A coloring method for bean products

Abstract

The present invention provides a method for coloring soy products, which comprises: heating and dissolving eumelanin and then adding auxiliary factors to prepare a composite coloring liquid. Adjusting the pH of the composite coloring liquid and heating it to a suitable temperature, placing the material to be colored after washing and draining the water, and soaking it in a heat-insulating state for 10 to 70 minutes until the coloring effect is achieved, thereby completing the coloring. The coloring method of the present invention uses eumelanin as a coloring pigment, and achieves different coloring effects by regulating the type of auxiliary factors, the pH value of the composite coloring liquid, the coloring temperature and time. The colored soy products can appear brown, reddish brown or ink black, meeting the coloring effect requirements of different soy products. The soy products colored by the method of the present invention not only have stable color, but also have improved water retention. The eumelanin used in the present invention is safe for human use and has biological activities such as anti-oxidation, free radical scavenging and anti-radiation, which are beneficial to human health.

Description

Technical Field

[0001] The present invention relates to a method for coloring bean products, and in particular to a method for coloring bean products including dried tofu, tofu blanks, tofu skins and the processed bean products using eumelanin as a main raw material. Background Art

[0002] Soy products are rich in nutrients such as plant protein, dietary fiber, and polyunsaturated fatty acids, making them a health food recommended by nutritionists. There are many varieties of soy products, including tofu, dried tofu, and dried bean curd sheets. Tofu can be further processed into fermented tofu and stinky tofu, while dried bean curd sheets can be made into vegetarian sausages, vegetarian meat, and other soy products in a variety of shapes and flavors. These soy products are popular with consumers due to their low calorie and high protein content. Some soy products, such as stinky tofu, dried tofu, vegetarian sausages, and vegetarian meats, require coloring during production to achieve the desired flavor. For example, Changsha stinky tofu, recognized by consumers for its "inky black color, crispy on the outside, and tender on the inside," has become a nationally renowned classic snack. Dried tofu is processed to a tan color, while vegetarian sausages and vegetarian meats are typically reddish-brown.

[0003] The current coloring process for Changsha stinky tofu involves soaking the tofu in a FeSO4 solution, adding tea extract to enhance the color, and then soaking it in brine to produce the inky black stinky tofu. This process results in unstable color and easily oxidizes and fades when exposed to air. Furthermore, the black substance produced by the reaction of FeSO4 with tea extract or brine is indigestible, posing a safety risk. Some consumers have also experienced allergic reactions such as itching and rashes after consuming foods containing ferrous sulfate. Furthermore, the current stinky tofu production process also poses a risk because the raw tofu loses water during transportation and storage, causing it to dry out internally. This prevents the tofu from achieving its characteristic "cooked on the outside, tender on the inside" quality after frying.

[0004] Existing coloring processes for soy products like dried tofu and vegetarian sausage typically use soy sauce or red yeast rice pigment to achieve the desired color. However, since red yeast rice pigment is unstable to both natural light and high temperatures, simply adding red yeast rice pigment alone cannot meet the processing requirements. Large amounts of soy sauce are often required to achieve the desired color. However, the excessive use of soy sauce results in a high salt content and a strong flavor, which does not meet the healthy needs of modern people for a light diet.

[0005] In order to explore the coloring process of stinky tofu that can replace FeSO4 coloring technology, Lei Ziyi et al. (Lei Ziyi, Jiang Liwen, Huang Feng, et al. Optimization of process conditions for dyeing white tofu with euphorbia pulverata leaves [J]. Hunan Agricultural Sciences, 2019(06):90-93.) studied the coloring effect of the juice obtained by filtering the euphorbia pulverata leaves after adding water and mashing on the white tofu blank used for making stinky tofu. Under the optimal process conditions, that is, the euphorbia pulverata leaf juice with a material-liquid ratio of 1:4 for white tofu was dyed in a constant temperature water bath at 80 ℃ for 75 minutes, the dyeing effect reached the best. The color of the dyed tofu was close to ink black, which provided a reference for the development of a new stinky tofu dyeing method. However, a study by Lu Zhijuan et al. (Lu Zhijuan, Zhou Bing, Li Yinghua. Optimization of water extraction process of black rice components from Candida altissima leaves and study on environmental stability of pigments [J]. Anhui Agricultural Sciences, 2014, 42(02): 367-368+394.) showed that the coloring components in Candida altissima leaf extracts are unstable under heat and alkaline conditions. Since stinky tofu is soaked in brine and fried at high temperature during the later processing, the pH of the brine is alkaline, and the frying temperature is far above 100°C, which will lead to color loss in Candida altissima leaf-dyed stinky tofu. In addition, Candida altissima leaf resources are limited, and the picking process must be entirely manual, which cannot meet the needs of industrial large-scale production.

