Preparation method of yoghourt rich in Chinese wolfberry dietary fibers

Abstract

The invention discloses a preparation method of a yoghourt beverage rich in Chinese wolfberry dietary fibers, which is characterized by comprising the following steps: mixing whole milk powder, cane sugar, Chinese wolfberry dietary fibers and water according to the mass ratio of 12: 6.5: (1-4): 79.5, fully dissolving, homogenizing at 4000-5000rpm for 1-6 minutes, and sterilizing to obtain sterile mixed emulsion; performing ultrasonic treatment on the sterile mixed emulsion under the conditions of 0-400W and 50-100kHz for 5-45min, inoculating a basic fermentation agent consisting of streptococcus thermophilus and lactobacillus bulgaricus and a mixed fermentation strain consisting of lactobacillus casei and phytobacterium plantarum subsp.plantarum according to the proportion of 0.1% by volume, fully stirring, standing for fermentation, and performing after-ripening, so as to obtain the fermented milk. The yoghourt rich in the Chinese wolfberry dietary fibers has the advantages of being rich in the Chinese wolfberry dietary fibers, excellent in texture, good in taste and rich in flavor.

Description

Technical Field

[0001] The invention belongs to the technical field of fermented dairy products, and in particular relates to a method for preparing yogurt rich in wolfberry dietary fiber. Background Art

[0002] Ultrasound (US) is a typical non-thermal food processing technology. It causes changes in the composition of dairy products through its chemical and physical effects, thereby improving the quality of milk and dairy products. Therefore, it has attracted great attention in the dairy industry. Ultrasound is defined as high-frequency sound waves that are higher than the human hearing threshold (20kHz). Ultrasound applications can be divided into high-intensity-low-frequency (ultrasound intensity = 10–1000W / cm 2 and frequency = 20–100kHz) and low intensity-high frequency (ultrasound intensity < 1W / cm 2 Ultrasonic technology has the potential to be used in fermented milk. For example, ultrasonic treatment can significantly reduce the casein particle size and improve some of its physical, chemical and functional properties. Existing scholars have refined milk protein and fat particles through ultrasonic treatment, thereby improving the texture and stability of fermented milk. However, improper treatment may lead to premature decomposition or even denaturation of protein, affecting the flavor and nutrition of fermented milk.

[0003] Wolfberry is one of the traditional food and medicine ingredients. Its nutritional and medicinal value comes from its rich polysaccharides, organic acids, fatty acids, carotenoids, amino acids, proteins, vitamins and essential elements. Fresh wolfberry fruit is not easy to store and transport. Dried wolfberry fruit is the main product form. Other processed wolfberry products mainly include: wolfberry puree, wolfberry beverage and wolfberry yogurt. Wolfberry dietary fiber is purified from wolfberry pomace and peel residues produced during the processing of wolfberry puree, wolfberry beverage and other products, which is a secondary use of production waste during the processing process.

[0004] The existing wolfberry yogurt products simply add wolfberry dry homogenate directly to yogurt. The ingredients in the wolfberry homogenate are complex, the dietary fiber content is low and unstable, and it is difficult to accurately control. At the same time, there are inevitably large particles or crude fibers in the wolfberry homogenate, which affects the uniformity and taste of the yogurt. Directly adding wolfberry dietary fiber can effectively avoid the above shortcomings. Wolfberry dietary fiber is a functional ingredient extracted and purified, and the addition amount can be more accurately controlled to ensure the functional effect of yogurt. Powdered dietary fiber can be dispersed in yogurt to improve texture and stability, avoid granularity, and at the same time have a flavor similar to wolfberry homogenate. Therefore, exogenous addition of wolfberry dietary fiber is more advantageous than directly adding wolfberry homogenate. However, there are some disadvantages in using this dietary fiber in yogurt products. For example, due to the presence of antibacterial substances such as wolfberry polysaccharides and pigments, the number of live lactic acid bacteria may be affected and the coagulation period may be extended, as well as the solubility of dietary fiber in cow's milk. Although it is a water-soluble dietary fiber, it cannot be completely dissolved in cow's milk and is not evenly distributed, which makes the color of fermented yogurt uneven, which may affect consumers' appetite to a certain extent. Summary of the invention

[0005] The technical problem to be solved by the invention is to provide a method for preparing a yogurt drink rich in wolfberry dietary fiber, which is rich in wolfberry dietary fiber, has excellent texture, good taste and strong flavor.

