Preparation method and application of konjac fine powder

Abstract

The application provides a preparation method and application of konjac fine powder. The preparation method of the konjac fine powder comprises the following steps: A1. konjac powder obtained by crushing konjac dry is mixed with water at 60-75 DEG C to perform infiltration swelling, a pulse electric field treatment is applied at 15-30 kV / cm, and then solid-liquid separation is performed to obtain wet konjac slurry; A2. the wet konjac slurry is subjected to dynamic countercurrent extraction by using supercritical CO2 containing ethanol under the conditions of 45-60 DEG C and 25-35 MPa, and then sulfides and pigments are removed to obtain extracted wet konjac slurry; and A3. the extracted wet konjac slurry is subjected to low-temperature vacuum drying and airflow crushing to obtain konjac fine powder. Through the dissolution capacity of supercritical CO2 and the cell wall breaking and bleaching effects of the pulse electric field, the konjac fly powder, sulfides and colored substances are efficiently physically stripped and removed without using any harmful chemical reagent, and the natural molecular structure and viscosity of konjac glucosaccharide are retained.

Description

Technical Field

[0001] This invention relates to the field of texture engineering of konjac food, and in particular to a method for preparing konjac flour and its application. Background Technology

[0002] Konjac flour is a natural high-molecular-weight polysaccharide product extracted from konjac tubers. Its main component is konjac glucomannan (KGM), which is widely used in food, medicine, cosmetics, and biomaterials. The core evaluation indicators for its product quality focus on three aspects: whiteness, viscosity (characterizing the integrity and purity of KGM molecules), and sulfur dioxide (SO2) residue (reflecting safety). Whiteness is the primary appearance factor affecting market acceptance and product grade; high viscosity directly determines its effectiveness as a thickener, gelling agent, and other functional applications; and low SO2 residue is a rigid requirement to meet food safety regulations. For a long time, to simultaneously achieve the two mutually restrictive goals of "high whiteness" and "high viscosity," the global konjac processing industry has generally adopted sulfur fumigation processes. This process utilizes SO2 gas generated from sulfur combustion to perform reductive bleaching of konjac materials, effectively inhibiting the enzymatic and non-enzymatic browning of polyphenols, thus achieving a good color. Simultaneously, the reducing environment of SO2 can, to some extent, slow down the oxidative degradation of KGM during processing, indirectly maintaining viscosity. However, this process results in a large amount of SO2 irreversibly binding to konjac flour in the form of sulfites, causing severe SO2 residue exceeding the standard in the final product (often reaching hundreds or even thousands of mg / kg). This not only poses health risks such as allergies and asthma but also fails to meet the increasingly stringent limits for SO2 residue in China's GB 2760 National Food Safety Standard for the Use of Food Additives and international markets such as the EU, the US, and Japan (typically ≤10–50 mg / kg). Furthermore, with consumers increasingly pursuing "clean label" and "chemical-free" products, process-related chemical residues have become a significant negative factor for products. To break free from dependence on sulfur fumigation, the industry has tried various alternative technological paths, but all have fundamental drawbacks that are difficult to overcome: one is chemical oxidation desulfurization. This method uses strong oxidants such as hydrogen peroxide and sodium hypochlorite for bleaching and desulfurization. While it can effectively remove bound sulfides, the strong oxidizing environment indiscriminately attacks active sites such as hydroxyl groups on the KGM molecular chain, leading to main chain breakage and depolymerization, resulting in a significant decrease in intrinsic viscosity (often exceeding 30%–50%), severely weakening its functional value. Simultaneously, this process may introduce new pollutants such as chlorates and heavy metals, and generate high-COD chlorine / oxygen-containing organic wastewater, placing a heavy burden on environmental treatment. Secondly, the physical-chemical washing method. This method attempts to remove sulfides through multi-stage water washing, acid washing (such as citric acid and hydrochloric acid), and chelating agent extraction (such as EDTA), using a combination of physical diffusion and chemical complexation. This method causes less viscosity damage, but to achieve deep desulfurization, it requires large amounts of water, high-concentration acid solutions, and specialized chemicals. The process is lengthy, costly, and generates large amounts of complex acidic wastewater, resulting in significant environmental pollution problems. Essentially, it still relies on specific chemical reagents. Thirdly, the biological enzymatic hydrolysis method.This method utilizes sulfite oxidase, polyphenol oxidase, or hemicellulase to specifically degrade SO2 precursors or pigments. Although the reaction conditions are mild and the theoretical selectivity is high, it faces three major industrialization bottlenecks in practical applications: first, food-grade high-activity enzyme preparations are expensive; second, enzyme activity is easily affected by temperature, pH, and material composition, resulting in poor process stability and low batch repeatability; and third, the reaction cycle is long, making it difficult to integrate efficiently with existing continuous and large-scale production equipment, leading to low production efficiency.

