Lactococcus lactis, compound dairy leavening agent and application of compound dairy leavening agent

Abstract

The invention provides lactococcus lactis, a composite dairy leavening agent and application of the lactococcus lactis, the lactococcus lactis has the effects of increasing sweetness and aroma, the classification name of the lactococcus lactis is lactococcus lactis, the preservation number of the lactococcus lactis is CGMCC No.18950, and the preservation date of the lactococcus lactis is November 13, 2019. The strain has the remarkable sweetness increasing and aroma increasing characteristics, and in the fermentation process, the content of the lactococcus lactis in the fermentation process is increased, and the content of the composite dairy leavening agent is increased. Strong FOS (fructo-oligosaccharide) and acetaldehyde generating capacity is shown, endogenous sweet and fragrant flavor substances can be provided for the fermented milk, and the taste and flavor of the fermented milk are remarkably improved. The lactococcus lactis C15, the lactobacillus casei AU9077 and the lactobacillus rhamnosus AU9260 are compounded to obtain the composite dairy leavening agent, the composite dairy leavening agent has a remarkable synergistic effect in the aspect of releasing fructo-oligosaccharide, a stable endogenous sweetening effect is continuously provided for fermented milk, and the requirements of consumers for healthy and flavored dairy products are met while the quality of dairy products is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of microbial fermentation, and particularly relates to a Lactococcus lactis strain, a compound dairy fermentation agent and their applications. Background Art

[0002] In the field of food fermentation, especially in the dairy fermentation industry, improving the flavor and taste of products has always been the focus of research and the goal pursued. Consumers' requirements for the flavor of fermented dairy products such as yogurt are increasing day by day, and they expect to obtain a richer, more intense and unique sensory experience, which has prompted scientific researchers and enterprises to continuously explore new fermentation strains and technologies.

[0003] At present, during the yogurt fermentation process, there are a wide variety of commonly used lactic acid bacteria, but they have certain limitations in endowing yogurt with fragrance and sweetness. The flavor substances produced by some traditional lactic acid bacteria are relatively single, and can only provide a basic yogurt flavor, which is difficult 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. Although it can make yogurt have a basic sour taste and fermentation aroma, it lacks richness and uniqueness in fragrance and cannot create a multi-layered flavor experience.

[0004] In terms of sweetening, the existing technology mainly relies on adding exogenous sweeteners to achieve, such as sucrose, glucose, fructose syrup, etc. However, the extensive use of these sweeteners not only increases the calorie content of the product, but also may bring health risks, such as causing blood sugar fluctuations, obesity and other problems, which does not conform to the current trend of consumers' pursuit of healthy foods. And the research on naturally producing sweet substances through the fermentation process is still insufficient. The reported fermentation strains with sweetening effects are very limited, and the sweetening ability is weak, making it difficult to meet the requirements of actual production applications.

[0005] In terms of enhancing fragrance, although some lactic acid bacteria can produce some fragrance substances, such as acetaldehyde, diacetyl, etc., the yield is low and cannot fully enhance the fragrance of yogurt. In addition, the types and proportions of fragrance substances produced by different lactic acid bacteria vary greatly, resulting in unstable product fragrance and making it difficult to achieve standardized production. For example, some adverse flavor substances produced by lactic acid bacteria during fermentation, such as bitter taste, rancid taste, etc., will affect the overall flavor quality of yogurt. Summary of the Invention

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

[0007] The object of the present invention is achieved by the following technical solutions:

[0008] In the first aspect of the present invention, a Lactococcus lactis C15 with the functions of enhancing sweetness and fragrance is provided. The Lactococcus lactis is classified and named as Lactococcus lactis. This strain is deposited in the China General Microbiological Culture Collection Center, with the deposit number CGMCC No. 18950 and the deposit date of November 13, 2019.

[0009] In the second aspect of the present invention, a compound dairy ferment starter with the functions of enhancing sweetness and fragrance is provided. The compound dairy ferment starter includes Lactococcus lactis C15 as described in claim 1, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260;

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

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

[0012] Further, the viable bacteria content ratio of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 in the dairy ferment starter is 2:1 to 2:1 to 2.