[0006] To achieve the above purpose, the technical method adopted by the present invention is as follows: a method for preparing a yogurt drink rich in wolfberry dietary fiber, comprising the following steps:

[0007] Step 1, after mixing whole milk powder, sucrose, wolfberry dietary fiber and water in a mass ratio of 12:6.5:1-4:79.5, placing in a 50-65° C. water bath and stirring to fully dissolve, homogenizing at 4000-5000 rpm for 1-6 min, and sterilizing to obtain a sterile mixed emulsion;

[0008] Step 2, after ultrasonically treating the sterile mixed emulsion for 5-45 minutes under the conditions of 0-400W and 50-100kHz, 0.1% by volume of a basic fermentation agent consisting of thermophilic streptococcus and bulgaricus and 2% of a mixed fermentation strain consisting of Lactobacillus casei and Lactobacillus plantarum subspecies are added to the mixture, the mixture is fully stirred, and the mixture is allowed to stand for fermentation and then ripened to obtain a yogurt rich in wolfberry dietary fiber.

[0009] Furthermore, in step 1, the mass ratio of the whole milk powder, sucrose, wolfberry dietary fiber and water is 12:6.5:2:79.5.

[0010] Furthermore, in step 2, the ultrasonic power is 100 and the ultrasonic time is 20 minutes.

[0011] Furthermore, in step 2, the Lactobacillus plantarum subspecies is deposited in the General Microbiology Center of China National Microorganism Culture Collection Committee with a deposit number of CGMCC No.15953; the Lactobacillus casei is deposited in the General Microbiology Center of China National Microorganism Culture Collection Committee with a deposit number of CGMCC No.15956.

[0012] Furthermore, the mixed fermentation strain is OD 600nm =1 Lactobacillus casei culture and OD 600nm =1 and Lactobacillus plantarum subspecies plantarum bacterial liquid are mixed in a volume ratio of 1:1.

[0013] Furthermore, in step 2, the static fermentation conditions are static fermentation in a fermentation box at 40°C to 44°C for 4-6 hours.

[0014] Furthermore, in step 2, the post-ripening conditions are post-ripening at 0-4°C for 8-16 hours.

[0015] Compared with the prior art, the advantages of the present invention are: a preparation method of yogurt rich in wolfberry dietary fiber, wolfberry dietary fiber powder is added to the yogurt formula, ultrasonic treatment is used in the yogurt preparation process to improve the texture and taste of the yogurt, and ultrasonic treatment promotes the dissolution of dietary fiber. The present invention uses ultrasound and wolfberry dietary fiber synergistic treatment in the yogurt preparation method to promote the full dissolution and uniform distribution of wolfberry dietary fiber in milk, and in addition to the starter, binary probiotics: Lactobacillus casei and Lactobacillus plantarum subspecies plantarum are additionally added as fermentation bacteria, and the mixed fermentation of Lactobacillus plantarum and Lactobacillus casei can increase the number of live bacteria in yogurt through the synergistic effect of metabolic pathways and shorten the coagulation period. In addition, the mixed fermentation of binary probiotics has also been shown to have a positive effect on the flavor of fermented milk. Thus, a yogurt drink rich in wolfberry dietary fiber, excellent texture, good taste and rich flavor is prepared.

[0016] The above-mentioned Lactobacillus casei is a GYX-1 strain, classified and named Lactobacillus casei, and was deposited in the General Microbiology Center of the China Culture Collection Administration on June 19, 2018, with a deposit number of CGMCC No.15956, and the deposit address is No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, Institute of Microbiology, Chinese Academy of Sciences.

[0017] The above-mentioned Lactobacillus plantarum subsp. plantarum is a PDD-1 strain, which is classified and named as Lactobacillus plantarum subsp. plantarum. It was deposited in the General Microbiology Center of China Microorganism Culture Collection on June 19, 2018, with a deposit number of CGMCC No.15953, and the deposit address is No. 3, Yard 1, Beichen West Road, Chaoyang District, Beijing, Institute of Microbiology, Chinese Academy of Sciences. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a comparison diagram of the actual effects of the finished yogurt prepared in Example 1;

[0019] Figure 2 The water holding capacity measurement results of yogurt products with different amounts of wolfberry dietary fiber added and the water holding capacity measurement results of yogurt products with different ultrasonic powers in Example 2;

[0020] Figure 3 The water holding capacity test results of yogurt products with different ultrasonic times in Example 3

[0021] Figure 4 The water holding capacity of yogurt products at different ultrasonic temperatures in Example 4 is measured

[0022] Figure 5 The water holding capacity test results of the yogurt products obtained by different treatment methods in Example 5;

[0023] Figure 6 The rheological properties of the yogurt products obtained by different treatment methods in Example 7 are measured, wherein A is the storage modulus (G') and loss modulus (G") of the yogurt, and B is the apparent viscosity change of different yogurt samples in the shear rate range (1-100s-1);

[0024] Figure 7 The surface microstructure observation results of yogurt products obtained by different treatment methods in Example 8, where A is the CK group, B is the US group, C is the WDF group, and D is the UW group;

[0025] Figure 8 The results of low-field nuclear magnetic resonance water distribution measurement of yogurt products obtained by different treatment methods in Example 9, wherein A is a 3D waterfall diagram of the relative peak areas of different water states of yogurt, and B is a two-dimensional plane diagram of the relative peak areas of different water states of yogurt;

