Streptococcus thermophilus LA6-1-3 and application of streptococcus thermophilus LA6-1-3 in preparation of cream-flavored yoghourt

Abstract

The invention belongs to the field of microbial fermentation, and particularly relates to streptococcus thermophilus LA6-1-3 and application of the streptococcus thermophilus LA6-1-3 in preparation of cream-flavored yoghourt, the classification name of the streptococcus thermophilus LA6-1-3 is Streptococcus thermophilus LA6-1-3, the preservation number of the streptococcus thermophilus LA6-1-3 is CCTCC M 20242781, the preservation date of the streptococcus thermophilus LA6-1-3 is December 12, 2024, and the preservation unit of the streptococcus thermophilus LA6-1-3 is China Center for Type Culture Collection. The streptococcus thermophilus LA6-1-3 which is high in sensory evaluation score and has typical cream flavor is excavated, and it is confirmed that the content of two key flavor substances including 2, 3-butanedione and delta-decalactone in fermented milk of the streptococcus thermophilus LA6-1-3 is high, so that the streptococcus thermophilus LA6-1-3 is the main source of the cream flavor; after the streptococcus thermophilus LA6-1-3 is used for fermentation, the fermented milk can be endowed with obvious cream flavor, and the problem of yoghurt flavor homogenization can be solved from the source of the fermented milk strain. And when the streptococcus thermophilus LA6-1-3 and the lactobacillus bulgaricus are compounded and fermented, the performance of fermentation characteristic indexes is excellent, the sensory evaluation is relatively good, and the indexes such as texture and viscosity are not greatly different from those of commercially available products, so that the streptococcus thermophilus LA6-1-3 and the lactobacillus bulgaricus have marketization application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of microbial fermentation, and particularly relates to Streptococcus thermophilus LA6-1-3 and its application in the preparation of cream-flavored yogurt. Background Art

[0002] Fermented milk is a curd-like product prepared by fermenting lactic acid bacteria such as Streptococcus thermophilus. After fermentation, the protein in dairy products itself is more easily absorbed. At the same time, various flavor substances produced during the fermentation process also endow fermented milk with a unique taste. Fermented milk that combines nutrition and taste is also highly sought after in the consumer market.

[0003] Flavor is an important indicator that can determine the quality of fermented dairy products and consumers' willingness to purchase. With the increasing market share of fermented milk, in order to effectively meet consumers' strong demand for diversified flavors of fermented milk and achieve flavor differences among different categories to make up for the deficiencies in the flavors fermented by strains, each manufacturer often adds flavoring agents or fruit ingredients to fermented milk to varying degrees. As a result, the taste of most commercially available fermented milk products is similar, not prominent, and the flavor homogenization phenomenon is serious. The main flavors are mainly traditional original flavor, fruit flavor, and cheese flavor, etc. There is a lack of production and sales of fermented milk with special flavors such as cream flavor and milk flavor in the market, which seriously restricts the long-term growth and rapid expansion of the fermented milk market.

[0004] With the improvement of people's living standards and the formation of the concept of "advocating nature and returning to nature", green and natural products without added flavors have become an important trend in the development of the food industry. Therefore, how to explore aroma-producing strains to increase the yield of their aroma components to increase the flavor of fermented milk without adding additional substances such as flavors is a problem to be solved. Summary of the Invention

[0005] In view of the need for strains with increased aroma components in the prior art, the present invention provides LA6-1-3 and its application in the preparation of cream-flavored yogurt. The specific technical solutions are as follows:

[0006] 2. First, the present invention provides Streptococcus thermophilus LA6-1-3. The taxonomic name of Streptococcus thermophilus LA6-1-3 is Streptococcus thermophilus LA6-1-3, the deposit number is CCTCC M 20242781, the deposit date is December 12, 2024, and the deposit unit is China Center for Type Culture Collection.

[0007] Second, the present invention provides a bacterial agent for yogurt fermentation, and the bacterial agent includes Streptococcus thermophilus LA6-1-3 as claimed in claim 1.

[0008] Furthermore, the bacterial agent also includes Lactobacillus bulgaricus.

[0009] Further, the Lactobacillus delbrueckii subsp. bulgaricus is L9-8.

[0010] Further, the ratio of Streptococcus thermophilus LA6-1-3 to Lactobacillus delbrueckii subsp. bulgaricus is 1 to 1000:1.

[0011] Further, the ratio of Streptococcus thermophilus LA6-1-3 to Lactobacillus delbrueckii subsp. bulgaricus is 10 to 100:1.

