Streptococcus salivarius subsp. thermophilus, fermented dairy product and method of preparation

Abstract

This invention relates to the field of dairy processing technology, specifically to a strain of Streptococcus salivarius thermophilus, fermented dairy products, and a preparation method. The invention provides a method for preparing fermented dairy products, comprising: fermenting cream to an acidity of 20-25°T; mixing raw milk with a portion of the fermented cream, and then mixing this mixture with the remaining raw milk; after sterilization and cooling, inoculating with Streptococcus salivarius thermophilus strain FC10 (preservation number CGMCC NO.31796) for fermentation; and cooling and packaging after demulsification. This method, by limiting key parameters such as the acidity of the cream, the mixing ratio, and the mixing temperature, and through the synergistic effect of a specific strain, produces a product that, without the need for stabilizers, has a significant milky texture, a smooth mouthfeel, and lacks the grainy and viscous texture of traditional yogurt.

Description

Technical Field

[0001] This invention relates to the field of dairy processing technology, specifically to a strain of Streptococcus salivarius thermophilus, fermented dairy products, and preparation methods. Background Technology

[0002] *Streptococcus salivarius* subsp. *thermophilus* is one of the dominant microbial strains in yogurt fermentation and is widely used in the industrial production of fermented dairy products. This strain possesses excellent acid and aroma production capabilities and can independently complete the fermentation process of cow's milk, making it a core microbial resource for fermented milk production. However, most *Streptococcus salivarius* subsp. *thermophilus* produce large amounts of exopolysaccharides (EPS) during fermentation, resulting in a high viscosity in the fermented milk. Mucilage-producing strains can not only reduce whey separation and strengthen the gel structure but also impart the characteristic thick texture of traditional yogurt to the product.

[0003] With the deepening trends of consumption upgrading and health consciousness, consumers' demands for the taste of fermented dairy products are becoming increasingly diversified. The thick texture of traditional high-viscosity yogurt can no longer meet the needs of all consumption scenarios, and refreshing, liquid, low-viscosity fermented milk products are gradually gaining market favor. However, the preparation of existing low-viscosity fermented milk usually requires the addition of stabilizers, thickeners, or the use of complex physical processing techniques, which not only increases production costs but also contradicts consumers' pursuit of "clean" labels and health benefits. In addition, some low-viscosity products achieve their low-viscosity texture by adding colloidal stabilizers and emulsifiers, but the use of additives affects the product's natural properties, and the taste still differs from the natural texture of cow's milk. Therefore, developing a fermented milk product that does not require the addition of stabilizers and has a smooth texture like natural cow's milk has become a technical challenge that the industry urgently needs to solve.

[0004] Although existing technologies have included screening for low-yielding *Streptococcus thermophilus* subspecies, the fermentation characteristics of these strains often fail to meet the requirements of coagulation state, mouthfeel, and process stability for fermented milk with a milky texture. Screening for a fast-acidifying, non-viscous strain that matches the processing conditions of this product is crucial for developing a refreshing fermented milk without added stabilizers. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies that rely on food additives or complex post-processing techniques to improve the quality and texture of fermented dairy products, and to provide a preparation scheme based on a two-step fermentation process and the synergistic effect of a dedicated fermentation strain. Specifically, it relates to a Streptococcus salivarius thermophilus subsp. FC10, a fermented dairy product, and a preparation method thereof.

[0006] In a first aspect, the present invention provides a strain of Streptococcus salivarius subsp. thermophilus FC10, which was deposited on September 2, 2024 at the China General Microbiological Culture Collection Center (CGMCC), at No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, with accession number CGMCC NO.31796.

[0007] In one embodiment, the *Streptococcus salivarius* subsp. *thermophilus* FC10 is a natural strain isolated from yak yogurt from Jianziwan Mountain in Yajiang County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China.

[0008] In one embodiment, the colonies of Streptococcus salivarius subsp. thermophilus FC10 on M17 solid medium are round with neat edges and smooth surfaces. Under a microscope, the bacteria are spherical or oval, arranged in pairs or chains, and Gram-positive.

[0009] In one embodiment, the *Streptococcus salivarius* subsp. *thermophilus* FC10 ferments rapidly in skim milk, has a strong acid-producing capacity, and produces fermented milk with low viscosity, exhibiting a thin and smooth milk texture.

[0010] Secondly, the present invention provides a fermented dairy product, wherein the viscosity of the fermented dairy product is 5-15 mPa·s.

[0011] This viscosity range is significantly lower than that of traditional fermented dairy products, reflecting the product's refreshing and smooth texture.

[0012] In one embodiment, the viscosity of the fermented dairy product is 5-10 mPa·s.

[0013] In one embodiment, the secondary structure of the protein in the fermented dairy product has a β-sheet ratio of 15-20%, an α-helix ratio of 24-28%, and a β-turn ratio of 25-30%. These characteristics of the protein secondary structure reflect the loose conformation of the protein chains in the product and are closely related to the product's smooth, low-viscosity sensory properties.

