High-protein long shelf-life milk and method for preparing the same

Abstract

The application provides high-protein long-shelf-life milk and a preparation method thereof. The preparation method comprises a step-by-step washing and filtering process, a continuous process without buffer storage, and a combination of ceramic membranes, centrifugal sterilization and pasteurization, prevents the loss of calcium ions in the concentration process, adjusts the lactose content to improve the taste, ensures high protein content, high active protein content, rich and sweet taste of the product, and improves the safety of the milk. Even in a more stringent 10-15 DEG C storage environment, the shelf life can reach 15 days.

Description

Technical Field

[0001] This invention relates to the field of dairy product technology, and in particular to a high-protein, long-shelf-life milk and its preparation method. Background Technology

[0002] Milk, often called "white blood," is rich in heat-sensitive nutrients and serves as a breeding ground for microorganisms. Furthermore, with consumers increasingly favoring high-protein products, the dairy industry faces a common challenge: how to ensure safety while maximizing the preservation of milk's valuable, highly active nutrients, achieving a longer shelf life, and reaching a wider market. Summary of the Invention

[0003] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a high-protein, long-shelf-life milk and its preparation method, so as to solve the problem that fresh milk in the prior art cannot simultaneously take into account the types and quantities of active substances and the long shelf life.

[0004] To achieve the above and other related objectives, the present invention provides a method for preparing high-protein, long-shelf-life milk, comprising the following steps:

[0005] (1) The raw milk is skimmed to obtain skim milk and light cream;

[0006] (2) The skim milk is pre-concentrated by ultrafiltration to reach the preset concentration ratio to obtain ultrafiltration permeate and ultrafiltration retentate. The ultrafiltration permeate and ultrafiltration retentate are washed and filtered in steps until the lactose content in the obtained retentate is less than 0.3% to stop washing and filtering, and the washing permeate and washing retentate are obtained. The washing retentate is concentrated again to obtain concentrated retentate.

[0007] (3) The washing permeate is concentrated by nanofiltration to obtain nanofiltration concentrate;

[0008] (4) Sterilize the light cream; sterilize the nanofiltration concentrate; pass the concentrated retentate through a microfiltration membrane and centrifuge to remove bacteria; then mix the sterilized light cream, the sterilized nanofiltration concentrate and the sterilized concentrated retentate to obtain a mixture.

[0009] (5) Homogenize and sterilize the mixture to obtain high-protein, long-shelf-life milk.

[0010] The present invention also provides a high-protein, long-shelf-life milk, prepared according to the preparation method described above.

[0011] As described above, the high-protein, long-shelf-life milk and its preparation method of the present invention have the following beneficial effects:

[0012] This invention addresses the issues of retaining active proteins and extending shelf life by combining ceramic membranes, centrifugal sterilization, and pasteurization. It employs a buffer-free, continuous process design to solve safety concerns, resulting in a shelf life more than twice that of existing fresh milk (3-7 days). A step-by-step washing and filtration process solves the problems of ceramic membrane clogging and calcium ion loss, and allows for adjustment of lactose content to improve taste. While ensuring high protein content and rich flavor, the optimized and limited concentration ratio guarantees both high protein and active protein content, while also improving milk safety. Even under more stringent storage conditions of 10-15℃, the shelf life can reach 15 days. Attached Figure Description

[0013] Figure 1 The diagram shows the pressure change of the ceramic microfiltration membrane through the concentrated retentate in Example 3 and Comparative Example 4 of this invention. Detailed Implementation

[0014] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0015] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0016] The first aspect of this invention provides a method for preparing high-protein, long-shelf-life milk, comprising the following steps:

[0017] (1) The raw milk is skimmed to obtain skim milk and light cream;

[0018] (2) The skim milk is pre-concentrated by ultrafiltration to reach the preset concentration ratio to obtain ultrafiltration permeate and ultrafiltration retentate. The ultrafiltration permeate and ultrafiltration retentate are washed and filtered in steps until the lactose content in the obtained retentate is less than 0.3% to stop washing and filtering, and the washing permeate and washing retentate are obtained. The washing retentate is concentrated again to obtain concentrated retentate.

