A method for detecting gel properties of exogenous milk protein and a method for evaluating quality of yogurt

Abstract

The present application belongs to the technical field of dairy product processing and quality monitoring, and particularly relates to a method for detecting gel properties of exogenous milk protein, and a method for evaluating yogurt quality based on gel properties of the exogenous milk protein. The method for detecting gel properties of exogenous milk protein can detect gel properties of different types of milk protein, quickly analyze gel strength of the protein, and well reflect the performance of the exogenous milk protein. The method for evaluating yogurt quality based on gel properties of exogenous milk protein can analyze the correlation between gel strength of the milk protein and water holding capacity and rheological properties of the yogurt, and construct a regression curve of gel strength, water holding capacity and shear viscosity of the yogurt, so that the water holding capacity and product taste of the yogurt can be predicted through the gel strength of the protein.

Description

Technical Field

[0001] This invention belongs to the field of dairy product processing and quality monitoring technology, specifically relating to a method for detecting the gel properties of exogenous milk proteins, and a method for evaluating the quality of yogurt based on the gel properties of the exogenous milk proteins. Background Technology

[0002] Milk protein is an excellent protein source, with a digestibility rate exceeding 90% in humans, providing a large amount of essential and non-essential amino acids. Milk proteins are diverse and structurally complex, primarily composed of casein and whey protein. With advancements in dairy processing technology, milk proteins have been concentrated and separated into various protein products, overcoming regional and seasonal supply and demand imbalances. Milk proteins are widely used in dairy products such as yogurt. On one hand, they increase the protein content of yogurt, enhancing its nutritional value; on the other hand, they fully utilize the water-retention and gelling properties of milk proteins to improve the water-holding capacity of yogurt, achieving a longer shelf life, and on the other hand, they increase the shear viscosity of yogurt, giving the product a different texture.

[0003] Currently, bulk milk protein ingredients can be categorized into milk protein concentrate (MPC), casein micelles, whey protein (WPC), and micronized whey protein, among others. These different types of protein ingredients exhibit significant differences in protein composition and characteristics. Milk protein concentrate and casein micelles are primarily composed of casein. Casein is heat-stable but acid-sensitive; as the pH decreases, minerals within the micelles gradually become released. With further dissolution of calcium phosphate in the micelles, the solubility of casein gradually decreases. When the pH drops to the isoelectric point, a milk gel is formed. Whey protein, on the other hand, exhibits characteristics quite different from casein. Whey protein is a heat-sensitive protein. When heated, its helical structure gradually unfolds, and protein molecules cross-link to form a three-dimensional network structure, thus forming a gel. This structure can lock water within the gel cavities, resulting in strong water-holding capacity. Traditional quality evaluation of dairy products such as yogurt is mostly based on testing and evaluation of finished dairy products. If the quality of products could be assessed in advance based on the performance of dairy raw materials, it could provide new ideas for the quality evaluation of dairy products such as yogurt and avoid more waste of finished raw materials. However, there are currently no reports on the quality evaluation of dairy products such as yogurt based on the characteristics of milk protein raw materials.

[0004] For example, Chinese patent CN106720302A discloses a method for improving the gel properties of milk protein concentrate. The method involves redissolving the MTG-modified milk protein concentrate in water, then simulating fermentation with GDL acidifier to characterize the gel properties of the milk protein concentrate, and measuring the gel strength using a CT3 physical property analyzer. Another example is the method for preparing modified whey protein gel disclosed in Chinese patent CN103564146A. This method involves preparing a 15% protein solution, adding phosphoric acid, and heating to obtain whey protein gel, and then measuring the protein gel strength at 25°C. Yet another example is the literature "Analysis of the Gel Properties of Potato Protein and Egg White Protein Mixture" [Food Science 2022, 43(02):34-40], which reports that potato protein and egg white protein were used as raw materials, and protein gel was prepared by heating and cooling. The gel strength of the sample was measured using a texture analyzer. For example, a study on the changes in the solubility, gel strength, and emulsifying properties of whey protein caused by free radical oxidation [Food Industry Technology, 2009(02):145-148] reported that whey protein isolate was added to a phosphate buffer solution and heated to form a gel, and the gel hardness was detected using a physical property analyzer in TPA mode. Although the above methods all involve the preparation and detection of protein gels, they are all aimed at obtaining protein gels with the expected properties, and are not applicable to the characterization and evaluation of the gel properties of milk protein raw materials.

[0005] This application aims to develop detection methods for the gel properties of different types of milk proteins, and to verify these methods through a yogurt testing system. The goal is to analyze the correlation between milk protein gel strength and the water-holding capacity and rheological properties of yogurt, thereby achieving the objective of predicting yogurt quality. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to provide a method for detecting the gel characteristics of exogenous milk proteins, the method being able to detect and evaluate the gel characteristics of exogenous milk proteins for subsequent quality evaluation of yogurt products;

[0007] The second technical problem to be solved by the present invention is to provide a method for evaluating the quality of yogurt based on the gel properties of the exogenous milk protein.

