A method for microwave-assisted enzymatic extraction of soluble dietary fiber from roselle residue

Abstract

This invention discloses a microwave-assisted enzymatic extraction method for soluble dietary fiber from roselle residue. The method includes: selecting roselle residue after water extraction of polyphenols, drying, pulverizing, sieving, adding PBS buffer solution, adjusting the pH to acidic, microwave to ensure uniform heating; adding a compound enzyme, enzymatic hydrolysis, enzyme inactivation, cooling, and centrifugation; adding 95% ethanol, stirring evenly, allowing to stand for alcohol precipitation, centrifuging, pre-freezing the precipitate, and drying to obtain the finished product of soluble dietary fiber from roselle residue. The soluble dietary fiber obtained by this invention retains good water-holding capacity, oil-holding capacity, and swelling capacity, and contains some bound active ingredients, enhancing the functional value of the product. This method is simple to operate, with easily controllable parameters, facilitating large-scale production. It not only enriches the sources of dietary fiber but also provides reliable technical support for the extension of the agricultural product deep processing industry, and has important practical significance for the development of the functional food field.

Description

Technical Field

[0001] This invention relates to a microwave-assisted enzymatic method for extracting soluble dietary fiber from roselle residue, belonging to the field of extracting soluble dietary fiber from roselle residue. Background Technology

[0002] Roselle, a natural plant used in both food and medicine, has calyxes rich in anthocyanins, polyphenols, and other active ingredients, making it widely used in food and pharmaceutical fields. However, the processing residue after water extraction of polyphenols is often discarded or incinerated, resulting in a waste of high-quality components such as dietary fiber and potentially causing environmental pollution, which contradicts the development needs for high-value utilization of agricultural products. Roselle residue is rich in dietary fiber, some of which is combined with polyphenols and anthocyanins, possessing both the physiological functions of dietary fiber and the antioxidant properties of polyphenols. It is an excellent raw material for extracting functional soluble dietary fiber, but its resource-based extraction and utilization has not yet been fully developed.

[0003] Currently, the main extraction methods for plant-derived soluble dietary fiber include enzymatic hydrolysis, alkaline hydrolysis, and microwave extraction, each with significant technical drawbacks. Enzymatic hydrolysis offers mild reaction conditions but suffers from low efficiency, long cycles, and limited cell wall damage, resulting in low extraction rates. Alkaline hydrolysis, while simple to operate, easily damages the functional structure of dietary fiber, and the alkaline wastewater it produces can cause secondary pollution. Microwave extraction, while rapidly breaking down cell walls and shortening extraction time, is susceptible to degradation of the dietary fiber macromolecular chains due to its strong thermal effect, leading to deterioration of key physicochemical properties such as water retention and poor extraction quality.

[0004] Current research on hibiscus residue mainly focuses on the simple extraction of insoluble dietary fiber. There are few reports on optimizing the extraction process for soluble dietary fiber, and an extraction technology that can balance extraction rate, product quality, process economy, and environmental friendliness has not yet been developed, failing to meet the needs of industrial production. Therefore, developing an extraction method that can realize the resource utilization of hibiscus residue while improving the extraction rate of soluble dietary fiber and preserving its excellent physicochemical properties is key to solving the problem of residue waste in the hibiscus processing industry and enriching the source of functional dietary fiber, and has significant industrial application value. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a microwave-assisted enzymatic method for extracting soluble dietary fiber from roselle residue. By leveraging the synergistic effect of microwaves and compound enzymes, this method enables the high-value utilization of roselle residue, improves the extraction rate and quality of soluble dietary fiber, reduces extraction costs, shortens the process cycle, and provides technical support for industrial production.

[0006] Technical Solution: This invention provides a method for microwave-assisted enzymatic extraction of soluble dietary fiber from roselle residue, comprising the following steps: (1) Select the hibiscus residue after water extraction of polyphenols, dry it, crush it, and sieve it to obtain hibiscus residue powder with uniform particle size. (2) Add PBS buffer solution to the roselle residue powder described in step (1), adjust the pH to acidic, microwave, and cool immediately after microwave treatment to obtain a mixture; (3) Add the complex enzyme to the mixture in step (2), perform enzymatic hydrolysis, inactivate the enzyme, cool to room temperature, centrifuge, and collect the supernatant; the complex enzyme is α-amylase and cellulase; (4) Add water-soluble organic solvent to the supernatant in step (3), stir evenly, let stand for alcohol precipitation, centrifuge, pre-freeze the precipitate, and dry it to obtain the soluble dietary fiber product of roselle residue.