[0019] (3) The washing permeate is concentrated by nanofiltration to obtain nanofiltration concentrate;

[0020] (4) Sterilize the light cream; sterilize the nanofiltration concentrate; pass the concentrated retentate through a microfiltration membrane and centrifuge to remove bacteria; then mix the sterilized light cream, nanofiltration concentrate and sterilized concentrated retentate to obtain a mixture.

[0021] (5) Homogenize and sterilize the mixture to obtain high-protein, long-shelf-life milk.

[0022] In the preparation method of the present invention, the raw milk in step (1) is conventional whole milk in the art.

[0023] The preparation method of this invention may include: defatting raw milk to provide cream and skim milk. Defatting generally refers to the removal of the milk fat portion from raw milk to provide a viscous product with a high milk fat content (e.g., cream) and a liquid product containing virtually no milk fat (e.g., skim milk). Appropriate parameters are typically required during defatting. For example, the temperature of the material during defatting may be 50–51°C, 51–53°C, or 53–55°C, preferably 50–53°C. As another example, defatting may be performed by centrifugal defatting, in which the fat in the material is fully removed, and the fat content in the skim milk may be ≤0.03%.

[0024] The protein content of the raw milk mentioned in step (1) is 3.0% to 4.0%. For example, it is 3.0% to 3.2%, 3.2% to 3.4%, 3.4% to 3.6%, 3.6% to 3.8%, or 3.8% to 4.0%.

[0025] The fat content of the raw milk mentioned in step (1) is 3.3% to 4.5%. For example, it is 3.3% to 3.5%, 3.5% to 3.7%, 3.7% to 3.9%, 3.9% to 4.1%, 4.1% to 4.5%, or 4.3% to 4.5%.

[0026] In the preparation method of the present invention, the fat content of the skim milk in step (1) is ≤0.03%. If the content is higher than this, the fat content of the concentrated effluent after concentration in step (2) will be higher than 0.1%, which will clog the microfiltration membrane in step (4).

[0027] In the preparation method of the present invention, the preset concentration ratio in step (2) is 1.1 to 1.3. For example, it is 1.1 to 1.2 or 1.2 to 1.3.

[0028] In the preparation method of the present invention, the membrane temperature for ultrafiltration pre-concentration in step (2) is 2-30°C. For example, it is 2-5°C, 5-10°C, 10-15°C, 15-20°C, 20-25°C, or 25-30°C.

[0029] In the preparation method of the present invention, the membrane material for ultrafiltration preconcentration in step (2) is selected from one of polyethersulfone, polysulfone, polyacrylonitrile, regenerated cellulose, cellulose acetate, polyamide, and polyvinyl alcohol.

[0030] In the preparation method of the present invention, the molecular weight cutoff of the ultrafiltration pre-concentration in step (2) is 10-30 kDa. For example, it is 10-15 kDa, 15-20 kDa, 20-25 kDa or 25-30 kDa.

[0031] In the preparation method of the present invention, the stepwise washing and filtration in step (2) includes the following steps:

[0032] (21) Keep the concentration ratio of ultrafiltration pre-concentration constant, and circulate the ultrafiltration permeate and ultrafiltration retentate back into the feed liquid skim milk for 10-15 min, 10-11 min, 11-12 min, 12-13 min, 13-14 min or 14-15 min to obtain retentate I and permeate I;

[0033] (22) Retained liquid I is refluxed again, with the concentration ratio remaining unchanged. RO water is added to the feed liquid until the lactose content in the obtained retained liquid reaches below 0.3%, at which point washing and filtration are stopped, yielding retained liquid II and permeate II. Retained liquid II is the washing retentate. Permeate I and permeate II are mixed to obtain the washing permeate.

[0034] The RO water is reverse osmosis water.

[0035] The amount of RO water added in step (22) is the same as the amount of permeate I.

[0036] The washing and filtration process adopts a step washing and filtration process. In step (21), the ultrafiltration permeate and ultrafiltration retentate are recycled back to the feed liquid to build a protein layer on the surface of the ultrafiltration membrane core to prevent calcium loss.