[0008] To solve the above-mentioned technical problems, the present invention provides a method for detecting the gel properties of exogenous milk protein, which includes a step of routinely detecting the protein content of the milk protein, a step of promoting protein denaturation to form a gel, and a step of detecting the strength of the gel.

[0009] Specifically, the method for detecting the gel properties of exogenous milk proteins includes the following steps for detecting the gel properties of milk proteins with thermogelation characteristics:

[0010] (1) Protein content determination: The protein content of the milk protein powder was determined using conventional methods;

[0011] (2) Protein solution preparation: Heat deionized water and add the milk protein powder while stirring to prepare a protein solution. Turn off the stirring and hydrate and dissolve the protein powder. Cool down to eliminate bubbles and set aside.

[0012] (3) Gel thermal denaturation: The protein solution is heated to 80-82℃ for thermal denaturation, and the resulting gel is cooled and equilibrated for later use.

[0013] (4) Gel strength test: The gel was tested for gel rupture strength using the weak gel test module of the texture analyzer, and the force value at the first gel rupture point was taken as the protein gel strength.

[0014] Specifically, in step (1), the protein content of the milk protein powder is detected according to GB 5009.5.

[0015] Specifically, in step (2), the protein concentration in the protein solution is 10-12 wt%.

[0016] The deionized water is heated at a temperature of 40-45℃.

[0017] Specifically, the stirring step involves a stirring speed of 250-350 rpm / min, controlling the protein powder to settle and disperse for 15-20 minutes. After turning off the stirring, the protein powder is hydrated and dissolved for 20-40 minutes, and then the temperature is lowered to about 4°C to dissolve the air bubbles.

[0018] Specifically, in step (3), the heating rate is 8-12℃ / min.

[0019] Specifically, in step (4), the gel strength test step uses a cylindrical probe of 12.700mm ± 0.013mm. The test parameters include: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, and Tigger Force 2g.

[0020] Specifically, the method for detecting the gel properties of exogenous milk proteins in the milk protein yogurt model includes the following steps:

[0021] (1) Protein content determination: The protein content of the milk protein powder was determined using conventional methods;

[0022] (2) Preparation of protein solution: Heat skim milk and add the milk protein powder while stirring. Turn off the stirring and hydrate and dissolve the protein powder. Homogenize and sterilize the resulting protein solution, and then cool it to remove bubbles to obtain a sterile solution for later use.

[0023] (3) Inoculation and fermentation: Take the sterile liquid and inoculate it with lactic acid bacteria for constant temperature fermentation until the endpoint pH 4.40±0.05. Collect the gel product and cool and equilibrate it for later use.

[0024] (4) Gel strength test: The gel was tested for gel rupture strength using the weak gel test module of the texture analyzer, and the force value at the first gel rupture point was taken as the protein gel strength.

[0025] Specifically, in step (1), the protein content of the milk protein powder is detected according to GB 5009.5.

[0026] Specifically, in step (2), the total protein concentration in the protein solution is 4.5-5 wt%.

[0027] The skim milk is heated to a temperature of 40-50°C.

[0028] The temperature of the homogenization step is 58-60℃, and the total homogenization pressure is 130-150 bar.

[0029] The sterilization step is performed at a temperature of 80-90℃.

[0030] Specifically, the stirring speed is 250-350 rpm / min, the stirring and dispersion is controlled for 10-15 minutes, the stirring is turned off and hydration is carried out for 15-25 minutes, and then the mixture is quickly cooled to 4°C to cool down and dissipate air bubbles.

[0031] Specifically, in step (3), the lactic acid bacteria include a compound fermentation agent of Lactobacillus bulgaricus and Streptococcus thermophilus;

[0032] The temperature of the constant temperature fermentation step is 40-45℃.

[0033] Specifically, the inoculation amount of the lactic acid bacteria is controlled at 1×10⁻⁶. 8 CFU / kg.

[0034] Specifically, in step (4), the gel strength test step uses a cylindrical probe of 12.700mm ± 0.013mm. The test parameters include: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 20mm, and Tigger Force 5g.

[0035] The present invention also discloses a method for evaluating the quality of yogurt based on the gel properties of exogenous milk proteins, including the step of detecting the gel strength index of exogenous milk proteins according to the method, and the step of evaluating the quality of yogurt based on the gel strength index.

[0036] Specifically, in the method for evaluating yogurt quality based on the gel properties of exogenous milk proteins, the yogurt quality evaluation steps include:

[0037] By constructing a functional relationship between the gel strength value of the milk protein exhibiting thermogelation properties and the centrifugal water loss rate of yogurt, the regression curve equation obtained by fitting is: y = 2E-07x 2 -0.0003x+0.2241, where R2=0.9904, the higher the protein gel strength, the stronger the ability to bind water, the lower the water loss rate of yogurt after centrifugation, and the better the water retention of the product.

[0038] or,

[0039] Using the gel strength value of milk protein and the shear viscosity of yogurt constructed from the milk protein yogurt model, a regression curve equation was obtained by fitting: y = -0.3466x 2 +28.442x-165.41, where R2=0.9848, is used to predict the mouthfeel viscosity of yogurt using the gel strength of the milk protein yogurt model.

