NL2035199B1

NL2035199B1 - Casein gel, and preparation method and application therefor

Info

Publication number: NL2035199B1
Application number: NL2035199A
Authority: NL
Inventors: Zhang Feng; Luo Liming; Wang Jing; Chen Zuguo; Hu Xiaofang; Zhang Zhen; Li Jiamin; Chen Chong; Jiang Yuanyuan
Original assignee: Chongqing Tianyou Dairy Co Ltd
Priority date: 2023-01-06
Filing date: 2023-06-28
Publication date: 2024-02-27
Also published as: CN118303613A; NL2035199A

Abstract

A preparation of protein gel, in particular casein gel, and a preparation method and application therefor. Raw materials of the casein gel include casein and/or casein sodium salt, sodium alginate, a calcium ion chelating agent and an acidification sustained-release agent. Under the action of the calcium ion chelating agent slowly releasing calcium ions, the casein and/or casein sodium salt and the sodium alginate are mutually interpenetrated to form a dual network structure, and at the same time, under the action of slow acidification of a pH sustained-release agent, the casein and/or casein sodium salt and the sodium alginate are acidized to an isoelectric point of the casein, so as to cause a reaction system to form the casein gel of the present application. The casein gel is excellent in rheological characteristic, high in hardness, strong in water-holding capacity, and stable in structure.

Description

CASEIN GEL, AND PREPARATION METHOD AND APPLICATION

THEREFOR

TECHNICAL FIELD

3 The present application belongs to the technical field of preparation of protein gel, and specifically relates to casein gel, and a preparation method and an application therefor.

BACKGROUND

Casein or a sodium salt of the casein is a protein derived from cow milk with rich nutritional values and special functions, consists of four casein monomers of a, B, y and x, and contains various essential amino acids for a human body. Furthermore, the casein or the sodium salt of the casein has a stable nature, and has the effect of protein fortification, thickening and foaming, may participate in normal metabolism of the human body after being absorbed by the human body after hydrolysis, is a high-quality protein source, and may be added to various types of food as a high protein nutritional fortifier. In addition, the casein or the sodium salt of the casein also has a good gel property, and may enhance food utilization and stability, improve taste, and embed flavor substances and probiotics, such that the casein or the sodium salt of the casein is currently widely applied to the field of dairy processing.

However, the casein or sodium salt gel of the casein is flimsy in structure and is prone to whey separation, which limits the application of the casein or the sodium salt gel of the casein in gel food. Therefore, preparation of the casein or the sodium salt gel of the casein with an excellent mechanical property without changing the original characteristics of the casein or the sodium salt gel of the casein has been a hot research topic at home and abroad. Current methods for improving the structure of the casein or the sodium salt gel of the casein mainly include Glutamine Transaminase (TG) covalent crosslinking, blending of the casein or the sodium salt gel of the casein and high and low acyl gellan gum and agar, or chemical modification. These methods have the defects of weak binding force, poor water-holding capacities, flimsy structures and poor stability. Moreover, the chemical modification method has the disadvantage of changing the original characteristics of sodium caseinate gel.

Therefore, there is a need for a different method to modify sodium caseinate so as to obtain the sodium caseinate gel with a good water-holding capacity, a compact structure and desirable stability.

SUMMARY

In view of the above problems, one of the objectives of the present application is to provide casein gel which is excellent in rheological characteristic, high in hardness, strong in water-holding capacity, and stable in structure.

In order to achieve the above purpose, the present application may use the following technical solutions.

One aspect of the present application provides casein gel. Raw materials of the casein gel may include casein and/or casein sodium salt, sodium alginate, a calcium ion chelating agent and an acidification sustained-release agent.

Another aspect of the present application provides a method for preparing the casein gel.

The method may include: mixing a casein and/or casein sodium salt solution and a sodium alginate solution, so as to obtain a mixed solution; mixing the mixed solution and a calcium ion chelating agent and an acidification sustained-release agent to perform a reaction; and performing the reaction until pH reaches 3.5-4.6, so as to obtain the casein gel.

Still another aspect of the present application provides an application of the casein gel in preparation of fermented milk.

The beneficial effects of the present application at least include the following. (1) The casein gel provided in the present application is excellent in rheological characteristic, high in hardness, strong in water-holding capacity, and stable in structure; compared with sodium alginate-sodium caseinate bi-continuous phase gel with no slow-released calcium ions, the hardness of the casein gel is improved by approximately 68%, the water-holding capacity of the casein gel is improved by approximately 36%, and the structure stability of the casein gel is obviously improved; and compared with sodium caseinate-calcium ion gel with no sodium alginate, the hardness of the casein gel is improved by approximately 265%, the water-holding capacity of the casein gel is improved by approximately 44%, and the structure stability of the casein gel is obviously improved. (2) The method for preparing the casein gel provided in the present application is simple and mild in condition, thereby facilitating industrial production.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an appearance diagram of (0. 1%Alg-CN/Ca?*) gel according to Embodiment 1.