[0014] In one embodiment, the fermented dairy product contains *Streptococcus thermophilus* subsp. FC10, with accession number CGMCC NO.31796. This strain is a natural strain isolated from yak yogurt from Jianziwan Mountain, Yajiang County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China. Its fermentation characteristics can produce a significant synergistic effect with the preparation process of the present invention, giving the product a unique smooth taste and milky texture.

[0015] Thirdly, the present invention provides a method for preparing fermented dairy products, comprising the following steps:

[0016] S1. Preparation of fermented butter: After sterilizing and cooling butter with a fat content of 40-45%, add a starter culture and ferment. When the acidity reaches 20-25°T, cool it for later use. S2. Mixing and Sterilization: The raw milk and the fermented cream obtained in step S1 are mixed for the first time to obtain a primary mixture; the primary mixture is then mixed with the raw milk for the second time to obtain a mixed liquid; the mixed liquid is sterilized; in step S2, the proportions of the raw milk and fermented cream are as follows: in the first mixing, the raw milk is 30-35 parts and the fermented cream is 10-15 parts; in the second mixing, the raw milk is 50-60 parts. S3. Fermentation: Cool the sterilized mixture and inoculate it with Streptococcus salivarius subsp. thermophilus FC10 for fermentation. S4. Post-processing: After fermentation reaches the endpoint, the milk is broken, cooled, homogenized under air pressure, filled, and then ripened to obtain the fermented dairy product.

[0017] The core of the above preparation method lies in the synergistic effect of a two-step mixing process and a specific strain. By pre-fermenting the fat phase to a specific acidity, then fully integrating it with the raw milk through stepwise mixing, and finally using the thermophilic subsp. salivarius FC10 strain isolated and screened in this invention for fermentation, the product can naturally form a low-viscosity, smooth milk texture without relying on stabilizers.

[0018] In one embodiment, in step S1, the cream is derived from defatting raw milk; the sterilization conditions are 87-95°C for 5-10 minutes; and the cooling is to cool to 6-10°C for later use.

[0019] In one embodiment, in step S2, the temperature of the first mixing is 25-30°C and the time is 10-15 min; the temperature of the second mixing is 8-12°C.

[0020] In one embodiment, in step S2, the homogenization pressure is 180-220 bar; the sterilization conditions are 90-98°C and the time is 240-360 seconds.

[0021] In one embodiment, in step S3, the cooling temperature is 40-44°C; the fermentation endpoint is demulsification when the acidity reaches 60-70°T.

[0022] Fourthly, the present invention also provides the application of the above-mentioned fermented dairy products in the food field.

[0023] Because the fermented dairy products provided by this invention have natural low viscosity, smooth taste and milky texture, and do not contain any stabilizers, they are particularly suitable for direct consumption as high-end fermented dairy products, and can also be used as a base material for processing other dairy products.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention provides a strain of Streptococcus salivarius thermophilic subspecies FC10 (CGMCC NO.31796), which is a natural strain isolated from yak yogurt in Ganzi, Sichuan. It produces acid quickly during milk fermentation, and the product has low viscosity and a refreshing and smooth taste after fermentation. It is particularly suitable for the preparation of low viscosity refreshing fermented milk.

[0025] (2) The present invention provides a fermented dairy product with a viscosity of 5-15 mPa·s, which has a smooth texture like natural milk, without the stickiness and graininess of traditional yogurt, and does not require the addition of any stabilizers, which is in line with the consumer trend of clean label.

[0026] (3) This invention provides a method for preparing fermented dairy products. By pre-fermenting the fat phase to a specific acidity (20-25°T), and then fully integrating it with raw milk through a stepwise mixing process, and fermenting it with Streptococcus thermophilus subsp. FC10 strain, the product can naturally form a low-viscosity, smooth milk texture without relying on any additives. This method is stable, highly operable, and has good prospects for industrial application. Attached Figure Description

[0027] Figure 1 Colony morphology and microscopic images of Streptococcus salivarius subsp. thermophilus FC10 on M17 solid medium; Figure 2 The growth curve and acid production curve of Streptococcus salivarius subsp. thermophilus FC10 are shown. Figure 3 This is a comparison diagram of the microstructure of the products of Example 1 and Comparative Examples 3-4 of the present invention; Figure 4 This is a comparison diagram of the flow states of the fermented milk product of the present invention (left), cow's milk (middle), and fermented milk of Comparative Example 1 (right). Detailed Implementation

[0028] The present invention will now be described in further detail with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0029] Source of materials: Raw milk: Commercially available fresh milk.