[0040] The exogenous milk protein gel property detection method described in this embodiment is developed for different types of milk proteins. By adjusting and controlling the parameters of the gel sample preparation steps, the gel property of exogenous milk proteins can be detected. The method is simple to operate, highly repeatable, and provides accurate detection results. It can quickly analyze the gel strength of proteins, reflects the performance of exogenous milk proteins well, and has wide applicability.

[0041] The present invention describes a method for evaluating yogurt quality based on the gel properties of exogenous milk proteins. This method predicts and evaluates yogurt quality based on the gel properties of exogenous milk proteins. By analyzing the correlation between milk protein gel strength and yogurt's water-holding capacity and rheological properties, and constructing regression curves between gel strength and yogurt's water-holding capacity and shear viscosity, the water-holding capacity and taste of yogurt can be predicted through protein gel strength, thereby achieving the goal of predicting yogurt quality. The accuracy of the evaluation method is verified through a yogurt testing system. Attached Figure Description

[0042] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein...

[0043] Figure 1 This is a flowchart of the method for detecting the gel properties of exogenous milk proteins according to the present invention;

[0044] Figure 2 The graph shows the functional relationship between the thermal gel strength of the milk protein and the water loss rate of the yogurt during centrifugation.

[0045] Figure 3 The graph shows the functional relationship between the gel strength of the milk protein yogurt model and the shear viscosity of the yogurt. Detailed Implementation

[0046] In the following embodiments of the present invention, the characteristics of the exogenous milk protein raw materials tested are detailed in Table 1 below. The protein content was determined according to the first method of GB 5009.5 to detect the protein content of milk protein powder.

[0047] Table 1 Characteristics of exogenous milk protein raw materials

[0048] A Whey protein WPC-Leprino Davidlin International Trading Co., Ltd. 76.0％ B Wheyco Whey Protein (WPC) Davidlin International Trading Co., Ltd. 76.0％ C Whey protein WPC-7550 Arla Foods 71.4％ D Micronized whey protein WPC-8075 Arla Foods 77.3％ E Casein micelles MCC1900 Davidlin International Trading Co., Ltd. 85.0％ F Milk protein MPC-8250 Arla Foods 75.2％ G Milk protein MPC-4882 Fonterra Co-operative Group 82.5％ H Whey protein WPC-5215 Arla Foods 46.6％ I Whey Protein WPC-GermanProt 8000 Marcus Trading Co., Ltd. 73.3％ J Whey protein WPC80 Leckertpe, Germany 75.5％ K Milk protein MPC-4861 Fonterra Co-operative Group 81.8％ L Whey protein 550 Fonterra Co-operative Group 79.2％

[0049] Example 1

[0050] like Figure 1 The flowchart shown illustrates how this embodiment tests the strength characteristics of the milk protein gel with thermogelation properties, specifically including the following steps:

[0051] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0052] (2) Protein solution preparation: Weigh 100g of deionized water, heat to 40℃, calculate the amount to be added according to the actual protein content of the protein powder AD, so that the protein concentration in each solution reaches 10g / 100g; add the protein powder while stirring, control the speed at 300rpm / min, let the protein powder settle and disperse for 15min, turn off the stirring, let the protein powder hydrate and dissolve for 30min, then cool to 4℃ and dissolve the bubbles by cooling. Three parallel samples need to be prepared with the same raw materials.

[0053] (3) Preparation of thermal gel: The above uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C, the heating rate was controlled at 10°C / min, and the heat denaturation was carried out for 45 min. The protein solution was then taken out and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0054] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g. The force value at the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0055] Example 2

[0056] like Figure 1 The flowchart shown illustrates how this embodiment tests the strength characteristics of the milk protein gel with thermogelation properties, specifically including the following steps:

[0057] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0058] (2) Protein solution preparation: Weigh 100g of deionized water and heat to 45℃. Calculate the amount to add based on the actual protein content of the protein powder AD, ensuring a protein concentration of 12g / 100g in each solution. Add the protein powder while stirring, controlling the stirring speed at 300rpm / min. Allow the protein powder to settle and disperse for 20min, then turn off the stirring and allow the protein powder to hydrate and dissolve for 30min. Afterward, cool to 4℃ to dissipate air bubbles. Three parallel samples should be prepared using the same raw materials.

[0059] (3) Preparation of thermal gel: The above uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 82°C, the heating rate was controlled at 10°C / min, and the heat denaturation was carried out for 45 min. The protein solution was then taken out and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0060] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g; the force value of the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0061] Comparative Example 1 (Preparation of thermogels using lower protein concentration solutions)

[0062] This comparative method tests the strength properties of the milk protein gel with thermogelation characteristics, specifically including the following steps:

[0063] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0064] (2) Protein solution preparation: Weigh 100g of deionized water and heat it to 40℃. Calculate the amount to be added based on the actual protein content of protein powder A, so that the protein concentration of the solution reaches 8g / 100g. Add the protein powder while stirring, control the speed at 300rpm / min, let the protein powder settle and disperse for 15min, turn off the stirring, let the protein powder hydrate and dissolve for 30min, and then cool it down to 4℃ to dissolve the bubbles. Three parallel samples need to be prepared with the same raw materials.