Fig. 2 is an appearance diagram of (0.2%4Alg-CN/Ca?*) gel according to Embodiment 2.

Fig. 3 is an appearance diagram of (0.3%Alg-CN/Ca?*) gel according to Embodiment 2.

Fig. 4 is an appearance diagram of (0.3%Alg-CN) gel according to Comparative example

I.

Fig. 5 is an appearance diagram of (CN/Ca**) gel according to Comparative example 2.

Fig. 6 is an appearance diagram of (0.4%Alg-CN/Ca*") gel according to Comparative example 3.

Fig. 7 is a microstructure diagram of (0.1%Alg-CN/Ca®") gel according to Embodiment 1.

Fig. 8 is a microstructure diagram of (0.2%Alg-CN/Ca?*) gel according to Embodiment 2.

Fig. 9 is a microstructure diagram of (0.3%Alg-CN/Ca?*) gel according to Embodiment 2.

Fig. 10 is a microstructure diagram of (0.3%Alg-CN) gel according to Comparative example 1.

Fig. 11 is a microstructure diagram of (CN/Ca?*) gel according to Comparative example 2.

Fig. 12 is a microstructure diagram of (0.4%Alg-CN/Ca?*) gel according to Comparative example 3.

Fig. 13 is a rheological curve of (0.1%Alg-CN/Ca?*) gel during fermentation according to Embodiment 1.

Fig. 14 is a rheological curve of (0.2%Alg-CN/Ca**) gel during fermentation according to Embodiment 2.

Fig. 15 is a rheological curve of (0.3%Alg-CN/Ca?*) gel during fermentation according to Embodiment 2.

Fig. 16 is a rheological curve of (0.3%Alg-CN) gel during fermentation according to

Comparative example 1.

Fig. 17 is a rheological curve of (CN/Ca®") gel during fermentation according to

Comparative example 2.

Fig. 18 is a rheological curve of (0.4%Alg-CN/Ca’*) gel during fermentation according to Comparative example 3.

Fig. 19 shows water-holding capacities of gel in embodiments and comparative examples.

Fig. 20 shows the hardness of gel in embodiments and comparative examples.

In the drawings, 0.1%Alg-CN/Ca** is the gel according to Embodiment I; 0.2%Alg-CN/Ca*" is the gel according to Embodiment 2; 0.3%%Alg-CN/Ca?” is the gel according to Embodiment 3; 0.3%Alg-CN is the gel according to Comparative example 1;

CN/Ca?* is the gel according to Comparative example 2; 0.4%Alg-CN/Ca?* is the gel according to Comparative example 3; and 0.3%Alg-CN is the gel according to Comparative example 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments are given to better describe the present application, but the content of the present application is not limited only to the embodiments given. Therefore, non-essential improvements and adjustments to the embodiments made by a person skilled in the art in accordance with the content of the above present application still fall within the scope of protection of the present application.

The terms used herein are only intended to describe specific embodiments and are not intended to limit the present disclosure. Expressions in the singular form include those in the plural form unless the expressions have a distinctly different meaning in the context. As used herein, it is to be understood that terms such as "include", "have", "contain", and the like are intended to indicate the presence of features, figures, operations, components, parts, elements, materials, or combinations. The terms of the present application are disclosed in the specification and are not intended to exclude the possibility that one or more other features, figures, operations, components, parts, elements, materials, or combinations thereof may exist or may be added. As used here, "/" may be interpreted as "and" or "or", as appropriate.

The term "calcium ion chelating agent" in the present application refers to a chelate that may slowly release calcium ions; and the term “acidification sustained-release agent” refers to a substance that may slowly acidize a system.

An embodiment of the present application provides casein gel. Raw materials of the casein gel include casein and/or casein sodium salt, sodium alginate, a calcium ion chelating agent and an acidification sustained-release agent. It is to be noted that, in the present application, the calcium ion chelating agent acts as a calcium ion sustained-release agent, while the acidification sustained-release agent induces slow acidification of a sodium caseinate-sodium alginate dispersion system to an isoelectric point of the casein so as to form the casein gel, the calcium ion chelating agent induces calcium ions in calcium citrate to release and form calcium alginate gel with the sodium alginate, so as to form casein-calcium alginate interpenetrating dual network gel during acidization. That is, the casein gel is formed by a reaction system when, under the action of the calcium ion chelating agent slowly releasing the calcium ions, the casein and/or casein sodium salt and the sodium alginate are mutually interpenetrated to form a dual network structure, and at the same time, under the action of slow acidification of a pH sustained-release agent, the casein and/or casein sodium salt and the sodium alginate are acidized to an isoelectric point of the casein.