[0006] CN202010602516.9 provides a method for coloring stinky tofu blanks using a melanin fermentation product of Eurotium cristatum. However, the preparation process of the melanin fermentation product of Eurotium cristatum used in this method requires steps such as strain activation, expansion culture, and fermentation preparation of the fermentation product. The production cycle is long, and the fermentation preparation of the fermentation product alone requires more than 5 days. In addition, the fermentation process is prone to infection with miscellaneous bacteria, and industrial large-scale production requires high investment and high risk. Moreover, the melanin fermentation product of Eurotium cristatum is a complex mixture, and its components are still unclear, making it difficult to demonstrate safety.

[0007] Eumelanin is a macromolecular polymer produced by the oxidative polymerization of tyrosine under the action of tyrosinase. It is widely found in animal and plant food raw materials such as black-bone chicken, black-bone sheep, cuttlefish, black sesame, and fungi such as black fungus and shiitake mushrooms. Its structural formula is:

[0008]

[0009] Eumelanin can be extracted from plant and animal sources or synthesized enzymatically. Its wide availability, short production cycles, and high efficiency make it suitable for large-scale industrial production. Many plants and animals containing eumelanin are traditional foods, and their safety is widely recognized. Enzymatically synthesized eumelanin has the same composition as that found in plant and animal food sources, complying with the principle of "substantial equivalence." Therefore, its safety can be treated the same as that of eumelanin from plant and animal sources. Compared to plant and animal sources, enzymatic synthesis makes it easier to obtain high-purity eumelanin, resulting in high yields, low costs, and a broader market prospect. Despite its well-defined composition, high safety, and widespread availability, eumelanin's application in food coloring is rare, and its use in soy products has been reported to be particularly unseen. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to address the above-mentioned shortcomings in the prior art and provide a method for coloring soy products. This method uses eumelanin as the coloring pigment. By selectively adding different cofactors and adjusting the pH of the coloring solution, the coloring temperature, and the coloring time, the soy products can be colored to the corresponding tan, reddish-brown, or inky black color, depending on the characteristic color of the soy product. This method is highly safe and meets the needs of a healthy diet. Furthermore, the coloring process is fast and easy to operate. Not only does the coloring maintain a stable color, but it also helps improve the water retention of the soy products, preventing them from losing water and drying out.

[0011] The technical solution adopted to solve the technical problem of the present invention is:

[0012] A method for coloring soy products, characterized by comprising the following steps:

[0013] (1) Preparation of eumelanin solution: add eumelanin to water, adjust the pH value to 7.5-13.0 with alkaline solution, and heat and stir to promote the dissolution of eumelanin to obtain eumelanin solution;

[0014] (2) Preparation of composite coloring solution: Add the auxiliary factor to the eumelanin solution, mix well to obtain the composite coloring solution, and control the eumelanin concentration in the composite coloring solution to be 0.05-0.5 g / 100 mL;

[0015] (3) Adjusting the pH value of the composite coloring solution: Adjust the pH value of the composite coloring solution to 5.5-12.0;

[0016] (4) Pretreatment of the soy products to be colored: soak the soy products to be colored in clean water for 5 to 30 minutes, remove them after washing, and drain the water;

[0017] (5) Coloring of soy products: Place the soy products pretreated in step (4) into the composite coloring liquid, completely immerse the soy products in the coloring liquid, and color them at a coloring temperature of 40°C to 70°C for 10 min to 70 min. After achieving the desired coloring effect, remove the colored material, drain the water, and quickly cool it to complete the coloring.

[0018] Preferably, in step (2), the auxiliary factor is one or more of polyethylene glycol (PEG), sorbitol, maltitol, and potassium sorbate.

[0019] Preferably, different auxiliary factors are selected in step (2), the pH of the composite coloring liquid is controlled in step (3), and the coloring temperature and coloring time are controlled in step (5) so that different types of soy products are colored with corresponding characteristic colors.