[0030] Streptococcus salivarius thermophilic subspecies FC10: This invention is a natural strain isolated from yak yogurt from Jianziwan Mountain, Yajiang County, Ganzi Tibetan Autonomous Prefecture, Sichuan Province, China. It is deposited at the China General Microbiological Culture Collection Center with accession number CGMCC NO.31796.

[0031] Commercial strains A and B: Commonly available yogurt starter cultures.

[0032] 1. Isolation, preservation and identification of Streptococcus salivarius subsp. thermophilus FC10 Weigh 25 g of the collected traditional fermentation sample and add it to a homogenization bag containing 225 mL of sterile physiological saline. Mix thoroughly using a beater homogenizer to prepare 10... -1 Bacterial suspension; serially diluted 10-fold with sterile physiological saline, selecting 10... -5 10 -6 10 -7 Three dilution gradients were used, with 100 μL of each diluted bacterial suspension spread onto M17 agar medium using the plating method. The culture was incubated at 37 ℃ inverted for 48 h. After incubation, single colonies with significantly different morphologies were picked and streaked for purification at least three times. The purified single colonies were then inoculated onto M17 liquid medium and incubated at 37 ℃ for 18 h to obtain activated bacterial suspension. An equal volume of the bacterial suspension was thoroughly mixed with 60% (v / v) sterile glycerol, aliquoted into cryovials, and stored at -80 ℃ for long-term preservation.

[0033] like Figure 1 As shown, FC10 colonies on M17 agar medium are milky white, with raised surfaces and smooth, intact edges; after Gram staining, optical microscopy reveals that the bacteria are Gram-positive and arranged in short, spherical chains.

[0034] Genomic DNA was extracted from the strain using the bacterial genomic DNA rapid extraction kit from Sangon Biotech (Shanghai) Co., Ltd.; the 16S rDNA gene of the strain was amplified by PCR using the universal bacterial primers 27F / 1492R; after the amplified product bands were verified to be qualified by agarose gel electrophoresis, Sangon Biotech was commissioned to complete the first-generation sequencing; the sequenced gene sequence was uploaded to the NCBI database for BLAST homology sequence comparison, and the taxonomic position of the strain was determined by combining the sequence similarity.

[0035] The full-length 16S rDNA sequence of strain FC10 is shown in SEQ ID NO.1.

[0036] Based on the physiological and biochemical characteristics of the strain and the molecular biological 16S rDNA identification results, the strain FC10 of this invention is a thermophilic subsp. salivarius, deposited at the China General Microbiological Culture Collection Center, accession number: CGMCC No. 31796.

[0037] 2. Growth and acid production curves of Streptococcus salivarius subsp. thermophilus FC10 Seed culture activated for three generations in liquid medium was inoculated into M17 liquid medium at an inoculum volume of 2% and cultured at 42°C for 24 hours. The culture was then analyzed using the Finnish Bioscreen assay. The C-type fully automated growth curve analyzer records the OD value every 2 hours and plots the growth curve.

[0038] Streptococcus salivarius subsp. thermophilus FC10 was inoculated into 10% (w / v) sterile skim milk at an inoculum rate of 3% (v / v) and fermented at 37°C for 20 h. The pH value was measured every 1 h, and an acid production curve was plotted with the culture time (h) as the x-axis and the pH value as the y-axis.

[0039] like Figure 2 As shown, the acid production curve of *Streptococcus salivarius* subsp. *thermophilus* FC10 generally follows the same trend as its growth curve, with the pH value decreasing rapidly during 5-10 hours and stabilizing after 10 hours. This strain coagulates in skim milk after approximately 5-6 hours of fermentation, indicating its potential for industrial-scale rapid fermentation applications.

[0040] 3. Testing Methods (1) Acidity determination: Phenolphthalein indicator titration method (refer to the section on acidity determination of milk and dairy products in GB 5009.239-2016 "National Food Safety Standard for Determination of Acidity of Food").

[0041] The results are expressed in units of acidity (°T), which is the number of milliliters of 0.1000 mol / L NaOH solution required to neutralize 100 mL of milk sample.

[0042] (2) Viscosity measurement: The Brookfield DV2T viscometer, combined with an ultra-low viscosity adapter, was used at 15°C. The standard rotor was selected and the viscosity of the sample was measured at 60 rpm. The unit is mPa·s.

[0043] (3) Microstructure observation The fermented milk was rapidly frozen in liquid nitrogen at -170°C. The sample bath was transferred to the sublimation chamber using a frozen sample preparation transfer system. The emulsion was cut to produce a flat cross-section and sublimated at -90°C for 10 min. After removing moisture, the sample was sputtered with gold at a current of 5 mA for 70 s and observed in the observation chamber.

[0044] (4) Sensory evaluation: Ten trained sensory evaluators were organized to score the product’s smoothness, graininess, milky texture, and overall acceptability (1-10 points).

