Single-bacterium fermented yoghurt oil with high quality and nutritional value and preparation method of single-bacterium fermented yoghurt oil

Abstract

The invention discloses single-bacterium fermented yoghurt oil with high quality and nutritional value and a preparation method of the single-bacterium fermented yoghurt oil. According to the method, lactococcus lactis grx602 is used for fermenting butter-based single cream added with exopolysaccharides from lactobacillus rhamnosus grx603. The lactobacillus rhamnosus grx603-sourced exopolysaccharide is added into the butter-based single cream, so that the hydrophobicity and the water-holding capacity of the fermented yoghurt oil are obviously improved, the whey separation condition of the fermented yoghurt oil is remarkably reduced, the flavor of the fermented yoghurt oil is richer, positive influences are exerted on the rheological property and the texture property of the yoghurt oil, and the fermented yoghurt oil has the advantages that the fermented yoghurt oil can be used for preparing the yogurt oil. The quality characteristics of the fermented yoghurt oil are improved, the in-vitro oxidation resistance of the fermented yoghurt oil can be improved, and the fermented yoghurt oil is endowed with more excellent functional characteristics.

Description

Technical Field

[0001] The present invention belongs to the technical field of sour cream processing, and particularly relates to a single-strain fermented sour cream with high quality and nutritional value and a preparation method thereof. Background Art

[0002] As an alternative name for fermented cream, sour cream belongs to the category of fermented dairy products with high nutritional value and is prepared by acidifying pasteurized cream with lactic acid bacteria. Among them, the titratable acidity index measured in terms of lactic acid shall not be less than 0.5%. Sour cream contains rich nutritional components, bioactive substances, essential fatty acids and unsaturated fatty acids required by the human body, and has extremely high nutritional value, and is widely used in the fields of baking, flavoring and ready-to-eat foods. With the improvement of consumers' requirements for food quality, the optimization of sour cream in terms of texture stability, sensory characteristics and functionality has become a research hotspot. However, problems such as whey separation, rough texture and high acidity that are prone to occur during its production and storage still restrict the further improvement of product quality.

[0003] Lactic acid bacteria exopolysaccharide (EPS) is a high-molecular polysaccharide secreted by specific strains during metabolism. Research shows that EPS can enhance the water-holding capacity of milk protein gel by forming a three-dimensional network structure, and at the same time regulate the viscoelasticity and taste of the product. In addition, the biological activities of EPS (such as antioxidant and immunomodulatory effects) also provide a new direction for the functional development of fermented dairy products. However, current research on EPS mostly focuses on yogurt or cheese systems, and the application research on the special matrix of sour cream with high fat and low pH value is still relatively scarce.

[0004] At present, there are few technological innovation achievements in sour cream, and problems such as uneven texture, whey separation and / or high acidity are prone to occur in the products. In actual production, other auxiliary materials are generally added to sour cream to improve the texture, inhibit syneresis or extend the shelf life, such as rennet, nutritive sweeteners, salt or some seasonings containing pigments. As consumers increasingly favor green and healthy foods, therefore, developing high-quality and stable sour cream has broad market prospects. Summary of the Invention

[0005] Aiming at the problems of whey separation, rough texture and high acidity that are prone to occur in the existing sour cream during storage, the present invention provides a single-strain fermented sour cream with high quality and nutritional value and a preparation method thereof. This method performs single-strain fermentation by adding exopolysaccharide (EPS-603) derived from Lactobacillus rhamnosus grx603 to cream. The prepared sour cream has excellent and stable quality and improved nutritional value after fermentation and within 28 hours of storage.

[0006] The technical solution of the present invention is as follows:

[0007] Method for preparing single-strain fermented sour cream with high quality and nutritional value, using Lactococcus lactis grx602 to ferment butter-based cream added with extracellular polysaccharide (EPS-603) derived from Lactobacillus rhamnosus grx603.

[0008] The butter-based cream described in the present invention is cream containing butter, and is composed of the following components: skim milk powder, butter, carrageenan, sodium caseinate, Tween 80, soy lecithin, polyglycerol fatty acid ester, deionized water.

[0009] Preferably, the butter-based cream is composed of the following components by mass percentage: 5% skim milk powder, 18% butter, 0.02% carrageenan, 1.1% sodium caseinate, 0.05% - 0.15% Tween 80, 0.10% - 0.30% soy lecithin, 0.10% - 0.30% polyglycerol fatty acid ester, and the balance is deionized water.

[0010] In a specific embodiment of the present invention, the butter-based cream is composed of the following components by mass percentage: 5% skim milk powder, 18% butter, 0.02% carrageenan, 1.1% sodium caseinate, 0.1% Tween 80, 0.15% soy lecithin, 0.1% polyglycerol fatty acid ester, and the balance is deionized water.

[0011] Preferably, the addition amount of EPS-603 is 0.1% - 0.5% of the mass of the butter-based cream, and more preferably 0.5%.

[0012] The EPS-603 described in the present invention is obtained through the following steps: inoculating Lactobacillus rhamnosus grx603 into a semi-synthetic medium SCDM, culturing, collecting the supernatant, precipitating with trichloroacetic acid and ethanol, dialyzing, and freeze-drying to obtain EPS-603.

[0013] Preferably, the composition of the semi-synthetic medium SCDM is: glucose 20.0 g / L, magnesium sulfate 0.1 g / L, tryptone 10.0 g / L, manganese sulfate 0.05 g / L, sodium acetate 5.0 g / L, dipotassium hydrogen phosphate 2.0 g / L, ammonium citrate 2.0 g / L, Tween 80 0.6 g / L, yeast nitrogen source 5.0 g / L, water.