[0065] (3) Preparation of thermal gel: The uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C at a rate of 10°C / min, and the heat was maintained for 45 min for thermal denaturation. The protein solution was then removed and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0066] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g; the force value of the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0067] Comparative Example 2 (Preparation of thermogels using higher protein concentration solutions)

[0068] This comparative method tests the strength properties of the milk protein gel with thermogelation characteristics, specifically including the following steps:

[0069] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0070] (2) Protein solution preparation: Weigh 100g of deionized water and heat it to 40℃. Calculate the amount to be added based on the actual protein content of protein powder C, so that the protein concentration of the solution reaches 14g / 100g. Add the protein powder while stirring, with the stirring on, and the speed is 300rpm / min. The protein powder settles and disperses for 15min. Turn off the stirring and let the protein powder hydrate and dissolve for 30min. Then cool down to 4℃ to dissipate bubbles. Three parallel samples need to be prepared with the same raw materials.

[0071] (3) Preparation of thermal gel: The uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C, the heating rate was controlled at 10°C / min, and the heat denaturation was maintained for 45 min. The protein solution was then removed and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0072] (4) Gel strength test: After the obtained gel temperature returned to room temperature, the gel rupture strength was tested using the weak gel test module of a texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g. The force value at the first gel rupture point was taken as the protein gel strength, and the average value of three parallel samples was taken.

[0073] Comparative Example 3 (without cooling and defoaming steps)

[0074] This comparative method tests the strength properties of the milk protein gel with thermogelation characteristics, specifically including the following steps:

[0075] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0076] (2) Protein solution preparation: Weigh 100g of deionized water and heat it to 40℃. Calculate the amount to be added based on the actual protein content of protein powder A, so that the protein concentration of the solution reaches 10g / 100g. Add the protein powder while stirring is on, with a speed of 300rpm / min. Let the protein powder settle and disperse for 15min. Turn off the stirring and let the protein powder hydrate and dissolve for 30min. Three parallel samples need to be prepared with the same raw materials.

[0077] (3) Preparation of thermogel: The uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C, the heating rate was controlled at 10°C / min, and the heat denaturation was carried out for 45 min. The protein solution was then taken out and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0078] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g. The force value of the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0079] Comparative Example 4 (different gel strength test parameters)

[0080] This comparative method tests the strength properties of the milk protein gel with thermogelation characteristics, specifically including the following steps:

[0081] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0082] (2) Protein solution preparation: Weigh 100g of deionized water and heat it to 40℃. Calculate the amount to be added based on the actual protein content of protein powder A, so that the protein concentration of the solution reaches 10g / 100g. Add the protein powder while stirring is on, with a speed of 300rpm / min. Let the protein powder settle and disperse for 15min. Turn off the stirring and let the protein powder hydrate and dissolve for 30min. Then cool down to 4℃ to dissolve the bubbles. Three parallel samples need to be prepared with the same raw materials.

[0083] (3) Preparation of thermogel: The uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C, the heating rate was controlled at 10°C / min, and the heat denaturation was carried out for 45 min. The protein solution was then taken out and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0084] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel rupture strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​2.00mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 10mm, TiggerForce 5g. The force value at the first gel rupture point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0085] The results of the thermogel strength test of milk protein in Examples 1-2 and Comparative Examples 1-4 are shown in Table 2 below.

[0086] Table 2 Results of milk protein thermogel strength test

[0087]

[0088] Because whey protein is a dense globular protein, heating causes it to fold and unfold. The free sulfhydryl groups on cysteine ​​residues undergo irreversible intermolecular cross-linking through covalent interactions, resulting in gel formation. The strength of the gel formed by heat-induced denaturation of whey protein is related to the whey protein composition, protein abundance, and the temperature and time of heat treatment.

[0089] As shown in Table 2 above, in Examples 1 and 2, whey protein thermogels were prepared by controlling the protein concentration in the protein solution to 10-12 g / 100 g. Whey proteins A, B, and C were ordinary whey proteins derived from sweet whey. Due to differences in whey protein composition and processing, their gel strengths varied significantly. However, within a reasonable concentration range, increasing the protein content could improve their gel strength. Whey protein D, on the other hand, was a micronized whey protein that had undergone heat and shear modification. The spherical whey protein chains unfurled, forming aggregates and losing their thermogel properties. Therefore, the protein concentration range for gel preparation protected by this patent is suitable for analyzing the gel properties of whey proteins with different gel strengths.

[0090] As shown in Table 2 above, the results of Comparative Example 1 are unsuitable because the method uses a low protein concentration solution to prepare the thermogel. Due to the weak gel properties of whey protein A, the thermogel has a paste-like texture and is unstable, making it impossible to detect its gel breaking strength. Comparative Example 2 uses a high protein concentration solution to prepare the thermogel. Whey protein C has a high gel strength and elasticity; the probe pressure did not puncture the gel, making it impossible to detect its gel breaking strength. Therefore, this condition is also unsuitable. In Comparative Example 3, the lack of a cooling and defoaming process resulted in a solidified bubble layer on the top of the prepared gel. The gel strength test showed multiple interfering peaks, making it impossible to determine the accurate gel strength. Therefore, a cooling and defoaming process is essential. In Comparative Example 4, different test parameters were used. Whey proteins B and C have relatively high gel strength, requiring faster probe penetration and deeper penetration, resulting in deep gel breakage. The test program failed to capture the effective breakage point.