[0045] (5) Secondary structure determination The freeze-dried yogurt was ground into powder, and 2.5 mg of the powder was weighed and mixed with 197.5 mg of potassium bromide. The mixture was then compressed into transparent tablets using a tablet press at a pressure of 2.5 MPa. The parameters were set as follows: temperature 25 ℃, wavelength scanning range 500–4000 cm⁻¹. -1 60 scans were performed.

[0046] Example 1 Preparation of fermented dairy products according to the present invention Preparation of fermented butter: Fresh milk is skimmed to obtain cream with a fat content of 42%.

[0047] Sterilize the butter at 90°C for 8 minutes. After sterilization, cool it to 28°C, add an appropriate amount of commercial starter culture, and ferment it. When the acidity reaches 23°T, quickly cool it to 8°C for later use.

[0048] Mixing and sterilization: Mix 33 parts raw milk (4°C) with 12 parts fermented cream (8°C), heat to 28°C, and stir for 13 minutes to obtain a primary mixture. Mix the primary mixture with 55 parts raw milk (approximately 4°C) at 10°C and stir until homogeneous.

[0049] The mixture was homogenized at 200 bar and then sterilized at 95°C for 300 seconds.

[0050] Fermentation: The sterilized liquid was cooled to 42°C and inoculated with *Streptococcus salivarius* subsp. *thermophilus* FC10 isolated in this invention. Fermentation was carried out at a constant temperature of 42°C, with acidity monitored during the process. Fermentation was stopped when the acidity reached 65°T.

[0051] Post-processing: Demulsification, homogenization, cooling to below 10°C, and filling. After ripening at 4°C for 24 hours, the final product is obtained.

[0052] Product performance test results: viscosity is 5 mPa·s. Secondary structure analysis showed a β-sheet ratio of 18.32%, an α-helix ratio of 25.67%, and a β-turn ratio of 27.90%, indicating a loose protein structure. In sensory evaluation, 10 trained evaluators gave it a smoothness score of 9.5, describing the texture as smooth, like milk, refreshing and non-sticky. No graininess was noted. Figure 4The figures shown are comparisons of the flow states of the fermented milk product of this invention (left), cow's milk (middle), and fermented milk of Comparative Example 1 (right). The fermented milk product of this invention has good flowability and a cow's milk texture, which contrasts sharply with the viscous state of traditional fermented milk.

[0053] Example 2 This embodiment is basically the same as Embodiment 1, except that the final acidity of the fermented cream in step S1 is controlled at 20°T to verify the effect of this lower limit on product performance.

[0054] Preparation of fermented cream: Fresh milk is skimmed to obtain cream with a fat content of 42%. The cream is sterilized at 90°C for 8 minutes, then cooled to 28°C. An appropriate amount of commercial starter culture is added for fermentation. When the acidity reaches 20°T, it is rapidly cooled to 8°C for later use.

[0055] Mixing and sterilization: Take 33 parts of raw milk (approximately 4°C) and 12 parts of the above-mentioned fermented cream (approximately 8°C), and mix them at 28°C for 13 minutes to obtain a primary mixture. Mix this primary mixture with 55 parts of raw milk (approximately 4°C) at 10°C and stir until homogeneous. Homogenize the mixture at 200 bar, and then sterilize it at 95°C for 300 seconds.

[0056] Fermentation: The sterilized liquid was cooled to 42°C and inoculated with Streptococcus salivarius subsp. thermophilus FC10 (preservation number CGMCC NO.31796). Fermentation was carried out at a constant temperature of 42°C, and the acidity was monitored during the process. Fermentation was stopped when the acidity reached 70°T.

[0057] Post-processing: Demulsification, homogenization, cooling to below 10°C, and filling. After ripening at 4°C for 24 hours, the final product is obtained.

[0058] The product performance test results are as follows: the viscosity is 12 mPa·s, significantly lower than that of traditional fermented dairy products and higher than that of Example 1 (5 mPa·s). Secondary structure analysis shows that the β-sheet ratio is 19.56%, the α-helix ratio is 24.83%, and the β-turn ratio is 28.15%. The protein structure remains relatively loose, consistent with the trend of Example 1 but with a slight tendency towards compaction. In terms of sensory evaluation, the smoothness score given by 10 trained evaluators is 8.5, and the mouthfeel is described as "smooth, close to the texture of milk, but slightly thicker than Example 1". The particle texture evaluation is "virtually no particles".

[0059] The above results show that when the acidity of fermented cream is controlled at the lower limit of 20°T within the range of the present invention, fermented dairy products with a milky texture can still be obtained. The smoothness and freshness are slightly inferior to those of Example 1, but significantly better than all comparative products, indicating that 20°T is still within the effective range.

[0060] Example 3 This embodiment is basically the same as Embodiment 1, except that the final acidity of the fermented cream in step S1 is controlled at 25°T to verify the effect of this upper limit on product performance.

