar plates, then inoculated (2%) into a modified MRS liquid medium for scale-up culture (37℃, 36 h). The cells are collected by centrifugation, resuspended in reconstituted skim milk, and the effective viable cell concentration is adjusted to [value missing]. The bacterial culture was obtained by lyophilizing CFU / mL, and then the culture was freeze-dried to obtain Lactococcus lactis C15 starter culture (C15).

[0055] Example 2

[0056] This embodiment provides a compound dairy starter culture, comprising: Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260, and the preparation method includes the following steps:

[0057] (1) Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 were activated on MRS agar plates, and then inoculated (2%) into modified MRS liquid medium for scale-up culture (37℃, 36h). The cells were collected by centrifugation, resuspended in reconstituted skim milk, and the effective viable cell concentration was adjusted to [value missing]. The bacterial solution was obtained by freeze-drying to produce three bacterial powders: Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260.

[0058] (2) The three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 are mixed in a ratio of 2:1:1 to obtain the compound dairy fermentation agent (C15+AU9077+AU9260).

[0059] Comparative Example 1

[0060] This comparative example provides a compound dairy starter, comprising: Lactococcus lactis C15 and Lactobacillus casei AU9077.

[0061] The preparation method of the compound dairy fermentation agent is the same as that in Example 2, except that in step (2), the two bacterial powders of Lactococcus lactis C15 and Lactobacillus casei AU9077 are mixed in a ratio of 1:1 to obtain the compound dairy fermentation agent (C15+AU9077).

[0062] Comparative Example 2

[0063] This comparative example provides a compound dairy starter, comprising: Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus plantarum W-4.

[0064] The preparation method of the compound dairy fermentation agent is the same as that in Example 2, except that in step (2), the three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus plantarum W-4 are mixed in a ratio of 2:1:1 to obtain the compound dairy fermentation agent (C15+AU9077+W-4).

[0065] Comparative Example 3

[0066] This comparative example provides a compound dairy starter, comprising: Lactococcus lactis C15, Lactobacillus casei AU9077, Lactobacillus rhamnosus AU9260, and Lactobacillus plantarum W-4.

[0067] The preparation method of the compound dairy starter is the same as in Example 2, except that in step (2), four bacterial powders, namely Lactococcus lactis C15, Lactobacillus casei AU9077, Lactobacillus rhamnosus AU9260 and Lactobacillus plantarum W-4, are mixed in a ratio of 2:1:1:1 to obtain the compound dairy starter (C15+AU9077+AU9260+W-4).

[0068] Example 3

[0069] This embodiment provides an endogenous sweetened and flavored yogurt, prepared by the following method:

[0070] (1) Raw material preparation: Raw milk is processed through sterilization separation, defatting, degassing, standardization, pasteurization and membrane treatment system to obtain standardized raw milk with a protein content of 3.2 / 100g.

[0071] (2) Preheating and homogenization: Add sucrose (3%) to standardized raw milk, heat to 55-65℃, and then homogenize using a high-pressure homogenizer. The homogenization pressure is set to 15-25 MPa.

[0072] (3) Sterilization: The homogenized milk is rapidly heated to 90-95℃ and held for 5-10 minutes for high-temperature sterilization. After sterilization, the milk is rapidly cooled to 40-43℃.

[0073] (4) Inoculation with fermenting agent: The activated lactococcus lactis C15 fermenting agent described in Example 1 is inoculated into the cooled milk at an inoculation amount of 0.2%, and stirred thoroughly to ensure that the fermenting agent is evenly dispersed.

[0074] (5) Constant temperature fermentation: After inoculation, the milk is divided into fermentation containers, sealed, and placed in a constant temperature incubator for fermentation at 37-42℃ for 6-12 hours. During the fermentation process, the acidity and pH value of the yogurt can be monitored regularly. When the pH value of the yogurt drops to 4.5-4.6, fermentation is terminated.

[0075] Example 4

[0076] This embodiment provides another type of endogenous sweetened and flavored yogurt. The preparation method is basically the same as that in Example 3, except that the starter in step (4) is the compound dairy starter (C15+AU9077+AU9260) described in Example 2.