[0026] Fig. 9 This is a clustering heat map of the differential volatile flavor substances of yogurt products obtained by different treatment methods in Example 10;

[0027] Fig.10The PCA analysis diagram of the yogurt products obtained by different treatment methods in Example 10;

[0028] Fig.11 These are the comparison results of the volatile flavor substances of the yogurt products obtained by different treatment methods in Example 10, wherein A is the comparison of the volatile flavor substances between the CK group and the US group, B is the comparison of the volatile flavor substances between the CK group and the WDF group, C is the comparison of the volatile flavor substances between the CK group and the UW group, D is the comparison of the volatile flavor substances between the US group and the WDF group, E is the comparison of the volatile flavor substances between the US group and the UW group, and F is the comparison of the volatile flavor substances between the WDF group and the UW group. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below with reference to the accompanying drawings.

[0030] Embodiment 1, a preparation method of wolfberry dietary fiber-rich yogurt, comprising the following steps:

[0031] Whole milk powder, sucrose, wolfberry dietary fiber (purchased from Snort Biotechnology Co., Ltd.) and water were mixed in a mass ratio of 12:6.5:2:79.5, placed in a 50-65°C water bath and stirred to fully dissolve, homogenized at 4600rpm for 4min, and sterilized at 95°C for 10min to obtain a sterile mixed emulsion; the sterile mixed emulsion was ultrasonically treated at 100W and 80kHz for 20min, and then 0.1% of the basic fermentation agent (composed of thermophilic Streptococcus and Bulgarian Lactobacillus) and 2% of the mixed fermentation strains (OD 600nm =1 with the Lactobacillus casei culture liquid of CGMCC No.15953 and OD 600nm =1 and Lactobacillus plantarum subspecies plantarum with a preservation number of CGMCC No.15953 in a volume ratio of 1:1), fully stirred, fermented in a 42°C fermentation box for 4-6 hours, and then post-ripened at 4°C for 12 hours to obtain a yogurt rich in wolfberry dietary fiber, which was recorded as UW group.

[0032] The yogurt prepared in the above preparation process without adding wolfberry dietary fiber and without ultrasonic treatment was recorded as CK group.

[0033] The yogurt prepared in the above preparation process without adding wolfberry dietary fiber was recorded as US group.

[0034] The yogurt prepared in the above preparation process without ultrasonic treatment was recorded as WDF group.

[0035] Finished yogurt products Figure 1As shown, it can be observed that the yogurt with added wolfberry dietary fiber (WDF group and UW group) presents a pleasant light orange color, and compared with the WDF group with only wolfberry dietary fiber added, the color of the yogurt in the UW group treated with the synergistic treatment is more uniform, which indicates that ultrasonic treatment can effectively improve the dissolution rate of dietary fiber and promote its uniform distribution in yogurt.

[0036] Example 2: Determination of the optimal ultrasonic power and the optimal amount of wolfberry dietary fiber added.

[0037] The water holding capability (WHC) of yogurt was used as the screening standard to conduct a single factor experiment to determine the optimal ultrasonic power and the optimal amount of wolfberry dietary fiber. The experimental method for determining the optimal amount of wolfberry dietary fiber using WHC as the screening standard was as follows: When the ratio of milk powder and sucrose remained unchanged, four groups of mixed emulsions with a weight percentage of 0%, 1%, 2%, and 3% of wolfberry dietary fiber were prepared, with three parallels in each group. After the mixed emulsions were fully dissolved by stirring in a water bath at 50-65°C, they were homogenized at 4600rpm for 4min and sterilized at 95°C for 10min. 0.1% of the starter and 2% of the laboratory mixed strains were added by volume and stirred thoroughly. After being fermented in a fermentation box at 42°C for 4-6h, they were placed in a 4°C environment for post-ripening for 12h to obtain four groups of yogurts with a wolfberry dietary fiber content of 0%, 1%, 2%, and 3%, respectively. The water holding capacity of yogurt samples was measured by centrifugation. 20 g of yogurt sample was weighed into a 50 mL centrifuge tube, covered and centrifuged at 1000 × g for 10 minutes at 5 ° C. The whey was carefully removed and weighed. The WHC calculation formula is as follows: WHC (%) = [(precipitate weight - empty tube weight) / sample weight] * 100%.

[0038] Experimental method for determining the optimal ultrasonic power using water holding capacity as the screening standard: milk powder, sucrose, wolfberry dietary fiber and water were mixed in a mass ratio of 12:6.5:2:79.5, and the dissolution, homogenization and sterilization steps were the same as above. Under the condition that the other conditions remained unchanged, the emulsion was divided into 5 groups, each with 3 parallels, and ultrasonic treatment was performed in groups with ultrasonic powers of 0W, 100W, 200W, 300W and 400W respectively. The inoculation, fermentation and post-ripening steps were the same as above. The water holding capacity of each group was measured.