[0013] In the third aspect of the present invention, a preparation method of the compound dairy ferment starter described in the second aspect is provided, including the following steps:

[0014] (1) Respectively activate the strains of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260, perform large-scale cultivation in a liquid medium, centrifuge to collect the bacterial cells, resuspend them in reconstituted skim milk, adjust the effective viable bacteria concentration, mix with a protective agent, and freeze-dry to respectively obtain three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260;

[0015] (2) Mix the three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 in proportion to obtain the compound dairy ferment starter.

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

[0017] In the fourth aspect of the present invention, there is provided the use of the Lactococcus lactis C15 described in the first aspect, or the compound dairy fermentation agent described in the second aspect, or the compound dairy fermentation agent prepared by the method described in the third aspect in the preparation of a fermentation agent or fermented dairy products.

[0018] In the fifth aspect of the present invention, there is provided an endogenous sweetening and flavor-enhancing fermented dairy product, wherein the raw materials for preparing the fermented dairy product include the Lactococcus lactis C15 described in the first aspect, or the compound dairy fermentation agent described in the second aspect, or the compound dairy fermentation agent prepared by the method described in the third aspect.

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

[0020] Strain preservation information:

[0021] Lactococcus lactis C15, classified and named as Lactococcus lactis, preservation institution: China General Microbiological Culture Collection Center (CGMCC); address: No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, postal code: 100101; preservation number: CGMCC No. 18950, preservation date: November 13, 2019.

[0022] Lactobacillus casei AU9077, classified and named as Lactobacillus casei, preservation institution: China General Microbiological Culture Collection Center (CGMCC); address: No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, postal code: 100101; preservation number: CGMCC No. 21663, preservation date: January 18, 2021.

[0023] Lactobacillus rhamnosus AU9260, classified and named as Lactobacillus rhamnosus, preservation institution: China General Microbiological Culture Collection Center (CGMCC); address: No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, postal code: 100101; preservation number: CGMCC No. 21662, preservation date: January 18, 2021.

[0024] The beneficial effects of the present invention compared with the prior art are as follows:

[0025] 1. A Lactococcus lactis C15 strain with unique metabolic characteristics was screened in the present invention. This strain has the dual functions of efficiently synthesizing fructooligosaccharide (FOS) and acetaldehyde. Through experimental determination, under optimized fermentation conditions, Lactococcus lactis C15 exhibits excellent FOS synthesis ability, and its yield can reach 57.7 g / L, significantly higher than that of conventional lactic acid bacteria strains. At the same time, this strain can also synchronously produce the characteristic flavor substance acetaldehyde, with the highest yield reaching 16.2 mg / L, providing a unique technical advantage for dairy fermentation. This characteristic makes it have important application value in the development of functional dairy products.

[0026] 2. In the present invention, Lactococcus lactis C15 was scientifically compounded with Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 to develop a compound dairy starter. Through experimental verification, this compound starter has a significant synergistic effect in releasing fructooligosaccharide, 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below in conjunction with the drawings and embodiments:

[0028] Figure 1 It is a microscopic image of the cells of Lactococcus lactis C15 after Gram staining;

[0029] Figure 2 It shows the FOS yield results of Lactococcus lactis C15 at different fermentation times. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The examples given are for better illustration of the present invention, but the content of the present invention is not limited only to the examples given. Therefore, those skilled in the art who make non-essential improvements and adjustments to the implementation solutions based on the above-mentioned invention content still fall within the protection scope of the present invention.

[0031] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

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

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

[0034] Raw milk (fat content: 3.66%, protein content: 3.14%, total solid content: 12.27%) was obtained from Hunan New Hope Nanshan Liquid Milk Co., Ltd., stored at 4°C, and used up within 1 day.

[0035] Modified MRS liquid medium: 10 g of peptone, 5 g of yeast extract powder, 15 g of glucose, 15 g of sucrose, 10 g of beef extract, 2 g of K2HPO4, 2 g of diammonium citrate, 0.58 g of MgSO4·7H2O, 0.25 g of MnSO4·4H2O, 5 g of anhydrous sodium acetate, 1 mL of Tween 80, 0.5 g of cysteine hydrochloride, made up to 1 L with distilled water, adjusted to pH 6.5, and sterilized at 121°C for 15 min.