[0020] Preferred:

[0021] When coloring the stinky tofu blank, PEG and maltitol are selected as auxiliary factors in step (2), the pH of the composite coloring liquid is controlled to be 8-12 in step (3), and the coloring temperature is controlled to be 45-55°C and the coloring time is controlled to be 20-70 minutes in step (5), so that the stinky tofu blank is colored in a characteristic color, ink black;

[0022] When coloring the dried bean curd, sorbitol and potassium sorbate are selected as auxiliary factors in step (2), the pH of the composite coloring solution is controlled to be 7-10 in step (3), and the coloring temperature is controlled to be 45-55°C and the coloring time is controlled to be 10-30 minutes in step (5), so that the dried bean curd is colored in a characteristic yellow-brown color.

[0023] When coloring the bean curd skin, sorbitol is selected as an auxiliary factor in step (2), the pH of the composite coloring solution is controlled to be 5.5-7.0 in step (3), and the coloring temperature is controlled to be 40-50°C and the coloring time is controlled to be 30-50 minutes in step (5), so that the bean curd skin is colored in the characteristic reddish brown color.

[0024] Preferably, in step (1), the alkaline solution is a Na2CO3 solution.

[0025] Preferably, when PEG, sorbitol, maltitol or potassium sorbate is selected, their usage is calculated based on 100 mL of the composite coloring solution as follows: PEG 0.2-1.2 g; sorbitol 1.0-10.0 g; maltitol 1.0-10.0 g; potassium sorbate 0.05-0.1 g.

[0026] Preferably, the PEG is PEG6000 or PEG1000.

[0027] Preferably, when the selected auxiliary factor includes PEG, the PEG is first dissolved in an appropriate amount of hot water or ultrasonically assisted dissolution, and then the dissolved PEG is added to the eumelanin solution prepared in step (1).

[0028] Preferably, in step (3), Na2CO3 solution or HCl solution is used to adjust the pH of the composite coloring solution.

[0029] Preferably, the soy products include dried tofu, tofu blanks, tofu skins and soy products formed after further processing thereof.

[0030] 1) The present invention uses eumelanin as a coloring pigment and adds auxiliary factors to prevent the colored dried tofu, tofu, tofu skin, and stinky tofu, vegetarian sausage, and other soy products made from them after being processed from them from oxidation when exposed to air. At the same time, the color after coloring is also relatively stable to light and heat. The subsequent soaking, steaming, frying, seasoning, and other processing of the colored materials will not affect the coloring effect of the present invention. In addition, the applicant unexpectedly discovered that by adding auxiliary factors, not only can the soy products be colored, but the water retention of the colored soy products can also be increased to reduce the water loss of the products during transportation or storage, which is conducive to maintaining the quality characteristics of the stinky tofu after frying, which is "cooked on the outside and tender on the inside". In summary, the present invention not only solves the technical problems that black stinky tofu colored by existing processes will oxidize and fade when exposed to air, and that vegetarian sausages and soy meat colored with red yeast rice pigments are easily faded under heating or light conditions, but also solves the problem of soy products drying out due to water loss.

[0031] 2) In the present invention, if an inappropriate alkaline solution (such as NaHCO3) is used to adjust the pH, the skin of the stinky tofu will fall off and break after frying. However, the applicant unexpectedly discovered that by using Na2CO3 instead of other alkaline solutions to adjust the pH in steps (1) and (3), the shape of the stinky tofu blocks after coloring can be kept intact after frying, thereby improving the sensory quality of the stinky tofu after frying.

[0032] 3) The present invention uses only one coloring pigment, eumelanin, and a set of process schemes. By selecting appropriate auxiliary factors and regulating factors such as the pH value, coloring temperature, and coloring time of the composite coloring liquid, the present invention can meet the coloring requirements of the yellow-brown, red-brown, and ink-black characteristic colors corresponding to the raw materials of traditional soy products such as dried tofu, tofu skin, and stinky tofu, without changing the coloring pigment or replacing the process scheme. This solves the problem that in the existing soy product coloring process, in order to achieve different color effects, multiple substances such as FeSO4, tea extract, red yeast rice pigment, soy sauce, etc. need to be used in combination with different process schemes, and there are problems such as a large number of raw materials required and cumbersome operations.