[0014] Furthermore, the above preparation method specifically includes the following steps:

[0015] Skim milk powder, butter, carrageenan, sodium caseinate, Tween 80, soy lecithin, polyglycerol fatty acid ester, and deionized water are formulated into butter-based cream according to a ratio. Then, EPS-603 is added, homogenized, sterilized, and cooled to room temperature. Activated Lactococcus lactis grx602 is added for fermentation to obtain sour cream.

[0016] Preferably, the room temperature is 25 °C, and the fermentation time is 9 - 18 h.

[0017] Preferably, the inoculation amount of Lactococcus lactis grx602 is 10.6 - 10.8 lg cfu / ml.

[0018] Preferably, the activation method of Lactococcus lactis grx602 is: inoculating Lactococcus lactis grx602 into MRS medium and fermenting for 24 - 48 h.

[0019] Preferably, the number of homogenization times is 2, and the pressure is 15 MPa; the sterilization method is sterilizing at 85 °C for 15 min.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] The present invention first ferments sour cream by adding EPS-603 to butter-based cream, and it is found that it has a significant impact on the secondary structure of the protein in fermented sour cream. Moreover, the network structure of the fermented sour cream with a high concentration of EPS-603 added is more compact and regular, and the particle size distribution is more concentrated and uniform. Furthermore, it affects the hydrophobicity and water-holding capacity of sour cream, significantly improving both, and significantly reducing the whey separation in fermented sour cream; adding EPS-603 also affects the composition of volatile flavor substances in fermented sour cream, making the flavor of fermented sour cream more rich; it has a positive impact on both the rheological properties and texture properties of sour cream, improving the quality characteristics of fermented sour cream, and can also enhance the in vitro antioxidant capacity of its digestion products, endowing fermented sour cream with more excellent functional characteristics. Within 28 days of storage, the variation ranges of parameters such as pH value, acidity, viable cell count, and water-holding capacity are smaller, and the rheological and texture properties have good stability. The sour cream prepared by adding EPS-603 shows better physical and chemical properties, sensory indexes, and stability during storage.

[0022] Explanation of the attached drawings

[0023] Figure 1 It is for the changes in pH value (A) and acidity (B) during the sour cream fermentation process;

[0024] Figure 2 It is for the influence of the EPS-603 addition amount on the viable cell count of sour cream;

[0025] Figure 3 It is for the secondary structure composition of sour cream protein, and different lowercase letters indicate significant differences (P < 0.05);

[0026] Figure 4 Particle size distribution diagram of fermented sour cream;

[0027] Figure 5 Microstructure of fermented sour cream samples with different EPS-603 addition amounts, where A, B, and C represent SC-1 sample, SC-2 sample, and SC-3 sample respectively, and 1, 2, and 3 represent magnifications of 600, 2000, and 5000 times respectively;

[0028] Figure 6 Effect of EPS-603 addition amount on the apparent viscosity of fermented sour cream;

[0029] Figure 7 Effect of temperature on the storage modulus (G′) (A) and loss modulus (G″) (B) of each group of fermented sour cream, and effect of frequency on the storage modulus (C) and loss modulus (D) of each group of fermented sour cream;

[0030] Figure 8 Effect of EPS-603 addition amount on the texture properties of fermented sour cream, where A, B, C, and D are hardness, adhesiveness, cohesiveness, and elasticity respectively;

[0031] Figure 9 Fat coalescence rate of each group of fermented sour cream;

[0032] Figure 10 Hydrophobicity of each group of fermented sour cream;

[0033] Figure 11 Water holding capacity of each group of fermented sour cream;

[0034] Figure 12 Whey separation situation of each group of fermented sour cream after fermentation;

[0035] Figure 13 Cluster heat map of volatile flavor substances of each group of fermented sour cream;

[0036] Figure 14 DPPH scavenging rate (A), hydroxyl radical scavenging rate (B), and total antioxidant capacity (C) of digestion products of each group of fermented sour cream at different digestion stages;

[0037] Figure 15 Changes in pH (A) and acidity (B) of each group of fermented sour cream during storage;

[0038] Figure 16 Changes in viable count of each group of fermented sour cream during storage;

[0039] Figure 17 Changes in water holding capacity of each group of fermented sour cream during storage;

[0040] Figure 18 For each group of fermented sour cream, the apparent viscosity during storage changes with the shear rate. Among them, A is the SC-1 sample, B is the SC-2 sample, and C is the SC-3 sample;

[0041] Figure 19 For each group of fermented sour cream, the frequency sweep curve during storage; among them, A, B, and C are the storage modulus of the SC-1, SC-2, and SC-3 samples respectively, and D, E, and F are the loss modulus of the SC-1, SC-2, and SC-3 samples respectively.

[0042] Figure 20 For each group of fermented sour cream, the changes in texture characteristics during storage. Among them, A, B, C, and D are hardness, adhesiveness, elasticity, and cohesiveness respectively. Detailed implementation mode

[0043] The present invention will be further described in detail below in conjunction with specific embodiments, drawings, and tables.

[0044] Lactococcus lactis grx602 of the present invention, with the preservation number of CGMCC No. 22692, has been fully disclosed in Chinese Patent 202110960058.0 previously applied by the inventor's laboratory. Lactobacillus rhamnosus grx603, with the preservation number of CGMCC No. 22693, has been fully disclosed in Chinese Patent 202111220369.X previously applied by the inventor's laboratory.

[0045] The relevant culture media, reagent formulas, and testing methods in the following examples are as follows.

