SM / LA dual gel and application thereof in improving yoghourt quality

Abstract

The invention provides SM / LA dual gel and application thereof in improving yoghourt quality, and a preparation method of the SM / LA dual gel comprises the following steps: removing fat in milk through centrifugation to obtain skim milk SM; adding gellan gum LA into the skim milk SM, so that the final concentration of the LA in the mixed system is 0.01-0.1%; the preparation method comprises the following steps: adding LA and SM into milk, then immediately adding glucolactone GDL, incubating for 3-5 hours at the fermentation temperature of 37 DEG C or above, and respectively inducing LA and SM to form gel by endogenous Ca < 2 + > and GDL in the milk, so as to prepare the SM / LA dual gel. The solution-gel transition temperature higher than or lower than LA is adopted for the first time to prepare the SM / LA dual gel with different gel sequences, it is clear that the optimal fermentation temperature can greatly improve the texture of yogurt produced by a milk / gellan gum mixed system, and therefore the quality of the yogurt is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of fermentation engineering, and particularly relates to an SM / LA double gel and application thereof in improving yogurt quality. Background Art

[0002] Yogurt is an acid-induced milk protein gel. Due to its high nutritional value and unique taste, it is deeply loved by consumers. Casein accounts for about 80% of milk protein and is the material basis for acid-induced milk gel formation. During the acidification process, the negative charge in the casein molecule gradually decreases as the pH decreases, resulting in a continuous decrease in the electrostatic repulsion between casein molecules; the insoluble colloidal calcium phosphate (CCP) inside the casein micelle structure gradually becomes soluble Ca 2+ Therefore, casein micelles undergo dissociation, aggregation and rearrangement, and finally form a three-dimensional network, and the three-dimensional network structure determines the texture of yogurt and consumer acceptance. However, yogurt often has defects such as the destruction of the curd state and whey separation, which greatly reduces consumer acceptance of yogurt and hinders its application range.

[0003] In order to prevent whey separation and enhance the texture of yogurt, factories often add stabilizers such as polysaccharides during the production of yogurt. Among stabilizers, gellan gum (LA) is widely used in yogurt production due to its unique gel properties and was approved by the FDA in 1992. However, after adding LA to commercially available yogurt, the quality often varies. How to further improve the quality of yogurt is currently an urgent problem to be solved. Summary of the invention

[0004] In order to overcome the problems of the prior art, the present invention proposes an SM / LA double gel and its application in improving the quality of yogurt, which helps manufacturers determine the appropriate acidification temperature in the milk / LA mixture system during the production process, thereby improving the quality of yogurt.

[0005] Gellan gum (LA) is a linear anionic polysaccharide produced by Sphingomonas. According to the content of acyl substituents, gellan gum is divided into low acyl gellan gum and high acyl gellan gum. In order to prevent whey separation and enhance the texture of yogurt when preparing yogurt, gellan gum is added by the existing technology without considering the solution-gel transition temperature of gellan gum. If the milk system is lower than the solution-gel transition temperature of gellan gum, gellan gum will form coagulation directly after being added to milk, which will reduce the quality of yogurt. In addition, the existing technology does not take into account the effect of gel temperature on yogurt quality, resulting in uneven quality of commercial yogurt products. Therefore, the inventors studied the addition conditions of gellan gum. The gelation of LA is a typical cold solidification (sol-gel transition temperature between 30 and 45°C) gelation, which requires divalent ions to form a gel. The solution-gel transition temperature of LA is close to the fermentation temperature used in yogurt production (42°C for thermophilic fermentation and 37°C for mesophilic fermentation). In addition, about 71% of the calcium (23.1mM) is insoluble CCP in casein micelles, and the remaining 29% is soluble calcium (9.4mM) in skim milk (SM). Therefore, acid and endogenous calcium will cause SM / LA to form a double gel. The different gelation orders of SM and LA will lead to different double gel microstructures, and the microstructure is the key to the water retention and three-dimensional network structure stability of yogurt. However, the quality of commercially available yogurt is uneven, and there is no research on the effect of milk gel and LA gel order on yogurt quality. Based on this, when further exploring how to obtain high-quality yogurt, the inventors found that the gel order of milk and LA complex gel depends on the solution-gel transition temperature of LA and the fermentation temperature during yogurt production. Therefore, this application uses a solution-gel transition temperature higher or lower than LA for the first time to prepare SM and LA double gels with different gel orders, and analyzes the overall visual appearance, water holding capacity, rheological properties and microstructure of SM / LA double gels induced by acid and endogenous calcium. In view of this, the inventors proposed the following technical solutions of the present invention.