[0033] 4) The eumelanin used in this invention is found in natural food ingredients such as cuttlefish, silkie chicken, and black fungus. Its composition is well-defined and highly safe, addressing the safety concerns associated with using FeSO₄ to color stinky tofu. The eumelanin used in this invention has no inherent taste or odor, and does not affect the flavor of the product. The coloring method of this invention can be used not only for soy products but also for coloring gluten, cereals, and eggs. DETAILED DESCRIPTION

[0034] The present invention is further described below in conjunction with specific embodiments. It should be pointed out that the following embodiments are merely illustrative of the present invention in the form of examples, but the scope of protection of the present invention is not limited thereto. All equivalent replacements made by those skilled in the art to the present invention in the spirit of the present invention fall within the scope of protection of the present invention. Example 1

[0035] A method for coloring a soy product comprises the following steps:

[0036] (1) Preparation of eumelanin solution: eumelanin is added to water, and the pH value is adjusted to 7.5-13.0 with an alkaline solution, while heating and stirring to promote the dissolution of eumelanin to obtain a eumelanin solution; the alkaline solution is a Na2CO3 solution with a concentration of 1-2 mol / L;

[0037] (2) Preparation of composite coloring solution: Add the auxiliary factor to the eumelanin solution, mix well to obtain the composite coloring solution, and control the eumelanin concentration of the composite coloring solution to be 0.05-0.5 g / 100 mL;

[0038] Wherein, the auxiliary factor is selected from one or more of PEG, sorbitol, maltitol, and potassium sorbate; when PEG, sorbitol, maltitol or potassium sorbate are selected, their dosages are calculated based on 100 mL of the composite coloring solution as follows: PEG 0.2-1.2 g; sorbitol 1.0-10.0 g; maltitol 1.0-10.0 g; potassium sorbate 0.05-0.1 g; the PEG is PEG6000 or PEG10000; when the auxiliary factor selected includes PEG, the PEG is first dissolved in an appropriate amount of hot water or ultrasonically assisted, and then the dissolved PEG is added to the eumelanin solution prepared in step (1);

[0039] (3) Adjusting the pH value of the composite coloring solution: adjusting the pH value of the composite coloring solution to 5.5-12.0; wherein, a Na2CO3 solution with a concentration of 1-2 mol / L or a HCl solution with a concentration of 2.0-5.0 mol / L is used to adjust the pH value of the composite coloring solution;

[0040] (4) Pretreatment of the soy products to be colored: soak the soy products to be colored in clean water for 5 to 30 minutes, remove them after washing, and drain the water;

[0041] (5) Coloring of soy products: Place the soy products pretreated in step (4) into the composite coloring liquid, completely immerse the soy products in the coloring liquid, and color them at a coloring temperature of 40°C to 70°C for 10 min to 70 min. After achieving the desired coloring effect, remove the colored material, drain the water, and quickly cool it to complete the coloring;

[0042] Based on the above steps, when coloring different types of soy products, different auxiliary factors can be selected in step (2), the pH of the composite coloring solution can be controlled in step (3), and the coloring temperature and coloring time can be controlled in step (5) to color different types of soy products with corresponding characteristic colors. Specifically:

[0043] When coloring the stinky tofu blank, PEG and maltitol are selected as auxiliary factors in step (2); in step (3), the pH of the composite coloring liquid is controlled to be 8-12, preferably 8.5; in step (5), the coloring temperature is controlled to be 45-55°C and the coloring time is controlled to be 20-70 min, preferably 50°C and the coloring time is controlled to be 30 min, so that the stinky tofu blank is colored in a characteristic color, ink black;

[0044] When coloring the dried bean curd, sorbitol and potassium sorbate are selected as auxiliary factors in step (2); in step (3), the pH of the composite coloring solution is controlled to be 7 to 10, preferably 9; in step (5), the coloring temperature is controlled to be 45 to 55°C and the coloring time is controlled to be 10 to 30 minutes, preferably 60°C and the coloring time is controlled to be 15 minutes, so that the dried bean curd is colored in a characteristic yellow-brown color;

[0045] When coloring the tofu skin, sorbitol is selected as an auxiliary factor in step (2); in step (3), the pH of the composite coloring liquid is controlled to be 5.5-7.0, preferably 6.0; in step (5), the coloring temperature is controlled to be 40-50°C and the coloring time is controlled to be 30-50 min, preferably 45°C and the coloring time is controlled to be 40 min, so that the characteristic color of the stinky tofu raw material is reddish brown. Example 2

[0046] Coloring of stinky tofu blanks: The eumelanin in this example was synthesized by Hunan Fulaige Biotechnology Co., Ltd. using an enzymatic method, the white tofu blanks and stinky tofu brine used were provided by Hunan Huashuo Food Co., Ltd., and the other reagents were all food-grade commercially available reagents.