[0046] 1. Experimental culture media

[0047] (1) SCDM medium: glucose 20.0 g / L, magnesium sulfate 0.1 g / L, tryptone 10.0 g / L, manganese sulfate 0.05 g / L, sodium acetate 5.0 g / L, dipotassium hydrogen phosphate 2.0 g / L, ammonium citrate 2.0 g / L, Tween 80 0.6 g / L, yeast nitrogen source 5.0 g / L, water.

[0048] (2) Liquid MRS medium: glucose 20.0 g, peptone 10.0 g, anhydrous sodium acetate 5.0 g, ammonium citrate 2.0 g, dipotassium hydrogen phosphate 2.0 g, magnesium sulfate heptahydrate 0.2 g, manganese sulfate 0.05 g, Tween-80 1.0 ml, beef extract 10.0 g, yeast extract 5.0 g, water 1.0 L.

[0049] Solid MRS medium: On the basis of the above liquid MRS medium, add agar 15.0 g.

[0050] 2. Main determination methods

[0051] (1) Determination of viable count: Refer to GB 4789.35—2016 to determine the number of lactic acid bacteria in light cream.

[0052] (2) Determination of pH: Measure using a precision pH meter (PHS-3E).

[0053] (3) Determination of acidity: Refer to GB 5009.239—2016 (phenolphthalein indicator method) for determination.

[0054] (4) Determination of titratable acidity: Determine the titratable acidity of the sample according to GB 5009.239—2016, "National Food Safety Standard - Determination of Food Acidity".

[0055] (5) Determination of the hydrophobicity of sour cream: Dilute the fermented sour cream sample with phosphate buffer (0.1 mol / L, pH 7.0) to a protein concentration of 0.1 g / L. Mix 3 mL of the diluted solution with 50 μL of ANS solution (8 mmol / L), vortex to make the system uniform, incubate in a light-shielded environment for 20 min, and measure using a fluorescence spectrophotometer. The main parameters are: slit width 5 nm, excitation wavelength 390 nm, emission wavelength range 400 - 600 nm.

[0056] (6) Determination of water-holding capacity: Take about 20 g of the ripened yogurt into a 50 mL centrifuge tube. The mass of the sample is m, and the mass of the centrifuge tube is m2. Centrifuge at 4000 r / min at 4 °C for 20 min. Discard the supernatant, invert the centrifuge tube for 5 min, and the mass of the centrifuge tube and the sample after centrifugation is m1. The water-holding capacity is calculated as shown in formula (1):

[0057]

[0058] In the formula: m1 is the total mass of the sample and the centrifuge tube after centrifugation, m2 is the mass of the centrifuge tube, and m is the mass of the sample.

[0059] (7) Determination of viscosity: Determine using a Brookfield DV-1 VISCOMETER viscometer.

[0060] (8) Determination of texture: Use a food texture analyzer to measure and analyze the texture of the fermented sour cream, mainly including texture characteristics such as hardness, elasticity, cohesiveness, and adhesiveness. Each sample is measured three times. The measurement parameters are as follows: detection mode is two compressions; pre-measurement speed is 1.0 mm / s; measurement speed is 1.0 mm / s; post-measurement speed is 1.0 mm / s; trigger force is 0.02 N; probe diameter is 5 mm; probe type is P / 5.

[0061] (9) Determination of rheological properties: The rheological properties of fermented sour cream were determined using a Kinexus Pro rotational rheometer. The CP4 / 40 rotor was selected to measure the change in the apparent viscosity of sour cream with the shear rate. The measurement temperature was 25 °C, and the shear rate was 0.01 - 100 s -1 , and the number of data acquisitions was 5.

[0062] (10) Determination of protein secondary structure: The secondary structure of fermented sour cream was determined by far-ultraviolet circular dichroism (190 - 250 nm) at room temperature. The protein concentration of the sample to be measured was 0.1 mg / mL, the optical path was 0.2 cm, and the measurement was scanned three times to record the spectral changes and calculate the proportion of each secondary structure.

[0063] (11) Determination of aroma components: The GC conditions, MS conditions, and SPME conditions are shown in Table 1. The volatile flavor substances of sour cream were determined according to the method in Table 1, and qualitative and quantitative analyses were performed on the measurement results.

[0064] Table 1 Experimental methods for the determination of flavor substances

[0065]

[0066]

[0067] (12) Determination of the fat coalescence rate: Oil red O pigment solution was used as the detection medium. First, the oil red O pigment solution was prepared and stored in the dark to ensure its chemical stability. Subsequently, the prepared oil red O pigment solution was thoroughly mixed with the cream sample to be measured, and the absorbance of the mixed solution was measured after centrifugation. The measured parameters were substituted into the formula:

[0068]

[0069] where: Φd - fat coalescence rate, i.e., the proportion of free fat in total fat; Φ - mass fraction of fat in the emulsion; m0 - mass of the added oil red O pigment solution, g; mg - mass of the emulsion, g; A1 - absorbance value of the oil red O pigment solution before centrifugation; A2 - absorbance value of the oil red O pigment solution after centrifugation.

[0070] (13) Determination of particle size and zeta potential: The sample to be measured was diluted to a suitable concentration, and a ES90 Nano type Malvern laser particle size analyzer was used with deionized water as the dispersion medium to measure the particle size distribution and zeta potential value of the fermented sour cream sample.

[0071] (14) Determination of in vitro digestion characteristics: Construction of the in vitro digestion model: The oral simulated digestive fluid, gastric simulated digestive fluid, and small intestine simulated digestive fluid are shown in Table 2.