[0037] This invention employs a stepwise washing and filtration process, which not only prevents some proteins from passing through the membrane and being lost during filtration, but also rapidly reduces lactose content, shortens washing and filtration time, and improves washing and filtration efficiency. This is because prolonged low-temperature membrane treatment can lead to the proliferation of psychrophilic bacteria in milk, affecting milk quality and flavor. Furthermore, this washing and filtration process can reduce the sodium ion content in the final washing effluent, but it cannot be used to remove sodium ions, as this would result in a significant loss of lactose in the washing permeate.

[0038] In the preparation method of the present invention, the further concentration in step (2) is to increase the concentration ratio to 2 to 3 times the concentration ratio of ultrafiltration pre-concentration.

[0039] In the preparation method of the present invention, the concentration ratio of nanofiltration concentration in step (3) is 10 to 15. For example, it is 10 to 11, 11 to 12, 12 to 13, 13 to 14 or 14 to 15.

[0040] In the preparation method of the present invention, the nanofiltration concentration temperature in step (3) is 2-15°C. For example, it is 2-4°C, 4-6°C, 6-8°C, 8-10°C, 10-12°C, 12-14°C, or 14-15°C.

[0041] In the preparation method of the present invention, the molecular weight cutoff of the nanofiltration concentration in step (3) is 150-300 kDa. For example, it is 150-160 kDa, 160-170 kDa, 170-180 kDa, 180-190 kDa, 190-200 kDa, 200-210 kDa, 210-220 kDa, 220-230 kDa, 230-240 kDa, 240-250 kDa, 250-260 kDa, 260-270 kDa, 270-280 kDa, 280-290 kDa, or 290-300 kDa.

[0042] In the preparation method of the present invention, the sterilization temperature of the light cream in step (4) is 110-125°C. For example, it is 110-115°C, 115-120°C, or 120-125°C.

[0043] The sterilization time of the cream in step (4) is 10-15 seconds. For example, it is 10-12 seconds, 12-14 seconds, or 14-15 seconds. In a preferred embodiment of the present invention, the sterilization time of the cream in step (4) is 15 seconds.

[0044] In the preparation method of the present invention, the sterilization temperature of the nanofiltration concentrate in step (4) is 75-85°C. For example, it is 75-80°C or 80-85°C.

[0045] The sterilization time of the nanofiltration concentrate in step (4) is 15-17 seconds, for example, 15-16 seconds or 16-17 seconds. In a preferred embodiment of the present invention, the sterilization time of the nanofiltration concentrate in step (4) is 15 seconds.

[0046] In the preparation method of the present invention, the microfiltration membrane in step (4) is a ceramic microfiltration membrane, and the average pore size of the ceramic microfiltration membrane is 1.2–1.6 μm. For example, it is 1.2–1.3 μm, 1.3–1.4 μm, 1.4–1.5 μm, or 1.5–1.6 μm. In a preferred embodiment of the present invention, the average pore size of the ceramic microfiltration membrane is 1.4 μm.

[0047] In the preparation method of the present invention, the proportion of nanofiltration concentrate in the mixture in step (4) is 28-35%. For example, it is 28-29%, 29-30%, 30-31%, 31-32%, 32-33%, 33-34%, or 34-35%.

[0048] In the preparation method of the present invention, the sterilization in step (5) is pasteurization, and the pasteurization temperature is 71-76℃, 71-71.5℃, 71.5-71.75℃, 71.75-72℃, 72-72.5℃, 72.5-73℃, 73-73.5℃, 73.5-74℃, 74-74.5℃, 74.5-75℃, 75-75.25℃, 75.25-75.5℃, or 75.5-76℃, and the pasteurization time is 15-17s, 15-16s, or 16-17s. In a preferred embodiment of the present invention, the pasteurization time is 15s.

[0049] In step (4), the sterilized cream, the sterilized filtrate concentrate, and the sterilized filtrate retentate are all mixed online without being cached. The resulting mixture is then directly proceeded to step (5) without being cached. The mixing ratio of the sterilized cream, the sterilized nanofiltration concentrate, and the sterilized retentate concentrate can be adjusted according to product design requirements, such as high-protein whole milk, high-protein partially skimmed milk, or high-protein skimmed milk.

[0050] In the preparation method of the present invention, after sterilization in step (5), the product is cooled and filled at a temperature of 2-8°C. For example, the temperature is 2-3°C, 3-4°C, 4-5°C, 5-6°C, 6-7°C, or 7-8°C.