[0039] The results are as follows Figure 2 As shown in the figure, when the wolfberry dietary fiber content is 2% and the ultrasonic power is 100 W, the water holding capacity of yogurt is the highest. Therefore, the optimal dietary fiber addition amount is determined to be 2%, and the optimal ultrasonic power is determined to be 100 W.

[0040] Example 3: Determination of the Optimal Ultrasonic Time

[0041] Referring to the implementation method of Example 2, the concentration of wolfberry dietary fiber was determined to be 2%, and the ultrasonic power was determined to be 100 W. Differently, when other conditions remained unchanged, the aseptic mixed emulsion was set to 10 groups with ultrasonic duration as a variable, and the ultrasonic temperatures were 0 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, and 45 min, respectively. The fermentation steps were the same as above. The water holding capacity was measured for each group.

[0042] The results are as follows Figure 3 As shown in the figure, when the ultrasound time is between 20-45 min, the water holding capacity of yogurt reaches the highest, and there is no significant difference in the water holding capacity of yogurt within this range. Therefore, the ultrasound time will be set to 20 min in subsequent experiments.

[0043] Example 4: Determination of the Optimal Ultrasonic Temperature

[0044] Referring to the implementation method of Example 3, the concentration of wolfberry dietary fiber was determined to be 2%, the ultrasonic power was determined to be 100 W, and the ultrasonic time was determined to be 20 min. Under the condition that other conditions remain unchanged, the aseptic mixed emulsion was set to 5 groups with ultrasonic temperature as a variable, and the ultrasonic temperatures were 10° C., 20° C., 30° C., 40° C., and 50° C. respectively. The fermentation steps were the same as above. The water holding capacity of each group was measured.

[0045] The results are as follows Figure 4 As shown in the figure, there is no significant difference among different experimental groups, so the ultrasonic temperature has no significant effect on the water holding capacity of yogurt. Therefore, the ultrasonic temperature can be adjusted according to the room temperature in subsequent experiments.

[0046] Example 5: Analysis of water holding capacity of yogurt in different treatment groups.

[0047] The water holding capacity (WHC) of yogurt samples was measured by centrifugation. 20 g of yogurt sample was weighed into a 50 mL centrifuge tube, capped and centrifuged at 1000 × g for 10 minutes at 5 ° C. The whey was carefully removed and weighed. The WHC calculation formula is as follows: WHC (%) = [(precipitate weight - empty tube weight) / sample weight] * 100%.

[0048] The results are as follows Figure 5As shown in the results, the water holding capacity of CK, US and WDF groups were 61.75%, 73.6% and 76.2% respectively, while the water holding capacity of UW group was significantly increased to 86.2% (p < 0.05). Compared with the water holding capacity of traditional fermented milk, single ultrasonic treatment or addition of wolfberry dietary fiber increased the water holding capacity by 19.28% and 23.5%, while ultrasonic treatment and addition of wolfberry dietary fiber synergistically increased the water holding capacity of yogurt, which was 39.7% higher than the water holding capacity of traditional fermented milk and 17.12% and 13.12% higher than single treatment, respectively. This phenomenon shows that the synergistic effect of ultrasonic treatment and wolfberry dietary fiber is the key factor in improving water holding capacity. Wolfberry dietary fiber mainly includes pectin, polysaccharides and β-glucan, which have strong hydrophilicity and high water holding capacity. In particular, polysaccharide molecules are usually composed of anionic hydrophilic colloids, which can interact with proteins in yogurt (especially casein) to form a three-dimensional network structure. These ingredients in wolfberry dietary fiber help improve the water holding capacity of yogurt, make the yogurt gel structure more compact, and prevent water from being lost, thereby enhancing the hydration and water holding capacity of yogurt. In addition, the addition of dietary fiber can increase the stability of yogurt gel, reduce water precipitation, and thus increase the overall texture characteristics of yogurt. Especially during the yogurt fermentation process, dietary fiber can affect the final water holding capacity by changing the hydration inside the yogurt. The cavitation effect of ultrasound can trigger the bursting of tiny bubbles in the liquid, thereby releasing energy. This process can not only promote the friction and collision between protein particles in the emulsion, but also increase the binding force between protein and water, and reduce the precipitation of whey. In this way, ultrasound can reduce the dehydration and coagulation of whey in yogurt, thereby improving the water holding capacity of yogurt. Ultrasound may also change the microstructure of yogurt, allowing casein micelles to form a more stable network at a higher density, thereby reducing the separation of whey and maintaining better hydration.

[0049] When ultrasonic pretreatment is used together with wolfberry dietary fiber, the effect of improving water holding capacity is particularly significant. The ultrasonic cavitation effect makes the protein in yogurt and wolfberry dietary fiber (especially the polysaccharides therein) more tightly bound. This tight binding forms a more stable three-dimensional network structure that can better capture water and reduce whey precipitation. Under the action of ultrasound, the exposure of hydrophilic sites on the protein surface allows water molecules to bind to more proteins. Wolfberry dietary fiber interacts with protein through its polysaccharide components, further enhancing the binding force between protein and water, thereby improving the water holding capacity of yogurt. The synergistic effect of ultrasound and dietary fiber may also reduce whey separation. Wolfberry dietary fiber can interact with components in whey and limit its precipitation, while ultrasound further reduces whey precipitation and water loss by increasing the interaction between protein and water molecules in the emulsion.