[0072] Table 2 Formulation of simulated in vitro digestive fluids

[0073]

[0074]

[0075] Simulate the oral digestion stage. Adjust the pH of the simulated oral digestive juice to 6.9 (±0.05) with 1M HCl and 1M NaHCO₃. Add α-amylase to the simulated oral digestive juice to make the enzyme activity 100 U / mL. After fully dissolving, filter it with a 0.22 μm sterile filter membrane. Mix 5 g of the fermented sour cream sample with 5 ml of SSF. Digest in a water bath at a temperature of 37 °C and a rotation speed of 150 rpm for 2 min.

[0076] Simulate the gastric digestion stage: Adjust the pH of the simulated gastric digestive juice to 2.0 (±0.05) with 6M HCl. Add pepsin to make the pepsin activity 800 U / mL. After fully dissolving, filter it with a 0.22 μm sterile filter membrane. Add 10 mL of the processed simulated gastric digestive juice to the sample that has completed the simulated oral digestion. Digest in a water bath at a temperature of 37 °C and a rotation speed of 150 rpm for 2 h.

[0077] Simulate the small intestine digestion stage: Adjust the pH of the simulated small intestine digestive juice to 6.5 (±0.05). Add trypsin and bile salts to the simulated small intestine digestive juice to make the final concentration of trypsin 200 U / mL and the final concentration of bile salts 20 mmol / L. After fully dissolving, filter it with a 0.22 μm sterile filter membrane. Add the simulated gastric juice digest to 20 mL of the processed small intestine digestive juice. Digest in a water bath at a temperature of 37 °C and a rotation speed of 150 rpm for 2 h.

[0078] After simulating digestion in various parts in vitro, centrifuge the mixture of each group of sour cream digestion at 9000 rpm and 4 °C for 5 minutes, then collect the supernatant and store it at -80 °C for the next study.

[0079] (15) Determination of the antioxidant capacity of the simulated in vitro digestive juice: The DPPH free radical scavenging ability, hydroxyl free radical, and total antioxidant capacity are all determined using a kit.

[0080] Example 1

[0081] 1. Preparation of sour cream

[0082] (1) Preparation of EPS-603: The freeze-dried Lactobacillus rhamnosus grx603 was transferred to SCDM medium for recovery and cultured continuously for two generations at 37°C. The inoculum was inoculated into SCDM medium at a rate of 5% and statically cultured at 37°C for 20 hours. Subsequently, the mixture was centrifuged at a centrifugal force of 10,000 g for 15 minutes, and the supernatant was collected. This process was repeated twice to ensure complete removal of the bacteria. The obtained supernatant was concentrated to 10% of the initial volume by rotary evaporation. Under continuous stirring, 80% (w / w) trichloroacetic acid solution was added dropwise to the concentrate to a final concentration of 20%. After the mixture was allowed to stand at 4°C overnight, it was centrifuged at 12,000 g for 15 minutes to obtain the supernatant. Three volumes of 95% (v / v) ethanol solution were added to the supernatant, and after standing at 4°C for 24 hours, the precipitate was collected by centrifugation again at 12,000 g for 15 minutes. The precipitate was resuspended in an appropriate amount of deionized water and placed in a dialysis bag with a molecular weight cutoff of 8,000 to 14,000 Da. Dialysis was performed at room temperature for 48 hours, with the dialysate replaced every four hours. Finally, the dialyzed product was freeze-dried to obtain EPS-603.

[0083] (2) Strain activation: Lactococcus lactis grx602 was inoculated into MRS medium at a culture temperature of 37°C and fermented for 48 h.

[0084] (3) Preparation of sour cream

[0085] 1) Preparation of Butter-Based Cream: 5% skim milk powder, 18% butter, 0.02% carrageenan, 1.1% sodium caseinate, 0.1% Tween 80, 0.15% soy lecithin, and 0.1% polyglycerol fatty acid ester were mixed, with deionized water to make up to 100%, to prepare a butter-based cream. The butter used was Anchor butter, with a fat content of 82.9 wt%, unsalted, purchased from Fonterra, New Zealand.

[0086] 2) Homogenization and sterilization: EPS-603 was added to the butter-based cream, homogenized twice at a pressure of 15 MPa, and then sterilized at 85°C for 15 min.

[0087] 3) Inoculation and fermentation: The material obtained in step (2) was cooled to room temperature, and 1% of fermentation bacteria Lactococcus lactis grx602 was added, and fermented at 25° C. for 18 h.

[0088] 4) Storage: Refrigerate the material obtained in step (3) at 4°C.

[0089] 2. Determination of physical and chemical properties of sour cream

[0090] To study the effect of EPS-603 on the quality of fermented sour cream, taking the one without EPS-603 as the control (sample SC-1), 0.1% EPS-603 (sample SC-2) and 0.5% EPS-603 (sample SC-3) were respectively added to the butter-based cream. After inoculating Lactococcus lactis grx602 and fermenting, the fermentation parameters (pH, acidity, viable cell count) during the fermentation process, the structural characteristics (microstructure, particle size), physical characteristics (rheology, texture), stability characteristics (potential, fat coalescence rate, hydrophobicity, water holding capacity), sensory characteristics (flavor substances), antioxidant properties of digestion products and other indicators of sour cream after fermentation were measured respectively.