[0012] Further, the ratio of Streptococcus thermophilus LA6-1-3 to Lactobacillus delbrueckii subsp. bulgaricus is 100:1.

[0013] In a third aspect, the present invention provides the use of the above-mentioned Streptococcus thermophilus LA6-1-3, or the above-mentioned bacterial agent in the preparation of creamy-flavored yogurt.

[0014] In a fourth aspect, the present invention provides a method for preparing creamy-flavored yogurt, comprising: activating the bacterial agent as described in claim 4, adding it to sterilized pure milk for fermentation, stopping fermentation at the end of fermentation, and performing after-ripening to obtain creamy-flavored yogurt.

[0015] Further, the volume ratio of the bacterial agent to pure milk is 1 to 5:100.

[0016] Further, during the fermentation process, the fermentation temperature is 37 to 42 °C.

[0017] Further, the end of fermentation is: after the pure milk is fermented, the pH value reaches 4.5.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention has discovered a Streptococcus thermophilus LA6-1-3 with a high sensory evaluation score and typical creamy flavor, and confirmed that the contents of two key flavor substances, 2,3-butanedione and δ-decalactone, in its fermented milk are relatively high, which are the main sources of creamy flavor; after fermentation with Streptococcus thermophilus LA6-1-3, the fermented milk can be given an obvious creamy flavor, which can solve the problem of homogenization of yogurt flavor from the source of fermented milk strains. And when Streptococcus thermophilus LA6-1-3 and Lactobacillus delbrueckii subsp. bulgaricus are used for compound fermentation, their fermentation characteristic indexes have excellent performance, the sensory evaluation is good, and indexes such as texture and viscosity are not much different from those of commercially available products, having a market application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a morphological diagram of the LA6-1-3 strain.

[0021] Figure 2It is a radar chart of the aroma typing evaluation results of 8 strains of S. thermophilus fermented milk.

[0022] Figure 3 It is the growth curve of S. thermophilus LA6-1-3 strain cultured in M17 medium at 37°C.

[0023] Figure 4 It is the pH change curve of S. thermophilus LA6-1-3 single-strain fermented milk.

[0024] Figure 5 It is a radar chart of the aroma typing evaluation of different compound fermented milk.

[0025] Figure 6 It is a radar chart of the sensory evaluation of different compound fermented milk.

[0026] Figure 7 It is the fingerprint spectrum of flavor components in fermented milk (where the left labels 6-13, 10, 100 represent group 6-13, group F2, and group F3 respectively. Among them, F2 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:10, F3 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:100, and LA6-1-3 is fermented by S. thermophilus LA6-1-3 alone).

[0027] Figure 8 It is a schematic diagram of the viscosity value of fermented milk; among them, F2 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:10, F3 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:100, and LA6-1-3 is fermented by S. thermophilus LA6-1-3 alone.

[0028] Figure 9 It is a schematic diagram of the syneresis rate of fermented milk; among them, F2 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:10, F3 is fermented with the ratio of S. thermophilus LA6-1-3 to Lactobacillus bulgaricus being 1:100, E5 is a commercially available starter, and LA6-1-3 is fermented by S. thermophilus LA6-1-3 alone. Detailed implementation methods

[0029] To enable those skilled in the art to better understand the solution of the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. It should be noted that the following detailed description is exemplary and only a part of the embodiments of the present invention, rather than all of the embodiments.

[0030] Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0031] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs. The experimental materials used in the embodiments of the present invention are all conventional experimental materials in the art and can be obtained through commercial channels. The experimental methods without detailed conditions are carried out according to conventional experimental methods or according to the operation manuals recommended by the suppliers.

[0032] Example 1 Isolation and Identification of Strains

[0033] 1. Strain isolation: The commercially available starter culture collected was added to the milk medium, mixed evenly, and cultured at 37°C for 4 h to facilitate the recovery of the strain's viability; 1 mL of the medium and bacterial powder mixture was added to normal saline for gradient dilution, 100 μL of the diluted solution was taken and spread evenly on the M17 solid medium, and cultured at 37°C for 48 h. Colonies with smooth surfaces were selected for Gram staining, and the bacterial morphology was observed under an oil immersion microscope. Colonies with coccus morphology were selected and inoculated into the M17 liquid medium, cultured at 37°C for 24 h. When the culture solution became turbid and there was precipitation at the bottom, the precipitate was resuspended, and a small amount of the bacterial solution was dipped with an inoculation loop and streaked on the M17 solid medium twice. Gram staining was performed to ensure pure bacteria. The colonies were taken into the M17 liquid medium and cultured at 37°C for 24 h. When the bacterial solution was turbid and there was flocculent precipitation at the bottom of the test tube, the strain was preserved and identified.