[0050] Example 6: Analysis of texture characteristics of yogurt in different treatment groups.

[0051] The texture of yogurt, including hardness, consistency, cohesion, gelatin and chewiness, was determined using a texture property analyzer (Stable Micro System, abbreviated as UK). The yogurt products were stored at 4°C for 16 hours, 40g of yogurt products were placed in a matching cup, and tested using an A / BE probe with a diameter of 35mm. The pre-test speed was 1mm / s, the test speed was 1mm / s, the post-test speed was 1mm / s, and the test depth was 10mm. The hardness, adhesion, elasticity, cohesion, gelatin, chewiness and resilience of the yogurt products were measured and analyzed. Each group had three parallel samples, and each sample was tested only once.

[0052] Table 1 Results of texture characteristics of yogurt fermented with different treatments

[0053]

[0054] Table 1 shows the specific test results of the texture characteristics of yogurt fermented with different treatment methods (CK, US, WDF and UW). These texture characteristics directly determine the taste of yogurt and consumer acceptance. The results showed that the ultrasonic pretreatment (US) and wolfberry dietary fiber (WDF) alone had a significant effect on the texture characteristics of yogurt (p < 0.05), while the synergistic treatment group UW further optimized the texture properties of yogurt.

[0055] Ultrasonic treatment (US) and the addition of wolfberry dietary fiber (WDF) significantly reduced the hardness of yogurt, while the hardness of the synergistic treatment (UW) group was the lowest (66.47±0.82g), indicating that ultrasonic treatment and dietary fiber have a significant synergistic effect in reducing the hardness of yogurt. The reduction in hardness means that the texture of yogurt is smoother and the palatability is significantly improved. In addition, chewiness also decreases with the reduction in hardness, and the chewiness of the UW group is the lowest, indicating that the yogurt tastes smoother and is easier to swallow, which has important positive significance for consumers' sensory experience.

[0056] The adhesion of the UW group was significantly higher than that of the control group (-75.92±2.12g), indicating that the synergistic treatment was effective in enhancing the adhesion of yogurt. The interaction between polysaccharides and proteins in wolfberry dietary fiber may be the main reason for the improvement in adhesion. These polysaccharide components can not only fill the gaps in the yogurt gel network, but also combine with casein micelles to form a more compact structure. In addition, ultrasonic treatment enhances the binding ability of whey protein and dietary fiber in yogurt through its cavitation effect, thereby further improving adhesion.

[0057] In summary, the UW group achieved the best balance in terms of texture properties such as hardness, adhesion, resilience and cohesion through the synergistic effect of ultrasonic treatment and wolfberry dietary fiber. This improvement in texture properties not only makes yogurt smoother and easier to swallow when eaten, but also meets consumers' dual needs for soft taste and good structural stability. In addition, the improvement in adhesion and enhanced resilience make yogurt less likely to stratify or precipitate whey during mixing and transportation, further improving its commercial value.

[0058] Example 7: Determination of rheological properties of yogurt in different treatment groups.

[0059] 4 mL of each of the UW, CK, US, and WDF groups were taken and the rheological properties were evaluated by using a Discovery HR-2 rheometer equipped with a 40 mm diameter parallel plate (plate gap of 1 mm). Frequency sweep analysis was performed in the range of 0.01–100 Hz while maintaining a constant strain of 0.05% and a shear rate range of 0.01 to 100 rad / s. All measurements were performed at a temperature of 4 °C.

[0060] like Figure 6 Figure A shows the storage modulus (G') and loss modulus (G") of yogurt. G' represents the stress generated by the deformation of the sample during the impact process and is a measure of the elasticity of the sample; the loss modulus (G") represents the energy lost by the sample during the deformation cycle and reflects the viscosity of the yogurt. In the entire angular frequency range (1–80 rad / s), G' of all treatment groups was greater than G", indicating that the four groups of yogurt all showed typical weak gel properties (G' / G">1) and had a stable gel network. The G' and G" of the UW group (ultrasound + wolfberry dietary fiber) were significantly higher than those of the other groups, indicating that the yogurt samples in this treatment group had higher elasticity and viscosity. Ultrasonic treatment promoted the redistribution and aggregate formation of whey protein and casein through its cavitation effect, thereby strengthening the protein network structure. The addition of WDF further acted as a filler in the gel network, and the interactions between the polysaccharides in wolfberry dietary fiber and protein molecules (such as hydrogen bonds and hydrophobic interactions) enhanced the stability and strength of the protein gel. The WDF group showed a certain effect in improving G' and G". This suggests that wolfberry dietary fiber participates in the construction of protein networks through its polysaccharide structure, thereby improving the elasticity and viscosity of yogurt, but its effect is not as significant as that of the UW group.

