Method for improving the stability of zero-fat fermented milk and use thereof

Abstract

The application provides a method for improving the stability of zero-fat fermented milk and application thereof, and particularly, the method comprises the following steps: dividing skimmed milk in raw materials of zero-fat fermented milk into two parts, using the first part of skimmed milk to perform heat pretreatment on whey protein to obtain a mixture A, mixing the mixture A with the second part of skimmed milk to obtain a mixture B, and adding TG enzyme for enzyme treatment in the mixing process; the mass ratio of casein and whey protein in the mixture B is (1.5-3):1; and the mixture B is subjected to homogenization, sterilization and fermentation to prepare zero-fat fermented milk. The method can improve the water holding capacity of zero-fat fermented milk without adding additional stabilizers, and ensures the texture stability of zero-fat fermented milk.

Description

Technical Field

[0001] This invention belongs to the field of dairy product preparation technology, specifically relating to a method for improving the stability of zero-fat fermented milk and its application. Background Technology

[0002] As consumers increasingly demand healthier diets, the demand for zero-fat products is growing. However, when fermented milk products contain no added fat, not only is flavor and aroma lost, but the microstructure of the fermented milk becomes more porous, viscosity and hardness decrease, and whey separation becomes severe, indicating that fat content has a significant impact on the textural stability of fermented milk. To improve the textural stability of zero-fat milk, concentration and separation processes can be used to increase the milk solids content and reduce water content, and stabilizers such as starch and colloids can be added to maintain water-holding capacity. However, these methods are complex and increase production costs. Summary of the Invention

[0003] To address the issue of poor textural stability in zero-fat fermented milk, this invention aims to provide a method for improving its stability. This invention adjusts the ratio of casein to whey protein to within the range of (1.5–3:1) by adding whey protein, ensuring the solids content of skim milk remains between 8–15%. Combined with adjustments to feed parameters and TG enzyme (transglutaminase) treatment to increase protein cross-linking, this method enhances the water-holding capacity of zero-fat fermented milk and ensures its textural stability without the addition of additional stabilizers.

[0004] To achieve the above objectives, the present invention provides a method for improving the stability of zero-fat fermented milk, which includes dividing the skim milk in the raw materials of zero-fat fermented milk into two parts, using the first part of skim milk to heat-pretreat whey protein to obtain mixture A, and then mixing mixture A with the second part of skim milk to obtain mixture B, adding TG enzyme for enzyme treatment during the mixing process; the mass ratio of casein to whey protein in mixture B is (1.5-3):1; mixture B is homogenized, sterilized, and fermented to prepare zero-fat fermented milk.

[0005] In some implementations, the mass ratio of the first portion of skim milk to the second portion of skim milk is (1-3):1.

[0006] In some embodiments, the whey protein is whey protein powder, and the whey protein powder has a whey protein content of 50-80%. As a preferred embodiment, the whey protein powder has a whey protein content of 50%, 60%, or 80%, more preferably 50%.

[0007] In some implementations, the heating pretreatment conditions are: 65–70°C for 30–50 min.

[0008] In this invention, whey protein preheated at 65–70°C for 30–50 minutes can fully expand its structure, making it easier for TG enzyme to process and bind with casein.

[0009] In some implementations, the enzyme treatment conditions are: 50-58°C, 60-80 min.

[0010] In this invention, under conditions of 50-58℃ and 60-80min, TG enzyme can increase the cross-linking degree of whey protein and casein through deamidation, effectively improving the gel efficiency and strength of the protein structure and enhancing its water-holding capacity.

[0011] In some implementations, the amount of TG enzyme added is 0.4-0.5‰, calculated based on 100% of the weight of the zero-fat fermented milk.

[0012] In some embodiments, the TG enzyme is a commonly used raw material in the food industry. In a specific embodiment of the present invention, the activity of the TG enzyme is 100 U / g.

[0013] This invention adds TG enzyme during the mixing process to promote the cross-linking of whey protein and casein, and finally ferments according to conventional fermentation process to obtain zero-fat fermented milk with better texture and stability.

[0014] In some embodiments, the method further includes adding ingredients to the second portion of skim milk, preferably including a sweetener.

[0015] In some embodiments, the sweetener is selected from one or more of granulated sugar, erythritol, and xylitol. As a preferred embodiment, the sweetener is granulated sugar.