[0077] Comparative Example 4

[0078] This comparative example provides a yogurt, the preparation method of which is basically the same as that of Example 3, except that the starter in step (4) is the compound dairy starter (C15+AU9077) described in Comparative Example 1.

[0079] Comparative Example 5

[0080] This comparative example provides a yogurt, and the preparation method is basically the same as that of Example 3. The only difference is that the starter in step (4) is the compound dairy starter (C15+AU9077+W-4) described in Comparative Example 2.

[0081] Comparative Example 6

[0082] This comparative example provides a yogurt, and the preparation method is basically the same as that of Example 3. The only difference is that the starter in step (4) is the compound dairy starter (C15+AU9077+AU9260+W-4) described in Comparative Example 3.

[0083] Test case

[0084] This test case focuses on the changes in the content of fructooligosaccharides (FOS) and acetaldehyde in yogurt products prepared with different starter cultures. The endogenous sweetened and flavored yogurts prepared in Examples 3 and 4, as well as the yogurts prepared in Comparative Examples 4 to 6, were measured. The content of fructooligosaccharides (FOS) and acetaldehyde in each product at the fermentation endpoint was determined.

[0085] The results are shown in Tables 2 and 3, respectively.

[0086]

[0087]

[0088] The above test results show that, regarding the fructooligosaccharide (FOS) content, the yogurt from Example 3 (fermented alone by Lactococcus lactis C15) contained 41.87 ± 1.22 g / L. In Example 4, the combined fermentation of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 significantly increased the FOS content to 59.17 ± 0.74 g / L. This indicates that the three strains synergistically promoted the retention or production of FOS, possibly due to their metabolic processes reducing FOS consumption or activating certain FOS-producing pathways. The increased FOS content not only reflects the potential synergistic effect among the three strains but also has significant implications from the perspective of FOS production during fermentation and providing endogenous sweetness. On the one hand, fructooligosaccharides (FOS), as a functional oligosaccharide, are not easily broken down by digestive enzymes in the human body, providing yogurt with a natural sweetness and reducing reliance on traditional sweeteners such as added sucrose, thus aligning with current consumer demand for healthy, low-sugar foods. On the other hand, the fermentation process of these strains to produce FOS not only enriches the sources of sweetness in yogurt but also enhances its functionality. FOS has probiotic functions such as regulating gut microbiota and promoting the proliferation of bifidobacteria, making yogurt more nutritionally and healthily superior.

[0089] The fructooligosaccharide content in the yogurt of Comparative Example 4 (Lactococcus lactis C15 + Lactobacillus casei AU9077) was 47.21 ± 0.79 g / L, which was significantly lower than that of Example 4 but higher than that of Example 3. This indicates that co-fermentation of Lactobacillus casei AU9077 and Lactococcus lactis C15 has a certain effect on increasing the fructooligosaccharide content. However, the absence of Lactobacillus rhamnosus AU9260 weakened the synergistic effect, and its ability to provide oligosaccharides in fermentation products and endogenous sweetness was relatively weak.

[0090] The fructooligosaccharide (FOS) content in the yogurt of Comparative Example 5 (Lactococcus lactis C15 + Lactobacillus casei AU9077 + Lactobacillus plantarum W-4) was only 24.87±1.10 g / L, which was significantly lower than that of other groups. This indicates that the addition of Lactobacillus plantarum W-4 greatly reduced the FOS content. It is possible that Lactobacillus plantarum W-4 consumed a large amount of FOS for its own growth and metabolism, thereby reducing the amount of FOS that could be retained in the yogurt. This not only reduced the ability of the yogurt to provide endogenous sweetness through FOS, but also weakened the probiotic function of the yogurt.

[0091] The fructooligosaccharide content in the yogurt of Comparative Example 6 (Lactococcus lactis C15 + Lactobacillus casei AU9077 + Lactobacillus rhamnosus AU9260 + Lactobacillus plantarum W-4) was 48.77±0.72 g / L, which was lower than that of Example 4, but higher than that of Comparative Example 5. This indicates that Lactobacillus rhamnosus AU9260 alleviated the consumption of fructooligosaccharides by Lactobacillus plantarum W-4 to some extent.