[0006] In order to achieve the above-mentioned object of the invention, the first aspect of the present invention provides a method for preparing a SM / LA double gel, comprising the following steps:

[0007] 1) removing fat from milk by centrifugation to obtain skimmed milk SM;

[0008] 2) Add gellan gum LA solution to skim milk SM to make the final concentration of LA in the mixed system 0.01-0.1%; then immediately add gluconolactone GDL and incubate at a fermentation temperature below 37°C or above 42°C for 3-5h to reduce the endogenous Ca in milk. 2+ The SM / LA double gel was prepared by inducing LA and SM to form gels with GDL respectively.

[0009] Furthermore, in step 2), the mass concentration of the gellan gum LA solution is 1-3%, and the final concentration of LA in the mixed system is 0.05%.

[0010] Furthermore, in step 2), the final concentration of gluconolactone GDL in the mixed system is 1.5%.

[0011] Furthermore, in step 2), the fermentation temperature is 35°C to 37°C or 42°C to 45°C.

[0012] Furthermore, in step 2), the fermentation temperature is 37° C. or 42° C., and the incubation is 4 hours.

[0013] The second aspect of the present invention provides a SM / LA double gel prepared by the method described in the first aspect.

[0014] Furthermore, the water holding capacity of the SM / LA double gel is at least 90.96%.

[0015] The third aspect of the present invention provides the use of the SM / LA double gel described in the second aspect in the preparation of dairy products.

[0016] Furthermore, the dairy product includes yogurt.

[0017] The advantages and beneficial effects of the present invention are:

[0018] The present invention is the first to use a solution-gel transition temperature higher or lower than LA (below 37°C or above 42°C) to prepare SM / LA double gels with different gelation orders, and analyzes the overall visual appearance, water holding capacity, rheological properties and microstructure of the SM / LA double gels induced by acid and endogenous calcium; experiments confirm that the present invention forms SM / LA double gel yogurt by controlling the acidification temperature, and the original gellan gum improves the gelation properties of yogurt by 1.9 times, and the quality of yogurt is significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] Figure 1 Shown at 0 mM Ca 2+ (A) and 32.5 mM Ca 2+ (B) Variation of storage modulus (G′) and loss modulus (G″) with temperature under the presence of 0.05% LA;

[0021] Figure 2 The change patterns of storage modulus (G′) and loss modulus (G″) of SM-37 gel, SM-42 gel, SM / LA-37 double gel and SM / LA-42 double gel with fermentation time are shown;

[0022] Figure 3 The pH variation of SM-37 gel, SM-42 gel, SM / LA-37 double gel and SM / LA-42 double gel with fermentation time is shown;

[0023] Figure 4 The apparent shapes of SM-37 gel, SM-42 gel, SM / LA-37 double gel and SM / LA-42 double gel are shown;

[0024] Figure 5 The water holding capacity of SM-37 gel, SM-42 gel, SM / LA-37 double gel and SM / LA-42 double gel is shown;

[0025] Figure 6 The curves showing the variation of G′ with frequency for SM-37 gel, SM-42 gel, SM / LA-37 double gel and SM / LA-42 double gel;

[0026] Figure 7 SEM images of SM-37 gel, SM-42 gel, SM / LA-37 double gel, and SM / LA-42 double gel are shown. DETAILED DESCRIPTION

[0027] The implementation of the present invention is not limited to the following embodiments, and any form of modification and / or change made to the present invention will fall within the scope of protection of the present invention. In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. Unless otherwise specified, the methods used in the embodiments are common techniques in the art.

