Preparation method and application of low-lactose yak milk powder

Abstract

The preparation method comprises the following steps: S1, pretreating raw materials; s2, performing ultrasonic-assisted enzymolysis; s3, nanofiltration membrane separation regulation and control; s4, carrying out microencapsulation embedding; and S5, spray drying. The method has the advantages that compared with a traditional method, the low-temperature ultrasonic technology has the advantages that the retention rate of heat-sensitive immune globulin IgG is increased by 30% or above, and the problem that the activity of the Shangri yak milk is prone to loss is solved; bitter ions and excessive monosaccharide causing bad flavor are effectively removed in the nanofiltration step; the microencapsulation embedding technology effectively prevents the wall sticking phenomenon during spray drying.

Description

Technical Field

[0001] This invention relates to the field of dairy processing technology, and in particular to a method for preparing and applying low-lactose yak milk powder. Background Technology

[0002] Yak milk is hailed as "liquid gold of the plateau," especially that from the high-altitude region of Shangri-La, Yunnan. Due to its pristine growing environment and abundant pasture, yak milk from this region boasts significantly higher levels of milk protein, milk fat, and conjugated linoleic acid (CLA) than ordinary Holstein milk, and is rich in unique immunoglobulins (IgG) and lactoferrin. However, the high fat content of yak milk from this region leads to a very low glass transition temperature (Tg) after hydrolysis, resulting in severe adhesion to the drying walls during spray drying. Furthermore, the yak milk from this region contains unique volatile branched-chain fatty acids due to its grazing on wild grasses, giving it a strong "wild" or muttony flavor. Traditional enzymatic hydrolysis amplifies this flavor and adds to the bitterness produced by hydrolysis. The core value of yak milk from this region lies in its highly active immune factors; traditional 50-55℃ enzymatic hydrolysis processes cause irreversible denaturation of these heat-sensitive proteins. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for preparing low-lactose yak milk powder and its application.

[0004] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0005] A method for preparing low-lactose yak milk powder, characterized by comprising the following steps:

[0006] S1. Raw material pretreatment: Fresh yak milk is purified and standardized to obtain standardized emulsion;

[0007] S2. Ultrasonic-assisted enzymatic hydrolysis: The standardized emulsion is heated to the reaction temperature, neutral lactase is added, and a constant-temperature enzymatic hydrolysis reaction is carried out under the assistance of ultrasound until the lactose hydrolysis rate reaches the preset value to obtain the enzymatic hydrolysate.

[0008] S3. Nanofiltration membrane separation and control: The enzymatic hydrolysate is pumped into the nanofiltration membrane separation system for filtration, the permeate containing glucose and monovalent inorganic salts is discharged, and the retentate rich in protein and milk fat is collected.

[0009] S4. Microencapsulation: Add wall material to the retentate obtained in step S3, dissolve it and then homogenize it under high pressure to form a microencapsulated emulsion with a core-shell structure.

[0010] S5. Spray drying: The microencapsulated emulsion is spray dried at low temperature to obtain low-lactose yak milk powder.

[0011] Preferably, in step S2, the temperature of the ultrasound-assisted enzymatic hydrolysis reaction is controlled at 38℃-40℃, the amount of neutral lactase added is 2000-3000 NLU / L of milk, and the enzymatic hydrolysis reaction time is 1.5-2.0 hours.

[0012] Preferably, in step S2, the ultrasonic-assisted processing adopts an intermittent pulse working mode, with an ultrasonic frequency of 20-28kHz, an ultrasonic power density of 0.3-0.5W / cm², and a pulse period of 3-5 seconds for operation and 3-5 seconds for interval.

[0013] Preferably, in step S3, the nanofiltration membrane separation system uses a nanofiltration membrane with a molecular weight cutoff of 300-400 Da, an operating pressure of 1.0-1.5 MPa, and an operating temperature of 40-45℃.

[0014] Preferably, in step S3, the discharge volume of the permeate is 15%-25% of the feed volume; the retentate retains immunoglobulins, lactoferrin, and galactooligosaccharides with a molecular weight greater than 400 Da.

[0015] Preferably, in step S4, the wall material is composed of maltodextrin and sodium caseinate, wherein the amount of maltodextrin added is 8%-12% of the dry matter mass in the retentate, and the amount of sodium caseinate added is 2%-3% of the dry matter mass in the retentate.

[0016] Preferably, in step S4, the high-pressure homogenization adopts a two-stage homogenization, with the first-stage homogenization pressure being 30-40 MPa and the second-stage homogenization pressure being 5-10 MPa; the homogenization temperature is 55-60℃.