It is further to be noted that, in recent years, a mixed acid of the sodium alginate and the casein is induced to form composite gel, such that the property of the casein gel is greatly improved. However, in the composite gel, the sodium alginate achieves electrostatic recombination with the casein by being only used as a polymer, so as to realize its function. In the present application, in combination of the physical characteristics of polysaccharide (sodium alginate) and a protein (the casein or a sodium salt gel of the casein), a multi-layered or layered binary system mixed gel microstructure is produced. Binary system mixed gel may finally form the interpenetrating network gel by independently forming three-dimensional network structures on the basis of two polymers, respectively. The interpenetrating network gel has more excellent gel properties, such as the strength, toughness and hardness of the gel; and adding an appropriate second polymer network (the calcium ion chelating agent) may 5 achieve equilibrium between the strength and the toughness by means of adjusting interactions and structures in two networks, thereby further improving the overall mechanical properties of the gel. It is widely known that, the sodium alginate is an anionic, linear, polysaccharide-based natural polymer derived from brown algae; a molecular chain of the polymer is formed by means of block linear polymerization of a-L-guluronic acid (G unit) and B-D-mannuronic acid (M unit); and the G unit easily form an “egg box” structure with some divalent cations (generally, Ca?*), so as to induce the casein to form solid gel. Therefore, the sodium alginate is blended with the casein, and while the gel is formed by the casein, the sodium alginate induces formation of the calcium alginate gel by means of the calcium ions, such that compared with casein-sodium alginate composite gel, the formed casein-calcium alginate dual gel has more excellent gel properties.

The above indicates that, in the absence of Ca?’ slow release, electrostatic repulsion exists between sodium caseinate and the sodium alginate, and the sodium caseinate and the sodium alginate are poor in compatibility, resulting in poor gel water-holding capacity and poor stability; and in the absence of the sodium alginate, the stability of the casein gel is relatively poor. That is to say, in the casein gel in the present application, under the action of calcium ion slow release, the casein or the sodium salt of the casein and the sodium alginate are mutually interpenetrated to form a dual network structure, such that the water-holding capacity and stability of the sodium caseinate gel are improved.

In addition, it is further to be emphasized that, in the present application, the calcium ion chelating agent is selected instead of the calcium ions. If an inorganic calcium salt, such as

Calcium Chloride (CaCls), is selected, since the CaCl: has a high solubility in water, at the moment when a pure CaCl; solution comes into contact with a mixed solution of the casein or the sodium salt of the casein and the sodium alginate, sediments may be produced rapidly, affecting the formation of the gel, such that the calcium ion chelating agent needs to be selected, so as to control the releasing of the calcium ions during a reaction with the casein or the sodium salt of the casein and the sodium alginate, thereby slowing down a gel rate.

It should be understood that, the raw materials of the casein gel may include the casein, the sodium alginate, the calcium ion chelating agent and the acidification sustained-release agent, or may include the casein sodium salt, the sodium alginate, the calcium ion chelating agent and the acidification sustained-release agent, or may include the casein, the casein sodium salt, the sodium alginate, the calcium ion chelating agent and the acidification sustained-release agent.

In some embodiments, in the casein gel, a mass ratio of the casein and/or casein sodium salt in the raw materials to the sodium alginate is 6:(0.1-1.0); and the mass concentration of the sodium alginate is 0.1%-1.0%. It should be understood that, in the raw materials of the casein gel, the mass ratio of the casein to the sodium alginate may be 6:(0.1-1.0), or the mass ratio of the casein sodium salt to the sodium alginate may be 6:(0.1-1.0), or the mass ratio of a combination of the casein and the casein sodium salt to the sodium alginate may be 6: (0.1-1.0). In some specific embodiments, the mass ratio of the casein and/or casein sodium salt to the sodium alginate may be 6:0.15, 6:0.2, 6:0.25, etc. It is to be noted that, in the mass ratio of the casein and/or casein sodium salt to the sodium alginate, if the mass of the casein and/or casein sodium salt is too much relative to the sodium alginate and exceeds the maximum mass ratio, the gel formed is the casein gel with the defects of a flimsy structure and whey separation that has occurred, which is not a dual gel system, such that the performance of the gel in the present application is not reflected. Likewise, in the mass ratio of the casein and/or casein sodium salt to the sodium alginate, if the mass of the sodium alginate relative to the casein and/or casein sodium salt is less and is less than the minimum mass ratio, the calcium ions cause intramolecular coordination of the sodium alginate and tangling of molecular chains of the sodium alginate, resulting in large pore sizes, thus affecting the gel stability.

The mass concentration of the sodium alginate may be 0.15%, 0.2% or 0.25% etc. It is to be noted that, the mass concentration of the sodium alginate should not be too low or too high.