[0061] Figure 6Figure B shows the changes in apparent viscosity of different yogurt samples within the shear rate range (1–100s-1). All samples showed typical shear thinning behavior, that is, the apparent viscosity decreased with increasing shear rate. This behavior is mainly due to the breakage of protein aggregates and fiber superaggregates in yogurt under shear, resulting in reduced fluid resistance. The apparent viscosity of the UW group was higher than that of other treatment groups in the entire shear rate range, indicating that the synergistic effect of ultrasonic treatment and wolfberry dietary fiber significantly improved the viscosity of yogurt. The mechanism of this synergistic effect may be that ultrasonic treatment destroys the secondary and tertiary structures of protein molecules, increases their exposed hydrophobic sites, and provides more action sites for the binding of wolfberry dietary fiber, thereby further enhancing the network strength and viscosity of yogurt. The viscosity of the ultrasonic treatment group (US) and wolfberry dietary fiber group (WDF) alone at low shear rates was higher than that of the control group (CK), indicating that both have a positive effect on improving the viscosity of yogurt. However, as the shear rate increases, the apparent viscosity of the WDF group and the US group is close, indicating that there are certain limitations in improving the viscosity of yogurt using these two treatments alone. The high viscosity of the UW group indicates that the synergistic effect is more significant. The UW group has a significantly high viscosity at low shear rates, indicating that it has a good gel network structure and high resistance to fluidity under static conditions. However, with the increase in shear rate, the apparent viscosity of the UW group decreases less, indicating that its network structure can still maintain a certain stability under high shear force. This performance is of great significance for the industrial processing and transportation of yogurt.

[0062] Example 8: Determination of tissue microstructure of yogurt in different treatment groups.

[0063] Yogurt blocks were carefully extracted from the center of the four groups of yogurt (CK, US, WDF, and UW), immersed in a 2.5wt% glutaraldehyde solution, and fixed at 4°C for 12h. After fixation, the blocks were rinsed twice with sterile saline for 15min each time, and then washed and dehydrated with an ethanol gradient (50%, 60%, 70%, 80% and 90%) for 10min each. Then, the sample blocks were washed three times with 100% ethanol for 10min each time. After precooling at -80°C for 12h, freeze-dried using a freeze dryer. Finally, the obtained samples were sprayed with gold, and the surface microstructure of the yogurt freeze-dried blocks was examined using a scanning electron microscope (S-3400N, Japan).

[0064] The results are as follows Figure 7As shown in the figure, it can be seen that the whey pores (dark pores) around the casein micelles in the CK group account for a larger proportion than those in the other three groups, presenting a rougher gel microstructure. The whey pores of the yogurt with exogenous fiber addition were significantly shrunk, and microstructural analysis showed that the matrix was densified. This may be because the swelling ability of dietary fiber in the aqueous phase enhances its interaction with casein micelles, and the dispersed fibers in the gel network structure act as spatial stabilizers and "fillers". On the other hand, the oscillation waves and local microjets generated by ultrasonic cavitation may lead to significant differences in microstructure, which is also reflected in the observation of smaller proteins in both US and WDF treated with ultrasound. Ultrasound causes the decomposition of protein aggregates, resulting in the release of discrete protein molecules that are more easily dispersed and dissolved in water. At the same time, ultrasound reduces the particle size of milk fat globules in bovine milk, which may be the reason why the yogurt gel system treated with US is more uniform and dense. Furthermore, the free casein (mainly κ-casein) and small-volume milk fat globules caused by US are more likely to enter the pores of the yogurt gel matrix as fillers, which also explains why the pores in the gel system are still reduced without adding wolfberry dietary fiber to the US group. These microstructural changes will directly affect the gel properties of each group of fermented milk, such as WHC, texture properties and rheological properties.

[0065] Example 9: Determination of moisture distribution of yogurt in different treatment groups.

[0066] Using the NMI20 low-field nuclear magnetic resonance imager, 5 mL of the yogurt sample to be tested was placed in a sample tube, and the MN120-060H-1-25mm magnet-probe was used. The pulse sequence scan selected was CPMG to measure the spin-relaxation time (T2) of the sample. The experimental parameters were set as follows: the sampling frequency was 200kHz, the waiting time was 2000ms, the sampling delay time was 0.002ms, the number of echoes was 8000, and the forward gear was 1. Low-field nuclear magnetic resonance uses non-destructive technology to determine the T2 relaxation time according to different proton mobilities to detect the water distribution. The proton relaxation signal is converted into the relationship between signal intensity and relaxation time through Laplace transform. The shorter the transverse relaxation time, the more tightly bound the water is.