[0028] For numerical ranges, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be deemed to be specifically disclosed herein.

[0029] The detection methods involved in the following embodiments include:

[0030] 1. Water holding capacity

[0031] The SM / LA-37, SM / LA-42 double gels, SM-37 and SM-42 gels prepared in the following examples and comparative examples were centrifuged at 2826×g for 10 min to measure the water displacement of the gel. The water holding capacity (WHC) of the gel was calculated as follows: WHC (%) = (gel weight - water displacement) / gel weight × 100%.

[0032] 2. Rheological test

[0033] The gel formation process was evaluated by rheometer, using cup and circular geometries. The different gel samples prepared in the following examples and comparative examples were transferred into concentric cylinders. The time sweep experiments were performed at 37°C or 42°C, 0.1% strain, and 1 Hz frequency for 4 h. The frequency experiments were performed after the samples were cooled to 25°C. The frequency sweep range was 0.1 to 100 Hz at 0.1% strain.

[0034] 3. Microstructure

[0035] The complex microstructure of the gel was observed under a TM 4000Lius scanning electron microscope. The gel was frozen in liquid nitrogen for rapid freezing. Subsequently, the free water in the sample was sublimated and the sample was quickly broken and placed in an observation chamber. The gel network structure was analyzed by scanning electron microscopy.

[0036] Example 1: Preparation of SM / LA double gel

[0037] This embodiment provides a method for preparing SM / LA double gel, comprising the following steps:

[0038] 1) Remove fat from milk by centrifugation (3000 rpm, 15 min) to obtain skim milk SM.

[0039] 2) Gellan gum LA solution (2.0%, w / w) was added to skim milk SM to make the final concentration of LA in the mixed system 0.05%; and then gluconolactone GDL was immediately added to make the final concentration of gluconolactone GDL in the mixed system 1.5%.

[0040] After incubation at 37℃ or 42℃ for 4h, the endogenous Ca in milk 2+ LA and SM were induced to form gels with GDL respectively, thereby preparing two SM / LA double gels, namely SM / LA-37 double gel and SM / LA-42 double gel.

[0041] Comparative Example: Preparation of SM gel

[0042] This comparative example provides a method for preparing SM gel, comprising the following steps:

[0043] 1) Remove fat from milk by centrifugation (3000 rpm, 15 min) to obtain skim milk SM.

[0044] 2) Adding gluconolactone GDL to skim milk SM, the final concentration of gluconolactone GDL in the mixed system is 1.5%.

[0045] The SM-37 and SM-42 gels were prepared as control groups by incubating at a fermentation temperature of 37°C or 42°C for 4 h.

[0046] Example 2: Solution-gel transition temperature of LA

[0047] In order to explore the solution-gel transition temperature of LA, LA (0.05%, w / w) and 32.5mmol / LCaCl were prepared in this example. 2 The mixture, as well as a single LA solution (0.05%, w / w), were temperature scanned in the scanning range of 60-20°C and a cooling rate of 1.0°C / min. The highest temperature point when the storage modulus (G′) was ≥1 was the solution-gel transition temperature of LA.

[0048] As a gelling polysaccharide, LA undergoes a transformation in Ca 2+ A weak gel is formed in the presence of Figure 1 A shows that in the absence of Ca 2+ In the case of Ca, LA (0.05%, w / w) did not undergo sol-gel transition even when the solution temperature was reduced from 60°C to 20°C. 2+ In the presence of LA, LA forms a gel when cooled to the gelling temperature ( Figure 1 Figure B). As shown in Figure B, the liquid-solid phase transition temperature of LA is 38°C (G'≥1), that is, below 38°C, LA is in a gel form (helix); above 38°C, LA is in a liquid form (coil). Therefore, the present application selects the LA transition temperature above 42°C and below 37°C as the acidification temperature to prepare the SM-LA double gel.