[0061] Preparation of fermented cream: Fresh milk is skimmed to obtain cream with a fat content of 42%. The cream is sterilized at 90°C for 8 minutes, then cooled to 28°C. An appropriate amount of commercial starter culture is added for fermentation. When the acidity reaches 25°T, it is rapidly cooled to 8°C for later use.

[0062] Mixing and sterilization: Take 33 parts of raw milk (approximately 4°C) and 12 parts of the above-mentioned fermented cream (approximately 8°C), and mix them at 28°C for 13 minutes to obtain a primary mixture. Mix this primary mixture with 55 parts of raw milk (approximately 4°C) at 10°C and stir until homogeneous. Homogenize the mixture at 200 bar, and then sterilize it at 95°C for 300 seconds.

[0063] Fermentation: The sterilized liquid was cooled to 42°C and inoculated with Streptococcus salivarius subsp. thermophilus FC10 (preservation number CGMCC NO.31796). Fermentation was carried out at a constant temperature of 42°C, and the acidity was monitored during the process. Fermentation was stopped when the acidity reached 68°T.

[0064] Post-processing: Demulsification, homogenization, cooling to below 10°C, and filling. After ripening at 4°C for 24 hours, the final product is obtained.

[0065] The product performance test results are as follows: the viscosity is 6 mPa·s, which is still at an extremely low level, slightly higher than 5 mPa·s in Example 1 but lower than 12 mPa·s in Example 2. Secondary structure analysis shows that the β-sheet ratio is 18.89%, the α-helix ratio is 25.12%, and the β-turn ratio is 27.95%, with the protein structure being very loose, similar to Example 1. In terms of sensory evaluation, the smoothness score is 9.0, and the taste is described as "smooth, with a distinct milky texture, refreshing and non-sticky." The graininess evaluation is "no grains."

[0066] The above results show that when the acidity of fermented cream is controlled within the upper limit of 25°T of the present invention, fermented dairy products with excellent milk texture can still be obtained. The product performance is very close to that of Example 1, and the smoothness and freshness are significantly better than those of Comparative Example 1 (18°T) and Comparative Example 2 (27°T). Combining the results of Examples 1-3, it can be confirmed that the technical effects of the present invention can be achieved within the acidity range of 20-25°T for fermented cream, with 23°T being the optimal value.

[0067] Example 4 Preparation of fermented dairy products made from a combination of Streptococcus salivarius subsp. thermophilus FC10 and Lactobacillus rhamnosus HF01 This embodiment verifies the feasibility of preparing fermented dairy products by combining Streptococcus salivarius thermophilus subsp. FC10 with probiotics.

[0068] The compound strain *Lactobacillus rhamnosus* HF01 was deposited at the China Center for Type Culture Collection (CCTCC) on October 25, 2021, with accession number CCTCC NO. M20211319. Based on in vitro experiments, probiotic strains with weight-loss functions were screened. Through tolerance experiments using simulated gastric and intestinal fluids, strains with good tolerance were obtained. Ingestion of the strain and its fermentation products significantly increased serum PYY levels, reduced food intake, effectively improved fat accumulation in the liver of obese mice and reduced abnormal blood glucose and insulin levels caused by obesity, and decreased body weight in high-fat diet-induced obese mice.

[0069] The preparation of fermented butter was the same as in Example 1. Mixing, homogenization, and sterilization were performed as in Example 1. The sterilized liquid was cooled to 42°C and simultaneously inoculated with Streptococcus salivarius subsp. thermophilus FC10 (inoculum size 1×10⁻⁶). 6 CFU / mL) and Lactobacillus rhamnosus HF01 (1×10 7 Fermentation was carried out at a constant temperature of 42°C (CFU / mL) until the acidity reached 65°T, at which point fermentation was stopped. Post-processing was the same as in Example 1. The resulting product combines a low-viscosity, smooth texture with the potential weight-loss benefits of *Lactobacillus rhamnosus* HF01.

[0070] Example 5 Preparation of fermented dairy products made from a combination of Streptococcus salivarius subsp. thermophilus FC10 and Bifidobacterium animalis subsp. lactis. This embodiment further verifies the feasibility of preparing fermented dairy products by combining Streptococcus salivarius thermophilus subsp. FC10 with other probiotics.

[0071] The compound strains are selected from *Bifidobacterium animalis* subsp. *lactis* B-1, with accession number CGMCC No. 38186, deposited at the China General Microbiological Culture Collection Center on March 30, 2026; or *Bifidobacterium animalis* subsp. *lactis* B-6, with accession number CGMCC No. 38187, deposited at the China General Microbiological Culture Collection Center on March 30, 2026. These strains can be used to develop fermented milk products with constipation-relieving effects.