If the mass concentration of the sodium alginate is too low, the sodium alginate cannot form a compact structure with the casein and/or sodium caseinate, thus affecting the casein gel to achieve optimal stability; and if the mass concentration of the sodium alginate is too high, there is a phase separation between the sodium alginate and the casein or the sodium salt of the casein, breaking the compactness between the sodium alginate and the casein or the sodium salt of the casein, thus also affecting the stability of the casein gel. Under the ratio of the present application, the casein gel has desirable stability and water-holding capacity.

In some specific embodiments, in the casein gel, when the mass concentration of the casein or the sodium salt of the casein in the casein gel is 6% and the mass concentration of the sodium alginate is 0.3%, compared with the gel with no Ca?*, the water-holding capacity of the prepared casein gel is increased by approximately 36%, and the hardness is increased by approximately 68%; compared with the gel with no sodium alginate, the water-holding capacity of the casein gel is increased by approximately 44%, and the hardness is increased by approximately 265%; and compared with the gel prepared in other concentrations and mass ratios, the water-holding capacity of the casein gel is increased by at least 7%, and the hardness is increased by at least 20%.

In some embodiments, in the casein gel, the calcium ion chelating agent in the raw materials is formed by chelating a calcium salt and a chelating agent. The calcium salt may be selected from those known in the art to be soluble in water and neutral in solution, such as calcium chloride or calcium gluconate; the chelating agent is selected from those known in the art, such as sodium citrate, sodium acetate, or sodium hexametaphosphate; and the casein sodium salt is known in the art, such as sodium caseinate.

In some embodiments, in the casein gel, the mass concentration of the calcium ion chelating agent in the raw materials may be 25 mmol/L-35 mmol/L, for example, 28 mmol/L, 30 mmol/L, or 32 mmol/L. It is to be noted that, in the present application, the role of calcium ions is that the Ca?* crosslinks with phosphate residues of serine on a-, s- and B-casein in the casein or sodium salt molecules of the casein, and the Ca** ligates with -COO- of the sodium alginate, so as to form an “egg box” structure; then during preparation, as the pH of a solution changes, the casein gradually releases partial Ca** and forms calcium alginate with the sodium alginate; and when a pH value reaches the isoelectric point (pH=4.6, approximately) of the casein, the sodium alginate and the casein form more compact dual network gel under the action of Ca?’. However, the concentration of the calcium ions should not be too high or too low; when the concentration is too low, only the viscosity of a sodium alginate solution is increased, and the casein or the sodium salt of the casein is not induced to form the dual network gel with the sodium alginate; and when the concentration of the calcium ions is too high, intramolecular coordination of the sodium alginate further occurs on the basis of intermolecular coordination, and the molecular chain of the sodium alginate is tangled by means of intramolecular coordination and intermolecular coordination together, resulting in large pore sizes, thus affecting the stability of sodium caseinate gel. At the concentration of the calcium ion chelating agent in the present application, the Ca?” induces the casein or the sodium salt of the casein to form the cohesive and solid gel.

In some embodiments, in the casein gel, the acidification sustained-release agent in the raw materials may be an acidification sustained-release agent (a substance that may slowly acidize a system) known in the art, for example, L-malic acid, glucono-delta-lactone, or a strain (such as a lactobacillus bulgaricus, a streptococcus thermophilus, and other strains that may undergo acidification). It is to be noted that, the acidification sustained-release agent is intended to induce a sodium caseinate dispersion system to be slowly acidized to a PI point, so as to form a gel state, such that inorganic acids such as hydrochloric acid cannot be directly added to reduce a pH value; and if the reduction of pH is too fast, gel precipitation and whey precipitation are easily caused. In addition, since the pH value of the system cannot be too low (not less than 3.5), the amount of the acidification sustained-release agent needs to be adjusted according to an initial pH value of the system. For example, when the initial pH is 6.6-7, the mass fraction of the acidification sustained-release agent may be 2.0%-3.5%, for example, 2.2%, 2.4% or 3.3%. In addition, when the glucono-delta-lactone acts as a protein coagulant for dairy products, the glucono-delta-lactone is hydrolyzed in an aqueous solution to produce gluconic acid, and reduces the pH value of the solution at a relatively-slow speed, such that enough time may be provided for aggregation of protein molecules into a gel structure. It is further to be noted that, in some embodiments, in order to conveniently control the amount of the acidification sustained-release agent, a reaction system may be first added to an alkaline solution (such as a sodium hydroxide solution) to adjust the pH to a fixed point, for example, any pH point among 6.6-7.

Another embodiment of the present application provides a method for preparing the casein gel. The method may include: mixing a casein and/or casein sodium salt solution and a sodium alginate solution, so as to obtain a mixed solution; mixing the mixed solution and a calcium ion chelating agent and an acidification sustained-release agent to perform a reaction; and performing the reaction until pH reaches 3.5-4.6, so as to obtain the casein gel.