[0051] The second fermentation method of the present invention provides a high-protein, long-shelf-life milk, which is prepared according to the preparation method described above.

[0052] The raw milk described in the following examples is sourced from Bright Dairy & Food Co., Ltd.

[0053] The detection methods for protein content, immunoglobulin content, lactoferrin content, and lactoperoxidase content used in the following examples refer to: T / CSCA 110059-2020 "Pasteurized Milk - Fresh Milk", where lactoferrin is determined according to GB5009.299-2024.

[0054] Method for detecting calcium content: GB 5009.268-2016 Method II

[0055] Method for determining bacterial colonies: GB 4789.2-2022

[0056] Example 1

[0057] Preparation of high-protein, long-shelf-life milk:

[0058] (1) Raw milk (protein content 3.0%, fat content 3.3%) was defatted to obtain skim milk (protein content 3.1%, fat content 0.03%) and light cream.

[0059] (2) Skim milk is pre-concentrated by ultrafiltration at a concentration ratio of 1.3 times and a membrane temperature of 2°C (molecular weight cutoff 10-30 kDa). After reaching the concentration ratio, the obtained ultrafiltration permeate and ultrafiltration retentate are washed stepwise: First, while maintaining the concentration ratio and temperature, the obtained ultrafiltration permeate and ultrafiltration retentate are circulated back into the feed solution to build a protein layer on the surface of the ultrafiltration membrane core to prevent calcium loss. The circulation is performed for 10 minutes to obtain retentate I and permeate I; Second, the retentate... Flow liquid I continues to flow back, while permeate I no longer flows back. The concentration ratio remains unchanged. RO water is added to the feed liquid. The amount of RO water added in the washing process is the same as the amount of permeate I. Washing is stopped when the lactose content in the retentate is 0.3%. Retentate II and permeate II are obtained. Permeate I and permeate II are collected and mixed to obtain washing permeate, which is temporarily stored. Retentate II is the washing retentate. The ultrafiltration concentration ratio is increased to 3 times, and the washing retentate is concentrated again to obtain concentrated retentate, which is temporarily stored.

[0060] (3) The washing permeate is concentrated by nanofiltration at a concentration ratio of 15 and a concentration temperature of 2℃ (with a molecular weight cutoff of 150-300 kDa) to obtain nanofiltration concentrate.

[0061] (4) The light cream was sterilized at 110°C for 15 seconds; the nanofiltration concentrate was sterilized at 75°C for 15 seconds; the concentrated retentate was sterilized by centrifugation through a 1.4μm ceramic microfiltration membrane; none of the three were buffered and all were immediately mixed online. The mixed solution was not buffered and the mixing ratio was concentrated retentate: nanofiltration concentrate: light cream = 72:28:0.

[0062] (5) Homogenize the mixture, pasteurize (71.25℃, 15s), cool to 2℃ and fill to obtain the finished product.

[0063] Example 2

[0064] Preparation of high-protein, long-shelf-life milk

[0065] (1) Raw milk (4.0% protein and 4.5% fat) was defatted to obtain skim milk (4.1% protein and 0.03% fat) and light cream.

[0066] (2) Skim milk is pre-concentrated by ultrafiltration at a concentration ratio of 1.1 times and a membrane temperature of 30°C (molecular weight cutoff 10-30 kDa). After reaching the concentration ratio, the obtained ultrafiltration permeate and ultrafiltration retentate are washed stepwise: First, keeping the concentration ratio and temperature constant, the obtained ultrafiltration permeate and ultrafiltration retentate are circulated back into the feed solution to build a protein layer on the surface of the ultrafiltration membrane core to prevent calcium loss. The circulation is carried out for 15 minutes to obtain retentate I and permeate I; Second, Retained liquid I is still refluxed, while permeate I is no longer refluxed. The concentration ratio remains unchanged. RO water is added to the feed liquid. The amount of RO water added in the washing process is the same as the amount of permeate I. Washing is stopped when the lactose content in the retained liquid is 0.3%. Retained liquid II and permeate II are obtained. Permeate I and permeate II are collected and mixed to obtain washing permeate, which is temporarily stored. Retained liquid II is the washing retentate. The ultrafiltration concentration ratio is increased to 2 times, and the washing retentate is concentrated again to obtain concentrated retentate, which is temporarily stored.