[0036] Some of the detection methods involved in the following examples include:

[0037] The detection of the content of fructooligosaccharides refers to the high-performance liquid chromatography method in GB / T 23528.2-2021 "Quality Requirements for Oligosaccharides - Part 2: Fructooligosaccharides". The specific conditions include: liquid chromatograph LC-6A (Shimadzu, Japan), amino chromatographic column Agilent Zorbax NH2 column (5μm, 4.6mm×250mm×208mm, Agilent), mobile phase: acetonitrile + water (volume ratio 75:25), injection volume 10 μl, column temperature 35°C, detector temperature 40°C, flowing at a flow rate of 1.0 ml / min, and determining the content of each sugar component in the product according to the retention time and peak area.

[0038] The determination method of 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 3500 g for 10 min, and take the clear supernatant for standby.

[0040] (2) Accurately measure 5.00 ml of 1% NaHSO3 solution into a 250 ml conical flask, add 25 ml of the supernatant of the treated sample, shake well, leave it at room temperature for 1 h, add 1 ml of 1% starch solution, titrate with 0.1 mol / L iodine solution until close to the end point, and then titrate with 0.01 mol / L iodine solution to the end point (light blue-violet color, no fading for 30 s is the end point), do not count. Then add 20 ml of 1 mol / L NaHCO3 solution, shake well for 0.5 min (the blue color of the solution disappears), titrate with 0.01 mol / L iodine (1 / 2I2) standard solution to the end point (light blue-violet color), record the volume of the standard iodine solution consumed, and at the same time do a blank test.

[0041] At least 3 parallel titrations should be done for each sample, and take the average value.

[0042] The calculation formula for acetaldehyde content is:

[0043]

[0044] In the formula, V2 is the volume (mL) of the I2 standard solution consumed in the blank titration, V1 is the volume (mL) of the I2 standard solution consumed in the sample titration, and C is the concentration (mol / L) of the 1 / 2I2 standard solution.

[0045] Example 1

[0046] A strain was isolated and purified from natural fermented yogurt. Its morphology was observed under a microscope as Figure 1 shown. After further analysis and identification, this strain was Lactococcus lactis, named Lactococcus lactis C15, and sent to a preservation institution for strain preservation. Preservation institution: China General Microbiological Culture Collection Center (CGMCC); Address: No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, Zip Code: 100101; Preservation number is CGMCC No. 18950, and the preservation date is November 13, 2019.

[0047] Carry out the cultivation of Lactococcus lactis C15 and the detection of fructooligosaccharides:

[0048] (1) Strain activation: Inoculate Lactococcus lactis C15 onto MRS plate medium and incubate at 37 °C for 24 hours;

[0049] (2) Seed culture: Inoculate the activated strain into the modified MRS liquid medium and incubate at 37 °C with shaking at 180 rpm for 36 h;

[0050] (3) Flask shaking culture: Inoculate the seed liquid into the fermentation medium at an inoculation amount of 2%, and incubate at 37 °C with shaking at 180 rpm for 48 h;

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

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

[0053] Table 1 Determination results of acetaldehyde content in fermentation broth at different fermentation times (n = 3, X±D) mg / L

[0054] Fermentation time 12h 24h 36h 48h Acetaldehyde <![CDATA[8.99±0.07 a > <![CDATA[11.22±0.11 b > <![CDATA[16.2±0.09 c > <![CDATA[15.9±0.10 d >

[0055] Note: The data with different letter superscripts are significantly different (P < 0.05).

[0056] In this example, Lactococcus lactis C15 was used to provide a dairy starter. The preparation method was as follows: Lactococcus lactis C15 was activated on an MRS plate medium, and then inoculated (2%) into a modified MRS liquid medium for scale-up culture (37 °C, 36 h). The cells were collected by centrifugation and resuspended in reconstituted skim milk, and the effective viable cell concentration was adjusted to 2.0×10 10 CFU / mL to obtain a bacterial solution, which was freeze-dried to prepare the Lactococcus lactis C15 starter (C15).

[0057] Example 2

[0058] This example provided a composite dairy starter, including: Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260. The preparation method included the following steps:

[0059] (1) Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 were respectively activated on an MRS plate medium, and then inoculated (2%) into a modified MRS liquid medium for scale-up culture (37 °C, 36 h). The cells were collected by centrifugation and resuspended in reconstituted skim milk, and the effective viable cell concentration was adjusted to 2.0×10 10 CFU / mL to obtain a bacterial solution, which was freeze-dried to respectively prepare three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260.