[0003] In summary, existing methods are caught in a three-way dilemma: "desulfurization inevitably damages viscosity, cleaning leads to low efficiency, and high efficiency leads to high pollution." There is no process that can simultaneously achieve ultra-low SO2 residue (≤10 mg / kg), high whiteness (Hunter whiteness ≥85), high viscosity retention rate (≥95%), and no chemical additives throughout the entire process. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a physical refining method for konjac flour based on the synergistic effect of supercritical fluid extraction and pulsed electric field. This method utilizes the unique dissolving power of supercritical CO2 and the cell-wall breaking and bleaching effect of the pulsed electric field to achieve efficient physical stripping and removal of konjac flour (impurities), sulfides, and colored substances without using any harmful chemical reagents, while maximizing the preservation of the natural molecular structure and viscosity of konjac glucomannan.

[0005] To achieve the above objectives, the present invention provides a method for preparing konjac flour, comprising the following steps: A1. Konjac flour slurry is mixed with water at 60~75℃ and soaked and swollen. Then, a pulsed electric field of 15~30kV / cm is applied to treat it, and the solid and liquid are separated to obtain wet konjac powder. A2. The wet konjac powder is subjected to dynamic countercurrent extraction with supercritical CO2 containing ethanol at 45~60℃ and 25~35 MPa to remove sulfides and pigments and obtain the extracted wet konjac powder. A3. The extracted wet konjac powder is dried under low temperature and vacuum, and then air-jet pulverized to obtain konjac refined powder.

[0006] The method for preparing konjac flour provided by this invention organically integrates pulsed electric field physical modification with supercritical CO2 green extraction technology, achieving a synergistic improvement in product safety, functionality, and appearance without the use of any chemical bleaching agents, desulfurizers, or enzyme preparations. Specifically, it is reflected in the following aspects: (1) Significantly improves product color and achieves efficient physical bleaching. Pulsed electric field treatment can induce reversible electroporation of konjac cell membranes through high-energy electric pulses without raising the system temperature, while destroying polyphenol oxidase activity and pigment precursors, thus playing an electrochemical bleaching role; subsequently, supercritical CO2, with its high diffusivity and solubility selectivity, efficiently extracts fat-soluble pigment components. The synergistic effect of the two results in a white and uniform konjac flour color, fully meeting the stringent requirements of high-end food for the appearance of raw materials, and without relying on sulfur fumigation or chemical oxidants; (2) Deeply removes sulfur dioxide residue and ensures food safety. With the assistance of ethanol as an entrainer, supercritical CO2 exhibits excellent extraction ability for polar-nonpolar mixed impurities such as sulfites and bound SO2 precursors, and can selectively remove them from the konjac matrix under mild conditions; (3) It retains the viscosity of konjac glucomannan to a high degree, maintaining the core functional value. The entire process is controlled in a low-temperature environment without strong oxidation, strong reduction or extreme pH conditions, avoiding thermal degradation, oxidative breakage or acid-base hydrolysis of konjac glucomannan (KGM) molecular chains.

[0007] According to an embodiment of the present invention, the temperature at which the konjac flour slurry and water are mixed, soaked, and swollen is 60~75°C. The soaking and swelling temperature can be, for example, any value between 60°C, 65°C, 70°C, 75°C, or 60~75°C.

[0008] According to an embodiment of the present invention, the electric field strength of the pulsed electric field treatment is 15~30 kV / cm. The electric field strength can be, for example, any value between 15 kV / cm, 20 kV / cm, 25 kV / cm, 30 kV / cm, or 15~30 kV / cm.