[0067] Depend on Figure 8 Middle A, Figure 8As can be seen in Figure B, a major bound water signal (T2 < 1ms) was detected in all four groups of yogurt, and the peak intensity of the UW group (ultrasound + wolfberry dietary fiber treatment) was the highest. This shows that the combination of ultrasound treatment and dietary fiber significantly increased the amount of bound water in yogurt and enhanced the binding capacity of water molecules in yogurt. Ultrasonic treatment destroys the higher-order structure of proteins, exposing more hydrophilic groups, and providing more binding sites for polysaccharide molecules in WDF. This protein-polysaccharide combination further enhances the ability of the yogurt network to capture water and increases the content of bound water. Wolfberry dietary fiber further acts as a filler and constructs a multi-level gel network structure with the protein network. This composite network structure can not only bind water molecules more efficiently, but also reduce water migration and improve water stability. The combination of ultrasound and WDF reduces the ratio of free water and unstable bound water by constructing a dense network structure, thereby improving the storage properties of yogurt.

[0068] Example 10: Analysis of flavor components of yogurt in different treatment groups.

[0069] Differential volatile flavor substances in yogurt were extracted and determined using headspace solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS). Volatile flavor substances in yogurt were extracted using HS-SPME. 5 g of yogurt sample was added to the extraction bottle, and the sample was extracted at 50 ° C and 350 × g for 1 h. The volatile compounds in the extract were analyzed using GC-MS with a model of Agilent 8890GCSystem+5977B / MSD. GC-MS determination was performed using helium as the carrier gas with a flow rate of 1.0 mL / min and an inlet temperature of 250 ° C. The starting temperature was 35 ° C and maintained for 5 minutes, then increased to 140 ° C at a rate of 5 ° C / min and maintained for 2 minutes. The final increase to 250 ° C was set at a rate of 10 ° C / min and maintained for 3 minutes.

[0070] Fig. 9The heat maps of volatile metabolites of the four sample groups were depicted, and then cluster analysis was performed. A total of 49 volatile metabolites were detected in the four groups of yogurt samples, including carbonyl compounds, alcohols, aldehydes, acids, aromatic compounds, and ketones. The CK group (CK1\CK2\CK3) and the US group (US1\US2\US3) clustered together, indicating that the volatile flavor characteristics of the two groups of fermented milk were similar. The volatile flavor substances of the WDF group (WDF1\WDF2\WDF3) and the UW (UW1\UW2\UW3) group did not cluster in the same cluster as the CK group and the US group, indicating that the exogenous addition of wolfberry dietary fiber affects the ability of the strain to produce flavor substances in fermented milk. The three parallel clusters of the UW group are clustered together, indicating that the repeatability of the samples within the group is high and the sample flavor is stable. At the same time, it can be seen from the cluster heat map that the content of flavor substances in the UW group is higher than that of the other three groups, indicating that the combination of US and WDF is conducive to the formation of yogurt flavor and improves the flavor quality of yogurt.

[0071] like Fig.10 As shown in the figure, PC1 and PC2 were 37.8% and 26.4%, respectively. The distance between the CK group and the US group was close, indicating that ultrasonic treatment had no significant effect on the composition of volatile flavor substances in yogurt without the addition of wolfberry dietary fiber. In contrast, the distance between the WDF group and the UW group was larger than that between the CK group and the US group, indicating that the addition of wolfberry dietary fiber significantly affected the volatile flavor of yogurt. At the same time, the WDF group and the UW group were also distinguished, which showed that ultrasonic treatment synergistically promoted the production of flavor substances in yogurt with the addition of wolfberry dietary fiber.

[0072] The difference volatile flavor compounds map (VIP map) between different groups was drawn with the standard of p value ≤ 0.05, VIP ≥ 1, fold change ≥ 1.5 or ≤ 0.667. Fig.11 As shown in Figure A, four different flavor compounds were identified between the CK group and the US group. Compared with the US group, the CK group had upregulated 3-hydroxy-2-butanone (Acetoin) and benzoic acid (Benzoic acid), while hexadecane and n-hexadecanoic acid were downregulated. Among them, 3-hydroxy-2-butanone had a buttery aroma, and benzoic acid belonged to aromatic acids, with a faint aroma like benzoin.

[0073] Depend on Fig.11From Figure B, we can see that seven different flavor compounds were identified between the CK group and the WDF group. Compared with the CK group, the WDF group had higher content of flavor compounds. Hexanoic acid is mainly used in cheese, cream and fruit flavors, with a faint smell of coconut oil, 4-ethylphenol (Phenol, 4-ethyl-) has a strong wood-phenol flavor and a slightly sweet aroma, Ethanol, 2-(2-ethoxyethoxy)-) has a faint fruity flavor, with a mild and special smell, Propanoic acid, 2-hydroxy-, ethyl ester has the aroma of rum, fruit and cream, 2-ethylhexanol (1-Hexanol, 2-ethyl-) has a sweet and light floral scent, and Acetic acid has the smell of acetic acid. The increase of these typical aromatic odors makes the yogurt flavor richer and more likely to be favored by consumers.