[0049] Example 3: Changes in gel time and pH during acidification

[0050] In order to evaluate the gelation order of LA and SM, the gelation time and pH value changes of SM-37, SM-42, SM / LA-37 or SM / LA-42 gels prepared in the examples and comparative examples during the acidification process were measured in this example. G′≥1 indicates gel formation, which has the characteristics of weak viscoelastic gel.

[0051] from Figure 2It can be seen that the gelation time of SM-37 is 55 min, while the G′ of SM / LA-37 is greater than 1 at the beginning. When the fermentation temperature is 37°C, a weak network has been formed at the beginning. Therefore, in SM / LA-37, LA forms gel first, and SM forms gel later. The gelation time of SM / LA-42 (23.01 min) is close to that of SM-42 (25.51 min), indicating that when the pH of SM / LA-42 drops to 5.0, SM forms gel first, and when the sample temperature is lower than the liquid-gel transition temperature, LA forms gel.

[0052] The pH changes of SM / LA double gel after adding GDL are shown in Figure 2. Figure 3 As shown. After adding GDL, the pH value of the sample continued to decrease. From the rheological results, it can be seen that after adding GDL, SM-37 began to gel at pH 5.0 (60min). At pH ≤ 5.0, the net negative charge on casein is significantly reduced, which leads to a reduction in the electrostatic repulsion between casein molecules, thereby forming a gel; while the gel time of SM / LA-37 is 0min, indicating that LA and endogenous calcium constitute the first network of the double gel, and when the pH value of the SM / LA double gel drops to 5.0, SM constitutes the second network of the double gel; about 30min after adding GDL, SM-42 and SM / LA-42 began to gel at pH 5.0, and when the pH of SM / LA-42 dropped to 5.0, SM formed the first network of the double gel, with the decrease of pH value and the change of mineral balance, when the temperature of SM / LA-42 is lower than the liquid-gel transition temperature, LA and endogenous calcium constitute the second network of the double gel. Therefore, the present application prepared SM / LA double gels with different gel sequences by different acidification temperatures.

[0053] Example 4: Gel appearance, water holding capacity, rheological properties and microstructure

[0054] In latex gels, overall visual appearance, WHC, rheological properties and microstructure are important factors in evaluating yogurt. In this example, the SM / LA-37, SM / LA-42 double gels, SM-37 and SM-42 gels prepared in the above Example 1 and Comparative Example were subjected to the following measurements.

[0055] (1) Gel appearance and water holding capacity

[0056] from Figure 4 It can be seen that no matter the acidification temperature is 37℃ or 42℃, SM gel cannot form a stable gel and is accompanied by partial water separation. When LA is added, the surface of the binary (SM / LA) gel is smooth. In addition, compared with SM gel, the addition of LA to milk significantly reduces whey precipitation.

[0057] WHC is an important indicator for measuring yogurt quality, which represents the ability of hydrogels to retain water after centrifugation. When the fermentation temperature was 37℃ and 42℃, the WHC of GDL-induced SM gel was 60.28% and 60.86%, respectively. When LA was added, the WHC of SM / LA gel at fermentation temperatures of 37℃ and 42℃ increased to 95.20% and 90.96%, respectively ( Figure 5 ), indicating that the WHC of SM / LA gel is higher than that of SM gel, and the WHC of SM / LA gel formed at a fermentation temperature of 37°C is higher than that formed at 42°C. Therefore, the acidification temperature of SM / LA double gel is preferably 37°C.