[0067] (3) The washing permeate is concentrated by nanofiltration at a concentration ratio of 10 and a concentration temperature of 15°C (with a molecular weight cutoff of 150-300 kDa) to obtain nanofiltration concentrate.

[0068] (4) The light cream was sterilized at 125°C for 15 seconds; the nanofiltration concentrate was sterilized at 85°C for 15 seconds; the concentrated retentate was sterilized by centrifugation through a 1.4μm ceramic microfiltration membrane; none of the three were buffered and all were immediately mixed online. The mixed solution was not buffered and the mixing ratio was concentrated retentate: nanofiltration concentrate: light cream = 66:28:6.

[0069] (5) Homogenize the mixture, pasteurize (75℃, 15s), cool to 8℃ and fill to obtain the finished product.

[0070] Example 3

[0071] Preparation of high-protein, long-shelf-life milk:

[0072] (1) Raw milk (protein content 3.5%, fat content 4%) was defatted to obtain skim milk (protein content 3.6%, fat content 0.02%) and light cream.

[0073] (2) Skim milk is pre-concentrated by ultrafiltration at a concentration ratio of 1.2 times and a membrane temperature of 20°C (molecular weight cutoff 10-30 kDa). After reaching the concentration ratio, the obtained ultrafiltration permeate and ultrafiltration retentate are washed stepwise: First, while maintaining the concentration ratio and temperature, the obtained ultrafiltration permeate and ultrafiltration retentate are circulated back to the feed solution to build a protein layer on the surface of the ultrafiltration membrane core to prevent calcium loss. The circulation is carried out for 10 minutes to obtain retentate I and permeate I; Second, the retentate... Flow liquid I continues to flow back, while permeate I no longer flows back. The concentration ratio remains unchanged. RO water is added to the feed liquid, and the amount of RO water added in the washing process is the same as the amount of permeate I. Washing is stopped when the lactose content in the retentate is 0.3%, resulting in retentate II and permeate II. Permeate I and permeate II are collected and mixed to obtain washing permeate, which is temporarily stored. Retentate II is the washing retentate. The ultrafiltration concentration ratio is increased to 2.5 times, and the washing retentate is concentrated again to obtain concentrated retentate, which is temporarily stored.

[0074] (3) The washing permeate is concentrated by nanofiltration at a concentration ratio of 13 and a concentration temperature of 10°C (with a molecular weight cutoff of 150-300 kDa) to obtain nanofiltration concentrate.

[0075] (4) The cream was sterilized at 120°C for 15 seconds; the nanofiltration concentrate was sterilized at 80°C for 15 seconds; the concentrated retentate was sterilized by centrifugation through a 1.4μm ceramic microfiltration membrane; none of the three were buffered and they were all mixed online immediately. The mixed solution was not buffered and the mixing ratio was concentrated retentate: nanofiltration concentrate: cream = 61:30:9.

[0076] (5) Homogenize the mixture, pasteurize (72℃, 15s), cool to 6℃ and fill to obtain the finished product.

[0077] Example 4

[0078] Preparation of high-protein, long-shelf-life milk:

[0079] (1) Raw milk (protein content 3.3%, fat content 3.6%) was defatted to obtain skim milk (protein content 3.4%, fat content 0.01%) and light cream.

[0080] (2) Skim milk is pre-concentrated by ultrafiltration at a concentration ratio of 1.1 times and a membrane temperature of 10°C (molecular weight cutoff 10-30 kDa). After reaching the concentration ratio, the obtained ultrafiltration permeate and ultrafiltration retentate are washed stepwise: First, while maintaining the concentration ratio and temperature, the obtained ultrafiltration permeate and ultrafiltration retentate are circulated back into the feed solution to build a protein layer on the surface of the ultrafiltration membrane core to prevent calcium loss. The circulation is carried out for 15 minutes to obtain retentate I and permeate I; Second, ... Retained liquid I is still refluxed, while permeate I is no longer refluxed. The concentration ratio remains unchanged. RO water is added to the feed liquid. The amount of RO water added in the washing process is the same as the amount of permeate I. Washing is stopped when the lactose content in the retained liquid is 0.3%. Retained liquid II and permeate II are obtained. Permeate I and permeate II are collected and mixed to obtain washing permeate, which is temporarily stored. Retained liquid II is the washing retentate. The ultrafiltration concentration ratio is increased to 2 times, and the washing retentate is concentrated again to obtain concentrated retentate, which is temporarily stored.

