A method for preparing a low methoxyl pectin from hibiscus flowers

Abstract

The application discloses a preparation method of low-methoxyl pectin from hibiscus flowers, and comprises the following steps: S1, drying and pretreating fresh hibiscus flowers to obtain dried hibiscus flowers; S2, crushing and screening the dried hibiscus flowers to obtain hibiscus flower powder; S3, mixing the hibiscus flower powder with a citric acid aqueous solution, adjusting the pH value to 1.5-2.5, and extracting at 90-100 DEG C for 50-80 min to obtain an extract; S4, performing solid-liquid separation on the extract to collect a liquid phase component; and S5, mixing the liquid phase component with ethanol after concentration to perform precipitation, collecting the precipitate, and drying to obtain the low-methoxyl pectin from hibiscus flowers. The application first uses hibiscus flower organs as raw materials to prepare low-methoxyl pectin, develops a new raw material source for pectin extraction, and realizes comprehensive utilization of hibiscus flower resources.

Description

Technical Field

[0001] This invention relates to the field of natural product extraction methods, and in particular to a method for preparing low-methoxyl pectin from hibiscus flowers. Background Technology

[0002] Pectin is an acidic heteropolysaccharide widely found in the primary cell walls and mesoglea of ​​higher plants. It is a key structural component for maintaining cell adhesion, tissue rigidity, and cell wall integrity, and has wide applications in food, medicine, and functional materials. Commercial pectin is mainly derived from fruit and vegetable processing byproducts such as citrus peel and apple pomace. However, with the advancement of research into functional foods and natural polymer materials, more and more unconventional plant resources are being used for pectin development. Based on its degree of esterification (DE), pectin can be classified into high-methoxyl pectin (DE>50%) and low-methoxyl pectin (DE<50%). Low-methoxyl pectin can form an "egg carton" type gel network mediated by Ca²⁺ over a wider pH range, making it more suitable for low-sugar, low-calorie, and functional food systems.

[0003] Hibiscus ( Hibiscus syriacus Hibiscus (L.) is the floral organ of plants in the genus Hibiscus of the Malvaceae family. It possesses ornamental, edible, and medicinal value, and is a unique plant resource with significant development potential in recent years. Existing research indicates that Hibiscus flowers are rich in pectin-type acidic polysaccharides, with galacturonic acid (GalA) as the main component, exhibiting typical pectin structural characteristics and certain antioxidant activity. However, fresh Hibiscus flowers have a high water content and delicate tissues, making them highly susceptible to wilting, browning, and spoilage after harvesting, resulting in a short shelf life and difficulty meeting the demands of storage, transportation, and large-scale processing. Currently, research on systematic preparation methods for Hibiscus flower pectin is still lacking, especially regarding how to obtain high-quality Hibiscus flower pectin through reasonable raw material pretreatment and extraction processes; no mature technical solution has yet been found. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by providing a method for preparing low-methoxyl pectin from hibiscus flowers. This method fills the technological gap in the lack of hibiscus flower organs as raw materials for preparing low-methoxyl pectin, and also addresses the technical problem that existing pectin extraction methods are unable to directionally control the structure and function of pectin according to the needs of the target product.

[0005] To solve the above-mentioned technical problems, the present invention provides a method for preparing low-methoxyl pectin from hibiscus flowers, comprising the following steps: S1: Pre-dry fresh hibiscus flowers to obtain dried hibiscus flowers; S2: The dried hibiscus flowers obtained in S1 are crushed and sieved to obtain hibiscus pollen; S3: Mix the hibiscus pollen obtained in S2 with a citric acid aqueous solution, adjust the pH to 1.5~2.5, and extract at 90~100℃ for 50~80 min to obtain the extract; S4: Perform solid-liquid separation on the extract obtained in S3 and collect the liquid phase components; S5: The liquid phase component obtained in S4 is concentrated and mixed with ethanol to precipitate. The precipitate is collected, dried, and hibiscus low-methoxyl pectin is obtained.

[0006] In the above scheme, preferably, the drying pretreatment in S1 is microwave drying. The microwave drying adopts an intermittent heating program with a microwave power of 150~600 W, each heating for 10~60 s and a pause for 10~90 s, drying until the moisture content is ≤10%.

[0007] In the above scheme, preferably, the microwave power of the microwave drying is 250~350 W, each heating is 20~40 s and the rest is 20~40 s, and the moisture content is dried to ≤8%.