[0091] (1) Fermentation parameters of sour cream

[0092] It can be seen from Figure 1 that the sour cream samples of SC-1 and SC-2 reached the fermentation end point of 4.5 at 9 h, the pH value of sample SC-3 was 4.61 at 9 h, and reached the fermentation end point of 4.49 at 9.5 h ( Figure 1 A). Generally, when the pH value of sour cream reaches 4.5, it is the fermentation end point. Compared with the control (SC-1), the acidity after fermentation with 0.5% EPS-603 (SC-3) was 44 °T ( Figure 1 B). After fermentation, the viable cell counts in the three samples were 8.81, 8.87 and 9.01 log cfu / mL ( Figure 2 ), respectively, all higher than the viable cell count of lactic acid bacteria in probiotic foods (6.00 lg cfu / mL) stipulated by the Food and Agriculture Organization of the United Nations and the World Health Organization.

[0093] (2) Structural characteristics of sour cream

[0094] 1) Secondary structure of sour cream protein

[0095] The functional properties and stability of proteins largely depend on the composition of their secondary structure. α-helix, β-sheet, β-turn and random coil together constitute the main forms of protein secondary structure. It can be seen from Figure 3 that α-helix was not detected in all three groups of samples; adding 0.1% EPS-603 (sample SC-2) could increase the β-sheet content in sour cream, reduce the β-turn content, and had no significant effect on the random coil content; while adding 0.5% EPS-603 (sample SC-3) could increase the random coil content in sour cream, significantly reduce the β-sheet content, and had no significant effect on the β-turn content. An appropriate β-sheet content can make the product taste smoother. A high random coil content may increase the interaction between protein and water, improve the water holding capacity, but may lead to a loose gel structure.

[0096] 2) Particle size of sour cream

[0097] The particle size distribution of sour cream mainly reflects the size range and uniformity of its fat globules, directly affecting the texture, stability and sensory characteristics of the product.

[0098] As shown in Table 3, the particle size range of the samples in Group SC-1 is between 1990 - 4150 nm; the particle size range of the samples in Group SC-2 is between 3090 - 6440 nm; the particle size range of the samples in Group SC-3 is between 3580 - 6440 nm. It can be seen that Figure 4 the average particle size of the sour cream samples added with EPS-603 is larger than that of the sour cream samples without EPS-603.

[0099] Average particle size of sour cream in each group in Table 3

[0100]

[0101] 3) Microstructure of sour cream

[0102] Figure 5 It can be seen that there are relatively large voids between the network structures of the SC-1 samples and the SC-2 samples, and the structures are relatively loose. The protein micelles crosslink to form an irregular network skeleton with different pore sizes and poor network continuity. The network structure of the SC-3 sour cream samples is relatively tight and regular, with a more concentrated and uniform particle size distribution. Compared with the SC-1 samples and the SC-2 samples, a more stable network structure is formed in the SC-3 sour cream.

[0103] (3) Physical properties of sour cream

[0104] 1) Rheological properties of sour cream

[0105] It can be seen from Figure 6 that as the shear rate increases, the apparent viscosity of the three groups of sour cream samples shows a decreasing trend. The apparent viscosity of the SC-2 samples is the same as that of the SC-1 samples. The initial apparent viscosity of the fermented sour cream added with 0.5% EPS-603 is much lower than that of the other two groups of samples, significantly reducing the fat coalescence rate and the apparent viscosity at low shear.

[0106] The effects of adding EPS-603 on the storage modulus (G′) and loss modulus (G″) of sour cream with changes in temperature and frequency are as Figure 7 shown. The storage modulus and loss modulus of the three groups of sour cream decrease with the increase in temperature and increase with the increase in frequency. The storage modulus and loss modulus of the SC-1 and SC-2 samples are too high, which will make the texture of the sour cream samples rough and the quality decline. The storage modulus and loss modulus of the SC-3 samples are significantly lower than those of the SC-1 and SC-2 samples, within a reasonable range, with a uniform texture, delicate taste, and good spreadability.

[0107] 2) Texture properties of sour cream

[0108] The textural properties of sour cream are the core of its sensory quality, directly affecting the taste experience of consumers and the product application performance, such as Figure 8 shown, the difference in the textural properties of SC-1 and SC-2 samples is not obvious, while the hardness of SC-3 sample ( Figure 8 A), adhesiveness ( Figure 8 B), cohesiveness ( Figure 8 C) and elasticity ( Figure 8 D) are significantly lower than those of the other two groups. 0.5% EPS-603 enables sour cream to form a continuous flexible network, which can dilute the protein cross-linking effect and disperse fat globules, resulting in a decrease in hardness, elasticity, cohesiveness and adhesiveness, and improving its textural properties.

[0109] (4) Effect of EPS-603 on the stability properties of sour cream

[0110] 1) Sour cream potential

[0111] Zeta potential is a measurement index of the surface charge of particles in a colloidal dispersion system, reflecting the potential difference of the double electric layer around the particles. The higher the absolute value, the stronger the electrostatic repulsion force between the particles, which helps to stabilize the dispersion system and prevent coalescence. As can be seen from Table 4, the absolute value of the Zeta potential of the fermented sour cream samples added with EPS-603 is greater than that of the sour cream samples without EPS-603, and with the gradual increase of the EPS-603 concentration, the absolute value of the Zeta potential increases accordingly, improving the stability of sour cream.

[0112] Table 4 Zeta potential of sour cream in each group

[0113]

[0114] 2) Sour cream fat coalescence rate

[0115] The rate of aggregation or coalescence between fat globules during the production and storage of sour cream directly affects the physical properties, sensory characteristics and stability of the product. The fat coalescence rate of fermented sour cream is as Figure 9 shown. Compared with the SC-1 sample, there is no difference in the fat coalescence rate of SC-2, but the fat coalescence rate of SC-3 fermented sour cream is significantly reduced, decreasing by 9%. The fat globules of sour cream with a high coalescence rate are much larger, resulting in an obvious sandy texture and a decrease in smoothness of the sour cream. The fat with a low coalescence rate is more evenly dispersed, the fat globules remain small in size and narrow in distribution, giving the product a delicate texture and good spreadability.