[0091] In summary, this invention determines that the protein concentration level for preparing milk protein thermogels is 10-12%. Gels with lower protein content are softer and looser, making it impossible to test their gel breaking strength; gels with higher protein content are harder and less prone to breaking, making it impossible to test their gel strength using instruments.

[0092] Example 3

[0093] like Figure 1 The flowchart shown illustrates the process of detecting the strength characteristics of the milk protein gel in the milk protein yogurt model in this embodiment, specifically including the following steps:

[0094] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0095] (2) Protein solution preparation: Weigh 500g of skim milk with a protein content of 3.2g / 100g and heat it to 45℃. Calculate the amount to add based on the actual protein content of the protein powder (AG), so that the total protein concentration in each solution reaches 4.7g / 100g. Add the protein powder while stirring, at a speed of 300rpm / min, and stir for 15min. Then turn off the stirring and hydrate for 20min. Heat the solution to 58℃ for homogenization at a total pressure of 150bar. Then perform water bath sterilization at 85℃ for 5min, and rapidly cool to 4℃ to dissipate air bubbles.

[0096] (3) Inoculation and Fermentation: Divide the sterile liquid obtained above into 3 portions, each 100g, heat to 42℃, and inoculate with lactic acid bacteria (a compound fermentation agent of Lactobacillus bulgaricus and Streptococcus thermophilus, wherein the activity ratio of Streptococcus thermophilus to Lactobacillus bulgaricus is 7:3), with an inoculation amount of 1×10⁻⁶. 8CFU / kg, fermented at 42℃ until the final pH of 4.40, cooled to 4℃, and stored for equilibration for 8 hours;

[0097] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 20mm, TiggerForce 5g. The force value at the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0098] Comparative Example 5 (using GDL instead of lactic acid bacteria for fermentation)

[0099] This comparative example tested the gel strength properties of the milk protein in the milk protein yogurt model, specifically including the following steps:

[0100] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0101] (2) Protein solution preparation: Weigh 500g of skim milk with a protein content of 3.2g / 100g and heat it to 45℃. Calculate the amount to be added based on the actual protein content of protein powders B, E, and G, so that the total protein concentration in each solution reaches 4.7g / 100g. Add the protein powder while stirring, and stir at 300rpm / min for 15min. Then turn off the stirring and hydrate for 20min. Heat the solution to 58℃ for homogenization at a total pressure of 150bar. Then sterilize in a water bath at 85℃ for 5min and cool it down to 4℃ to dissipate air bubbles.

[0102] (3) Inoculation and fermentation: Divide the sterile liquid obtained above into 3 portions, each 100g, heat to 42℃, add 1.5% gluconate-δ-lactone, keep at 42℃ for fermentation until the endpoint pH 4.40, cool to 4℃, and store for equilibration for 8h.

[0103] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 20mm, TiggerForce 5g. The force value at the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0104] The gel strength test results of the milk protein yogurt model in Example 3 and Comparative Example 5 are shown in Table 3 below.

[0105] Table 3. Results of gel strength testing for the lactic acid yogurt model.

[0106] Example 3 23.6g 27.3g 29.1g 21.9g 40.2g 28.1g 41.1g Protein B Protein E Protein G Comparative Example 5 28.2g 45.5g 43.2g

[0107] Cow's milk naturally contains casein and whey protein. Adding exogenous proteins to cow's milk helps improve its nutritional value. On the other hand, changing the protein content and ratio also affects its gel structure and properties. Evaluating the gel strength of different types of milk proteins using a yogurt model helps analyze their impact on the system.

[0108] As shown in Table 3 above, the effects of whey proteins A, B, C, and D on the gel strength of yogurt are correlated with their own thermal gelation properties. Casein E and milk protein G contribute more to the gel strength of the yogurt system, indicating that an increase in casein helps the yogurt form a more robust gel network. Since milk protein F is a micronized milk protein that has undergone heat and shear modification, its contribution to the gel strength of the system is reduced.

[0109] In Comparative Example 5, the GDL-based method simulated the acidification process of lactic acid bacteria fermentation. The comparison revealed that GDL-acidified yogurt gels exhibited higher strength. Furthermore, the rate and magnitude of acidity decrease during GDL acidification and lactic acid bacteria fermentation were inconsistent within the milk system, leading to discrepancies in casein micelle dissociation and rearrangement times, resulting in different physicochemical properties in the products. Therefore, GDL acidification can only yield limited conclusions and cannot fully represent the lactic acid bacteria fermentation process.

[0110] Example 4: Predicting the effect of protein powder on the water-holding capacity of yogurt

[0111] In the processing of yogurt products, raw materials with thermogelation properties contribute to the water retention of the final yogurt product. Therefore, this embodiment evaluates the quality of yogurt by testing its water retention capacity.

[0112] The method for evaluating yogurt quality based on the gel properties of exogenous milk proteins described in this embodiment includes a step of detecting the gel strength index of exogenous milk proteins, and a step of evaluating yogurt quality (water-holding capacity) based on the gel strength index.