[0016] In some embodiments, the fat content of the skim milk is less than 0.5% by mass fraction.

[0017] Another aspect of the present invention provides a method for preparing zero-fat fermented milk, which includes a method for improving the stability of zero-fat fermented milk, i.e., including the steps of the method described above.

[0018] In some implementations, the ingredients of the zero-fat fermented milk per 1,000 parts by weight include 913.94-926.54 parts skim milk, 60-70 parts sweetener, 4.42-14.66 parts whey protein powder, 0.4-0.5 parts TG enzyme, and 0.5-1 parts starter culture.

[0019] In some embodiments, the sweetener is selected from one or more of granulated sugar, erythritol, and xylitol. As a preferred embodiment, the sweetener is granulated sugar.

[0020] In some embodiments, the whey protein powder has a whey protein content of 50-80%. As a preferred embodiment, the whey protein powder has a whey protein content of 50%, 60%, or 80%, more preferably 50%.

[0021] In some embodiments, the preparation method includes the following steps:

[0022] (1) Preparation of skim milk: Raw milk is centrifuged to reduce its fat content to less than 0.5% by mass fraction to obtain skim milk;

[0023] (2) Heat pretreatment of whey protein: The skim milk is divided into two parts. The first part of the skim milk is used to heat pretreat the whey protein to obtain mixture A;

[0024] (3) Enzyme treatment: Add ingredients to the skim milk in the second part, and then mix with mixture A to obtain mixture B. Add TG enzyme during the mixing process and then perform enzyme treatment.

[0025] (4) Homogenization and sterilization: Homogenize and sterilize the mixture B after enzyme treatment in step (3). The homogenization pressure is 30-40 / 180-200 bar, the sterilization temperature is 95-98℃, and the sterilization time is 300-350s.

[0026] (5) Inoculation and fermentation: Add fermenting agent to the homogenized and sterilized mixture B in step (4) to ferment and obtain zero-fat fermented milk.

[0027] In some implementations, fermentation is stopped in step (5) when it reaches 65–72°T.

[0028] In some embodiments, the preparation method further includes:

[0029] (6) Cooling and ripening: The zero-fat fermented milk obtained in step (5) is stirred and broken, and then cooled and ripened at 2-10°C and refrigerated.

[0030] The present invention also provides a zero-fat fermented milk, which is prepared by the above preparation method.

[0031] Beneficial effects:

[0032] (1) This invention increases the whey protein content in the formula milk system by adding whey protein, and controls the ratio of casein to whey protein in the formula milk to be within the range of (1.5 to 3):1. At the same time, the added whey protein is preheated in part of the formula milk. The whey protein preheated at 65 to 70°C for 30 to 50 minutes can fully expand its structure, making it easier to be treated by TG enzyme and bind with casein.

[0033] (2) The present invention treats preheated whey protein and casein with TG enzyme. When the TG enzyme is added in the range of 0.4-0.5‰, at 50-58℃ for 60-80 minutes, the TG enzyme can increase the cross-linking degree of whey protein and casein through deamidation, effectively improving the gel efficiency and strength of the protein structure and enhancing its water holding capacity. Attached Figure Description

[0034] Figure 1 This is a technical circuit diagram of the present invention.

[0035] Figure 2 The graph shows the protein structure uniformity results of the zero-fat fermented dairy products in each embodiment and comparative example. Detailed Implementation

[0036] Before further describing specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terminology used in the embodiments of the present invention is for describing specific embodiments and not for limiting the scope of protection of the present invention.

[0037] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. In addition to the specific methods, apparatus, and materials used in the embodiments, based on the knowledge of the prior art possessed by one of ordinary skill in the art and the description of this invention, any prior art methods, apparatus, and materials similar to or equivalent to those described, apparatus, and materials in the embodiments of this invention may be used to implement the present invention.

[0038] Unless otherwise stated, the experimental methods, detection methods, and preparation methods not described in detail in this invention all adopt conventional techniques in this technical field.

[0039] Figure 1 This is a technical circuit diagram of the present invention.

[0040] Unless otherwise specified, all percentages of content in the embodiments of this application are mass percentages.