[0017] Preferably, in step S5, the inlet air temperature of the low-temperature spray drying is controlled at 155℃-165℃, and the outlet air temperature is controlled at 70℃-75℃; the dried powder is cooled to below 25℃ by a fluidized bed before packaging.

[0018] A low-lactose yak milk powder prepared using the method described above, wherein the lactose hydrolysis rate of the yak milk powder is 85%-90%, and the retention rate of immunoglobulin IgG is ≥85%.

[0019] The present invention also provides the application of low-lactose yak milk powder in the preparation of food for people with lactose intolerance.

[0020] The low-lactose yak milk powder obtained by this invention has the comprehensive characteristics of low lactose, moderate sweetness, low sodium, and high active IgG. Nanofiltration removes some glucose, avoiding the "overly sweet" defect of traditional hydrolyzed milk powder, improving the sensory experience, and making it suitable for long-term consumption.

[0021] The beneficial effects of this invention are:

[0022] Compared to traditional methods, the low-temperature ultrasonic process of this invention increases the retention rate of thermosensitive immunoglobulin IgG by more than 30%, solving the problem of easy loss of activity in Shangri-La yak milk; the nanofiltration step effectively removes bitter ions and excess monosaccharides that cause unpleasant flavor; and the microencapsulation technology effectively prevents the phenomenon of sticking to the spray drying wall. Detailed Implementation

[0023] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0024] Example 1

[0025] First, fresh Shangri-La yak milk is taken, centrifuged at 4℃ to purify the milk, and then standardized before being pumped into an enzymatic hydrolysis tank.

[0026] Subsequently, ultrasound-assisted enzymatic hydrolysis was performed: the temperature of the standardized emulsion was raised to 38°C, and neutral lactase at a dosage of 2000 NLU / L was added. Simultaneously, the ultrasound-assisted device was turned on, with the ultrasound frequency set to 20 kHz, the power density to 0.3 W / cm², and a pulse working mode of 3 seconds on, 5 seconds off. The reaction was carried out under constant temperature and stirring conditions for 2.0 hours to obtain the enzymatic hydrolysate.

[0027] Next, nanofiltration membrane separation and control were performed: the above-mentioned enzymatic hydrolysate was pumped into the nanofiltration system, and a nanofiltration membrane with a molecular weight cutoff of 300 Da was selected. The operating pressure was controlled at 1.0 MPa, and the operating temperature was controlled at 40℃. During the filtration process, the volume of the discharged permeate was controlled to be 15% of the feed volume, and the retentate was collected.

[0028] Then, microencapsulation was performed: 8% (by dry matter) of low-DE maltodextrin and 2% (by dry matter) of sodium caseinate were added to the retentate. After heating and stirring to dissolve, the solution was heated to 55°C for two-stage high-pressure homogenization, with the first-stage homogenization pressure set at 30 MPa and the second-stage homogenization pressure set at 5 MPa, to form a microencapsulated emulsion.

[0029] Finally, spray drying is performed: the emulsion is pumped into a spray drying tower, with the inlet air temperature controlled at 155℃ and the outlet air temperature at 70℃. The dried powder is then cooled in a fluidized bed and packaged to obtain low-lactose yak milk powder product A.

[0030] The specific methods for centrifugal purification and standardization in the above steps are as follows: Cold purification is performed at 6500 r / min at 4℃ to remove mechanical impurities such as dust, hair, and somatic cells from the milk. Subsequently, the composition of the purified milk is analyzed using an online milk component analyzer. Considering the high fat content (average fat content >5.5%) of Shangri-La yak milk, centrifugal separation standardization technology is employed. The milk is pumped into a milk fat separator to separate a portion of the cream, uniformly adjusting the fat content, which varies seasonally, to 4.2% ± 0.1%, while ensuring that the non-fat milk solids (SNF) content is not less than 8.5%. The standardized milk is immediately cooled to 4℃ for temporary storage, ready for subsequent enzymatic hydrolysis. This step ensures a constant ratio of core material (fat) to wall material during subsequent microencapsulation, avoiding problems such as powder oxidation and poor solubility caused by excessive fat content. This method is used in all the following examples.

[0031] Example 2

[0032] First, fresh Shangri-La yak milk is taken, centrifuged at 4℃ to purify the milk, and then standardized before being pumped into an enzymatic hydrolysis tank.