It should be understood that, in the method for preparing the casein gel, the casein and/or casein sodium salt solution and the sodium alginate solution are both aqueous solutions. In addition, the casein and/or casein sodium salt solution may be a casein solution, or may be a casein sodium salt solution, or may be a combination solution of the casein and the casein sodium salt.

It is to be noted that, in the method for preparing the casein gel, the calcium ion chelating agent may be directly obtained by means of a reaction in the reaction system, or the calcium ion chelating agent may be directly added. For example, in some specific embodiments, after the casein and/or casein sodium salt solution and the sodium alginate solution are mixed, a calcium chloride solution, a sodium citrate solution and the acidification sustained-release agent may be added to the mixed solution, and mixed; and the reaction system is acidized to obtain the casein gel.

In some embodiments, in the method for preparing the casein gel, the reaction is performed at 40°C-44°C. Specifically, a constant temperature reaction is performed on a mixed system of all raw materials at 40°C-44°C, so as to obtain the casein gel. It is to be noted that, after the mixed solution and the calcium ion chelating agent are mixed, the acidification sustained-release agent may be added at 40°C-44°C for reaction, so as to obtain the casein gel;

or after the mixed solution, the calcium ion chelating agent and the acidification sustained-release agent are mixed, the reaction may be performed at 40°C-44°C, so as to obtain the casein gel.

In some specific embodiments, the method for preparing the casein gel may include the following steps: (1) dissolving the sodium caseinate with 70°C deionized water, and performing constant temperature stirring at 70°C for 4h, until the sodium caseinate is completely hydrated, so as to prepare a sodium caseinate solution; and dissolving the sodium alginate with the 70°C deionized water, and performing constant temperature stirring at 70°C for 4h, until the sodium alginate is completely hydrated, so as to prepare the sodium alginate solution; (2) mixing the sodium alginate solution and the sodium caseinate solution according to the above mass ratio, and replenishing the deionized water to a certain mass, so as to prepare a sodium alginate-sodium caseinate mixed solution; (3) adding the mixed milk solution in step (2) to a mixed solution of the calcium chloride and the sodium citrate, to cause the calcium ions to crosslink sodium alginate-sodium caseinate, so as to form a dual gel solution; and (4) under the condition of 42°C in step (3), adding the glucono-delta-lactone, performing full stirring for 20s, then preparing a sodium caseinate gel solution, and placing the sodium caseinate gel solution in a 42°C constant temperature incubator for fermentation for 4h, so as to obtain the sodium caseinate gel.

Still another embodiment of the present application provides an application of the casein gel in preparation of fermented milk. It is to be noted that, the casein gel may be used as a functional composition in the dairy product or a slow-release material of a flavor substance, so as to prevent the whey of the fermented milk from precipitating, or may be used as an embedding material of probiotics. On the basis of the casein gel in the present application being excellent in rheological characteristic, high in hardness, strong in water-holding capacity, and stable in structure, by means of applying the casein gel to preparation of the fermented milk, the rich functional compositions, the flavor substance or the probiotics can be added in the fermented milk insofar as the stability of the fermented milk is guaranteed, thereby enriching the categories of the fermented milk. In addition, the phosphate residues of the serine on o-casein and [-casein in the sodium caseinate may crosslink with the calcium ions, so as to form a calcium supplement substance that is easily absorbed and utilized by the human body, such that nutrient elements of the fermented milk may be enriched.

In order to better understand the present application, the content of the present application is further described below with reference to specific embodiments, but is not only limited to the following examples.

I Sodium caseinate gel solution and gel preparation

Embodiment 1 (0.1% Alg-CN/Ca’") gel 9.60g of sodium caseinate was weighed and placed in 71 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 80g, so as to prepare a 12%(w/w) sodium caseinate solution. 0.16g of sodium alginate was weighed and placed in 40 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 40g, so as to prepare a 0.4%(w/w) sodium alginate solution.

The sodium alginate solution and the sodium caseinate solution were mixed; constant temperature stirring was continuously performed at 70°C for 1 hour, then the deionized water was replenished until a system mass was 120g, and 13 mL of the deionized water was quickly added at 70°C to dissolve a mixed solution that was prepared by 2.38g of trisodium citrate dihydrate and 0.54g of anhydrous calcium chloride; and 1 mol/L of NaOH was added dropwise, so as to adjust a pH value of a mixed system to 6.8. 4.0g of glucono-delta-lactone was weighed and dissolved in 20 mL of the deionized water; vortex was performed for 10s, and the mixture was immediately added to the mixed system; and full stirring was performed for 20s, and then a gel solution was prepared.

The mass concentration of the sodium caseinate in a gel system was 6%, the mass concentration of the sodium alginate was 0.1%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Rheological analysis was performed on the gel solution; and the gel solution was placed ina42°C constant temperature incubator for fermentation for 4h, so as to form the gel.