[0007] In step (2), the power of the microwave is 350~450W and the duration is 150~210 s.

[0008] In step (2), the liquid-to-solid ratio of the PBS buffer solution and the roselle residue powder is 10-20:1.

[0009] In step (2), in addition to PBS buffer solution, other buffer solutions that can stabilize the pH value of the reaction system can be selected as needed to achieve the technical effect of the present invention.

[0010] The complex enzymes mentioned in step (3) are α-amylase and cellulase.

[0011] The ratio of α-amylase to cellulase is 1:2~4.

[0012] The preparation method of the roselle residue after water extraction of polyphenols in step (1) includes: taking dried roselle flowers, adding deionized water, water bath extraction, filtering to remove the filtrate from which polyphenols are extracted, and the resulting filter residue is the roselle residue after water extraction of polyphenols.

[0013] Among them, the dried roselle flowers are hibiscus flowers. This process is developed for this common commercial variety. If other roselle varieties are used, the content of the basic components of the raw materials will be slightly changed. The extraction rules, optimal process parameters and product performance change trends will remain consistent and will not affect the implementation of the core technical solution of this invention.

[0014] In step (2), the pH is adjusted to 4.5-6.0, with the optimal pH being 5.5. Within this range, the activity of the complex enzyme can be maintained at a high level, and the SDF extraction rate is stable. Deviating from this range will significantly reduce the enzymatic hydrolysis efficiency and the final yield.

[0015] The water-soluble organic solvents mentioned in step (4) include ethanol, methanol, and acetone, with 95% ethanol being preferred. Ethanol has high safety, low cost, and good separation effect, making it suitable for the preparation of food-grade dietary fiber.

[0016] The present invention also provides a method for increasing the yield of soluble dietary fiber in roselle residue using the aforementioned method, characterized by comprising the following steps: (1) Select the hibiscus residue after water extraction of polyphenols, dry it, crush it, and sieve it to obtain hibiscus residue powder with uniform particle size. (2) Add PBS buffer solution to the roselle residue powder described in step (1), adjust the pH to acidic, microwave, and cool immediately after microwave treatment to obtain a mixture; (3) Add the complex enzyme to the mixture in step (2), perform enzymatic hydrolysis, inactivate the enzyme, cool to room temperature, centrifuge, and collect the supernatant; the complex enzyme is α-amylase and cellulase; (4) Add water-soluble organic solvent to the supernatant in step (3), stir evenly, let stand for alcohol precipitation, centrifuge, pre-freeze the precipitate, and dry it to obtain the soluble dietary fiber product of roselle residue.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. This invention achieves high-value utilization of hibiscus residue through the synergistic effect of microwaves and compound enzymes, effectively solving the problem of waste from processing byproducts. Microwaves can rapidly destroy the cell wall structure of the residue, creating favorable conditions for enzymatic hydrolysis, while the compound enzymes specifically degrade relevant components, significantly improving the extraction efficiency of soluble dietary fiber and transforming waste into high-value functional components. The entire process requires no toxic or harmful reagents, operates under mild conditions, and has low energy and solvent consumption, conforming to the concepts of green production and resource recycling.

[0018] 2. This invention combines microwave and compound enzyme synergistic processing, which results in a higher yield of soluble dietary fiber and a shorter overall processing time compared to traditional single extraction processes, demonstrating a significant advantage in extraction efficiency.

[0019] 3. The entire process operates under mild reaction conditions. Microwaves only break down plant cell walls to release internal components, without excessively degrading dietary fiber molecules, thus preserving their functional structure intact.

[0020] 4. This method is simple to operate and the parameters are easy to control. The required equipment is the type commonly used in the food industry, which is conducive to large-scale production. It not only enriches the sources of dietary fiber, but also provides reliable technical support for the extension of the deep processing industry of agricultural products. It has important practical significance for the development of the functional food field.