[0081] (3) The washing permeate is concentrated by nanofiltration at a concentration ratio of 12 and a concentration temperature of 6°C (with a molecular weight cutoff of 150-300 kDa) to obtain nanofiltration concentrate.

[0082] (4) The light cream was sterilized at 115°C for 15 seconds; the nanofiltration concentrate was sterilized at 80°C for 15 seconds; the concentrated retentate was sterilized by centrifugation through a 1.4μm ceramic microfiltration membrane; none of the three were buffered and all were immediately mixed online. The mixed solution was not buffered and the mixing ratio was concentrated retentate: nanofiltration concentrate: light cream = 68:28:4.

[0083] (5) Homogenize the mixture, pasteurize (75.25℃, 15s), cool to 4℃ and fill to obtain the finished product.

[0084] Comparative Example 1

[0085] The difference between Comparative Example 1 and Example 1 is that in step 4), the ceramic microfiltration membrane is replaced with centrifugal sterilization, which is used in combination with the centrifugal sterilization in Example 1, i.e., double centrifugal sterilization. The remaining steps are the same.

[0086] Comparative Example 2

[0087] The difference between Comparative Example 2 and Example 2 is that in step 4), the ceramic microfiltration membrane is no longer centrifuged for sterilization after treatment, while the other steps are the same.

[0088] Comparative Example 3

[0089] The difference between Comparative Example 3 and Example 3 is that in step (4), after the cream is sterilized, the nanofiltration concentrate is sterilized, and the concentrated retentate is sterilized, it is temporarily stored for 1-2 hours before being mixed online. After mixing, it is temporarily stored for 1-2 hours before step (5) is carried out.

[0090] Comparative Example 4

[0091] The difference between Comparative Example 4 and Example 3 is that step (2) is removed, and the skim milk obtained in step (1) is directly subjected to nanofiltration concentration in step (3) with a concentration ratio of 1.5 to obtain nanofiltration concentrate and nanofiltration retentate; the nanofiltration retentate is subjected to the 1.4μm ceramic microfiltration membrane process in step (4), and the other steps are the same as in Example 3.

[0092] Comparative Example 5

[0093] The difference between Comparative Example 5 and Example 4 is that the first step in the stepwise washing and filtration in step (2) is changed to ultrafiltration intercepting liquid recirculation, and ultrafiltration permeate is not recirculated. The second step is performed after 15 minutes of circulation.

[0094] Comparative Example 6

[0095] The difference between Comparative Example 6 and Example 4 is that: in step (4), the concentrated retentate is not subjected to ceramic microfiltration membrane and centrifugation sterilization, but is directly mixed online with light cream and nanofiltration concentrate; in step (5), the pasteurization parameters are changed to 125°C and 15s.

[0096] Comparative Example 7

[0097] The difference between Comparative Example 7 and Example 4 is that: in step (4), the concentrated retentate is not subjected to ceramic microfiltration membrane and centrifugation sterilization, but is directly mixed online with light cream and nanofiltration concentrate; in step (5), the pasteurization parameters are changed to 157°C and 0.1s steam direct sterilization.

[0098] Results analysis:

[0099] (a) The permeate I after the first step of the stepwise washing and filtration in Example 4 and Comparative Example 5 was collected respectively, and the calcium content in it was determined. The results are shown in Table 1 below.

[0100] Table 1

[0101] Example 4 Comparative Example 5 Calcium content (mg / 100g) 0 30.4

[0102] As shown in Table 1, in Comparative Example 5, if the ultrafiltration permeate and ultrafiltration cut-off liquid are not simultaneously returned in the first step of the stepwise washing filtration, some calcium will be lost into the permeate. However, Example 1 uses a process of completely backfilling both the ultrafiltration cut-off liquid and the ultrafiltration permeate, which pre-forms a protein layer on the membrane and prevents the loss of calcium ions.