[0074] Depend on Fig.11 As shown in Figure C, seven different volatile flavor substances were identified between the CK group and the UW group, and the contents of all seven flavor substances were increased, among which 2-nonanone has a herbal earthy smell, and polyethylene glycol-adipate-diethylene glycol usually has a slight oily smell. The types and contents of flavor substances in yogurt with added wolfberry dietary fiber were significantly higher than those in the CK group.

[0075] Depend on Fig.11 As shown in Figure 3, 7 different volatile flavor compounds were upregulated between the US group and the WDF group, namely diethylene glycol ethyl ether, 4-ethylphenol, palmitic acid, caproic acid, ethyl lactate, 2-ethylhexanol, and lactic acid.

[0076] Depend on Fig.11 As shown in Figure E, seven different volatile flavor compounds were identified in the US group and the UW group, and all seven were significantly upregulated, namely hexanoic acid, n-hexadecene, 2-nonanone, ethyl lactate, 2-ethylhexanol, palmitic acid, and lactic acid. Therefore, it can be found that the content of volatile flavor substances in the yogurt of the WDF group and the UW group is higher than that of the yogurt treated with ultrasound alone, which further proves that the addition of WDF has a significant positive effect on the flavor of yogurt.

[0077] Depend on Fig.11From Figure F, we can see that there are ten different volatile flavor compounds in the comparison between the WDF group and the UW group, among which 3,5-di-tert-butyltoluene (Benzene,1,3-bis(1,1-dimethylethyl)-), 4-ethylphenol, 2-ethylhexanol and acetic acid are downregulated, while palmitic acid, diethylene glycol ethyl ether, polyethylene glycol azelaic acid, 2-nonanone, ethyl lactate and benzoic acid are upregulated, among which 3,5-di-tert-butyltoluene has a mild, slightly sweet aromatic hydrocarbon smell, and 4-ethylphenol and 2-ethylhexanol also have a pleasant smell.

[0078] The above research results show that the flavor of the yogurt rich in wolfberry dietary fiber in the UW group of Example 1 is significantly better than that of other groups. The combination of ultrasound and wolfberry dietary fiber more strongly promotes the formation of flavor substances in yogurt, especially acids such as caproic acid, heptanoic acid, 9-decenoic acid (9-Decenoic acid), nonanoic acid (Nonanoic acid); ketones such as acetophenone (Acetophenone), 2-pentadecanone (2-Pentadecanone), 2-undecanone (2-Undecanone); alcohols such as 1-octanol (1-Octanol); esters such as polyethylene glycol azelaic acid and other flavor substances, giving the yogurt a rich aroma.

[0079] The above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions or substitutions made by a person skilled in the art within the spirit and scope of the present invention shall also fall within the protection scope of the present invention.

Claims

1. A method for preparing a yogurt drink rich in wolfberry dietary fiber, characterized in that The steps include: Step 1, after mixing whole milk powder, sucrose, wolfberry dietary fiber and water in a mass ratio of 12:6.5:1-4:79.5, placing in a 50-65° C. water bath and stirring to fully dissolve, homogenizing at 4000-5000 rpm for 1-6 min, and sterilizing to obtain a sterile mixed emulsion; Step 2, after ultrasonically treating the sterile mixed emulsion for 5-45 minutes under the conditions of 0-400W and 50-100kHz, 0.1% by volume of a basic fermentation agent consisting of thermophilic streptococcus and bulgaricus and 2% of a mixed fermentation strain consisting of Lactobacillus casei and Lactobacillus plantarum subspecies are added to the mixture, the mixture is fully stirred, and the mixture is allowed to stand for fermentation and then ripened to obtain a yogurt rich in wolfberry dietary fiber.

2. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 1, characterized in that In step 1, the mass ratio of the whole milk powder, sucrose, wolfberry dietary fiber and water is 12:6.5:2:79.

5.

3. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 1, characterized in that In step 2, the ultrasonic power is 100 and the ultrasonic time is 20 minutes.

4. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 1, characterized in that In step 2, the Lactobacillus plantarum subspecies is deposited in the General Microbiology Center of China National Microorganism Culture Collection Committee with a deposit number of CGMCC No.15953; the Lactobacillus casei is deposited in the General Microbiology Center of China National Microorganism Culture Collection Committee with a deposit number of CGMCC No.15956.

5. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 4, characterized in that: The mixed fermentation strain is OD 600nm =1 Lactobacillus casei culture and OD 600nm =1 and Lactobacillus plantarum subspecies plantarum bacterial liquid are mixed in a volume ratio of 1:

1.

6. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 1, characterized in that In step 2, the static fermentation condition is static fermentation in a fermentation box at 40°C to 44°C for 4-6 hours.

7. The method for preparing a yogurt drink rich in wolfberry dietary fiber according to claim 1, characterized in that In step 2, the post-ripening conditions are post-ripening at 0-4°C for 8-16 hours.

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