Embodiment 2 (0.2% Alg-CN/Ca’") gel 9.60g of sodium caseinate was weighed and placed in 71 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 80g, so as to prepare a 12% (w/w) sodium caseinate solution. 0.32g of sodium alginate was weighed and placed in 60 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 40g, so as to prepare a 0.8%(w/w) sodium alginate solution.

The mass concentration of the sodium caseinate in a gel system was 6%, the mass concentration of the sodium alginate was 0.2%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Rheological analysis was performed on the gel solution; and the gel solution was placed ina42°C constant temperature incubator for reaction for 4h, so as to form the gel.

Embodiment 3 (0.3%Alg-CN/Ca’") gel 9.60g of sodium caseinate was weighed and placed in 51 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 16%(w/w) sodium caseinate solution. 0.48g of sodium alginate was weighed and placed in 60 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 0.8% (w/w) sodium alginate solution.

The mass concentration of the sodium caseinate in a gel system was 6%, the mass concentration of the sodium alginate was 0.3%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Comparative example 1 (0.3% Alg-CN) gel 9.60g of sodium caseinate was weighed and placed in 51 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 16%(w/w) sodium caseinate solution. 0.48g of sodium alginate was weighed and placed in 60 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 0.8%(w/w) sodium alginate solution.

The prepared sodium alginate solution and the sodium caseinate solution were mixed; constant temperature stirring was continuously performed at 70°C for 1 hour, then the deionized water was replenished until a system mass was 120g, and 13 mL of the deionized water was quickly added at 70°C to dissolve a mixed solution that was prepared by 2.38g of trisodium citrate dihydrate; and 1 mol/L of NaOH was added dropwise, so as to adjust a pH value of a mixed system to 6.8. 4.0g of glucono-delta-lactone was weighed and dissolved in 20 mL of the deionized water; vortex was performed for 10s, and the mixture was immediately added to the mixed system; and full stirring was performed for 20s, and then a gel solution was prepared.

The mass concentration of the sodium caseinate in a gel system was 6%, the mass concentration of the sodium alginate was 0.3%, the concentration of sodium citrate was 50 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Rheological analysis was performed on the gel solution; and the gel solution was placed ina 42°C constant temperature incubator for reaction for 4h, so as to form the gel.

Comparative example 2 (CN/Ca*") gel 9.60g of sodium caseinate was weighed and placed in 71 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 80g, so as to prepare a 12% (w/w) sodium caseinate solution.

The sodium alginate solution was replenished to make up to 120g by mass, constant temperature stirring was continuously performed at 70°C for 1 hour, then 13 mL of the deionized water was quickly added at 70°C to dissolve a mixed solution that was prepared by 2.38g of trisodium citrate dihydrate and 0.54g of anhydrous calcium chloride; and 1 mol/L of

NaOH was added dropwise, so as to adjust a pH value of a mixed system to 6.8. 4.0g of glucono-delta-lactone was weighed and dissolved in 20 mL of the deionized water; vortex was performed for 10s, and the mixture was immediately added to the mixed system; and full stirring was performed for 20s, and then a gel solution was prepared.

The mass concentration of the sodium caseinate in a gel system was 6%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Rheological analysis was performed on the gel solution; and the gel solution was placed in a 42°C constant temperature incubator for reaction for 4h, so as to form the gel.

Comparative example 3 (0.4% Alg-CN/Ca?*) gel 9.60g of sodium caseinate was weighed and placed in 51 mL of deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium caseinate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 16%(w/w) sodium caseinate solution. 0.64g of sodium alginate was weighed and placed in 60 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 64g, so as to prepare a 1.0%(w/w) sodium alginate solution.

The sodium alginate solution and the sodium caseinate solution were mixed; constant temperature stirring was continuously performed at 70°C for 1 hour, then the deionized water was replenished until a system mass was 120g, and 9 mL of the deionized water was quickly added at 70°C to dissolve a mixed solution that was prepared by 2.38g of trisodium citrate dihydrate and 0.54g of anhydrous calcium chloride; and 1 mol/L of NaOH was added dropwise, so as to adjust a pH value of a mixed system to 6.8. 4.0g of glucono-delta-lactone was weighed and dissolved in 20 mL of the deionized water; vortex was performed for 10s, and the mixture was immediately added to the mixed system; and full stirring was performed for 20s, and then a gel solution was prepared.

The mass concentration of the sodium caseinate in a gel system was 6%, the mass concentration of the sodium alginate was 0.4%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

Comparative example 4 (0.3%Alg/Ca’" gel) 0.48g of sodium alginate was weighed and placed in 60 mL of the deionized water; constant temperature stirring was performed at 70°C for 4h, until the sodium alginate was completely hydrated; and the deionized water was replenished to make up to 60g, so as to prepare a 0.8%(w/w) sodium alginate solution.