[0103] (ii) The high-protein, long-shelf-life milk prepared in Example 1 and Comparative Examples 1, 2, and 3 was placed at 10–15°C, and the change in total bacterial count was measured over 15 days. The results are shown in Table 2.

[0104] Table 2

[0105]

[0106] Based on the data from Example 1, Comparative Examples 1-3, and Table 2 above, it can be seen that under the more stringent storage conditions of 10-15°C (currently, the market storage temperature is 2-6°C), Comparative Example 1, using dual centrifugal sterilization, detected microorganisms on day D1; while Comparative Example 2, using only a ceramic membrane without centrifugal sterilization, could only be stored at 10-15°C for 11 days; Comparative Example 3, using a ceramic membrane, centrifugal sterilization, and pasteurization, but not using a buffer-free process design, still could not achieve a 15-day shelf life, only slightly better than the dual centrifugal sterilization process.

[0107] (III) Record the changes in liquid pressure when the concentrated retentate from Example 3 and the concentrated retentate from Comparative Example 4 pass through the ceramic microfiltration membrane. The results are shown in the appendix. Figure 1 As shown.

[0108] From the appendix Figure 1 It can be seen that in Comparative Example 4, after directly concentrating all solids in skim milk and then sterilizing it with a ceramic microfiltration membrane, the permeate pressure dropped rapidly, indicating that some substances blocked the ceramic membrane, resulting in low solids content and pressure drop at the permeate end of the ceramic membrane. However, the process of Example 3 of this invention can pass through the ceramic membrane smoothly without causing blockage.

[0109] (iv) The active substances of lactoperoxidase, lactoferrin, immunoglobulins, α-lactalbumin, and β-lactoglobulin in the high-protein, long-shelf-life milk prepared in Example 4, Comparative Example 6, and Comparative Example 7 were detected respectively. The results are shown in Table 3.

[0110] Table 3

[0111]

[0112] Comparative Examples 6 and 7 mainly employ existing fresh milk sterilization processes with a shelf life of 15 days. As shown in Table 3, although Comparative Examples 6 and 7 can achieve a shelf life of 15 days, compared with Example 4, there is a significant difference between Comparative Examples 6 and 7 and the process of the present invention in terms of both the types and content of retained active proteins. Therefore, the process of the present invention can not only completely retain active substances but also significantly extend the shelf life.

[0113] In summary, this invention addresses safety concerns through a buffer-free, continuous process design, extending the shelf life of fresh milk by more than double (3-7 days). A step-by-step washing and filtration process solves the problems of ceramic membrane clogging and calcium ion loss, while also allowing for adjustment of lactose content to improve taste. Furthermore, a combination of ceramic membrane, centrifugal sterilization, and pasteurization addresses the retention of active proteins and the extension of shelf life. In other words, this invention optimizes and limits the concentration ratio while maintaining high protein content and rich flavor during extended shelf life. This ensures both high protein and active protein content in the product and improved milk safety, achieving a shelf life of up to 15 days even under more stringent storage conditions of 10-15°C. Therefore, this invention effectively overcomes the shortcomings of existing technologies and possesses high industrial applicability.

[0114] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a high-protein long shelf-life milk, characterized in that, The method comprises the following steps: (1) defatting raw milk to obtain skim milk and cream; (2) pre-concentrating the skim milk by ultrafiltration to obtain an ultrafiltration permeate and an ultrafiltration retentate, and performing step-by-step washing of the ultrafiltration permeate and the ultrafiltration retentate until the lactose content in the obtained retentate is less than or equal to 0.3%, to obtain a washing permeate and a washing retentate, and re-concentrating the washing retentate to obtain a concentrated retentate; the step-by-step washing comprises the following steps: (21) keeping the concentration ratio of the pre-concentration by ultrafiltration unchanged, and recycling the ultrafiltration permeate and the ultrafiltration retentate into the feed liquid of the skim milk for 10-15 minutes to obtain a retentate I and a permeate I; (22) recycling the retentate I again, keeping the concentration ratio unchanged, and adding RO water into the feed liquid until the lactose content in the obtained retentate is less than or equal to 0.3% to stop the washing, to obtain a retentate II and a permeate II; the retentate II is the washing retentate; and the permeate I and the permeate II are mixed to obtain the washing permeate; (3) concentrating the washing permeate by nanofiltration to obtain a nanofiltration concentrate; (4) sterilizing the cream, sterilizing the nanofiltration concentrate, and sterilizing the concentrated retentate by microfiltration and centrifugation, and then mixing the sterilized cream, the sterilized nanofiltration concentrate and the sterilized concentrated retentate to obtain a mixed liquid; (5) homogenizing and sterilizing the mixed liquid to obtain the high-protein long-shelf-life milk; the sterilization is pasteurization, the pasteurization temperature is 71-76°C, and the pasteurization time is 15-17 seconds.