[0021] 5. The soluble dietary fiber obtained by this invention retains good water-holding capacity, oil-holding capacity and swelling power, and contains some bound active ingredients, retaining natural bound active substances. The overall quality of the product is excellent, enhancing its functional value. Attached Figure Description

[0022] Figure 1 The curve showing the relationship between microwave power and extraction rate; Figure 2 The curve showing the relationship between microwave time and extraction rate; Figure 3 The curve showing the relationship between the proportion of the compound enzyme and the extraction rate; Figure 4 The curve showing the relationship between the liquid-to-solid ratio and the extraction rate; Figure 5 Water retention under different treatment conditions; Figure 6 Oil retention under different processing conditions; Figure 7 Swelling force under different treatment conditions; Figure 8 SDF extraction rate under different treatment conditions; Figure 9 SEM images of SDF under different processing conditions; Figure 10 SDF values ​​of samples obtained by different processing methods. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0025] Example 1: Determination of Soluble Dietary Fiber Extraction Rate Step 1: Preparation of residue from water extraction of polyphenols: Select commercially available dried roselle flowers (hibiscus flowers), add deionized water at a liquid-to-solid ratio of 20:1 (mL / g), and extract twice in an 80℃ water bath for 1.5 h each time; combine the two extracts, and the remaining filter residue is the roselle residue after water extraction of polyphenols. Step 2: Drying and pulverizing the filter residue: Select the hibiscus residue after water extraction of polyphenols, place it in a 60℃ oven and dry for 12 hours to ensure that the moisture content of the material is less than 5%; after drying, pulverize it and pass it through a 60-mesh sieve to obtain hibiscus residue powder with uniform particle size, and seal it for storage in the dark for later use.

[0026] Step 3: Microwave Pretreatment: Accurately weigh 3 g of pretreated hibiscus residue powder into a 250 mL beaker. Add 45 mL of 0.01 mol / L PBS buffer solution (pH 7.4) at a liquid-to-solid ratio of 15:1 mL / g. Adjust the pH of the mixture to 5.5 with 6 mol / L HCl. Place the mixture in a microwave oven, set the microwave power to 350 W, and the treatment time to 150 s. Stir manually for 10 s every 60 s during the treatment to ensure uniform heating. Immediately after microwave treatment, place the mixture in an ice bath below 50°C.

[0027] Step 4: Compound enzymatic hydrolysis: Add 2% of the dry weight of the raw material (the mass ratio of α-amylase and cellulase is 1:1) of the compound enzyme to the cooled mixture. Cover the mouth of the beaker with plastic wrap and place it in a 60℃ constant temperature water bath for 1 hour for enzymatic hydrolysis. Shake and mix every 15 minutes during the process. After the process is completed, transfer the mixture to a boiling water bath to inactivate the enzyme for 10 minutes, and then cool it to room temperature in an ice water bath.

[0028] Step 5: Centrifugation: Dispense the cooled enzymatic hydrolysate into centrifuge tubes, centrifuge at 4000 rpm for 10 min, and collect the supernatant.

[0029] Step 6: Alcohol precipitation purification: Add 4 times the volume of 95% ethanol to the supernatant, stir well, and let it stand in a refrigerator at 4℃ for 12 h for alcohol precipitation. Then, centrifuge the mixture at 4000 rpm for 10 min and collect the precipitate.

[0030] Step 7: Freeze-drying: Pre-freeze the precipitate in an ultra-low temperature freezer at -80℃ for 8 hours, and then freeze-dry it at -80℃ for 48 hours to obtain the soluble dietary fiber product of roselle residue.

[0031] The formula for calculating the extraction rate (R) of soluble dietary fiber (SDF) is as follows:

[0032] Example 2: Effect of microwave power on extraction rate Under the conditions of maintaining a microwave time of 150 s, a compound enzyme ratio of 1:1, and a liquid-to-solid ratio of 15:1 mL / g, the effect of microwave power of 300 W, 350 W, 400 W, 450 W, and 500 W on SDF extraction rate was investigated, with three parallel samples set up for each level. The results are as follows: Figure 1 .

[0033] Depend on Figure 1It can be seen that as the microwave power increases, the extraction rate shows a significant trend of first increasing and then decreasing. When the microwave power increases from 300 W to 400 W, the SDF extraction rate increases from 16.57% to 21.00% and reaches its peak. As the power further increases to 450 W and 500 W, the extraction rate decreases to 16.03% and 15.47%, respectively. This trend indicates that microwave power can indirectly affect the enzymatic hydrolysis effect by regulating the physical state of the raw materials, and that there is an optimal microwave power range to achieve the highest SDF extraction efficiency. Therefore, in the orthogonal experiment, the microwave power should be selected at three levels: 350 W, 400 W, and 450 W.