[0034] 2. Identification of lactic acid bacteria strains

[0035] (1) Physiological and biochemical identification

[0036] After Gram staining, the LA6-1-3 strain was observed under an optical microscope at 100× oil immersion magnification for the strain morphology. The microscopic examination results of the strain are as Figure 1 shown. The morphology of this strain is spherical and distributed in short chains.

[0037] (2) The 16S rRNA sequencing of the strain was completed by Shanghai Sangon Biotech Co., Ltd. The sequencing results were compared for homology using BLAST on NCBI: The results showed that the 16S rRNA sequence of the strain (shown as SEQ ID NO.1) had a similarity of 99.9% with strains such as Streptococcus thermophilus 4602 (GenBank accession number MT545099.1) and 3781 (GenBank accession number MT538634.1), proving that the strain belongs to the genus Streptococcus thermophilus and has a high homology. However, there are still some differences in part of the sequences, and it is a new strain. Therefore, it was named Streptococcus thermophilus LA6-1-3.

[0038] SEQ ID NO.1:

[0039]

[0040] Example 2 Preparation and Analysis of Fermented Milk

[0041] Using pure milk as the raw milk, add 6% (wt / v) granulated sugar, heat in a water bath at 60 °C for 30 min, then sterilize at high temperature in a water bath at 95 °C for 5 min, and quickly cool in an ice bath to about 42 °C. In a laminar flow hood, add the activated different S. thermophilus strains to the sterilized raw milk at a ratio of 2% and stir well, then place it in an incubator at 42 °C for fermentation. Stop fermentation when the pH of the fermented milk drops to 4.5, and cool the sample. Store the fermented sample in a refrigerator at 4 °C for after-ripening. After 24 h of after-ripening, it is ready for sensory analysis and instrumental analysis.

[0042] (1) Aroma Typing Evaluation

[0043] Perform aroma typing evaluation on S. thermophilus single-strain fermented milk. The definitions and reference substances of 4 aroma attribute descriptors are shown in Table 1, and the scoring basis for evaluation is in accordance with Table 2. The specific evaluation results of aroma typing are shown in Table 3, and the aroma typing radar chart is as Figure 2 shown. It can be seen that compared with other strains, LA6-1-3 shows a stronger buttery flavor.

[0044] Table 1 Definitions and Reference Substances of 4 Aroma Attribute Descriptors

[0045]

[0046] Table 2 Scoring Basis for the Intensity of Aroma Attributes

[0047]

[0048]

[0049] Table 3 Evaluation Results of Aroma Typing of 8 Strains of S. thermophilus Fermented Milk

[0050]

[0051] Note: 1. The results of aroma typing analysis are averaged and rounded to an integer; 2. Different lowercase letters in the same row with superscripts indicate significant differences (p < 0.05).

[0052] (2) Analysis of Quantitative Results of Flavor Compounds by HS-SPME-GC-MS

[0053] Weigh 8.0 g of the fermented milk sample and put it into a headspace vial. Add 5 μL of 0.8093 mg / mL 2-methyl-3-heptanone solution as the internal standard. Place the headspace vial containing the sample and internal standard mixture into a solid-phase microextraction device, heat it at 55 °C for 20 min for equilibration and stir magnetically at a rotation speed of 600 r / min simultaneously. Insert the aged CAR / PDMS extraction head into the headspace above the headspace vial, and inject the sample after 40 min of headspace extraction. After the extraction is completed, insert the extraction fiber into the GC injection port for desorption for 6 min, and the desorption temperature is 250 °C. The chromatographic column is an Agilent DB-Wax capillary column (30 m × 0.25 mm, 0.25 μm). The front injection port adopts the splitless injection mode, and the solvent delay is set to 3.2 min. The carrier gas uses 99.99% high-purity helium gas, and its flow rate is 1 mL / min. The interface is at 250 °C, the connecting rod is at 150 °C, the ion source is at 230 °C, and the electron energy is 70 eV. The mass spectrometry scanning range is: m / z 30 - 350.