[0047] The implementation of this embodiment includes the following steps:

[0048] (1) Preparation of eumelanin solution: Weigh 0.5 g of eumelanin, put it into 70 mL of water, slowly add 1 mol / L Na2CO3 solution, while heating and stirring, and adjust the pH value of the solution to 10.0 to promote the dissolution of eumelanin.

[0049] (2) Preparation of composite coloring solution: Weigh 0.75 g of PEG6000 and add it to 20 mL of hot water at 80 °C. Ultrasonicate for 15 min to dissolve the PEG. Add the dissolved PEG solution to the melanin solution prepared in step (1). Then add 2.5 g of maltitol and adjust the total volume of the solution to 100 mL. Stir magnetically at 40 °C and 600 r / min for 20 min to prepare the composite coloring solution.

[0050] (3) Adjust the pH value of the composite coloring solution. Use 1 mol / L Na2CO3 or 2 mol / L HCl solution to adjust the pH value of the composite coloring solution to 8.5.

[0051] (4) Pretreatment of soy products to be colored: Soak the white stinky tofu to be colored in clean water for 5 minutes, remove it and drain the water.

[0052] (5) Coloring of soy products: The composite coloring liquid after pH adjustment in step (3) is heated to 50°C, and the tofu blank treated in step (4) is placed in the liquid. The tofu blank is kept warm and soaked at 50°C for 30 minutes. The white tofu blank turns into ink black, and the coloring is completed. The color difference between the stinky tofu blank after coloring in this embodiment and the commercially available stinky tofu blank is less than 0.5, and there is no obvious color difference between the two when observed with the naked eye.

[0053] After being colored, the stinky tofu dough is soaked in 50°C stinky tofu brine for 40 minutes to create the raw stinky tofu. It is then deep-fried and seasoned with chili sauce, cilantro, and other ingredients to taste, resulting in the finished stinky tofu, which is "inky black, crispy on the outside, and tender on the inside." Example 3

[0054] This example is a comparative example, comparing the changes in color and water loss rate of the stinky tofu dough prepared in Example 2 with commercially available stinky tofu dough colored using existing technology after being exposed to air for different periods of time.

[0055] The implementation steps of this embodiment are:

[0056] (1) The stinky tofu dough prepared in Example 2 and the commercially available stinky tofu dough were placed in clean open magnetic disks and allowed to stand at room temperature for 24 h.

[0057] (2) Observe the color changes of the stinky tofu dough prepared in Example 2 and the commercially available stinky tofu dough after standing for 4 h, 6 h, and 8 h, and record the color changes.

[0058] (3) Weigh the weights of the two tofu doughs at the beginning of the resting period, after 12 h of resting period, and after 24 h of resting period. Calculate the water loss rate using the following formula:

[0059] Water loss rate = ( W 1 - W 2 ) / W 1 ×100%

[0060] Where, W 1 is the weight of the raw tofu at the beginning of standing, W 2 It is the weight of the raw tofu after being left standing for a certain period of time.

[0061] The results recorded and measured in this embodiment are shown in Table 1.

[0062] Table 1 Comparison of color stability and water loss rate between the stinky tofu green body prepared in Example 2 and the commercially available stinky tofu green body

[0063]

[0064] As shown in Table 1, in terms of color change, the commercially available stinky tofu green dough prepared using existing techniques almost completely loses its black color and turns off-white after being exposed to air for 8 hours. However, the stinky tofu green dough prepared in Example 2 of the present invention maintains a stable color and does not fade under the same conditions. Regarding water loss, the commercially available stinky tofu green dough has poor water retention, with water loss rates of 34.8% and 61.2% after 12 hours and 24 hours of air exposure, respectively. The stinky tofu green dough prepared in Example 2 of the present invention has significantly improved water retention, with water loss rates of only 12.4% and 29.3% after 12 hours and 24 hours of air exposure, respectively. This effectively maintains the freshness and tenderness of the stinky tofu green dough, thereby maintaining a crispy exterior and tender interior after frying. Example 4

[0065] Coloring of Tofu Skin: In this example, two types of eumelanin were used: commercially available cuttlefish eumelanin and enzymatically synthesized eumelanin from Hunan Fulaige Biotechnology Co., Ltd. The tofu skin used for coloring was provided by Hunan Huashuo Food Co., Ltd., and all other reagents were food-grade commercially available reagents.