[0092] Regarding acetaldehyde content, the acetaldehyde content of the yogurt in Example 3 was 12.89±0.12 mg / L. In Example 4, the acetaldehyde content increased to 18.47±0.17 mg / L, indicating that the combined microbial agents promoted acetaldehyde production and enriched the yogurt flavor. The acetaldehyde content of Comparative Example 4 was 15.74±0.09 mg / L, falling between Examples 3 and 4, further demonstrating that the co-fermentation of *Lactobacillus casei* AU9077 and *Lactococcus lactis* C15 can increase acetaldehyde content, but the effect is not as good as the addition of *Lactobacillus rhamnosus* AU9260. The acetaldehyde contents of Comparative Examples 5 and 6 were 13.33±0.10 mg / L and 13.27±0.22 mg / L, respectively, with no significant difference from Example 3, indicating that the addition of *Lactobacillus plantarum* W-4 inhibited acetaldehyde production, and even with the presence of *Lactobacillus rhamnosus* AU9260, the acetaldehyde production could not be restored.

[0093] In summary, the starter culture combination in Example 4—Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260—performed the best results in increasing the content of fructooligosaccharides and acetaldehyde in yogurt and optimizing yogurt quality, providing a useful reference for developing high-quality, healthy yogurt products. The negative impact of Lactobacillus plantarum W-4 on the content of fructooligosaccharides and acetaldehyde in Comparative Examples 5 and 6 warrants attention. Future research could explore optimizing the strain ratio or screening for other strains to balance the composition and flavor of yogurt, providing stronger theoretical and practical evidence for optimizing yogurt starter cultures.

[0094] Finally, it should be noted that the above description is only used to illustrate the technical solutions of the present invention and is 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 present invention.

Claims

1. A strain of Lactococcus lactis C15 that has both sweetening and aroma-enhancing effects, characterized in that, The lactococcus is classified and named as Lactococcus lactis ( Lactococcus lactis (The accession number is CGMCC No. 18950).

2. A compound dairy starter with both sweetening and aroma-enhancing effects, characterized in that, The compound dairy starter includes Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 as described in claim 1; The Lactobacillus casei AU9077 is classified and named Lactobacillus casei ( Lactobacillus casei (The accession number is CGMCC No. 21663;) The Lactobacillus rhamnosus AU9260 was classified and named Lactobacillus rhamnosus ( Lactobacillus rhamnosus (The accession number is CGMCC No. 21662).

3. The compound dairy starter according to claim 2, characterized in that, The ratio of live bacteria content of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 in the dairy fermentation agent is 2:1~2:1~2.

4. A method for preparing the compound dairy starter as described in claim 2 or 3, characterized in that, The preparation method includes the following steps: (1) Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 were activated and cultured in liquid medium, the bacterial cells were collected by centrifugation, resuspended in reconstituted skim milk, the effective live bacteria concentration was adjusted and mixed with a protectant and freeze-dried to obtain three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 respectively; (2) The three bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 are mixed in proportion to obtain the compound dairy fermentation agent.

5. The preparation method according to claim 4, characterized in that, The protective agents include mannitol, trehalose, reconstituted skim milk, dimethyl sulfoxide, and polyvinylpyrrolidone.

6. The use of Lactococcus lactis C15 as described in claim 1 in the preparation of starter culture or fermented dairy products.

7. The application of the compound dairy starter as described in claim 2 or 3, or the compound dairy starter prepared by the method of claim 4 or 5, in the preparation of fermented dairy products.

8. A fermented dairy product with endogenous sweetening and flavor enhancement, characterized in that, The raw materials for preparing the fermented dairy products include Lactococcus lactis C15 as described in claim 1, or the compound dairy starter as described in claim 2 or 3, or the compound dairy starter prepared by the method of claim 4 or 5.

9. The fermented dairy product according to claim 8, characterized in that, The dairy products include yogurt and cheese.

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