[0054] Adopt the semi-quantitative method of internal standard method, and calculate the content of the compound in the fermented milk system by formula (1):

[0055]

[0056] In the formula, X 1 is the content of the compound to be determined in the fermented milk system; X 0 is the content of the internal standard in the 8.0 g fermented milk system; S 1 is the integral peak area of the compound; S 0 is the integral peak area of the internal standard.

[0057] Calculate the retention index RI of the compound by formula (2):

[0058]

[0059] In the formula, n is the number of carbon atoms, t n is the retention time of the n-alkane with n carbon atoms; t n+1 is the retention time of the n-alkane with n + 1 carbon atoms; t i is the retention time of compound i.

[0060] The results are shown in Table 4. It can be seen that the contents of δ-decalactone and 2,3-butanedione in LA6-1-3 fermented milk are higher than those of other strains.

[0061] Table 4 Standard curves of volatile compounds and quantitative results by external standard method

[0062]

[0063] Continued Table 4

[0064]

[0065]

[0066] (3) Qualitative and semi - quantitative analysis of volatile flavor compounds were carried out on LA6 - 1 - 3 with the most prominent buttery flavor and S4 - 2 - 8 with the most prominent fermented flavor.

[0067] The comprehensive determination of the volatility differences of each aroma compound was carried out by HS - SPME - GC - MS, and the results are shown in Table 5. It can be seen from Table 5 that the contents of ketones and lactones in the LA6 - 1 - 3 fermented milk sample were significantly higher than those in the S4 - 2 - 8 fermented milk (P < 0.05).

[0068] Table 5 Qualitative and semi - quantitative results of volatile flavor compounds in fermented milk

[0069]

[0070]

[0071]

[0072] Note: 1. Retention index of compounds separated by HP - INNOWax chromatographic column. 2. The reference value of the retention index refers to the NIST Chemistry WebBook website (https: / / webbook.nist.gov / chemistry / ). 3. Compound qualitative methods: Mass Spectrometry (MS), Retention Index (RI), and Standards (S). 4. "ND" means not detected. 5. The HS - GC - MS results are expressed as mean ± standard deviation (SD), and different letters in the same row indicate significant differences (p < 0.05).

[0073] Example 3 Strain growth characteristics and pH change of fermented milk

[0074] 1. Strain growth characteristics

[0075] S. thermophilus LA6 - 1 - 3 was cultured and grown in M17 medium at 37°C. Every 2 h, the activated S. thermophilus LA6 - 1 - 3 bacterial solution was taken out of the incubator, and the change in its absorbance value (Optical Density, OD) within 24 h was measured at 600 nm using a microplate reader. The data was recorded to construct the growth curve of S. thermophilus LA6 - 1 - 3, and the results are as Figure 2 shown.

[0076] FromFigure 3 It can be seen that the strain is in the lag phase within the 0 - 2h period, with a relatively slow growth rate. It is in the rapid growth phase within the 2 - 16h period. LA6 - 1 - 3 enters the exponential growth phase at the 4th hour and is still in the growth phase at the 24th hour, indicating its rapid growth and large reproduction amount.

[0077] 2. Change curve of the pH value of fermented milk

[0078] Fermented milk was fermented according to the method in Example 2, and the pH of the yogurt was measured according to the operation of the national standard "GB5009.239 - 2016 National Food Safety Standard Determination of Food Acidity". The results are as Figure 4 shown. It can be seen that within the 0 - 4h period, it is in the lag phase, with a relatively slow acid production rate. It is in the rapid acid production phase within the 4 - 18h period, and the acidity steadily increases. After 18h, it is in the acid production stable phase, and the change of the acidity value is relatively stable, and the acid production rate decreases. And it can be seen that the acid production amount of LA6 - 1 - 3 is relatively large.

[0079] Example 4. Co - fermentation of S. thermophilus LA6 - 1 - 3 and Lactobacillus bulgaricus

[0080] Co - fermentation of LA6 - 1 - 3 and Lactobacillus bulgaricus L9 - 8 (Ao L, Qiqi L, Yushan B, et al. Aroma classification and characterization of Lactobacillus delbrueckii subsp. bulgaricus fermented milk[J]. Food Chemistry: X, 2022, 15100385 - 100385.) was carried out according to the method described in Example 2, where the ratios of LA6 - 1 - 3 to Lactobacillus bulgaricus were 1:1 (Group F1), 1:10 (Group F2), 1:100 (Group F3), and 1:1000 (Group F4) respectively.