[0116] 3) Sour cream hydrophobicity

[0117] Hydrophobic interactions play a central role in the folding process by driving the aggregation of nonpolar residues within the protein interior. The strength of hydrophobic interactions directly determines the solubility characteristics and conformational stability of proteins. Excessive hydrophobicity can lead to structural defects and reduce water-holding capacity; moderate hydrophobicity can promote the formation of a gel network and improve water-holding capacity. As Figure 10 shown, compared with the control SC-1, the addition of EPS-603 increased the hydrophobicity of fermented sour cream, but there was little difference in the hydrophobicity of the two sour cream samples with 0.1% and 0.5% EPS-603 added.

[0118] 4) Water-holding capacity of sour cream

[0119] Water-holding capacity refers to the ability to maintain internal moisture. If the water-holding capacity of sour cream is insufficient, it will lead to loose tissue structure and increased whey separation, thereby significantly reducing the stability and sensory quality of sour cream. As Figure 11 shown, the water-holding capacity of the sour cream sample with 0.5% EPS-603 added was significantly higher than that of the sour cream samples without EPS-603 added and with 0.1% EPS-603 added. In addition, as Figure 12 shown, the SC-1 and SC-2 samples were prone to shrinkage and cracking and whey separation during refrigerated storage. The fermented sour cream with 0.5% EPS-603 added had a moderate consistency, a uniform system, and no whey separation, which was consistent with the above-mentioned measured structural characteristics.

[0120] (5) Flavor characteristics of sour cream

[0121] The volatile flavor compounds of the three groups of samples were determined by SPME-GC-MS method, and the results are shown in Table 5. The effects of calculating the aroma activity values OVA of the volatile flavor compounds in the three groups of samples are shown in Table 6.

[0122] Table 5 Volatile components of each group of sour cream

[0123]

[0124]

[0125]

[0126] Note: "-" indicates not detected, the same below.

[0127] Table 6 OAV of volatile flavor compounds of each group of sour cream

[0128]

[0129]

[0130] As shown in Table 5, a total of six major categories of volatile substances were detected in the three groups of samples, including 49 volatile substances such as acids, alcohols, ketones, aldehydes, esters, and other compound classes. In the fermented sour cream system, organic acid substances, as key flavor components, play a key role in the flavor of fermented sour cream. As shown in Table 6, caproic acid and caprylic acid were detected in all three groups of sour cream samples. Caproic acid synergistically enhances the buttery flavor with esters, and caprylic acid presents fresh fruit and caramel scents, enriching the aroma components of sour cream. The mass concentrations of caproic acid and caprylic acid in the sour cream sample added with 0.5% EPS-603 were significantly increased. Butyric acid and trans-2-undecenoic acid were also detected in the SC-3 sour cream sample. Butyric acid presents a buttery aroma, and trans-2-undecenoic acid presents a smell similar to that of a ripe pear or apple peel with a faint grassy note. These substances jointly contribute to the flavor composition of SC-3 sour cream.

[0131] Alcohol compounds were detected in all three sour cream products, mainly including isopentanol, n-hexanol, β-phenylethyl alcohol, 3,4-dimethylcyclohexanol, and 2-methyl-1-hexadecanol. Among them, isopentanol appeared in all three sour cream samples. Isopentanol may be produced by the further conversion of some organic acids and amino acids in sour cream after fermentation, which helps the formation of cheese flavor. The mass concentrations of n-hexanol, β-phenylethyl alcohol, and 3,4-dimethylcyclohexanol decreased with the increase in the EPS-603 concentration of sour cream. These three alcohol compounds were not detected in the SC-3 sour cream sample, and the richness of alcohol flavor substances was relatively low. However, excessive alcohol compounds would give sour cream an obvious alcoholic smell, masking its natural lactic fermentation aroma and leading to flavor imbalance. With the addition of EPS-603, 2-methyl-1-hexadecanol appeared in the sour cream sample. This alcohol compound presents a fresh smell similar to that after fresh mowing at a very low concentration, which can increase the richness of alcohol substances in sour cream.

[0132] A total of 10 ketone compounds were detected in the three groups of fermented sour cream samples. Among them, 2-heptanone, 2-nonanone, 3,5-octadien-2-one, and 2-undecanone were detected in all three groups of samples. The content of 2-heptanone was the highest in the sour cream sample added with 0.5% EPS-603. 2-Heptanone has a cheese aroma and can give fermented sour cream a strong milky flavor. 2-Heptanone is considered one of the key aroma compounds for the buttery and fresh flavors in fermented sour cream. Isophorone, 4-methyl-2-hexanone, and 2-nonadecanone appeared in SC-3, enriching the ketone compound flavor system of the SC-3 sour cream sample.

[0133] Aldehydes generally have a relatively low threshold and are key factors in the flavor composition of fermented sour cream. The content of nonanal in the SC-3 sample is the highest, and the flavor is the strongest. The content of decanal in SC-3 sour cream increases significantly, and decanal can impart a floral fragrance to the system. Phenylacetaldehyde, due to its unique aromatic structure, presents typical hyacinth floral fragrance characteristics and is accompanied by a delicate almond sweet fragrance. There is no significant difference in the types of aldehydes in the three groups of sour cream, but the mass concentration of most aldehydes in the SC-3 sour cream sample is higher than that in the SC-1 and SC-2 sour cream samples.