[0113] like Figure 1 The flowchart shown illustrates how this embodiment tests the strength characteristics of the milk protein gel with thermogelation properties, specifically including the following steps:

[0114] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0115] (2) Protein solution preparation: Weigh 100g of deionized water, heat to 40℃, calculate the amount to be added according to the actual protein content of protein powders A, B, C, H and I, and make the protein concentration in the solution reach 10g / 100g respectively; add protein powder with stirring on, speed 300rpm / min, let the protein powder settle and disperse for 15min, turn off the stirring, let the protein powder hydrate and dissolve for 30min, then cool to 4℃, and dissolve bubbles by cooling. Three parallel samples need to be prepared with the same raw materials.

[0116] (3) Preparation of thermal gel: The above uniformly dispersed protein solution was placed in a water bath, the water bath was heated to 80°C, the heating rate was controlled at 10°C / min, and the heat denaturation was maintained for 45 min. The protein solution was then taken out and cooled in an ice-water bath, and stored at 4°C for equilibration for 8 h.

[0117] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 5mm, TiggerForce 2g; the force value of the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0118] This embodiment verifies the application of the yogurt testing system. 910g of raw milk with a protein content of 3.2g / 100g was preheated to 45℃. While stirring, 12g of hydroxypropyl distarch phosphate, 1.5g of low-fat pectin, 1.5g of agar, 70g of white sugar, and milk protein powder (the amount added was calculated based on the actual protein content of protein powders A, B, C, H, and I to achieve a fermented milk protein content of 3.1g / 100g) were added. No milk protein powder was added to the control sample.

[0119] The above mixture was continuously stirred for 12 minutes and then heated to 60°C for homogenization at a pressure of 140 bar. The mixture was then heated to 85°C for sterilization for 5 minutes. The mixture was cooled to 42°C and inoculated with lactic acid bacteria to prepare fermented milk. Fermentation was carried out until the pH reached the endpoint of 4.50±0.5. After demulsification, the mixture was cooled to 18°C ​​to obtain a yogurt sample. The water-holding capacity of the yogurt sample was then tested.

[0120] The water-holding capacity of yogurt is characterized by centrifugal water loss rate. 10g of yogurt is added to a 15ml centrifuge tube, and the test conditions are 4000r / min for 15min. The centrifugal water loss rate is then calculated using the following formula:

[0121]

[0122] The results of the thermogel strength of milk protein and the corresponding centrifugal water loss rate of yogurt in this embodiment are shown in Table 4 below.

[0123] Table 4. Results of thermogel strength of milk proteins and corresponding centrifugal water loss rate of yogurt.

[0124]

[0125]

[0126] The thermogel strength of whey protein reflects its water-binding capacity; higher gel strength indicates a stronger ability to bind water, resulting in lower water loss during centrifugation of the yogurt and better water retention in the product. A functional relationship was established between the thermogel strength and the water loss rate during centrifugation of the yogurt, and a regression curve equation was obtained through fitting: y = 2E-07x 2 -0.0003x + 0.2241, where R² = 0.9904, the goodness of fit is close to 1, proving that the fit is good (see appendix for functional relationship). Figure 2 This regression equation can be used to predict the water-holding capacity of yogurt based on the thermogel strength of milk proteins.

[0127] The scheme and model of this embodiment were further validated. The thermogel strength of whey proteins J, D, and L was measured using the method described above. A yogurt test system was prepared using whey proteins J, D, and L, and the centrifugal water loss rate of the yogurt was detected. The theoretical centrifugal water loss rate was then calculated using the fitted regression equation. The results are shown in Table 5 below.

[0128] Table 5 Model Validation

[0129] whey protein J 415.3g 13.1％ 13.4％ Whey protein D Modified protein is non-gelatinous 27.3％ / Whey protein L Modified protein is non-gelatinous 25.6％ /

[0130] Comparative analysis of the data showed that the centrifugal water loss rate calculated by the J function of whey proteins with thermogel properties was very close to the centrifugal water loss rate detected by the actual test model, proving that the method and model have the ability and accuracy to predict the effect of proteins on the water-holding capacity of yogurt. However, whey proteins D and L, which have undergone thermal and shear denaturation polymerization, lose their gel properties and only have an inert filling effect on the system, without contributing to water-holding capacity. Therefore, their centrifugal water loss rate is higher, indicating that their test results are consistent with the predictive results of this scheme. It is evident that the thermogel strength test can quickly predict the effect of proteins on the water-holding capacity of the system, and thus, the performance and quality of the final product can be evaluated before the process based on the properties of the raw materials.

[0131] Example 5: Predicting the effect of protein powder on the shear viscosity of yogurt

[0132] In the processing of yogurt products, raw materials that exhibit higher gel strength in the yogurt mold usually contribute more to the taste and viscosity of the final yogurt product. Therefore, in this embodiment, the taste and performance of yogurt are measured by detecting rheological shear viscosity and extracting the viscosity value at a shear rate of 75 [1 / s].