[0072] The preparation of fermented butter was the same as in Example 1. Mixing and sterilization were performed as in Example 1. The sterilized liquid was cooled to 42°C and simultaneously inoculated with Streptococcus salivarius thermophilic subsp. FC10 (1×10⁻⁶). 6 CFU / mL) and Bifidobacterium animalis subsp. lactis B-1 (or B-6, 1×10⁻⁶) 7 Fermentation was carried out at a constant temperature of 42°C (CFU / mL) until the acidity reached 65°T. Post-processing was the same as in Example 1. The resulting product combines a low-viscosity, smooth texture with the potential constipation-relieving effects of Bifidobacterium animalis subsp. lactis.

[0073] Comparative Example 1 The purpose of this Comparative Example 1 is to verify the effect of fermented cream acidity being lower than the range of this invention (20-25°T) on the texture and taste of the final product.

[0074] Parameter settings: basically the same as in Example 1, except that in step S1, the final acidity of the cream is controlled at 18°T.

[0075] Fermented butter was prepared according to the method of Example 1, and immediately cooled to 8°C when the acidity reached 18°T for later use. The remaining mixing, homogenization, sterilization, fermentation and post-processing steps were the same as in Example 1.

[0076] Results: Product viscosity: 42 mPa·s, higher than that of Example 1 (5 mPa·s).

[0077] Secondary structure: β-sheet ratio 26.43%, α-helix ratio 21.05%, β-turn ratio 33.32%, indicating that the protein structure tends to be tightly stacked.

[0078] Sensory evaluation: The texture is noticeably powdery and thick, with a smoothness score of only 4.5 out of 10, and a noticeable grainy texture.

[0079] It is evident that when the acidity of fermented cream is too low, the pre-acidification is insufficient, which cannot effectively change the interaction between the fat phase and the protein phase. This leads to excessive cross-linking of proteins during fermentation, forming a dense gel. As a result, the product loses its smooth texture like milk and instead becomes powdery and thick.

[0080] Comparative Example 2 The purpose of setting up Comparative Example 2 is to verify the effect of fermented cream acidity exceeding the range of this invention (20-25°T) on the texture and taste of the product.

[0081] Parameter settings: basically the same as in Example 1, except that in step S1, the final acidity of the cream fermentation is controlled at 27°T.

[0082] Fermented butter was prepared according to the method in Example 1, and immediately cooled to 8°C when the acidity reached 27°T. The remaining steps remained unchanged.

[0083] result; Product viscosity: 36 mPa·s, still significantly higher than that of Example 1.

[0084] Secondary structure: β-sheet ratio 25.70%, α-helix ratio 20.56%, β-turn ratio 31.87%, also showing a compact protein structure.

[0085] Sensory evaluation: The texture is noticeably powdery and thick, with a smoothness score of 4.0 and a noticeable grainy texture.

[0086] Excessive acidity (27°T) in fermented cream leads to over-acidification of the fat phase, which disrupts the emulsification stability of fat and protein. During fermentation, protein aggregates too quickly, forming a coarse gel network. This results in a product with a rough, powdery texture that fails to achieve a smooth milky feel.

[0087] Table 1: Effects of different endpoint acidity levels on the performance of fermented butter

[0088] Comparative Examples 1 and 2 exhibit a higher β-sheet ratio and a lower α-helix content (>20%), indicating that the protein structure tends towards a more tightly stacked folded arrangement with stronger interchain hydrogen bonds. Example 1, on the other hand, shows a lower content of β-turns and β-sheets, indicating fewer bends and turns in the local conformation of its protein chain, resulting in a looser protein chain structure. These secondary structure data are also consistent with the sensory evaluation trends.

[0089] Comparative Example 3 Purpose of this study: To verify the effect on the microstructure and taste of the product when the proportion of raw milk in a single mixing process exceeds the range of this invention.

[0090] Parameter settings: basically the same as in Example 1, except that in step S2, the raw milk is mixed in 37 parts and the fermented cream is still 12 parts.

[0091] Following the method in Example 1, 37 parts of raw milk and 12 parts of fermented cream were mixed at 28°C for 13 minutes, and then mixed again with 51 parts of raw milk at 10°C. The remaining steps remained unchanged.

[0092] Test results; Product viscosity: 42 mPa·s.

[0093] Microstructure: The network structure is dense, with small pores and high continuity.

[0094] Sensory evaluation: The texture is thick and powdery, with a smoothness score of 5.5.

[0095] An excessively high proportion of raw milk dilutes the effect of pre-fermented cream, resulting in insufficient fat phase in the fermentation system, excessive cross-linking of the protein network, and the formation of a dense structure. Consequently, the product loses its refreshing and smooth feel and becomes thick and powdery.

[0096] Comparative Example 4 The effect of a lower proportion of raw milk in a single mixing process on the microstructure and taste of the product was verified.

[0097] Parameter settings: basically the same as in Example 1, except that in step S2, the raw milk is mixed in 28 parts and the fermented cream is mixed in 12 parts.