[0134] There are significant differences in the ester compounds among the three groups of fermented sour cream samples. Many ester compounds can impart good flavors to sour cream. Most esters have fruit and floral fragrances and are extremely important volatile compounds in fermented dairy products. 2-Methyl-2-butyl heptanoate and heptyl formate were not detected in the SC-3 sour cream sample. The content of δ-decalactone, methyl 10-octenoate, benzyl oleate, and methyl isostearate in SC-3 sour cream is the highest. Generally speaking, the content of ester compounds in SC-3 is higher and the flavor is better.

[0135] Among other compound types, most are alkane compounds, and the aroma threshold of alkane compounds is relatively high, so their impact on fermented sour cream is not obvious.

[0136] A substance clustering heat map was used to conduct a clustering analysis on the volatile flavor substances of fermented sour cream with different EPS-603 addition amounts. The results are as Figure 13 shown. From the first- and second-level classifications of the clustering heat map, it can be seen that the fermented sour cream with three different concentrations of EPS-603 addition amounts has similar flavor substances, and the content of most volatile flavor substances is higher in the SC-3 fermented sour cream. From this, it can be concluded that compared with the SC-1 and SC-2 fermented sour cream samples, the flavor of the SC-3 fermented sour cream is more complex and rich.

[0137] (6) Antioxidant properties of sour cream in vitro digestion

[0138] An in vitro digestion model was established to measure the DPPH scavenging rate, hydroxyl radical scavenging rate, and total antioxidant capacity of sour cream prepared with different EPS-603 addition amounts at different digestion stages. As Figure 14 shown, at the three simulated in vitro digestion stages, the DPPH scavenging rate, hydroxyl radical scavenging ability, and total antioxidant capacity of the digestion products of SC-3 fermented sour cream are significantly higher than those of the SC-1 and SC-2 samples.

[0139] In summary, the addition of EPS-603 had no significant effect on the changes in pH, acidity, and viable cell count during the sour cream fermentation process; it had a significant effect on the secondary structure of the protein in the fermented sour cream. The fermented sour cream with 0.5% EPS-603 added had a more compact and regular network structure, a more concentrated and uniform particle size distribution, significantly improved hydrophobicity and water-holding capacity of the sour cream, reduced the whey separation of the fermented sour cream; made the flavor of the fermented sour cream more rich; improved the quality characteristics of the fermented sour cream and the in vitro antioxidant capacity of its digestion products, endowing the fermented sour cream with more excellent functional characteristics.

[0140] 3. Stability of Sour Cream during Storage

[0141] Evaluate the change rules of the physicochemical properties, microbial community dynamics, rheological properties (apparent viscosity, frequency sweep effect, and temperature effect), and texture properties (hardness, cohesiveness, adhesiveness, etc.) of the three groups of sour cream during storage.

[0142] (1) pH Value and Acidity of Sour Cream during Storage

[0143] As can be seen from Figure 15 (A), the pH values of the three groups of sour cream samples showed a decreasing trend with the extension of storage time. The pH values of the SC-1, SC-2, and SC-3 samples decreased from 4.5, 4.53, and 4.53 to 4.33, 4.33, and 4.41 respectively. The pH change range of the SC-3 sample was small during storage; as can be seen from Figure 15 (B), the acidity change of the SC-3 sample was gentler than that of the SC-1 and SC-2 samples. Therefore, the addition of 0.5% EPS-603 helped to maintain the stability of the pH and acidity of the sour cream during storage.

[0144] (2) Viable Cell Count of Sour Cream during Storage

[0145] As can be seen from Figure 16 , with the extension of storage time, the viable cell count continued to decrease. The viable cell counts of the SC-1 and SC-2 samples decreased significantly, while that of the SC-3 sample was relatively gentle.

[0146] (3) Water-Holding Capacity of Sour Cream during Storage

[0147] As can be seen from Figure 17 , the water-holding capacity of the three groups of sour cream samples decreased with the increase of storage time during storage. The stability of the water-holding capacity of the SC-3 sour cream during storage was significantly better than that of the SC-1 and SC-2 groups. On the 28th day of storage, the SC-3 sour cream could still maintain 60% of its water-holding capacity, which was about 8% higher than that of the SC-1 and SC-2 groups.

[0148] (4) Rheological Properties of Sour Cream during Storage

[0149] 1) Influence of Apparent Viscosity during Storage on Shear Rate

[0150] As Figure 18 shown, the apparent viscosities of the three groups of samples decreased with the extension of the storage period. During the 28-day storage period, the apparent viscosity of the SC-1 sample decreased from 321.8 Pa·s to 122.3 Pa·s, the apparent viscosity of the SC-2 sample decreased from 305.2 Pa·s to 97.95 Pa·s, and the apparent viscosity of the SC-3 sample decreased from 114.5 Pa·s to 31.95 Pa·s. The apparent viscosity of the SC-3 sample decreased at a slower rate. The level of apparent viscosity affects the use of fermented sour cream. Sour cream with a high apparent viscosity is more suitable for direct consumption or spreading, while sour cream samples with a low apparent viscosity are more suitable for baking or sauces.