[0081] (1) Fermentation characteristics

[0082] The fermentation characteristic indexes of 5 groups of fermented milk samples are shown in Table 3. Among them, the fermentation time of Group F1 is the shortest, which is 5.4h. Followed by Group F3 and Group F2, the fermentation times are 5.6h and 5.8h respectively. The fermentation time of Group F4 is the longest, reaching 6.2h. When the 5 groups of fermented milk with different ratios reach the end point, that is, when the pH reaches about 4.5, the TA at the end point is above 60°T, and the total colony count is also relatively high (the highest in Group F3 is 9.24×10 8 CFU / g, followed by Group F2 with 8.65×10 8 CFU / g), all meeting the quality requirements of the national standard for fermented milk.

[0083] Table 6 Fermentation characteristic indexes of different compound fermented milk samples

[0084]

[0085] (2) Aroma classification evaluation of compound fermented milk

[0086] The results of aroma classification evaluation of 4 groups of compound fermented milk and the single-strain fermented milk LA6-1-3 with cream flavor are as Figure 5 shown. It can be seen that, compared with the single-strain fermented milk LA6-1-3, the aroma characteristics of the compound milk did not change significantly due to the different compounding ratios between the two, and still maintained the prominent cream flavor; the aromas of the 5 groups of samples were all concentrated in the high scores of cream fragrance, and on the contrary, the fermentation fragrance was the lightest. Among them, the aroma attribute score of cream flavor in group F3 was the highest at 8 points, followed by group F2 with a score of 7 points, and group F1 was the lowest at 4 points.

[0087] (3) Sensory evaluation of compound fermented milk

[0088] The results of sensory evaluation of fermented milk samples are as Figure 6 shown. It can be seen that the total score of the fermented milk in group F1 was 67.50 points, with a small amount of whey separated out, the color was milky white and dull yellow, the whole was slightly dull, and the taste was slightly sour. The drawability of group F2 was better, and the score of tissue state reached the highest score of 15.89 points, with an appropriate sweet and sour ratio and a very delicate taste. The total score of group F3 was the highest at 80.70 points, and the scores of color, smell and taste, tissue state and degree of preference reached 16.75 points, 13.57 points, 15.00 points, 15.88 points, and 17.50 points respectively. The surface of the fermented milk sample of this compound group was smooth and shiny, the color was milky white, accompanied by cream fragrance, tasted cool, the tissue was uniform and delicate, without bubbles and whey separation, and the taste was good. The total score of group F4 was the lowest, and the scores in terms of color, smell and taste, tissue state and degree of preference were 15.00 points, 12.20 points, 11.7 points, 13.12 points, and 10.80 points respectively, and the total score was only 62.82 points. It had more whey separation, a slightly sour taste and a not delicate enough taste.

[0089] (4) Contents of 2,3-butanedione and δ-decalactone in compound fermented milk samples

[0090] The contents of the key flavor substances with buttery aroma, 2,3-butanedione and δ-decalactone, in different compound fermented milks were determined, and the results are shown in Table 7. It can be seen from the table that there are differences in the yields of 2,3-butanedione in the four groups of different compound fermented milks. The yields of 2,3-butanedione in Group F3 and Group F2 are 22.61 μg / kg and 20.97 μg / kg respectively, which are significantly higher than 19.78 μg / kg in the single strain LA6-1-3 (p<0.05). In terms of the yield of δ-decalactone, the yields in Group F3 and Group F2 are 2.36 μg / kg and 2.26 μg / kg respectively, which are significantly higher than 2.16 μg / kg in the single strain LA6-1-3 (p<0.05); while the contents of 2,3-butanedione and δ-decalactone in Group F1 and Group F4 are relatively reduced.

[0091] Table 7 Contents of 2,3-butanedione and δ-decalactone in different compound fermented milks

[0092]

[0093] Note: Different lowercase letters in the same column superscript indicate significant differences (p<0.05).

[0094] (5) HS-GC-IMS analysis of compound fermented milk

[0095] The HS-GC-IMS analysis fingerprint of the compound fermented milk is as Figure 7 shown. Each integration point represents a specific substance, and its size and color intensity may be related to the relative content of the substance. It can be Figure 7 seen that the integration circle diagrams of 2,3-butanedione in the compound fermented milks of Group F2 and Group F3 are more obvious, indicating that its content is higher. The addition of L. bulgaricus helps to increase the production of 2,3-butanedione; however, the dimer of 2,3-butanedione has a higher content in LA6-1-3. The contents of ketone substances such as 2,3-pentanedione, 2-heptanone, 2-pentanone, and 2-butanone in Group F2 and Group F3 are significantly increased compared with LA6-1-3. These may all be due to the effect of the addition of L. bulgaricus in the compound. The content of ethyl acetate also increases significantly through compounding, but the dimer of ethyl acetate has a higher content in the fermented milk of LA6-1-3.