[0009] In some embodiments, the temperature of the supercritical CO2 dynamic countercurrent extraction is 45~60°C. The extraction temperature can be, for example, any value between 45°C, 50°C, 55°C, 60°C, or 45~60°C.

[0010] In some embodiments, the pressure of the supercritical CO2 dynamic countercurrent extraction is 25-35 MPa. The extraction pressure can be, for example, any value between 25 MPa, 30 MPa, 35 MPa, or 25-35 MPa.

[0011] In some embodiments, the ethanol mass fraction in the supercritical CO2 containing ethanol is 1-5%. The ethanol mass fraction can be, for example, any value between 1%, 2%, 3%, 4%, 5%, or 1-5%.

[0012] In some embodiments, the temperature of the low-temperature vacuum drying does not exceed 50°C. The drying temperature can be, for example, any value of 40°C, 45°C, 50°C, or not exceeding 50°C.

[0013] In some embodiments, the vacuum degree of the low-temperature vacuum drying does not exceed -0.095 MPa. The vacuum degree can be, for example, -0.08 MPa, -0.085 MPa, -0.09 MPa, or any value not exceeding -0.095 MPa.

[0014] According to an embodiment of the present invention, the konjac flour slurry is prepared by mixing konjac coarse flour and water at a mass ratio of 1:2 to 5.

[0015] According to an embodiment of the present invention, the immersion and swelling temperature is 55~65°C, and the immersion and swelling time is 20~40 min.

[0016] The konjac flour preparation method provided by this invention optimizes the wetting and swelling conditions, enabling the particles to swell fully and uniformly, which softens the cell structure, improves dielectric responsiveness, and avoids heat damage to glucomannan caused by high temperature or long-term processing.

[0017] According to an embodiment of the present invention, the pulse width of the pulse electric field treatment is 2~10μs and the pulse frequency is 50~200 Hz.

[0018] According to an embodiment of the present invention, the mass fraction of ethanol in the supercritical CO2 containing ethanol is 1-5%.

[0019] The konjac flour preparation method provided by this invention achieves efficient non-thermal physical modification of konjac tissue by precisely controlling key parameters of pulsed electric field treatment. This parameter window has been systematically optimized to form uniform, reversible microporous channels on the konjac cell membrane, while simultaneously promoting the release of water-soluble impurities such as bound sulfites, laying the foundation for subsequent warm water washing and supercritical CO2 extraction. According to an embodiment of this invention, the extraction time of the dynamic countercurrent extraction is 30-120 min.

[0020] According to an embodiment of the present invention, between step A1 and step A2, the wet konjac powder after pulse electric field treatment is washed with water at 40~50°C, and the washed wet konjac powder is obtained after solid-liquid separation.

[0021] The konjac flour preparation method provided by this invention further introduces a 40-50°C warm water washing operation between steps A1 and A2 to clean and separate the wet konjac powder after pulsed electric field treatment. After pulsed electric field treatment, the permeability of konjac cell membranes is significantly enhanced, and a large number of water-soluble impurities originally embedded in the cells (such as small molecule sugars, inorganic salts, and some free and weakly bound sulfites) are released to the surface or gaps of the material. If not removed in time, these impurities will not only interfere with the subsequent selective extraction of target sulfides by supercritical CO2, but may also be re-adsorbed or undergo Maillard reactions during the drying process, affecting the whiteness and purity of the product. Washing with 40-50°C warm water can effectively dissolve and wash away the above-mentioned water-soluble impurities, while avoiding local gelatinization or viscosity loss of konjac glucomannan caused by excessively high temperatures.

[0022] According to an embodiment of the present invention, the temperature of the low-temperature vacuum drying does not exceed 50°C and the vacuum degree does not exceed -0.09 MPa.

[0023] According to an embodiment of the present invention, in step A2, the specific process of dynamic countercurrent extraction is as follows: supercritical CO2 is introduced from the bottom of the extraction vessel and penetrates the material layer formed by the wet konjac powder from bottom to top. The enriched fluid carrying the solute is discharged from the top and enters the separator. After depressurization and separation, the CO2 is recovered and recycled, and the extract is collected.