[0041] Example 1

[0042] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, whey protein powder (whey protein content of 50%), TG enzyme (transglutaminase (raw material code Sternzym PT 100W), Sternzym Food Ingredients (Suzhou) Co., Ltd.), and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 924.68 parts skim milk, 70 parts granulated sugar, 4.42 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts starter culture. The technical method for improving the stability of zero-fat fermented milk in this embodiment further includes the following steps:

[0043] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0044] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 65°C for 40 min.

[0045] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). After adding TG enzyme to the mixture, circulate and mix at 50°C for 60 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 3:1.

[0046] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0047] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0048] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0049] Example 2

[0050] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, white sugar, whey protein powder (whey protein content of 50%), TG enzyme, and fermentation agent. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 924.46 parts skim milk, 65 parts white sugar, 9.54 parts whey protein powder, 0.5 parts TG enzyme, and 0.5 parts fermentation agent. The technical method for improving the stability of zero-fat fermented milk in this embodiment further includes the following steps:

[0051] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0052] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 68°C for 40 min.

[0053] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). After adding TG enzyme to the mixture, circulate and mix at 55°C for 70 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0054] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0055] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0056] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0057] Example 3

[0058] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, white sugar, whey protein powder (whey protein content of 50%), TG enzyme, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: skim milk 923.94 parts, white sugar 60 parts, whey protein powder 14.66 parts, TG enzyme 0.4 parts, and starter culture 1 part. The technical method for improving the stability of zero-fat fermented milk in this embodiment further includes the following steps:

[0059] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0060] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 70°C for 30 min.

[0061] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). After adding TG enzyme to the mixture, circulate and mix at 58°C for 60 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 1.7:1.

[0062] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0063] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0064] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0065] Example 4

[0066] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, white sugar, whey protein powder (whey protein content of 60%), TG enzyme, and fermentation agent. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 921.27 parts skim milk, 70 parts white sugar, 7.83 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts fermentation agent. The technical method for improving the stability of zero-fat fermented milk in this embodiment further includes the following steps:

[0067] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0068] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 68°C for 40 min.

[0069] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). After adding TG enzyme to the mixture, circulate and mix at 55°C for 60 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0070] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0071] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0072] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0073] Example 5

[0074] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, white sugar, whey protein powder (whey protein content of 80%), TG enzyme, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: skim milk 923.4 parts, white sugar 70 parts, whey protein powder 5.7 parts, TG enzyme 0.4 parts, and starter culture 0.5 parts. The technical method for improving the stability of zero-fat fermented milk in this embodiment further includes the following steps:

[0075] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0076] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 68°C for 40 min.

[0077] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). After adding TG enzyme to the mixture, circulate and mix at 55°C for 60 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0078] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0079] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0080] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0081] Comparative Example 1

[0082] A method for preparing zero-fat fermented milk, wherein the raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, and a starter culture, wherein per 1000 parts by weight of zero-fat milk, skim milk comprises 930 parts, granulated sugar comprises 70 parts, and starter culture comprises 0.5 parts. This comparative example also includes the following steps:

[0083] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0084] (2) Mixing: Add white sugar to the standardized skim milk obtained in step (1) and mix in a circulating manner at 55°C for 30 minutes;

[0085] (3) Homogenization and sterilization: The liquid obtained in step (2) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0086] (4) Inoculation and fermentation: Cool the mixed material in step (3) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0087] (5) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (4) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0088] Comparative Example 2

[0089] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, whey protein powder (whey protein content of 50%), TG enzyme, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 919.56 parts skim milk, 70 parts granulated sugar, 7.83 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts starter culture. This comparative method for improving the stability of zero-fat fermented milk further includes the following steps:

[0090] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0091] (2) Enzyme treatment: Add white sugar and whey protein to the standardized skim milk obtained in step (1), add TG enzyme to the mixture, and circulate and mix at 58°C for 60 min. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0092] (3) Homogenization and sterilization: The liquid obtained in step (2) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0093] (4) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0094] (5) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0095] Comparative Example 3

[0096] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, whey protein powder (whey protein content of 50%), TG enzyme, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 896.1 parts skim milk, 70 parts granulated sugar, 33 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts starter culture. This comparative method for improving the stability of zero-fat fermented milk further includes the following steps:

[0097] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0098] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 68°C for 40 min.