[0096] (6) Viscosity analysis of compound fermented milk

[0097] The viscosity measurement results of the compound fermented milk are as Figure 8 shown, in which the commercially available starter culture (yogurt starter culture E5, Danisco, France) is set as Group E5. It can be seen that the viscosity values of the initially screened excellent compound fermented milks from high to low are F3>F2>LA6-1-3>E5. Among them, the viscosity value of Group F3 is the highest at 5290 mPa·s, and the viscosity value of Group E5 is the lowest at 2950 mPa·s.

[0098] (7) Texture analysis of compound fermented milk

[0099] The texture analysis results of the compound fermented milk in groups F2, F3, LA6-1-3, and E5 are shown in Table 8. Further analysis shows that the initially screened excellent compound fermented milk, F2 and F3, are significantly higher than LA6-1-3 in terms of hardness, adhesiveness, cohesiveness, gumminess, and chewiness. At the same time, there is no significant difference between F3 and E5. The hardness and chewiness of F3 and E5 are both significantly higher than those of LA6-1-3. The texture analysis results show that compared with the single-strain fermentation of LA6-1-3, the texture characteristics of the compound fermented milk in groups F2 and F3 are significantly improved and are very close to those of the commercially available starter culture group E5, indicating a very high possibility of being used as a market-launched starter culture in the future.

[0100] Table 8 Texture analysis results of fermented milk

[0101]

[0102] Note: Different letters in each column represent significant differences between groups (p < 0.05)

[0103] (8) Syneresis rate of compound fermented milk

[0104] The syneresis rate results of the 4 groups of fermented milk samples are as Figure 9 shown. It can be seen that the syneresis rate of group F3 reaches 14.13%, which has no significant difference compared with 13.73% of the commercially available starter culture group E5, indicating that the less the whey separation rate, the better the quality. By comparing the cohesiveness results (Table 8), it is found that the change trend of the cohesiveness results is consistent with that of the syneresis rate results and also coincides with the results of the previous sensory evaluation of the tissue morphology. The syneresis rate of group LA6-1-3 reaches 22.5% as the highest, and that of group F2 is 17.67%. The water-holding capacity of group LA6-1-3 is poor and is weaker than that of the compound fermented milk, indicating that the addition of L. bulgaricus in the compound enhances the water-holding capacity of the fermented milk. The syneresis rate shows that the quality of the fermented milk in the best compound group F3 is close to that of the commercially available starter culture fermented milk in terms of water-holding ability, and it has a relatively high possibility of being used as a market-launched starter culture in the future.

Claims

1. Streptococcus thermophilus LA6-1-3, characterized in that The thermophilic Streptococcus LA6-1-3 is classified and named as Streptococcus thermophilus LA6-1-3, the preservation number is CCTCC M 20242781, the preservation date is December 12, 2024, and the preservation unit is China Center for Type Culture Collection.

2. A bacterial agent for yogurt fermentation, characterized in that: The bacterial agent includes the thermophilic Streptococcus LA6-1-3 described in claim 1.

3. The bacterial agent according to claim 2, characterized in that The bacterial agent also includes Lactobacillus bulgaricus.

4. The bacterial agent according to claim 3, characterized in that The Lactobacillus bulgaricus is L9-8.

5. Use of the thermophilic Streptococcus LA6-1-3 according to claim 1 or the bacterial agent according to any one of claims 2 to 4 in the preparation of cream-flavored yogurt.

6. A method for preparing cream-flavored yogurt, characterized in that: include: The bacterial agent as claimed in claim 4 is activated, added into sterilized pure milk for fermentation, and the fermentation is stopped when the fermentation end point is reached, and the milk is post-ripened to obtain cream-flavored yogurt.

7. The method according to claim 6, characterized in that The volume ratio of the bacterial agent to pure milk is 1 to 5:

100.

8. The method according to claim 6, characterized in that During the fermentation process, the fermentation temperature is 37-42°C.

9. The method according to claim 6, characterized in that The fermentation end point is: after the pure milk is fermented, the pH value reaches 4.5.

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