The sodium alginate solution and the deionized water of equal mass were mixed; constant temperature stirring was continuously performed at 70°C for 1 hour, then the deionized water was replenished until a system mass reached 120g, and 13 mL of the deionized water was quickly added at 70°C to dissolve a mixed solution that was prepared by 2.38g of trisodium citrate dihydrate and 0.54g of anhydrous calcium chloride; and 1 mol/L of

The mass concentration of the sodium alginate in a gel system was 0.3%, the concentration of sodium citrate was 50 mmol/L, the concentration of calcium chloride was 30 mmol/L, and the mass fraction of the glucono-delta-lactone was 2.5%.

The gel solution was placed in a 42°C constant temperature incubator for fermentation for 4h, so as to form the gel.

II Performance verification of casein-calcium alginate gel (1) Gel appearance

The appearance of the gel prepared in Embodiment 1 to Embodiment 3 and the gel prepared in Comparative example 1 to Comparative example 3 was recorded; microstructures of the gel prepared in Embodiment 1 to Embodiment 3 were shown in Fig. 1, Fig. 2 and Fig. 3; and the appearance of the gel prepared in Comparative example 1 to Comparative example 3 was shown in Fig. 4, Fig. 5 and Fig. 6. Specific analysis was as follows.

In Comparative example 1, specifically referring to Fig. 4, in the absence of Ca’, electrostatic repulsion existed between sodium caseinate and the sodium alginate; and the sodium caseinate and the sodium alginate were poor in compatibility, resulting in poor gel water-holding capacity and small appearance, and water was easily precipitated.

In Embodiment 1 to Embodiment 3, specifically referring to Fig. 1, Fig. 2 and Fig. 3, respectively, calcium ions released by calcium citrate crosslinked with phosphate residues in the sodium caseinate, while the acidification sustained-release agent induced slow acidification of a sodium caseinate-sodium alginate dispersion system to an isoelectric point of the casein, so as to form the casein gel, the casein gradually released partial Ca?*, which may ligate with -COO- of the sodium alginate to form calcium alginate gel, such that casein-calcium alginate interpenetrating dual network gel was formed during acidization. That is to say, when a pH value is reduced to 4.6, the sodium alginate and the sodium caseinate formed more compact dual network gel under the action of the Ca?”

In addition, when there was no sodium alginate, it was a mixed system of the sodium caseinate and CaCl:. In the gel formed by the system, only the calcium ions crosslinked with the sodium caseinate, such that it was not the dual network gel (see Fig. 11), and therefore, both the hardness and water-holding capacity of the system gel did not reach an optimal state (see Fig. 5). The sodium alginate was negatively charged; the sodium caseinate was partially positively charged when the pH value reduced to an isoelectric point of the sodium caseinate; and when the concentration of the sodium alginate in the system was too high, and the sodium caseinate and the calcium ions could not completely neutralize the negative charge of the sodium alginate, there was a phase separation between the sodium alginate and the sodium caseinate (see Fig. 12), resulting in slight reduction of the hardness and the water-holding capacity (see Fig. 6). (2) Microstructure of gel

The microstructures of the gel prepared in Embodiment 1 to Embodiment 3 and the gel prepared in Comparative example 1 to Comparative example 3 were observed; the microstructures of the casein-calcium alginate gel prepared in Embodiment 1 to Embodiment 3 were shown in Fig. 7, Fig. 8 and Fig. 9; and the microstructures of the gel prepared in

Comparative example 1, Comparative example 2 and Comparative example 3 were shown in

Fig. 10, Fig. 11 and Fig. 12. Comparative analysis was as follows.

From Embodiment 1 to Embodiment 3 and Comparative example 3, it may be learned that, specifically referring to Fig. 7, Fig. 8, Fig. 9 and Fig. 12, in the presence of the Ca?*, the sodium alginate and the sodium caseinate were interpenetrated with each other, and obvious crosslinking occurred, so as to form the dual network structure gel. With the increasing of the concentration of the sodium alginate, holes were gradually reduced, this was because when the low-concentration sodium alginate was added, after sites of intermolecular coordination of the sodium alginate were occupied by the Ca?*, the high-concentration Ca?’ might cause the sodium alginate to be further subjected to intramolecular coordination on the basis of intermolecular coordination, and the molecular chain of the sodium alginate was tangled by means of intramolecular coordination and intermolecular coordination together, resulting in large pore sizes. When the high-concentration sodium alginate was added, the Ca?* only neutralized surface charge of sodium alginate molecules, facilitating double-layer compression and tighter aggregation of the sodium alginate, such that a large-size polymer was formed. In Embodiment 3, specifically referring to Fig. 9, a mutually-interpenetrated dual network structure formed by the sodium alginate and the sodium caseinate was the most compact. However, in Comparative example 3, referring to Fig. 12, when the concentration of the sodium alginate was too high, there was a phase separation between the sodium alginate and the sodium caseinate, breaking the compactness between the sodium alginate and the sodium caseinate. (3) Rheological analysis of gel solution

Rheological analysis results of a gel solution prepared in Embodiment 1 to Embodiment 3 were shown in Fig. 13, Fig. 14 and Fig. 15; and rheological analysis results of a gel solution prepared in Comparative example 1 to Comparative example 3 were shown in Fig. 16, Fig. 17 and Fig. 18. Specific analysis was as follows.