[0034] Example 3: Effect of microwave time on extraction rate The effect of microwave power of 350 W, enzyme-to-solid ratio of 1:1, and liquid-to-solid ratio of 15:1 mL / g on SDF extraction rate was investigated. Three replicates were set up for each level. The results are shown below. Figure 2 .

[0035] Depend on Figure 2 It can be seen that the extraction rate of SDF generally follows a parabolic trend with the change of microwave treatment time. When the microwave time is extended from 120 s to 150 s, the SDF extraction rate decreases from 18.87% to 15.03%, a slight decrease with no significant difference. This is because the energy accumulation is insufficient in a shorter time, failing to effectively break down the cell wall and release SDF. When the microwave time is increased to 180 s, the SDF extraction rate increases to the highest value of 28.53% in the experiment, indicating that more sufficient energy input effectively breaks down the cell wall structure of the roselle residue and promotes SDF dissolution. At the same time, microwave radiation may enhance solvent penetration and improve SDF solubility. Continuing to extend the microwave time to 210 s, the extraction rate decreases slightly to 27.27%, indicating the emergence of factors unfavorable to SDF extraction, at which point the SDF release rate slows down. When the microwave time is further extended to 240 s, the SDF extraction rate drops sharply to 17.20%. This allows us to determine that the microwave time levels selected in subsequent orthogonal experiments are 150 s, 180 s, and 210 s.

[0036] Example 4 Effect of compound enzyme ratio on extraction rate The effect of the ratio of the compound enzyme (α-amylase:cellulase) at 1:1, 1:2, 1:3, 1:4, and 1:5 on SDF extraction rate was investigated while maintaining a microwave power of 350 W, a microwave time of 150 s, and a liquid-to-solid ratio of 15:1 mL / g. Three replicates were set up for each level. The results are shown below. Figure 3 .

[0037] Depend on Figure 3 It was found that when the total amount of the complex enzyme was fixed at 2%, the ratio of α-amylase to cellulase significantly affected the SDF extraction rate. The extraction rate was lowest at a ratio of 1:1 (16.43%); increasing to 1:2 significantly improved it to 19.67%; and reaching a peak of 22.13% at a ratio of 1:3. However, when the ratio was further adjusted to 1:4 and 1:5, the extraction rate decreased to 20.53% and 20.47%, respectively. This indicates that within the experimental range, the optimal enzyme ratio is close to 1:3, and insufficient cellulase will reduce extraction efficiency. Based on this, the optimal levels of the complex enzyme ratio in the orthogonal experiment were determined to be 1:2, 1:3, and 1:4.

[0038] Example 5 Effect of liquid-to-solid ratio on extraction rate The effects of liquid-to-solid ratios of 10:1, 15:1, 20:1, 25:1, and 30:1 on SDF extraction rate were investigated while maintaining a microwave power of 350 W, a microwave time of 150 s, and a complex enzyme ratio of 1:1. Three replicates were set up for each level. The results are shown below. Figure 4 .

[0039] Depend on Figure 4 It can be seen that when the liquid-to-solid ratio increases from 10:1 (mL / g) to 20:1 (mL / g), the extraction rate of SDF shows a significant difference, increasing from 10.01% to 22.15%, reaching the peak under the experimental conditions. When the liquid-to-solid ratio increases to 30:1, the extraction rate of SDF shows a sharp decline, dropping to 13.03%. The graph shows a parabolic shape that first rises and then falls; the peak of this parabola represents the optimal SDF extraction rate under the liquid-to-solid ratio condition of 20:1. Based on this, the liquid-to-solid ratio levels for subsequent orthogonal experiments were determined to be 10:1, 15:1, and 20:1.

[0040] Example 6: Optimization and Verification of Orthogonal Experiments The orthogonal experiment kept the mass of hibiscus residue used constant at 4 g, using L9(3) 4 An orthogonal array was used to design a four-factor, three-level experiment. Based on the results of single-factor experiments, factors and levels that significantly affected SDF extraction rate were selected. Four factors were chosen for the orthogonal experiment: microwave power (A), microwave time (B), the ratio of compound enzymes (α-amylase: cellulase) (C), and the liquid-to-solid ratio (D). Each factor had three levels. The experimental factors and levels are shown in Table 1.