[0151] 2) Elastic and viscous properties during storage period vary with frequency

[0152] Measuring the changes in the storage modulus and loss modulus of sour cream during storage with frequency can be used to predict the stability of sour cream during storage. G′ and G″ represent the storage modulus (elasticity) and loss modulus (viscosity) of the sample, respectively. As Figure 19 shown, the elasticity of the SC-1 sample ( Figure 19 A) and the elasticity of the SC-2 sample ( Figure 19 B) were of the same magnitude and change trend during the same period of storage, and both were significantly higher than the elasticity of the SC-3 sample ( Figure 19 C) during the same period; the storage modulus of the SC-3 sample was within a reasonable range on the first day of storage, and the texture of the sample was uniform and the elasticity was moderate. The viscosity of the SC-1 sample ( Figure 19 D) and the viscosity of the SC-2 sample ( Figure 19 E) were of the same magnitude and change trend during the same period of storage, and both were significantly higher than the viscosity of the SC-3 sample ( Figure 19 F). Adding 0.5% EPS-603 can reduce the elasticity and viscosity of sour cream during storage and slow down the reduction rate of viscosity and elasticity. The SC-3 sample is less significantly frequency-dependent than the SC-1 and SC-2 samples, indicating that the SC-3 sample has better long-term stability.

[0153] (5) Texture properties of sour cream during storage

[0154] The changes in the core parameters of the texture properties of the three groups of sour cream during storage are as Figure 20As shown in the figure, during the storage of sour cream, the hardness, adhesiveness, elasticity, and cohesiveness of the three groups of samples showed a gradually decreasing trend. At the same storage time, the hardness, adhesiveness, and elasticity of the SC-1 and SC-2 samples were higher than those of the SC-3 samples. The cohesiveness of the SC-1 and SC-2 samples was higher than that of the SC-3 samples before 14 days of storage, and lower than that of the SC-3 samples after 14 days of storage. Adding 0.5% EPS-603 can significantly improve the texture characteristics and stability of sour cream during storage.

[0155] In summary, during the 28-day storage period, compared with the SC-1 and SC-2 samples, the changes in parameters such as pH value, acidity, viable bacteria count, and water holding capacity of the sour cream SC-3 prepared with 0.5% EPS-603 were smaller; the apparent viscosity, storage modulus, and loss modulus were all within a reasonable range; the changes in hardness, adhesiveness, and cohesiveness during storage were small. The research results show that the fermented sour cream SC-3 exhibits better physicochemical properties, sensory properties, and stability during storage.

Claims

1. A method for preparing single-strain fermented sour cream with high quality and nutritional value, characterized in that, Butter-based light cream fermented with Lactococcus lactis ( Lactococcus lactis ), supplemented with exopolysaccharides derived from Lactobacillus rhamnosus ( Lactobacillus rhamnosus ).

2. The preparation method according to claim 1, wherein The butter-based cream consists of the following components: skim milk powder, butter, carrageenan, sodium caseinate, Tween 80, soy lecithin, polyglycerol fatty acid ester, and deionized water.

3. The preparation method according to claim 1, wherein, The butter-based cream consists of the following components by mass percentage: 5% skim milk powder, 18% butter, 0.02% carrageenan, 1.1% sodium caseinate, 0.05% - 0.15% Tween 80, 0.10% - 0.30% soy lecithin, 0.10% - 0.30% polyglycerol fatty acid ester, and the balance is deionized water.

4. The preparation method according to claim 1, characterized in that, The butter-based cream consists of the following components by mass percentage: 5% skim milk powder, 18% butter, 0.02% carrageenan, 1.1% sodium caseinate, 0.1% Tween 80, 0.15% soy lecithin, 0.1% polyglycerol fatty acid ester, and the balance is deionized water.

5. The preparation method according to claim 1, characterized in that, The addition amount of the exopolysaccharide derived from Lactobacillus rhamnosus grx603 is 0.1% - 0.5% of the mass of the butter-based cream. The exopolysaccharide derived from Lactobacillus rhamnosus grx603 is obtained through the following steps: inoculating Lactobacillus rhamnosus grx603 into the semi-synthetic medium SCDM, culturing and then collecting the supernatant, precipitating with trichloroacetic acid and ethanol, dialyzing, and freeze-drying to obtain the exopolysaccharide derived from Lactobacillus rhamnosus grx603.

6. The preparation method according to claim 5, characterized in that, The composition of the semi-synthetic medium SCDM is: glucose 20.0 g / L, magnesium sulfate 0.1 g / L, tryptone 10.0 g / L, manganese sulfate 0.05 g / L, sodium acetate 5.0 g / L, dipotassium hydrogen phosphate 2.0 g / L, ammonium citrate 2.0 g / L, Tween 80 0.6 g / L, yeast nitrogen source 5.0 g / L, and water.

7. The preparation method according to claim 1, characterized in that, Specifically, it includes the following steps: Formulating the skim milk powder, butter, carrageenan, sodium caseinate, Tween 80, soy lecithin, polyglycerol fatty acid ester, and deionized water into the butter-based cream according to the ratio, then adding the exopolysaccharide derived from Lactobacillus rhamnosus grx603, homogenizing and sterilizing, cooling to room temperature, and adding the activated Lactococcus lactis grx602 for fermentation to obtain sour cream.

8. The preparation method according to claim 7, wherein The room temperature is 25°C, the fermentation time is 9 - 18 h; the number of homogenization times is 2 times, and the pressure is 15 MPa; the sterilization method is sterilizing at 85°C for 15 min.

9. The preparation method according to claim 7, characterized in that, The inoculation amount of Lactococcus lactis grx602 is 10.6 - 10.8 lg cfu / ml; the activation method of Lactococcus lactis grx602 is: inoculating Lactococcus lactis grx602 into the MRS medium and fermenting and culturing for 24 - 48 h.

10. Sour cream prepared by the preparation method according to any one of claims 1 - 9.

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