[0060] (2) The three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 were mixed in a ratio of 2:1:1 to obtain the composite dairy starter (C15+AU9077+AU9260).

[0061] Comparative Example 1

[0062] This comparative example provides a compound dairy ferment, including: Lactococcus lactis C15 and Lactobacillus casei AU9077.

[0063] The preparation method of the compound dairy ferment refers to Example 2, with the difference that in step (2), two kinds of bacterial powders, Lactococcus lactis C15 and Lactobacillus casei AU9077, are mixed in a ratio of 1:1, thus obtaining the compound dairy ferment (C15 + AU9077).

[0064] Comparative Example 2

[0065] This comparative example provides a compound dairy ferment, including: Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus plantarum W-4.

[0066] The preparation method of the compound dairy ferment refers to Example 2, with the difference that in step (2), three kinds of bacterial powders, Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus plantarum W-4, are mixed in a ratio of 2:1:1, thus obtaining the compound dairy ferment (C15 + AU9077 + W-4).

[0067] Comparative Example 3

[0068] This comparative example provides a compound dairy ferment, including: Lactococcus lactis C15, Lactobacillus casei AU9077, Lactobacillus rhamnosus AU9260 and Lactobacillus plantarum W-4.

[0069] The preparation method of the compound dairy ferment refers to Example 2, with the difference that in step (2), four kinds of bacterial powders, Lactococcus lactis C15, Lactobacillus casei AU9077, Lactobacillus rhamnosus AU9260 and Lactobacillus plantarum W-4, are mixed in a ratio of 2:1:1:1, thus obtaining the compound dairy ferment (C15 + AU9077 + AU9260 + W-4).

[0070] Example 3

[0071] This example provides an endogenous sweetening and flavor-enhancing yogurt, and the preparation method is as follows:

[0072] (1) Raw material preparation: Raw milk is subjected to sterilization separation, defatting, deaeration, standardization, pasteurization and membrane treatment system to obtain standardized raw milk with a protein content of 3.2 / 100g.

[0073] (2) Preheating and homogenization: Sucrose (3%) is added to the standardized raw milk, heated to 55 - 65°C, and then homogenized by a high-pressure homogenizer. The homogenization pressure is set to 15 - 25 MPa.

[0074] (3) Sterilization: Rapidly heat the homogenized milk to 90 - 95 °C and maintain for 5 - 10 minutes for high-temperature sterilization. After sterilization, quickly cool the milk to 40 - 43 °C.

[0075] (4) Inoculation of starter culture: Inoculate the activated Lactococcus lactis C15 starter culture described in Example 1 into the cooled milk at an inoculation amount of 0.2%, and stir well to ensure uniform dispersion of the starter culture.

[0076] (5) Incubation fermentation: Divide the inoculated milk into fermentation containers, seal them, and place them in a constant-temperature incubator for fermentation at 37 - 42 °C for 6 - 12 hours. During fermentation, the acidity and pH value of the yogurt can be measured regularly. When the pH value of the yogurt drops to 4.5 - 4.6, terminate the fermentation.

[0077] Example 4

[0078] This example provides another type of endogenous sweetening and flavor-enhancing yogurt. The preparation method is basically the same as that of Example 3, except that in step (4), the starter culture is the compound dairy starter culture (C15 + AU9077 + AU9260) described in Example 2.

[0079] Comparative Example 4

[0080] This comparative example provides a yogurt. The preparation method is basically the same as that of Example 3, except that in step (4), the starter culture is the compound dairy starter culture (C15 + AU9077) described in Comparative Example 1.

[0081] Comparative Example 5

[0082] This comparative example provides a yogurt. The preparation method is basically the same as that of Example 3, except that in step (4), the starter culture is the compound dairy starter culture (C15 + AU9077 + W - 4) described in Comparative Example 2.

[0083] Comparative Example 6

[0084] This comparative example provides a yogurt. The preparation method is basically the same as that of Example 3, except that in step (4), the starter culture is the compound dairy starter culture (C15 + AU9077 + AU9260 + W - 4) described in Comparative Example 3.

[0085] Test Example

[0086] This test example focuses on the changes in the content of fructooligosaccharide and acetaldehyde in the products when preparing yogurt with different starter cultures. The fructooligosaccharide (FOS) and acetaldehyde contents in the products at the end of fermentation of the endogenous sweetening and flavor-enhancing yogurt prepared in Examples 3 and 4, and the yogurt prepared in Comparative Examples 4 - 6 were measured respectively.