[0041] Table 1. Factor Level Table for Orthogonal Experiment

[0042] The orthogonal experimental design and results are shown in Table 2. The experiment was conducted according to the conditions of the software design.

[0043] Table 2 Orthogonal Results Analysis Table

[0044] Table 2 shows that the order of influence of each factor on the SDF extraction rate is: D (liquid-to-solid ratio) > A (microwave power) > B (microwave time) > C (compound enzyme ratio). In terms of range, the liquid-to-solid ratio is the most significant factor affecting the SDF extraction rate, while microwave power, microwave time, and the compound enzyme ratio have relatively smaller effects. Analysis of the average values ​​reveals that the extraction rate at a liquid-to-solid ratio of 20:1 (D3) is significantly higher than that at 10:1 and 15:1, indicating that a high liquid-to-solid ratio can fully wet the raw material and enhance mass transfer efficiency. The extraction rate at a microwave power of 400W (A2) is better than that at 350W and 450W, indicating that this power can achieve a balance between cell disruption efficiency and thermal damage. The extraction rate at a microwave time of 180 s (B2) is slightly higher than that at 150 s and 210 s. The results showed that moderately extending the extraction time could increase the degree of cell wall rupture. The extraction rate at a complex enzyme ratio of 1:3 (C2) was close to that at 1:2, but better than that at 1:4, indicating that the enzyme ratio should pay particular attention to synergistic effects.

[0045] Based on the combined range and mean analysis, the theoretically optimal process combination is A2B2C2D3, namely, microwave power of 400 W, microwave time of 180 s, compound enzyme ratio of 1:3, and liquid-to-solid ratio of 20:1 mL / g. However, this combination was not directly included in the orthogonal experimental group. To further verify this, three replicate experiments were conducted according to the A2B2C2D3 conditions, and the average SDF extraction rate was measured to be 28.23%, which is lower than that of experiment 8 in the orthogonal table. This indicates that the actual optimal combination is A3B2C1D3, namely, microwave power of 450 W, microwave time of 180 s, compound enzyme ratio of 1:2, and liquid-to-solid ratio of 20:1 mL / g.

[0046] Example 7: Effects of different treatments on SDF properties After determining the optimal conditions for microwave-assisted enzymatic extraction of dietary fiber from hibiscus residue through single-factor and orthogonal experiments, the optimal conditions were fixed, and the experimental procedures were kept unchanged. SDF was extracted using three different treatment methods: microwave treatment, enzyme treatment, and microwave-assisted enzyme treatment. The control group used pretreated hibiscus residue powder as the raw material, but without microwave treatment or the addition of compound enzymes. All other procedures, including buffering, settling, centrifugation, alcohol precipitation, and lyophilization, were identical to the experimental group. The physicochemical properties and structure of the SDF samples obtained from these three treatment methods were measured and observed, including water-holding capacity, oil-holding capacity, swelling power, and electron microscopy analysis of the microstructure. The effects of different treatment methods on the physicochemical properties and structure of SDF in hibiscus residue were investigated. Results are as follows: Figures 5-10 As shown.

[0047] Figure 5 This is a comparison of the water-holding capacity of hibiscus dietary fiber obtained from different treatments. One-way ANOVA showed significant differences between groups (p < 0.05). The control group had the highest water-holding capacity, while the enzyme-treated group had a water-holding capacity of 5.67 g·g. -1 The water-holding capacity of the microwave-treated group alone was only 3.25 g·g. -1 The water-holding capacity of the microwave-assisted enzyme treatment group of this invention is 4.36 g·g. -1 This falls between the first two groups. It is evident that microwaves can enhance enzyme penetration efficiency, balancing this with the optimization effect of enzymatic hydrolysis on fiber structure, resulting in a finished product with moderate water-holding capacity.

[0048] Figure 6 This is a comparison of the oil-holding capacity of hibiscus dietary fiber obtained through different treatment methods. Compared with the control group, the oil-holding capacity of the enzyme-treated, microwave-treated, and microwave-assisted enzyme-treated groups of this invention was significantly improved, with values ​​of 2.54 g·g for the three groups, respectively. -1 2.25 g·g -1 2.33 g·g -1 There were no significant differences between the groups. Enzymatic hydrolysis can promote the dissolution of polyphenols and reduce the inhibition of oil-holding capacity by polyphenols. Microwave pretreatment can further optimize the fiber structure, ultimately giving the product of this invention excellent oil-holding capacity.