At the beginning of fermentation of the gel system, G” was higher than G’, and in this case, the gel system was in a liquid state; and as time goes by, G' showed a greater growth trend than G”, and the intersection of the gel was formed. In Comparative example 1, specifically referring to Fig. 16, a gel point was at about 1868s, this was because, in the absence of Ca2+, both the sodium alginate and the sodium caseinate were negatively charged, and there was a phase separation, resulting in a relatively backward gel point. In Comparative example 1 (see Fig. 16), Comparative example 2 (see Fig. 17), Embodiment 1 (see Fig. 13),

Embodiment 2 (see Fig. 14), Embodiment 3 (see Fig. 15) and Comparative example 3 (see

Fig. 18), as the addition of the sodium alginate increased, the gel point gradually moved forward, this was because, as the concentration of the sodium alginate increased, the viscosity of the gel system was increased, and also because, under the action of the Ca?*, the sodium alginate and the sodium caseinate were promoted to form the dual network structure, so as to achieve quick gelation of the gel system. (4) Water-holding capacity and hardness of gel

The water-holding capacity and hardness of the gel prepared in Embodiment 1 to

Embodiment 3 and Comparative example 1 to Comparative example 3 were detected; a water-holding capacity detection result was shown in Fig. 19; and a hardness detection result was shown in Fig. 20. Specific analysis was as follows.

With the increasing of the concentration of the sodium alginate, the water-holding capacity and hardness of the gel both showed a trend of first increasing and then decreasing; and the water-holding capacity and hardness of a Ca*" group were obviously higher than that of a Ca?’-free group. Since the sodium alginate and the sodium caseinate had conditions to form a bi-continuous phase, the Ca?” facilitates the formation of a dual network structure of the sodium alginate and the sodium caseinate gel. Therefore, in the gel system prepared in

Embodiment 3, when the concentration of the sodium alginate was 0.3% and the concentration of the Ca?’ was 30 mmol/L, the gel showed optimal water-holding capacity and hardness. Compared with the sodium alginate-sodium caseinate bi-continuous phase gel in the absence of the Ca?” in Comparative example 1, the water-holding capacity of the gel in

Embodiment 3 was increased by approximately 36%, and the hardness was increased by approximately 68%; and compared with the sodium caseinate-calcium ion gel in the absence of the sodium alginate-free in Comparative example 2, the water-holding capacity of the gel in Embodiment 3 was increased by approximately 44%, and the hardness was increased by approximately 265%.

Claims

CONCLUSIONS

1. Casein gel, characterized in that the raw materials contain casein and/or casein sodium salt, sodium alginate, calcium ion chelating agent and a sustained-release acidifying agent.

Casein gel according to claim 1, characterized in that the mass ratio of the casein and/or casein sodium salt to sodium alginate is 6: (0.1-1.0), and the mass concentration of the sodium alginate is 0.1% -1.0 % is.

Casein gel according to claim 1 or 2, characterized in that the calcium ion chelating agent is formed by the chelation of calcium salt and a chelating agent, wherein the calcium salt is selected from calcium chloride or calcium gluconate, and the chelating agent is selected from sodium citrate, sodium acetate or sodium hexametaphosphate.

Casein gel according to claim 1 or 2, characterized in that the mass concentration of the calcium ion chelator is 25mmol/L - 35mmol/L.

5. Casein gel according to claim 3, characterized in that the mass concentration of the calcium ion chelating agent is 25mmol/L - 35mmol/L.

Casein gel according to claim 1 or 2, characterized in that the sustained release acidifying agent is selected from L-malic acid, gluconic acid-ò-lactone or stem.

Casein gel according to claim 3, characterized in that the sustained release acidifying agent is selected from L-malic acid or gluconic acid-ò-lactone or strain.

Method for preparing casein gel according to any one of claims 1 to 7, characterized in that the method comprises: mixing the casein and/or casein sodium salt solution with a sodium alginate solution to obtain a mixed solution; mixing the mixed solution with the calcium ion chelating agent and the sustained release acidifying agent to effect a reaction; carrying out the reaction until a pH of 3.5-4.6 is obtained to obtain the casein gel.

A method for preparing casein gel according to claim 8, characterized in that the reaction is carried out at 40°C-44°C.

Use of casein gel according to any one of claims 1 to 7 in the preparation of fermented milk.