[0087] The results are shown in Tables 2 and 3 respectively.

[0088] Table 2 Determination results of the content of fructooligosaccharides in yogurt products of different groups (n = 3, X±D) g / L

[0089] Yogurt sample Fructooligosaccharide Example 3 (C15) <![CDATA[41.87±1.22 a > Example 4 (C15 + AU9077 + AU9260) <![CDATA[59.17±0.74 b > Comparative Example 4 (C15 + AU9077) <![CDATA[47.21±0.79 c > Comparative Example 5 (C15 + AU9077 + W-4) <![CDATA[24.87±1.10 d > Comparative Example 6 (C15 + AU9077 + AU9260 + W-4) <![CDATA[48.77±0.72 c >

[0090] Table 3 Determination results of the content of acetaldehyde in yogurt products of different groups (n = 3, X±D) mg / L

[0091] Yogurt sample Acetaldehyde Example 3 (C15) <![CDATA[12.89±0.12 a > Example 4 (C15 + AU9077 + AU9260) <![CDATA[18.47±0.17 b > Comparative Example 4 (C15 + AU9077) <![CDATA[15.74±0.09 c > Comparative Example 5 (C15 + AU9077 + W-4) <![CDATA[13.33±0.10 a > Comparative Example 6 (C15 + AU9077 + AU9260 + W-4) <![CDATA[13.27±0.22 a >

[0092] Note: Data with different letter superscripts are significantly different (P < 0.05).

[0093] The above test results show that in terms of the content of fructooligosaccharides, the fructooligosaccharide content of the yogurt in Example 3 (single fermentation of Lactococcus lactis C15) is 41.87±1.22 g / L. In Example 4, Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 were combined for fermentation, and the fructooligosaccharide content was significantly increased to 59.17±0.74 g / L, indicating that the three strains synergistically promoted the retention or production of fructooligosaccharides. It may be that their metabolic processes reduced the consumption of fructooligosaccharides or activated certain reaction pathways for the production of fructooligosaccharides. The increase in the fructooligosaccharide content not only reflects the possible synergistic effect among the three strains, but also has important significance from the perspective of fermenting to produce fructooligosaccharides and providing endogenous sweetness. On the one hand, as a functional oligosaccharide, fructooligosaccharides are not easily decomposed by digestive enzymes in the human body, can provide natural sweetness for yogurt, reduce the dependence on traditional sweeteners such as added sucrose, and meet the current consumers' pursuit of healthy low-sugar foods. On the other hand, the process of these strains fermenting to produce fructooligosaccharides not only enriches the sweetness source of yogurt, but also increases the functionality of yogurt. Fructooligosaccharides have probiotic functions such as regulating the intestinal flora and promoting the proliferation of Bifidobacterium, making yogurt more advantageous in terms of nutrition and health.

[0094] The fructooligosaccharide content in the yogurt of Comparative Example 4 (Lactococcus lactis C15 + Lactobacillus casei AU9077) is 47.21±0.79 g / L, which is significantly lower than that in Example 4 but higher than that in Example 3, indicating that the co-fermentation of Lactobacillus casei AU9077 and Lactococcus lactis C15 has a certain effect on increasing the fructooligosaccharide content, but the absence of Lactobacillus rhamnosus AU9260 weakens the synergistic effect and is relatively weak in the ability of fermented oligosaccharides and providing endogenous sweetness.

[0095] The content of fructooligosaccharide 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, significantly lower than that of other groups, indicating that the addition of Lactobacillus plantarum W-4 greatly reduced the content of fructooligosaccharide. It may be that Lactobacillus plantarum W-4 consumed a large amount of fructooligosaccharide for its own growth and metabolism, thereby reducing the fructooligosaccharide that could be retained in the yogurt. This not only reduced the ability of the yogurt to provide endogenous sweetness through fructooligosaccharide, but also weakened the probiotic function of the yogurt.

[0096] The content of fructooligosaccharide 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. Although it was lower than that of Example 4, it was higher than that of Comparative Example 5, indicating that Lactobacillus rhamnosus AU9260 alleviated the consumption of fructooligosaccharide by Lactobacillus plantarum W-4 to a certain extent.