[0099] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). Add TG enzyme to the mixture and circulate and mix at 58°C for 60 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 1:1.

[0100] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0101] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0102] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0103] Comparative Example 4

[0104] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, whey protein powder (whey protein content of 50%), TG enzyme, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 919.56 parts skim milk, 70 parts granulated sugar, 9.54 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts starter culture. This comparative method for improving the stability of zero-fat fermented milk further includes the following steps:

[0105] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0106] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 65°C for 40 min.

[0107] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). Add TG enzyme to the mixture and circulate and mix at 58°C for 50 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0108] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0109] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0110] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0111] Comparative Example 5

[0112] A technical method for improving the stability of zero-fat fermented milk and its application are disclosed. The raw materials of the zero-fat fermented milk mainly consist of skim milk, granulated sugar, whey protein powder (whey protein content of 50%), enzyme treatment, and a starter culture. Specifically, per 1000 parts by weight of zero-fat milk, the components are: 919.56 parts skim milk, 70 parts granulated sugar, 9.54 parts whey protein powder, 0.4 parts TG enzyme, and 0.5 parts starter culture. This comparative method for improving the stability of zero-fat fermented milk further includes the following steps:

[0113] (1) Preparation of skim milk: The raw milk is centrifuged to make the final fat content less than 0.5% and the protein content 3.2% (of which the mass ratio of casein to whey protein is 4:1);

[0114] (2) Heat pretreatment of whey protein: The standardized 1 / 2 skim milk obtained in step (1) and whey protein powder were heat pretreated at 65°C for 40 min.

[0115] (3) Enzyme treatment: Add white sugar to the remaining 1 / 2 of the standardized skim milk obtained in step (1), and add the whey protein solution pretreated in step (2). Add TG enzyme to the mixture and circulate and mix at 58°C for 90 minutes. The mass ratio of casein to whey protein in the resulting skim milk mixture is 2:1.

[0116] (4) Homogenization and sterilization: The liquid obtained in step (3) is homogenized at a homogenization pressure of 30 / 180 bar; the sterilization temperature is 95℃ and the sterilization time is 300s;

[0117] (5) Inoculation and fermentation: Cool the mixed material in step (4) to 41°C, inoculate with fermentation agent (Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus, with an initial viable count ratio of 1:2), and carry out constant temperature fermentation at 41°C until the titratable acidity is 65°T and then stop fermentation.

[0118] (6) Cooling and ripening: The nonfat yogurt obtained after fermentation in step (5) is stirred to break the milk, and then cooled and ripened at 2-10℃ and refrigerated.

[0119] Stability evaluation of zero-fat fermented milk

[0120] To evaluate the stability of zero-fat fermented dairy products, the water-holding capacity and viscosity of the zero-fat fermented products in the examples and comparative examples were verified. The results are shown in Table 1:

[0121] Table 1

[0122] Water holding capacity (%) Viscosity (mPa·s) Example 1 73.55 587.22 Example 2 75.24 654.19 Example 3 70.11 631.24 Example 4 68.21 558.11 Example 5 65.57 574.35 Comparative Example 1 35.44 325.14 Comparative Example 2 42.38 365.23 Comparative Example 3 53.68 346.45 Comparative Example 4 45.29 326.12 Comparative Example 5 50.14 357.24

[0123] Figure 2 The protein structure uniformity of the zero-fat fermented dairy products of each embodiment and comparative example is shown.

[0124] Water holding capacity determination method: Accurately weigh the mass M1 of the sample to be tested, centrifuge at 5000 r / min for 10 min using the centrifugal sedimentation method, obtain the mass M2 of the precipitate, and calculate the water holding capacity of the sample:

[0125] Viscosity measurement method: Anton Paar MCR302 rheometer was used, with a constant shear rate of 50 1 / s, data points were collected at 60 s 1 s / point, and the temperature was controlled at 20℃.

[0126] Method for determining structural uniformity: The microstructure of zero-fat fermented milk was observed using a laser confocal microscope and scanned at 100x magnification.