2. The method for preparing high-protein, long-shelf-life milk according to claim 1, characterized in that, In step (1), the protein content of the raw milk is 3.0-4.0%, and the fat content is 3.3-4.5%. In step (1), the fat content in the skim milk is less than or equal to 0.03%.

3. The method of claim 1, wherein the high-protein long shelf-life milk is prepared by adding 0.1 to 0.3% of the protein to 100 parts of the milk, and then adding 0.1 to 0.3% of the thickening agent to the milk. In step (2), the pre-concentration ratio is 1.1-1.

3. In step (2), the membrane temperature of the pre-concentration by ultrafiltration is 2-30°C. In step (2), the membrane material of the pre-concentration by ultrafiltration is selected from one of polysulfone, polyacrylonitrile, regenerated cellulose, cellulose acetate, polyamide and polyvinyl alcohol. In step (2), the molecular weight cut-off of the pre-concentration by ultrafiltration is 10-30 KDa.

4. The method of claim 3, wherein the high-protein long shelf-life milk is prepared by adding 0.1 to 0.3% of the protein to 100 parts of the milk, and then adding 0.1 to 0.3% of the thickening agent to the milk. In step (2), the membrane material of the pre-concentration by ultrafiltration is polyether sulfone.

5. The method of claim 1, wherein the high protein long shelf life milk is prepared by adding 0.5 to 1.5% of the whey protein concentrate to 100 parts of the milk. 5 In step (22), the amount of the RO water added is the same as the amount of the permeate I.

6. The method for preparing high-protein, long-shelf-life milk according to claim 1, characterized in that, In step (2), the re-concentration is to increase the concentration ratio to 2-3 times of the pre-concentration by ultrafiltration.

7. The method for preparing high-protein, long-shelf-life milk according to claim 1, characterized in that, In step (3), the concentration ratio of the nanofiltration concentration is 10-15. In step (3), the temperature of the nanofiltration concentration is 2-15°C. In step (3), the molecular weight cut-off of the nanofiltration concentration is 150-300 KDa.

8. The method of claim 1, wherein the high protein long shelf life milk is prepared by, In step (4), the sterilization temperature of the cream is 110-125°C. In step (4), the sterilization time of the cream is 10-15 seconds. In step (4), the sterilization temperature of the nanofiltration concentrate is 75-85°C. In step (4), the sterilization time of the nanofiltration concentrate is 15-17 seconds. And / or, the microfiltration membrane in step (4) is a ceramic microfiltration membrane, and the average pore size of the ceramic microfiltration membrane is 1.2-1.6 microns; And / or, the volume ratio of the nanofiltration concentrated solution in the mixed solution in step (4) is 28-35%.

9. The method according to claim 1, wherein the high-protein long-shelf-life milk is prepared by the method comprising the following steps: (1) preparing a milk base; (2) mixing the milk base with a protein source to obtain a mixed solution; (3) heating the mixed solution to a temperature of 80-95 DEG C; (4) cooling the mixed solution to a temperature of 20-30 DEG C; (5) sterilizing the mixed solution; (6) cooling the sterilized mixed solution to a temperature of 2-8 DEG C; and (7) filling the cooled sterilized mixed solution into a container. Step (5) is cooling and filling after sterilization, and the temperature of the cooling and filling is 2-8 DEG C.

10. A high protein long shelf life milk characterized in that, The high-protein long-shelf-life milk is prepared by the method according to any one of claims 1-9.

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