[0133] The method for evaluating yogurt quality based on the gel properties of exogenous milk proteins described in this embodiment includes a step of detecting the gel strength index of exogenous milk proteins, and a step of evaluating yogurt quality (yogurt texture and viscosity) based on the gel strength index.

[0134] like Figure 1 The flowchart shown illustrates the process of detecting the strength characteristics of the milk protein gel in the milk protein yogurt model in this embodiment, specifically including the following steps:

[0135] (1) Protein content determination: The protein content of milk protein powder was determined according to the first method of GB 5009.5;

[0136] (2) Preparation of protein solution: Weigh 500g of skim milk with a protein content of 3.2g / 100g, heat to 45℃, calculate the amount to be added according to the actual protein content of protein powders A, B, C, E, and G, so that the total protein concentration in each solution reaches 4.7g / 100g; add protein powder while stirring, rotate at 300rpm / min, stir for 15min, then turn off the stirring and hydrate for 20min; heat the solution to 58℃ for homogenization, with a total homogenization pressure of 150bar, then perform water bath sterilization, sterilization intensity 85℃, keep at 5min, and quickly cool to 4℃ to dissipate air bubbles by cooling;

[0137] (3) Inoculation and fermentation: Divide the obtained sterile liquid into 3 portions, each 100g, heat to 42℃, and inoculate with lactic acid bacteria (a mixture of Lactobacillus bulgaricus and Streptococcus thermophilus fermentation agent), with an inoculation amount of 1×10⁻⁶. 8CFU / kg, fermented at 42℃ until the final pH 4.40, the gel product was collected, cooled to 4℃, and stored for equilibration for 8 hours;

[0138] (4) Gel strength test: After the obtained gel temperature returns to room temperature, the gel breaking strength is tested by the weak gel test module of the texture analyzer. Test probe: cylindrical 12.700mm±0.013mm, test parameters: Pre-Test Speed ​​1.00mm / sec, Test Speed ​​0.50mm / sec, Post-Test Speed ​​10.00mm / sec, Distance 20mm, TiggerForce 5g. The force value at the first gel breaking point is taken as the protein gel strength, and the average value of three parallel samples is taken.

[0139] This embodiment verifies the application of the yogurt testing system. 910g of raw milk with a protein content of 3.2g / 100g was preheated to 45℃. While stirring, 12g of hydroxypropyl distarch phosphate, 1.5g of low-fat pectin, 1.5g of agar, 70g of white sugar, and milk protein powder (the amount added was calculated based on the actual protein content of protein powders A, B, C, E, and G to achieve a fermented milk protein content of 3.1g / 100g) were added. No milk protein powder was added to the control sample.

[0140] The above mixture was stirred continuously for 12 minutes, heated to 60°C for homogenization at a pressure of 140 bar; then heated to 85°C and sterilized for 5 minutes; after cooling to 42°C, lactic acid bacteria were inoculated and fermented until the final pH reached 4.50±0.5; after demulsification, the mixture was cooled to 18°C ​​to obtain a yogurt sample.

[0141] The rotational viscosity of the yogurt sample was tested using a rheometer at a test temperature of 25°C. The viscosity value was read at a shear rate of 75 [1 / s], and sensory tests were performed.

[0142] The results of the thermogel strength of milk protein and the corresponding yogurt shear viscosity in this embodiment are shown in Table 5 below.

[0143] Table 6. Gel strength of milk protein yogurt model and corresponding shear viscosity of yogurt

[0144] Protein Yogurt Model Gel Strength 23.6g 27.3g 29.1g 40.2g 41.1g plan Protein A Protein B Protein C Protein E Protein G Rheological shear viscosity at 75 [1 / s] 314 mPa·s 346 MPa·s 375 mPa·s 413 MPa·s 422 mPa·s

[0145] In this embodiment, the gel strength of a milk protein yogurt model is used to predict the yogurt mouth viscosity. The yogurt mouth viscosity is characterized by the viscosity value at a shear rate of 75 [1 / s] using a rheometer. Studies have shown that the viscosity value at a shear rate of 75 [1 / s] is most correlated with the oral perceived viscosity.

[0146] A functional relationship was established between the gel strength of the tested milk protein yogurt model and the shear viscosity of the yogurt. The regression curve equation was obtained through fitting: y = -0.3466x 2 +28.442x-165.41, where R² = 0.9848, the goodness of fit is close to 1, proving that the fit is good (the functional relationship is attached). Figure 3 This regression equation can be used to predict the texture and viscosity of yogurt using the gel strength of a milk protein yogurt model.

[0147] The scheme and model of this embodiment were further validated. The gel strength of the yogurt model of whey protein H, J and milk protein K was measured using the method described above. A yogurt test system was prepared using whey protein H, J and milk protein K, the rheological shear viscosity of the yogurt was detected, and the theoretical rheological shear viscosity was calculated using the fitted regression equation. The results are shown in Table 7 below.

[0148] Table 7 Model Validation

[0149]

[0150]

[0151] Through comparative analysis of the data, the rheological shear viscosity values ​​obtained by the function calculation are quite close to the rheological shear viscosity values ​​detected by the actual test model, proving that the method and model can predict the influence of protein on the mouthfeel viscosity of yogurt and have a certain degree of accuracy. The performance and quality of the final product can be evaluated before the process by evaluating the performance of the raw materials.