[0024] This invention constructs a new green refining pathway by coupling pulsed electric field treatment with supercritical CO2 extraction. In terms of color, the pulsed electric field effectively disrupts the conjugated structure of polyphenols and quinone pigments in konjac through high-energy short pulses, achieving non-thermal, reagent-free electrochemical bleaching. Simultaneously, supercritical CO2, with its excellent permeability and solubility, efficiently extracts fat-soluble pigments. The synergistic effect of these two processes significantly improves the whiteness of the product, resulting in a pure white konjac extract that fully meets the appearance requirements of the high-end market. Regarding safety, supercritical CO2 exhibits high selectivity for polar-nonpolar mixed impurities such as sulfites and bound SO2 precursors, and with the assistance of an ethanol entrainer, it can deeply remove them, ensuring that the sulfur dioxide residue in the final product is far below the food safety standard limit. In terms of functionality, the entire process is carried out at low temperatures and does not introduce any strong oxidants, reducing agents, or acid / alkali chemicals, avoiding the oxidative breakage or hydrolytic degradation of konjac glucomannan (KGM) molecular chains.

[0025] A second aspect of the present invention provides a konjac flour product, which is prepared by the above-described preparation method. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0028] To further illustrate the present invention, the following examples are provided: Example 1 A1.1: Take 100 kg of dried konjac slices that have been treated with sulfur fumigation, and crush them to 40 mesh to obtain coarse konjac powder; A1.2: Mix the above-mentioned konjac coarse powder with 65℃ hot water at a mass ratio of 1:4 to prepare a slurry. Soak the slurry at a constant temperature of 60℃ for 30 minutes to allow the particles to fully swell. Then, pass the slurry into a pulse electric field treatment device and apply a high voltage pulse with an electric field strength of 15 kV / cm, a pulse frequency of 100 Hz, and a pulse width of 30 μs. A total of 100 pulses are applied. A1.3: The slurry treated with the pulsed electric field is washed once with 45°C warm water to remove water-soluble impurities (such as small molecule sugars, some sulfites, etc.) released due to increased cell membrane permeability. Then it is filtered by a plate and frame filter press to obtain wet konjac powder. A2. The obtained wet konjac powder was loaded into a 100 L supercritical extraction vessel. Under the conditions of 30 MPa pressure and 50℃ temperature, supercritical CO2 containing 3% (v / v) anhydrous ethanol as an entrainer was introduced to penetrate the material layer from bottom to top and perform dynamic countercurrent extraction for 90 minutes. The enriched fluid carrying the solute was discharged from the top of the extraction vessel. After being depressurized by a separator, the CO2 was recovered and recycled, and the extract was collected and discarded. A3. The extracted wet konjac powder was placed in a vacuum drying oven and dried at 50°C and a vacuum of -0.095 MPa until the moisture content was ≤8%. Subsequently, an air jet mill was used for pulverization and classification, with the feed pressure controlled at 0.7 MPa and the classifier speed at 4000 rpm, to obtain a konjac refined powder product with uniform particle size.

[0029] Example 2 The difference between this embodiment and Embodiment 1 is that the electric field strength is increased from 15 kV / cm to 25 kV / cm, while the other conditions are the same as in Embodiment 1.

[0030] Example 3 The difference between this embodiment and Embodiment 1 is that the concentration of the ethanol entrainer is reduced from 3% to 1%, while the other conditions are the same as in Embodiment 1.

[0031] Example 4 The difference between this embodiment and Embodiment 1 is that the water washing step is omitted, while the other conditions are the same as in Embodiment 1.

[0032] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the pulsed electric field is replaced with conventional hot immersion, while the other conditions are the same.

[0033] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that H2O2 chemical oxidation was used instead of supercritical CO2 extraction, while the other conditions were the same.