[0098] Mix 28 parts raw milk with 12 parts fermented cream at 28°C for 13 minutes, then mix again with 60 parts raw milk at 10°C. The remaining steps remain the same.

[0099] Test Results: Product viscosity: 35 mPa·s. Microstructure: Dense network structure with small pores. Sensory evaluation: Thick texture with a powdery feel; smoothness score: 5.0.

[0100] If the proportion of raw milk is too low and the amount of fermented cream is too high, the fat will accumulate excessively. During the fermentation process, the protein network will be excessively disturbed, forming an uneven and dense structure. The product will have a thick and powdery texture.

[0101] Table 2: Effects of different raw milk to fat mixing ratios on product performance

[0102] like Figure 3 The figures show the microstructures of the products from Example 1 and Comparative Examples 3-4 of this invention. As can be seen from the figures, all three groups of yogurt exhibit a gel network composed of interwoven casein micelle chains with accompanying pores. However, the network structure of Comparative Examples 3 and 4 is significantly denser and more continuous than that of Example 1, with smaller pores and a denser overall network structure, resulting in a viscous and aggregated product. Example 1, on the other hand, is relatively loose, with more obvious pores and lower viscosity. The microstructure characteristics are consistent with the described taste trends.

[0103] Comparative Example 5 The effect of a single mixing time shorter than the range of this invention (10-15 min) on the product texture was verified.

[0104] It is basically the same as Example 1, except that in step S2, the mixing time is 5 min.

[0105] Mix at 28°C for only 5 minutes according to the proportions of Example 1, with the remaining steps unchanged.

[0106] Test results: Product viscosity: 19 mPa·s.

[0107] Sensory evaluation: It has a powdery feel, and the smoothness score is 5.5.

[0108] If the mixing time is too short, the fermented cream and raw milk will not be fully integrated, resulting in uneven distribution of the fat phase and uneven protein aggregation during fermentation, forming a localized dense structure, and the product will have a rough and thick texture.

[0109] Comparative Example 6 The effect of a single mixing time exceeding the range of this invention (10-15 min) on the product texture was verified.

[0110] Parameter settings: basically the same as in Example 1, except that in step S2, the mixing time is 20 min. Mix at 28°C for 20 min, and the other steps remain unchanged.

[0111] Product viscosity: 16 mPa·s.

[0112] Sensory evaluation: It has a powdery feel, and its smoothness score is 6.5.

[0113] Excessive mixing time leads to over-emulsification of some fat phases, damages the interface structure between protein and fat, causes abnormal protein network structure during fermentation, and results in a product with a thicker texture and a noticeable powdery feel.

[0114] Table 3: The impact of different single-mixing times on product performance

[0115] Comparative Example 7 Verify the effect on product texture when the mixing temperature is lower than the range of this invention (25-30°C).

[0116] The preparation method is the same as in Example 1, except that the mixing temperature in step 2 is 23°C.

[0117] Test results: Product viscosity: 25 mPa·s.

[0118] Sensory evaluation: The texture is distinctly powdery, and the smoothness score is 7.0.

[0119] If the mixing temperature is too low, the fat in the fermented butter will not be softened and dispersed sufficiently, resulting in uneven fusion of fat and protein. During fermentation, the protein network will be excessively cross-linked, resulting in a powdery texture in the product.

[0120] Comparative Example 8 Verify the effect on product texture when the mixing temperature is higher than the range of this invention (25-30°C).

[0121] The preparation method is the same as in Example 1, except that the mixing temperature in step 2 is 32°C. Mixing is performed at 32°C for 13 minutes, with the remaining steps unchanged.

[0122] Test results: Product viscosity: 34 mPa·s.

[0123] Sensory evaluation: The texture is noticeably powdery, and the smoothness score is 6.5.

[0124] If the mixing temperature is too high, some fats will over-melt or become unstable through emulsification. During fermentation, protein aggregation will accelerate, forming an uneven and dense network, resulting in a rough and thick product texture.

[0125] Table 4: Effect of different primary mixing temperatures on product performance

[0126] Comparative Example 9 Verify the irreplaceability of the strain FC10 specific to this invention.

[0127] Parameter settings: exactly the same as in Example 1, except that the FC10 strain in step S3 is replaced with commercially available strain A (single Streptococcus thermophilus subsp. ST446, from Sacco, Italy).

[0128] Following the method in Example 1, the same concentration of commercial strain A was inoculated and fermented until the acidity reached 65°T, with the rest remaining unchanged.

[0129] Test results; Product viscosity: 142 mPa·s.

[0130] Sensory evaluation: The texture is thick, with a smoothness score of 7.0, and it does not have a typical milky texture.