[0049] Figure 7 This is a comparison of the swelling power of hibiscus dietary fiber obtained from different treatments. The swelling power of all groups was lower than that of the control group, with the enzyme-treated group exhibiting the highest swelling power at 3.44 mL·g. -1 The microwave-treated group had the lowest swelling power, at only 2.79 mL·g. -1 The swelling power of the microwave-assisted enzyme treatment group of this invention is 3.01 mL·g. -1 This method is superior to microwave treatment alone. Microwave pretreatment can easily damage the porous structure of fibers; combining it with enzymatic hydrolysis can mitigate this negative impact, allowing the finished product to retain good water absorption and swelling properties.

[0050] Figure 8 This chart compares the extraction rates of soluble dietary fiber under different treatment methods. The extraction rate of the control group was 10.4%, the extraction rate of the enzyme-treated group alone was 15.70%, the extraction rate of the microwave-treated group alone was 15.92%, and the extraction rate of the microwave-assisted enzyme-treated group of this invention reached 22.72%, the highest among the four groups. Microwave pretreatment can destroy cell walls and improve enzymatic hydrolysis efficiency; the synergistic effect of the two significantly improves the yield of soluble dietary fiber.

[0051] Figure 9The images show the scanning electron microscope (SEM) microstructures of SDF under different treatment conditions (left column magnification 500x, right column magnification 1500x; first row: enzyme-treated group; second row: microwave-treated group alone; third row: microwave-assisted enzyme-treated group of this invention). The enzyme-treated group exhibits a dispersed, small, lamellar structure with a smooth surface and bulging cavities, which improves oil retention. However, the discretization of the fiber network reduces water retention and swelling performance. The microwave-treated group consists of thick, large lamellar sheets with a rough surface densely covered with fine micropores. The dense pores restrict water permeation, resulting in lower water retention and swelling capacity. The rough micropores can adsorb oil, improving oil retention. The microwave-assisted enzyme-treated group of this invention has thinner lamellar sheets with honeycomb-like pores and fragmented pieces on the surface. Microwave pretreatment disrupts the cell wall cross-linking structure to promote enzyme penetration. The composite structure balances the retention of hydrophilic groups and high specific surface area, achieving a balance between water retention, swelling, and oil retention properties.

[0052] Figure 10 Photos of soluble dietary fiber prepared by different treatment methods (from left to right: control group, enzyme-treated group, microwave-treated group, and microwave-assisted enzyme-treated group of this invention). All four samples are light red, indicating that the extracts retain a certain amount of anthocyanins, polyphenols, and polypeptide active substances. There are significant differences in the fluffiness of the powders among the groups. The powder obtained by microwave-assisted enzyme treatment has higher fluffiness, more uniform color, and better appearance.

[0053] Comparative Example 1: Adding only α-amylase After determining the optimal conditions for microwave-assisted enzymatic extraction of dietary fiber from roselle residue through single-factor and orthogonal experiments, these optimal conditions were fixed. Only an equal amount of α-amylase was added to the system, without adding cellulase, while all other process parameters remained unchanged. The resulting soluble dietary fiber extraction rate was 17.90%, and the product water-holding capacity was 3.82 g·g⁻¹. -1 The oil holding capacity is 1.91 g·g. -1 The swelling power is 2.35 mL·g. -1 A single α-amylase can only degrade the starch component in the raw material matrix, and it is difficult to destroy the dense fiber cell wall structure. A large amount of soluble dietary fiber is covered by the fiber skeleton and cannot be fully dissolved. Therefore, the extraction efficiency is low, and the fiber pore structure is not well developed, resulting in poor physicochemical properties.

[0054] Comparative Example 2: Adding only a single cellulase After determining the optimal conditions for microwave-assisted enzymatic extraction of dietary fiber from roselle residue through single-factor and orthogonal experiments, these optimal conditions were fixed. The system was then modified by adding only an amount of cellulase equal to the total amount added, without adding α-amylase, while keeping other process parameters unchanged. The resulting soluble dietary fiber extraction rate was 17.60%, and the product water-holding capacity was 3.75 g·g⁻¹. -1The oil holding capacity is 1.86 g·g. -1 The swelling force is 2.28 mL·g. -1 A single cellulase can only cleave the cell wall of fibers and cannot degrade starch particles in the residue. Residual starch will block the internal pores of the fibers and inhibit the dissolution and release of soluble dietary fiber. Not only is the extraction effect limited, but the resulting dietary fiber also has insufficient porous structure, and its water-holding, oil-holding and swelling properties are significantly lower.