[0033] Subsequently, ultrasound-assisted enzymatic hydrolysis was performed: the standardized emulsion was heated to 39°C, and neutral lactase at a dosage of 2500 NLU / L was added. Ultrasound assistance was activated, with the ultrasonic frequency set to 24 kHz, power density to 0.4 W / cm², and pulse mode set to 4 seconds on, 4 seconds off. The reaction was carried out at a constant temperature for 1.8 hours until the lactose hydrolysis rate reached the preset value.

[0034] Next, nanofiltration membrane separation and control were performed: a nanofiltration membrane with a molecular weight cutoff of 350 Da was selected to filter the enzymatic hydrolysate. The operating pressure was adjusted to 1.2 MPa, and the temperature was maintained at 42℃. The volume of the discharged permeate was controlled to be 20% of the feed volume, and the retentate was collected.

[0035] Then, microencapsulation was performed: 10% (by dry matter) of low-DE maltodextrin and 2.5% (by dry matter) of sodium caseinate were added to the retentate. After dissolution, the mixture was heated to 58°C for two-stage homogenization, with the first-stage homogenization pressure set at 35 MPa and the second-stage homogenization pressure at 8 MPa.

[0036] Finally, spray drying is performed: the inlet air temperature of the spray drying tower is controlled at 160℃, and the outlet air temperature is controlled at 72℃. After drying and cooling, low-lactose yak milk powder product B is obtained.

[0037] Example 3

[0038] First, fresh Shangri-La yak milk is taken, centrifuged at 4℃ to purify the milk, and then standardized before being pumped into an enzymatic hydrolysis tank.

[0039] Subsequently, ultrasound-assisted enzymatic hydrolysis was performed: the emulsion temperature was raised to 40°C, and neutral lactase at a dosage of 3000 NLU / L was added. The ultrasound frequency was set to 28 kHz, the power density to 0.5 W / cm², and the pulse mode to be 5 seconds on, 3 seconds off. The reaction was terminated after 1.5 hours.

[0040] Next, nanofiltration membrane separation and control were performed: a nanofiltration membrane with a molecular weight cutoff of 400 Da was selected. The operating pressure was increased to 1.5 MPa, and the temperature was controlled at 45°C. The volume of the discharged permeate was controlled to be 25% of the feed volume to achieve deep desalination and desaccharification.

[0041] Then, microencapsulation was performed: 12% (by dry matter) of low-DE maltodextrin and 3% (by dry matter) of sodium caseinate were added to the retentate. The mixture was heated to 60°C for two-stage homogenization, with the first-stage homogenization pressure set at 40 MPa and the second-stage homogenization pressure at 10 MPa.

[0042] Finally, spray drying is performed: the inlet air temperature is set to 165℃ and the outlet air temperature is set to 75℃. After drying and cooling, low-lactose yak milk powder product C is obtained.

[0043] Comparative Example 1

[0044] This comparative example uses traditional enzymatic hydrolysis and drying processes. Shangri-La yak milk was taken, heated to 55°C, and the same amount of neutral lactase as in Example 2 was added. Hydrolysis was performed for 2.5 hours without ultrasonic assistance. After hydrolysis, no nanofiltration separation was performed, and no microcapsule wall material was added. The mixture was directly concentrated and then spray-dried at an inlet air temperature of 180°C.

[0045] Comparative Example 2

[0046] The comparative example is basically the same as the process in Example 2, except that after the enzymatic hydrolysis is completed, the nanofiltration membrane separation step is skipped, and wall material is directly added to the enzymatic hydrolysate and homogenized and dried.

[0047] Comparative Example 3

[0048] The process of this comparative example is basically the same as that of Example 2, except that: after nanofiltration separation, maltodextrin and sodium caseinate are not added to the retentate, and the retentate is directly homogenized and then spray-dried.

[0049] The performance indicators of the yak milk powders prepared in Examples 1-3 and Comparative Examples 1-3 were tested, and the results are shown in the table below:

[0050] detection indicators Example 1 (Lower Limit) Example 2 (Intermediate Value) Example 3 (Upper Limit) Comparative Example 1 (Traditional) Comparative Example 2 (without nanofiltration) Comparative Example 3 (without embedding) IgG retention rate (%) 91.5 89.4 86.5 56.2 88.2 88 Powder yield (%) from spray drying 96.5 97.2 98.1 78.5 85 65.4 Moisture absorption weight gain rate (24h) 4.50% 4.10% 3.80% 8.50% 7.80% 12.60% Sweetness rating (0-10) 5.5 5.2 (Moderate) 4.5 (Light) 8.5 (Too sweet) 8.2 (Too sweet) 5.3 Goaty smell rating (0-10) 2.8 2.5 (Slight) 2.2 (Extremely Light) 6.5 (Strong) 3.5 2.6