[0066] The implementation of this embodiment includes the following steps:

[0067] (1) Preparation of eumelanin solution: Weigh 0.2 g of each of the two eumelanins and place them in two beakers containing 100 mL of water. While heating and stirring, add 1 mol / L Na2CO3 solution dropwise. Adjust the pH value of the melanin solution in both beakers to 9.5 to allow the eumelanin to fully dissolve.

[0068] (2) Preparation of composite coloring solution: Weigh 2 parts of 4.0 g of sorbitol and add them to the two eumelanin solutions prepared in step (1), respectively. Ultrasonic dissolution treatment is performed for 15 min, and then magnetic stirring is performed at 45 °C and 600 r / min for 15 min to prepare a composite coloring solution of two eumelanin.

[0069] (3) Adjust the pH value of the composite coloring solution: Use 1 mol / L Na2CO3 or 2 mol / L HCl solution to adjust the pH value of both composite coloring solutions to 6.0.

[0070] (4) Soak the tofu skin to be colored in clean water for 10 minutes, remove it and drain the water.

[0071] (5) Coloring of soy products: Heat the two composite coloring solutions after pH adjustment in step (3) to 45°C, add equal weights of bean curd skin treated in step (4) to each solution, and soak at 45°C for 40 min. The color of the bean curd skin in the two composite coloring solutions turns into a reddish brown color close to that of the sausage, and the coloring is completed.

[0072] The tofu skin colored in this embodiment can be used to make vegetarian sausages, vegetarian meat, and other products as needed. Compared to vegetarian sausages colored with red yeast rice pigment, the vegetarian sausages made with tofu skin colored in this embodiment have a more rosy color, do not fade under natural light, and maintain a stable color even after steaming at 80-100°C. Example 5

[0073] Coloring of Dried Tofu: The eumelanin in this example was synthesized by Hunan Fulaige Biotechnology Co., Ltd. using an enzymatic method, and the white dried tofu used for coloring was provided by Hunan Huashuo Food Co., Ltd. Other reagents were all food-grade commercially available reagents.

[0074] The implementation of this embodiment includes the following steps:

[0075] (1) Preparation of eumelanin solution: Weigh 0.08 g of eumelanin and put it into 100 mL of water. Slowly add 1 mol / L Na2CO3 solution while heating and stirring. Adjust the pH value of the solution to 8.0 to promote the dissolution of melanin.

[0076] (2) Preparation of composite coloring solution: Weigh 2.0 g of sorbitol and 0.06 g of potassium sorbate, add them to the eumelanin solution prepared in step (1), and use ultrasonic dissolution treatment for 20 min to prepare a composite coloring solution.

[0077] (3) Adjust the pH value of the composite coloring solution: Use 1 mol / L Na2CO3 or 2 mol / L HCl solution to adjust the pH value of the composite coloring solution to 9.0.

[0078] (4) Pretreatment of bean products to be colored: Soak the white dried bean curd to be colored in clean water for 8 minutes, remove and drain the water.

[0079] (5) Coloring of soy products: Heat the composite coloring liquid treated in step (3) to 60°C, add the white dried tofu treated in step (4), and soak at 60°C for 15 minutes. The dried tofu will be dyed into the yellow-brown color characteristic of dried tofu, and the coloring is completed.

[0080] The dried tofu colored by this embodiment has a stable color and only needs to be marinated for a short time during the later processing, without the need to add soy sauce or other seasonings to assist in coloring. It can be used to produce low-salt dried tofu products, which are suitable for the consumption needs of people who pursue a low-salt healthy diet. Example 6

[0081] This example is a comparative example of Example 2, and illustrates the role of auxiliary factors in coloring by comparison. Except that the auxiliary factors PEG and maltitol are not added in step (2), the other steps of this example are the same as those of Example 2.

[0082] The eumelanin in this example was synthesized by Hunan Fulaige Biotechnology Co., Ltd. using an enzymatic method, and other reagents were all food-grade commercially available reagents.