[0055] Comprehensive comparative analysis: This invention employs a 1:1 mass ratio of α-amylase and cellulase in a processing scheme. Under uniform microwave processing conditions, the soluble dietary fiber extraction rate can reach 29.60%, and the product water-holding capacity is 4.36 g·g. -1 The oil holding capacity is 2.33 g·g. -1 The swelling force is 3.01 L·g. -1 Under the same microwave pretreatment conditions, the soluble dietary fiber extraction rates of the two single-enzyme experimental groups were only 17.60%~17.90%, significantly lower than that of the compound enzyme system of this invention. The results indicate that the combination of α-amylase and cellulase can produce a significant synergistic degradation effect. α-amylase can decompose starch impurities to unclog fiber pores, while cellulase can fully lyse cell walls to release soluble dietary fiber. The substrate degradation functions of the two enzymes are complementary, overcoming the shortcomings of single enzymes in terms of substrate limitation and insufficient extraction efficiency. This demonstrates the superior technical advantages of the compound enzyme formulation scheme of this invention in both dietary fiber yield and physicochemical quality.

Claims

1. A method for microwave-assisted enzymatic extraction of soluble dietary fiber from hibiscus residue, characterized in that, Includes the following steps: (1) Select the hibiscus residue after water extraction of polyphenols, dry it, crush it, and sieve it to obtain hibiscus residue powder with uniform particle size. (2) Add PBS buffer solution to the roselle residue powder described in step (1), adjust the pH to acidic, microwave, and cool immediately after microwave treatment to obtain a mixture; (3) Add the complex enzyme to the mixture in step (2), perform enzymatic hydrolysis, inactivate the enzyme, cool to room temperature, centrifuge, and collect the supernatant; the complex enzyme is α-amylase and cellulase; (4) Add water-soluble organic solvent to the supernatant in step (3), stir evenly, let stand for alcohol precipitation, centrifuge, pre-freeze the precipitate, and dry it to obtain the soluble dietary fiber product of roselle residue.

2. The method according to claim 1, characterized in that, The power of the microwave in step (2) is 350~450W.

3. The method according to claim 1, characterized in that, The microwave duration in step (2) is 150~210 s.

4. The method according to claim 1, characterized in that, The liquid-to-solid ratio of the PBS buffer solution and roselle residue powder in step (2) is 10-20:

1.

5. The method according to claim 1, characterized in that, The complex enzymes mentioned in step (3) are α-amylase and cellulase.

6. The method according to claim 5, characterized in that, The ratio of α-amylase to cellulase is 1:2~4.

7. The method according to claim 1, characterized in that, The preparation method of the roselle residue after water extraction of polyphenols in step (1) includes: taking dried roselle flowers, adding deionized water, water bath extraction, filtering to remove the filtrate from which polyphenols are extracted, and the resulting filter residue is the roselle residue after water extraction of polyphenols.

8. The method of claim 1, wherein, The pH is adjusted to 4.5-6.0 as described in step (2).

9. The method of claim 1 wherein, The water-soluble organic solvents mentioned in step (4) include ethanol, methanol, and acetone.

10. A method of increasing the yield of soluble dietary fiber from Roselle pomace using the method of any one of claims 1 to 9. Includes the following steps: (1) Select the hibiscus residue after water extraction of polyphenols, dry it, crush it, and sieve it to obtain hibiscus residue powder with uniform particle size. (2) Add PBS buffer solution to the roselle residue powder described in step (1), adjust the pH to acidic, microwave, and cool immediately after microwave treatment to obtain a mixture; (3) Add the complex enzyme to the mixture in step (2), perform enzymatic hydrolysis, inactivate the enzyme, cool to room temperature, centrifuge, and collect the supernatant; the complex enzyme is α-amylase and cellulase; (4) Add water-soluble organic solvent to the supernatant in step (3), stir evenly, let stand for alcohol precipitation, centrifuge, pre-freeze the precipitate, and dry it to obtain the soluble dietary fiber product of roselle residue.

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