[0058] (2) Rheological properties

[0059] In order to further monitor the formation process of SM / LA dual gel, the changes of G′ and G″ during the acidification process were measured. Figure 2 It can be seen that the G′ in the SM / LA double gel is significantly higher than that in the SM gel, indicating that the addition of LA significantly improves the gel strength. This result is attributed to the endogenous Ca 2+ The LA gel and SM gel were formed with acid, thus forming SM / LA double gel. The addition of an appropriate second polymer network improves the mechanical properties). The G′ of the SM / LA double gel formed at 37°C is higher than that of the double gel formed at 42°C. Dynamic frequency sweeps were performed on all samples at 25°C ( Figure 6 ). Both G′ and G″ increased with increasing frequency. In single SM, the gel formed at lower temperature (37°C) had a weaker and rougher network than the gel formed at 42°C, which may be because the reduction in fermentation temperature increased the time required to reach the gel pH. The G′ and G″ values ​​of SM / LA gels were higher than those of SM gels, indicating that LA enhanced the structural properties of SM gels. The G′ of SM / LA double gels formed at 37°C was higher than that of double gels formed at 42°C. The G′ value indicates a higher hardness. These rheological results are consistent with the above results.

[0060] In summary, 37 °C is the preferred acidification temperature for the production of SM / LA double gels.

[0061] (3) Microstructure

[0062] In order to clarify the microstructure of SM gel, SM / LA double gel and LA gel, this example uses scanning electron microscopy (SEM) to study the three-dimensional network structure of the gel. The pores of SM gel are evenly distributed, and the pores are small and narrow ( Figure 7 After adding 0.05% LA, a significant structural transformation occurred. Compared with SM gel, the SM / LA-37 double gel transformed from a single porous network structure to two network structures: one is a porous network and the other is a dense network ( Figure 7 ), the formation of this network significantly improves the gel strength. Compared with the SM / LA-37 double gel, the SM / LA-42 double gel exhibits a unique gel network, which is mainly a dense network. The size of most pores in the 0.05% LA gel is significantly larger than the size of casein micelles, so at the beginning of acidification, the formed LA gel will not hinder the free diffusion of casein micelles; however, due to the small volume of SM gel, the formation of casein gel may weaken the LA gel strength, which may be the reason why the gel performance of SM / LA-37 is better than that of SM / LA-42.

[0063] Finally, it should be noted that the above is only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred arrangement scheme, a person skilled in the art should understand that the technical solution of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing SM / LA double gel, characterized in that: The preparation method comprises the following steps: 1) removing fat from milk by centrifugation to obtain skimmed milk SM; 2) Add gellan gum LA solution to skim milk SM to make the final concentration of LA in the mixed system 0.01-0.1%; then immediately add gluconolactone GDL and incubate at a fermentation temperature below 37°C or above 42°C for 3-5h to reduce the endogenous Ca in milk. 2+ The SM / LA double gel was prepared by inducing LA and SM to form gels with GDL respectively.

2. The method for preparing the SM / LA double gel according to claim 1, characterized in that: In step 2), the mass concentration of the gellan gum LA solution is 1-3%, and the final concentration of LA in the mixed system is 0.05%.

3. The method for preparing the SM / LA double gel according to claim 1, characterized in that: In step 2), the final concentration of gluconolactone GDL in the mixed system is 1.5%.

4. The method for preparing the SM / LA double gel according to claim 1, characterized in that: In step 2), the fermentation temperature is 35°C to 37°C or 42°C to 45°C.

5. The method for preparing the SM / LA double gel according to claim 4, characterized in that: In step 2), the fermentation temperature is 37° C. or 42° C., and the incubation is 4 hours.

6. A SM / LA double gel prepared by the method according to any one of claims 1 to 5.

7. The SM / LA double gel according to claim 6, characterized in that: The water holding capacity of the SM / LA dual gel is at least 90.96%.

8. Use of the SM / LA double gel as claimed in claim 7 in the preparation of dairy products.

9. The use according to claim 8, characterized in that: The dairy product includes yogurt.

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