[0131] Analysis and Conclusion: Commercial strain A produced a certain amount of extracellular polysaccharides in the specific matrix of this invention, resulting in a significant increase in product viscosity and a loss of the smooth, thin milky texture. This indicates that strain FC10 possesses unique fermentation characteristics highly adapted to the process of this invention and cannot be replaced by conventional strains.

[0132] Comparative Example 10 Further verification of the uniqueness and synergistic effect of the special strain FC10 of this invention.

[0133] Parameter settings: The preparation method is the same as in Example 1, except that the bacterial strain in step 3 is replaced with commercially available bacterial strain B (Denisco strain YO-MIX187: containing Streptococcus salivarius thermophilus subsp. and Lactobacillus delbrueckii subsp. bulgaricus).

[0134] Following the method in Example 1, the same concentration of commercial strain B was inoculated and fermented until the acidity reached 65°T, with the rest remaining unchanged.

[0135] Test results: Product viscosity: 168 mPa·s. Sensory evaluation: Thick texture, smoothness score 6.0, no milky texture.

[0136] Analysis and Conclusion: Commercial strain B, under the synergistic effect of multiple strains, produces a large amount of extracellular polysaccharides, further exacerbating the product's thick texture, completely deviating from the refreshing, smooth, milky texture sought by this invention. This further demonstrates that strain FC10 and the two-step fermentation process of this invention have a unique synergistic effect.

[0137] Table 5: Effects of different fermentation strains on product performance

[0138] The above examples and comparative examples fully demonstrate that the present invention, by precisely controlling the final acidity of fermented cream to 20-25°T, controlling the mixing ratio of raw milk and fermented cream within a specific range, precisely limiting the temperature and time of a single mixing step, and combining it with a specific subspecies of Streptococcus salivarius FC10, can synergistically produce excellent technical effects. Any deviation from key parameters (such as in comparative examples 1-8), or substitution of bacterial strains, will not yield a unique product with a smooth milky texture and no graininess. The method provided by the present invention successfully achieves a breakthrough improvement in the texture of fermented dairy products without relying on any additives.

[0139] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A strain of Streptococcus salivarius subsp. thermophilus FC10, characterized in that, The preservation number of the thermophilic subspecies of Streptococcus salivarius FC10 is CGMCC NO.31796.

2. A fermented dairy product, characterized in that, The fermentation viscosity of the fermented dairy product is 5-15 mPa·s.

3. The fermented dairy product according to claim 2, characterized in that, The viscosity of the fermented dairy product is 5-10 mPa·s.

4. The fermented dairy product according to claim 2, characterized in that, In the secondary protein structure of the fermented dairy product, the β-sheet ratio is 15-20%, the α-helix ratio is 24-28%, and the β-turn ratio is 25-30%.

5. The fermented dairy product according to claim 2, characterized in that, The fermented dairy product includes the Streptococcus salivarius thermophilic subsp. FC10 strain as described in claim 1, wherein the FC10 strain is deposited at the China General Microbiological Culture Collection Center with accession number CGMCC NO.31796 and accession date of September 2, 2024.

6. A method for preparing a fermented dairy product according to claim 2, characterized in that, Includes the following steps: S1. Preparation of fermented butter: After sterilizing and cooling butter with a fat content of 40-45%, add a starter culture and ferment. When the acidity reaches 20-25°T, cool it for later use. S2. Mixing and Sterilization: The raw milk and the fermented cream obtained in step S1 are mixed for the first time to obtain a primary mixture; the primary mixture is then mixed with the raw milk for the second time to obtain a mixed liquid; the mixed liquid is homogenized and sterilized; the proportions of raw milk and fermented cream are as follows: in the first mixing, the raw milk is 30-35 parts and the fermented cream is 10-15 parts, and in the second mixing, the raw milk is 50-60 parts. S3. Fermentation: Cool the sterilized mixture and inoculate it with Streptococcus salivarius subsp. thermophilus FC10 for fermentation. S4. Post-processing: After fermentation reaches the endpoint, the milk is broken, cooled, homogenized under air pressure, filled, and then ripened to obtain the fermented dairy product.

7. The method for preparing fermented dairy products according to claim 6, characterized in that, In step S1, the cream is derived from defatting the raw milk; the sterilization conditions are 87-95°C for 5-10 minutes; the cooling conditions are cooling to 6-10°C for later use.

8. The method for preparing fermented dairy products according to claim 6, characterized in that, In step S2, the temperature of the first mixing is 25-30°C and the time is 10-15 min; the temperature of the second mixing is 8-12°C.

9. The method for preparing fermented dairy products according to claim 6, characterized in that, In step S2, the homogenization pressure is 180-220 bar; the sterilization conditions are 90-98°C and the time is 240-360 seconds.

10. The method for preparing fermented dairy products according to claim 6, characterized in that, In step S3, the cooling temperature is 40-44°C; the fermentation endpoint is when the acidity reaches 60-70°T and the emulsion breaks down.

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