[0034] Test case Systematic tests were conducted on the products obtained from the examples and comparative examples to examine indicators such as sulfur dioxide residue. The tests were conducted according to GB 2760 National Food Safety Standard for the Use of Food Additives. The results showed that the SO2 residue in the products of Example 1 and Example 2 was 470 ppm and 510 ppm, respectively, meeting food safety requirements. However, in Comparative Example 1 (without a pulsed electric field, only conventional hot-soaking treatment), the SO2 residue was as high as 2000 ppm because the konjac cell structure was not effectively opened, making it difficult to release the internal bound sulfides. Furthermore, the strong oxidizing environment in Comparative Example 2 caused severe degradation of konjac glucomannan and may introduce potential safety risks such as peroxidation byproducts. In summary, the process of this invention achieves ultra-low sulfur residue while maintaining high viscosity and high whiteness, significantly superior to existing technical approaches.

[0035] Viscosity determination was performed according to the method in NY / T 494—2023 "Konjac Flour" standard. The specific procedure is as follows: Accurately weigh 0.01 g of konjac flour into a 500 mL beaker, add 50 mL of deionized water at 30℃, stir well, and then place in a (30±1)℃ constant temperature water bath. Stir at 150 r / min for 1 h to allow the konjac flour to fully swell and form a stable colloid. Then, transfer the mixture to the measuring cup of an NDJ-1 or NDJ-5S rotational viscometer, set the rotor type to No. 1, and the rotation speed to 12 r / min. After the reading stabilizes, record the initial viscosity value. Next, heat the sample to 90℃ and hold for 10 min. After cooling to room temperature, measure the viscosity again. Repeat this process three times and take the average value as the final result. Test results show that the konjac flour of Examples 1 and 2 had viscosity values ​​of 14500 mPa·s and 15200 mPa·s respectively under the same conditions, demonstrating excellent thickening properties and polymer integrity.

[0036] The above technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included in the patent protection scope of the present invention.

Claims

1. A method for preparing konjac flour, characterized in that, Includes the following steps: A1. After pulverizing dried konjac to obtain konjac powder, mix it with water at 60~75℃ and soak it to swell. Then apply a pulse electric field of 15~30kV / cm to treat it and separate the solid and liquid to obtain wet konjac slurry. A2. The wet konjac slurry is subjected to dynamic countercurrent extraction with supercritical CO2 containing ethanol at 45~60℃ and 25~35 MPa to remove sulfides and pigments and obtain the extracted wet konjac slurry. A3. The extracted wet konjac slurry is dried under low temperature and vacuum, and then air-jet pulverized to obtain konjac flour.

2. The method for preparing konjac flour according to claim 1, characterized in that, The konjac flour slurry is made by mixing coarse konjac flour and water at a mass ratio of 1:2 to 5.

3. The method for preparing konjac flour according to claim 1, characterized in that, The immersion and swelling temperature is 55~65℃, and the immersion and swelling time is 20~40min.

4. The method for preparing konjac flour according to claim 1, characterized in that, The pulse width of the pulsed electric field processing is 2~10μs and the pulse frequency is 50~200Hz.

5. The method for preparing konjac flour according to claim 1, characterized in that, The supercritical CO2 containing ethanol has an ethanol mass fraction of 1-5%.

6. The method for preparing konjac flour according to claim 1, characterized in that, The extraction time for the dynamic countercurrent extraction is 30~120 min.

7. The method for preparing konjac flour according to claim 1, characterized in that, Between step A1 and step A2, the process further includes: washing the wet konjac powder treated with the pulsed electric field with water at 40~50℃, and obtaining the washed wet konjac powder after solid-liquid separation.

8. The method for preparing konjac flour according to claim 1, characterized in that, The temperature of the low-temperature vacuum drying process does not exceed 50°C, and the vacuum degree does not exceed -0.09 MPa.

9. The method for preparing konjac flour according to claim 1, characterized in that, In step A2, the specific process of dynamic countercurrent extraction is as follows: supercritical CO2 is introduced from the bottom of the extraction vessel and penetrates the material layer formed by the wet konjac powder from bottom to top. The enriched fluid carrying the solute is discharged from the top and enters the separator. After depressurization and separation, the CO2 is recovered and recycled, and the extract is collected.

10. A konjac flour product, characterized in that, The konjac flour product is prepared by the preparation method according to any one of claims 1 to 9.