[0152] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. The application of a method for detecting the gel properties of exogenous milk proteins in yogurt quality assessment, characterized in that, The method for detecting the gel properties of exogenous milk proteins includes the following steps: (1) Protein content determination: The protein content of the milk protein powder was determined; (2) Preparation of protein solution: Heat deionized water, add the milk protein powder to prepare a protein solution, and hydrate and dissolve the protein powder. After cooling to eliminate air bubbles, the solution is ready for use. The protein concentration in the protein solution is 10-12 wt%. (3) Gel thermal denaturation: The protein solution is heated to 80-82℃ for thermal denaturation, and the resulting gel is cooled and equilibrated for later use; (4) Gel strength test: The gel is tested for gel rupture strength using the weak gel test module of the texture analyzer. The force value at the first gel rupture point is taken as the protein gel strength. In the gel strength test step, the test probe is cylindrical with a diameter of 12.700 mm ± 0.013 mm. The test parameters include: pre-test speed of 1.00 mm / sec, test speed of 0.50 mm / sec, post-test speed of 10.00 mm / sec, pressure distance of 5 mm, and trigger force of 2 g. The yogurt quality assessment steps include: A functional relationship was established between the gel strength value of milk proteins with thermogelation properties and the centrifugal water loss rate of yogurt. The higher the protein gel strength, the stronger the ability to bind water, and the lower the centrifugal water loss rate of yogurt, thus evaluating the better water retention of the product. In the yogurt, the total protein concentration was 4.5-5 wt%, homogenization was performed at 130-150 bar, and fermentation was carried out to the final pH of 4.40±0.

05.

2. The application according to claim 1, characterized in that, In step (2), the heating temperature of the deionized water is 40-45℃.

3. The application according to claim 1 or 2, characterized in that, In step (3), the heating rate is 8-12℃ / min.

4. The application of a method for detecting the gel properties of exogenous milk proteins in yogurt quality assessment, characterized in that, The method for detecting the gel properties of the milk protein in the milk protein yogurt model includes the following steps: (1) Protein content determination: The protein content of the milk protein powder was determined; (2) Preparation of protein solution: Heat skim milk, add the milk protein powder and mix, and then hydrate and dissolve the protein powder; The obtained protein solution was homogenized and sterilized, and then cooled to dissipate air bubbles to obtain a sterile solution for later use; the total protein concentration in the protein solution was 4.5-5 wt%, and the total homogenization pressure was 130-150 bar. (3) Inoculation and fermentation: Take the sterile liquid and inoculate it with lactic acid bacteria for constant temperature fermentation until the endpoint pH 4.40±0.

05. Collect the gel product and cool and equilibrate it for later use. (4) Gel strength test: The gel is tested for gel rupture strength using the weak gel test module of the texture analyzer. The force value at the first gel rupture point is taken as the protein gel strength. In the gel strength test step, the test probe is cylindrical with a diameter of 12.700 mm ± 0.013 mm. The test parameters include: pre-test speed of 1.00 mm / sec, test speed of 0.50 mm / sec, post-test speed of 10.00 mm / sec, pressure distance of 20 mm, and trigger force of 5 g. The yogurt quality assessment steps include: By constructing a function relationship between the gel strength value of milk protein and the shear viscosity of yogurt using a milk protein yogurt model, the mouthfeel viscosity of yogurt can be predicted through the gel strength of the milk protein yogurt model.

5. The application according to claim 4, characterized in that, In step (2), The skim milk is heated to a temperature of 40-50°C. The temperature of the homogenization step is 58-60℃; The sterilization step is performed at a temperature of 80-90℃.

6. The application according to claim 4 or 5, characterized in that, In step (3), the lactic acid bacteria include a compound fermentation agent of Lactobacillus bulgaricus and Streptococcus thermophilus; The temperature of the constant temperature fermentation step is 40-45℃.

7. A method for evaluating yogurt quality based on the gelation properties of exogenous milk proteins, characterized in that, The method includes the step of detecting the exogenous milk protein gel strength index according to the exogenous milk protein gel characteristic detection method according to any one of claims 1-6, and the step of evaluating the quality of yogurt based on the gel strength index.

8. The method for evaluating yogurt quality based on the gel properties of exogenous milk proteins according to claim 7, characterized in that, The yogurt quality assessment steps include: By constructing a functional relationship between the gel strength value of the milk protein exhibiting thermogelation properties and the centrifugal water loss rate of yogurt, the regression curve equation obtained by fitting is: y = 2E-07x 2 -0.0003x+0.2241, where R² = 0.9904. The higher the protein gel strength, the stronger the ability to bind water, the lower the water loss rate of the yogurt after centrifugation, and the better the water retention of the product. or, Using the gel strength value of milk protein and the shear viscosity of yogurt constructed from the milk protein yogurt model, a regression curve equation was obtained by fitting: y = -0.3466x 2 +28.442x-165.41, where R² = 0.9848, is used to predict the mouthfeel viscosity of yogurt using the gel strength of the milk protein yogurt model.

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