[0051] As can be seen from the table, compared with Comparative Examples 1-3, Example 1, using the lower limit values ​​of each parameter (38℃ low temperature, 300Da small pore membrane), showed that its IgG retention rate reached the highest of 91.5%, proving the superiority of this process boundary in terms of activity protection; Example 3, using the upper limit values ​​of each parameter (400Da large pore membrane, 25% permeability), showed that its moisture absorption rate was the lowest (3.8%) and its odor was the lightest (2.2 points), proving the advantages of this process boundary in terms of flavor improvement and physical stability; Example 2, as an intermediate value, showed balanced performance of each indicator.

[0052] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing low-lactose yak milk powder, characterized in that, Includes the following steps: S1. Raw material pretreatment: Fresh yak milk is purified and standardized to obtain standardized emulsion; S2. Ultrasonic-assisted enzymatic hydrolysis: The standardized emulsion is heated to the reaction temperature, neutral lactase is added, and a constant-temperature enzymatic hydrolysis reaction is carried out under the assistance of ultrasound until the lactose hydrolysis rate reaches the preset value to obtain the enzymatic hydrolysate. S3. Nanofiltration membrane separation and control: The enzymatic hydrolysate is pumped into the nanofiltration membrane separation system for filtration, the permeate containing glucose and monovalent inorganic salts is discharged, and the retentate rich in protein and milk fat is collected. S4. Microencapsulation: Add wall material to the retentate obtained in step S3, dissolve it and then homogenize it under high pressure to form a microencapsulated emulsion with a core-shell structure. S5. Spray drying: The microencapsulated emulsion is spray dried at low temperature to obtain low-lactose yak milk powder.

2. The method for preparing low-lactose yak milk powder according to claim 1, characterized in that: In step S2, the temperature of the ultrasound-assisted enzymatic hydrolysis reaction is controlled at 38℃-40℃, the amount of neutral lactase added is 2000-3000 NLU / L of milk, and the enzymatic hydrolysis reaction time is 1.5-2.0 hours.

3. The method for preparing low-lactose yak milk powder according to claim 2, characterized in that: In step S2, the ultrasonic-assisted processing adopts an intermittent pulse working mode, with an ultrasonic frequency of 20-28kHz, an ultrasonic power density of 0.3-0.5W / cm², and a pulse period of 3-5 seconds for operation and 3-5 seconds for interval.

4. The method for preparing low-lactose yak milk powder according to claim 1, characterized in that: In step S3, the nanofiltration membrane separation system uses a nanofiltration membrane with a molecular weight cutoff of 300-400 Da, an operating pressure of 1.0-1.5 MPa, and an operating temperature of 40-45℃.

5. The method for preparing low-lactose yak milk powder according to claim 4, characterized in that: In step S3, the discharge volume of the permeate is 15%-25% of the feed volume; the retentate retains immunoglobulins, lactoferrin, and galactooligosaccharides with a molecular weight greater than 400 Da.

6. The method for preparing low-lactose yak milk powder according to claim 1, characterized in that: In step S4, the wall material is composed of maltodextrin and sodium caseinate, wherein the amount of maltodextrin added is 8%-12% of the dry matter mass in the retentate, and the amount of sodium caseinate added is 2%-3% of the dry matter mass in the retentate.

7. The method for preparing low-lactose yak milk powder according to claim 1, characterized in that: In step S4, the high-pressure homogenization adopts a two-stage homogenization process, with the first-stage homogenization pressure being 30-40 MPa and the second-stage homogenization pressure being 5-10 MPa; the homogenization temperature is 55-60℃.

8. The method for preparing low-lactose yak milk powder according to claim 1, characterized in that: In step S5, the inlet air temperature of the low-temperature spray drying is controlled at 155℃-165℃, and the outlet air temperature is controlled at 70℃-75℃; the dried powder is cooled to below 25℃ by a fluidized bed before packaging.

9. A low-lactose yak milk powder prepared using the method described in any one of claims 1-8, characterized in that, The lactose hydrolysis rate of the yak milk powder is 85%-90%, and the retention rate of immunoglobulin IgG is ≥85%.

10. The use of the low-lactose yak milk powder according to claim 9 in the preparation of food products for lactose-intolerant individuals.