[0097] In terms of acetaldehyde content, the acetaldehyde content of the yogurt in Example 3 was 12.89 ± 0.12 mg / L. The acetaldehyde content in Example 4 increased to 18.47 ± 0.17 mg / L. The combined starter promoted the production of acetaldehyde and enriched the flavor of the yogurt. The acetaldehyde content of Comparative Example 4 was 15.74 ± 0.09 mg / L, which was between Example 3 and 4, further proving that the co-fermentation of Lactobacillus casei AU9077 and Lactococcus lactis C15 could increase the acetaldehyde content, but the effect was not as good as adding 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 the production of acetaldehyde, and even in the presence of Lactobacillus rhamnosus AU9260, the amount of acetaldehyde production could not be restored.

[0098] Generally speaking, the starter combination in Example 4 - Lactococcus lactis C15, Lactobacillus casei AU9077 and Lactobacillus rhamnosus AU9260 - showed the best performance in increasing the content of fructooligosaccharide and acetaldehyde in the yogurt and optimizing the yogurt quality, providing a useful reference for the development of high-quality and healthy yogurt products. The negative impacts of Lactobacillus plantarum W-4 on the content of fructooligosaccharide and acetaldehyde in Comparative Examples 5 and 6 need attention. Follow-up studies can explore optimizing the proportion of strains or screening other strains to balance the components and flavor of the yogurt, providing more powerful theoretical and practical bases for the optimization of yogurt starters.

[0099] Finally, it should be noted that the above is only used to illustrate the technical solution of the present invention and not to limit it. Although the present invention has been described in detail with reference to the preferred arrangement, those of ordinary skill in the art should understand that the technical solution of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. Lactococcus lactis C15 with the functions of enhancing sweetness and fragrance, characterized in that, The Lactococcus lactis is classified and named as Lactococcus lactis. This strain is preserved in the General Microbiological Center of the China Committee for Culture Collection of Microorganisms, with the preservation number of CGMCC No. 18950 and the preservation date of November 13, 2019.

2. A compound dairy ferment with the functions of enhancing sweetness and fragrance, characterized in that, The compound dairy starter culture includes Lactococcus lactis C15 as described in claim 1, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260. The Lactobacillus casei AU9077 is classified and named as Lactobacillus casei. This strain is preserved in the General Microbiological Center of the China Committee for Culture Collection of Microorganisms, with the preservation number of CGMCC No. 21663 and the preservation date of January 18, 2021. The Lactobacillus rhamnosus AU9260 is classified and named as Lactobacillus rhamnosus. This strain is preserved in the General Microbiological Center of the China Committee for Culture Collection of Microorganisms, with the preservation number of CGMCC No. 21662 and the preservation date of January 18, 2021.

3. The compound dairy starter according to claim 2, characterized in that, In the dairy starter culture, the viable cell content ratio of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 is 2:1 to 2:1 to 2.

4. A method for preparing the composite dairy starter culture according to claim 2 or 3, characterized in that, The preparation method includes the following steps: (1) Respectively activate the strains of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260, perform large-scale cultivation in liquid medium, collect the bacterial cells by centrifugation, resuspend them in reconstituted skim milk, adjust the effective viable cell concentration, then mix with the cryoprotectant and perform freeze-drying to respectively obtain three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260. (2) Mix the three kinds of bacterial powders of Lactococcus lactis C15, Lactobacillus casei AU9077, and Lactobacillus rhamnosus AU9260 in proportion to obtain the compound dairy starter culture.

5. The preparation method according to claim 4, wherein The cryoprotectant includes mannitol, trehalose, reconstituted skim milk, dimethyl sulfoxide, and polyvinylpyrrolidone.

6. Use of Lactococcus lactis C15 as described in claim 1, or the compound dairy starter culture as described in claim 2 or 3, or the compound dairy starter culture prepared by the method as described in claim 4 or 5 in the preparation of a starter culture or fermented dairy products.

7. An endogenous sweetening and flavor-enhancing fermented dairy product, 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 culture as described in claim 2 or 3, or the compound dairy starter culture prepared by the method as described in claim 4 or 5.

8. The fermented dairy product according to claim 7, characterized in that, The dairy products include yogurt, cheese, koumiss, and milk beverages.

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