[0127] The above results indicate that: Examples 1-5, by adding whey protein to adjust the casein to whey protein ratio to (1.5-3):1, and then subjecting the whey protein to preheating and enzymatic hydrolysis, all produced zero-fat fermented milk with a water holding capacity ≥60%, a viscosity ≥550 mPa·s, and good uniformity of microscopic protein structure with no large voids, significantly better than untreated zero-fat fermented milk (Comparative Example 1). However, Examples 4 (supplemented with 60% whey protein powder) and 5 (supplemented with 80% whey protein powder) showed inferior water holding capacity, viscosity, and protein structure compared to Examples 1-3 (supplemented with 50% whey protein powder), indicating that the lower the content of added whey protein powder, the better the effect on balancing the uniformity of the fermented milk.

[0128] Although Comparative Example 2 supplemented with whey protein and underwent TG enzyme treatment, the lack of heat pretreatment resulted in lower water retention and viscosity compared to Comparative Example 1. Comparative Example 3 supplemented with whey protein and underwent TG enzyme treatment, but the excessive amount of whey protein (casein to whey protein ratio of 1:1) led to an imbalance in the casein-to-whey protein ratio. Therefore, its water retention, viscosity, and protein structure uniformity were lower than those of Examples 1-5, indicating that a suitable whey protein to casein ratio can better maintain the stability of zero-fat fermented milk. Comparative Examples 4 and 5, due to excessively long or short enzyme treatment times, resulted in poor enzymatic hydrolysis, leading to lower water retention, viscosity, and protein structure uniformity compared to Examples 1-5. This indicates that when casein and whey protein are in an appropriate ratio, simultaneous and appropriate enzymatic hydrolysis can improve the stability of zero-fat fermented milk.

[0129] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for improving the stability of zero-fat fermented milk, comprising: The skim milk in the raw materials of zero-fat fermented milk is divided into two parts. The first part of skim milk is used to heat and pretreat whey protein to obtain mixture A. Then, mixture A is mixed with the second part of skim milk to obtain mixture B. During the mixing process, TG enzyme is added for enzyme treatment. The mass ratio of casein to whey protein in mixture B is (1.5~3):

1. Mixture B is homogenized, sterilized, and fermented to prepare zero-fat fermented milk.

2. The method according to claim 1, wherein, The conditions for heat pretreatment are: 65-70℃ for 30-50 min; the conditions for enzyme treatment are: 50-58℃ for 60-80 min.

3. The method according to claim 1, wherein, The mass ratio of the first portion of skim milk to the second portion of skim milk is (1-3):

1.

4. The method according to claim 1, wherein, Based on the weight of the zero-fat fermented milk as 100%, the amount of TG enzyme added is 0.4-0.5‰.

5. The method according to any one of claims 1-4, wherein, The method further includes adding ingredients to the second portion of skim milk, preferably including granulated sugar.

6. A method for preparing zero-fat fermented milk, comprising the steps of the method described in any one of claims 1-5.

7. The preparation method according to claim 6, wherein, In every 1000 parts by weight of zero-fat milk, the raw materials of the zero-fat fermented milk include: 913.94-926.54 parts of skim milk, 60-70 parts of sweetener, 4.42-14.66 parts of whey protein powder, 0.4-0.5 parts of TG enzyme, and 0.5-1 parts of starter culture.

8. The preparation method according to claim 7, wherein, The whey protein powder has a whey protein content of 50-80%, preferably 50%.

9. The preparation method according to any one of claims 6-8, comprising the following steps: (1) Preparation of skim milk: Raw milk is centrifuged to reduce its fat content to less than 0.5% by mass fraction to obtain skim milk; (2) Heat pretreatment of whey protein: The skim milk is divided into two parts. The first part of the skim milk is used to heat pretreat the whey protein to obtain mixture A; (3) Enzyme treatment: Add ingredients to the skim milk in the second part, and then mix with mixture A to obtain mixture B. Add TG enzyme during the mixing process and then perform enzyme treatment. (4) Homogenization and sterilization: Homogenize and sterilize the mixture B after enzyme treatment in step (3). The homogenization pressure is 30-40 / 180-200 bar, the sterilization temperature is 95-98℃, and the sterilization time is 300-350s. (5) Inoculation and fermentation: Add fermenting agent to the homogenized and sterilized mixture B in step (4) to ferment and obtain zero-fat fermented milk.

10. A zero-fat fermented milk, which is prepared by the preparation method according to any one of claims 6-9.

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