[0083] The implementation of this embodiment includes the following steps:

[0084] (1) Preparation of eumelanin solution: Weigh 0.5 g of eumelanin, put it into 70 mL of water, slowly add 1 mol / L Na2CO3 solution, while heating and stirring, and adjust the pH value of the solution to 10.0 to promote the dissolution of eumelanin.

[0085] (2) Preparation of composite coloring solution: The total volume of the solution in step (1) was fixed to 100 mL, and magnetic stirring was performed at 40 °C and 600 r / min for 20 min to ensure that the eumelanin solution was evenly dissolved.

[0086] (3) Adjust the pH value of the composite coloring solution: Use 1 mol / L Na2CO3 or 2 mol / L HCl solution to adjust the pH value of the eumelanin solution to 8.5.

[0087] (4) Pretreatment of soy products to be colored: Soak the white stinky tofu to be colored in clean water for 5 minutes, remove it and drain the water.

[0088] (5) Coloring of soy products: Heat the eumelanin solution treated in step (3) to 50°C, add the tofu blank treated in step (4), and soak at 50°C for 30 minutes. The color of the tofu blank is brown, and it does not become its characteristic color.

[0089] This example demonstrates that cofactors play an important role in regulating the coloring effect of eumelanin on the stinky tofu. The cofactors employed in the present invention are essential for achieving both the coloring and moisture retention effects of the soy product.

Claims

1. A method for coloring soy products, characterized in that The steps include: (1) Add eumelanin to water, adjust the pH value to 7.5-13.0 with an alkaline solution, and heat and stir to promote the dissolution of eumelanin to obtain a eumelanin solution; (2) Add the auxiliary factor to the eumelanin solution, mix well to obtain a composite coloring solution, and control the eumelanin concentration in the composite coloring solution to be 0.05-0.5 g / 100 mL; (3) Adjust the pH value of the composite coloring solution to 5.5-12.0; (4) Soak the soy products to be colored in clean water for 5 to 30 minutes, remove them after washing, and drain the water; (5) Place the soy product pretreated in step (4) into the composite coloring liquid, completely immerse the soy product in the coloring liquid, and color it at a coloring temperature of 40°C to 70°C for 10 min to 70 min. After achieving the desired coloring effect, remove the colored material, drain the water, and quickly cool it to complete the coloring; in: When coloring the stinky tofu blank, polyethylene glycol and maltitol are selected as auxiliary factors in step (2), the pH of the composite coloring liquid is controlled to be 8 to 12 in step (3), and the coloring temperature is controlled to be 45 to 55° C. and the coloring time is controlled to be 20 to 70 minutes in step (5), so that the stinky tofu blank is colored in a characteristic color, ink black; When coloring the dried bean curd, sorbitol and potassium sorbate are selected as auxiliary factors in step (2), the pH of the composite coloring solution is controlled to be 7 to 10 in step (3), and the coloring temperature is controlled to be 45 to 55° C. and the coloring time is controlled to be 10 to 30 minutes in step (5), so that the dried bean curd is colored in a characteristic yellow-brown color. When coloring the bean curd skin, sorbitol is selected as an auxiliary factor in step (2), the pH of the composite coloring solution is controlled to be 5.5-7.0 in step (3), and the coloring temperature is controlled to be 40-50°C and the coloring time is controlled to be 30-50 min in step (5), so that the bean curd skin is colored in the characteristic reddish brown color.

2. The method for coloring a soy product according to claim 1, wherein: In the step (1), the alkaline solution is a Na2CO3 solution.

3. The method for coloring a soy product according to claim 1, wherein: When polyethylene glycol, sorbitol, maltitol or potassium sorbate are selected, their usage is calculated based on 100 mL of the composite coloring solution as follows: polyethylene glycol 0.2-1.2 g; sorbitol 1.0-10.0 g; maltitol 1.0-10.0 g; potassium sorbate 0.05-0.1 g.

4. The method for coloring a soy product according to claim 1, wherein: The polyethylene glycol is PEG6000 or PEG1000.

5. The method for coloring a soy product according to claim 1, wherein: When the auxiliary factor selected includes polyethylene glycol, the polyethylene glycol is first dissolved with an appropriate amount of hot water or ultrasonically assisted dissolution, and then the dissolved polyethylene glycol is added to the eumelanin solution prepared in step (1).

6. The method for coloring a soy product according to claim 1, wherein: In the step (3), Na2CO3 solution or HCl solution is used to adjust the pH of the composite coloring solution.

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