[0008] In the above scheme, preferably, the drying pretreatment in S1 is vacuum freeze drying, which includes: pre-freezing fresh hibiscus flowers at -60~-80℃ for 4~12 h, and then transferring them to a vacuum freeze dryer, drying them to constant weight under the conditions of cold trap temperature -50~-90℃, vacuum degree ≤100 Pa, and partition temperature -10~30℃.

[0009] In the above scheme, preferably, the cold trap temperature of the vacuum freeze-drying is -70~-80℃, the vacuum degree is ≤5 Pa, and the partition temperature is 5~15℃.

[0010] In the above scheme, preferably, in step S3, before mixing the hibiscus pollen with the citric acid aqueous solution, the step of pretreating the hibiscus pollen with ethanol is further included: the ethanol pretreatment involves mixing the hibiscus pollen with an ethanol solution with a volume concentration of 40% to 90%, stirring or sonicating for 0.5 to 2 hours, separating the solid and liquid, collecting the filter residue, drying it, and using it for subsequent extraction.

[0011] In the above scheme, preferably, when the drying pretreatment of S1 is microwave drying, the ethanol pretreatment uses an ethanol solution with a volume concentration of 40%~70%, the treatment time is 10~40 min, and it is repeated 1~2 times; when the drying pretreatment of S1 is vacuum freeze drying, the ethanol pretreatment uses an ethanol solution with a volume concentration of 60%~90%, the treatment time is 0.5~2 h, and it is repeated 2~5 times.

[0012] In the above scheme, preferably, the material-to-liquid ratio of the citric acid aqueous solution in S3 is 1:20~1:30 g / mL, the pH value of the extraction is 1.8~2.2, the temperature is 92~98℃, and the time is 60~70 min; the concentration in S5 is concentrated under reduced pressure to 1 / 3~2 / 3 of the original volume, the ethanol is anhydrous ethanol or ethanol with a volume concentration ≥95%, the volume ratio of the liquid phase component to ethanol is 1:3~1:5, and the precipitation is carried out at 2~8℃ for 8~24 h.

[0013] In the above scheme, preferably, the hibiscus flower mentioned in S1 is Hibiscus syriacus (family Malvaceae, genus Hibiscus). Hibiscus syriacus The floral organs of the hibiscus (L.) are dried within 0.5 to 6 hours after harvesting.

[0014] A method for preparing hibiscus low-methoxyl pectin, wherein the hibiscus low-methoxyl pectin has an esterification degree <50%, a galacturonic acid content ≥60%, an HG domain content ≥58%, an R1 value ≥2.30, a weight-average molecular weight ≥620 kDa, and a polydispersity index ≤1.70.

[0015] The beneficial effects of this invention are: This invention is the first to prepare low-methoxyl pectin using hibiscus flower organs as raw materials, opening up a new source of raw materials for pectin extraction and realizing the comprehensive utilization of hibiscus flower resources.

[0016] This invention pre-treats hibiscus flowers by microwave drying or vacuum freeze-drying, elevating the drying method from a simple dehydration and preservation approach to a pre-regulation method for pectin structure and function. The microwave drying route yields raw materials with good phenol / flavonoid retention, suitable for the development of antioxidant-oriented products. The vacuum freeze-drying route yields structure-preserving pectin with high GalA content, high HG domain content, high molecular weight, and low polydispersity index.

[0017] Meanwhile, the present invention employs differentiated ethanol pretreatment strategies based on different drying routes before extraction. The microwave drying route uses low-concentration ethanol for short-time treatment to retain phenolic / flavonoid active ingredients, while the vacuum freeze-drying route uses high-concentration ethanol for multiple treatments to remove pigments, lipids, and small molecule impurities, thereby improving the quality of subsequent pectin products.

[0018] This invention uses citric acid aqueous solution for extraction, without the use of ultrasound-assisted extraction. The process is simple, requires low equipment, is suitable for industrial production, and avoids overlap with existing patents on ultrasound-assisted pectin extraction.

[0019] Furthermore, the hibiscus low-methoxyl pectin prepared by this invention has an esterification degree of <50%, belonging to low-methoxyl pectin, which is suitable for low-sugar gel systems; galacturonic acid content ≥60%, HG domain content ≥58%, and pectin linear structure is intact; weight-average molecular weight ≥620 kDa, polydispersity index ≤1.70, large molecular weight and uniform distribution, with good thickening, gelling and stability. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: Figure 1 .

[0022] Example 1

[0023] Select hibiscus flowers that are in full bloom, free from mold, browning, and pests. Hibiscus syriacus Remove flower stems and impurities, and process within 2 hours of harvesting. Place fresh hibiscus flowers in a single layer on a microwave drying tray, set the microwave power to 300 W, and use an intermittent heating program of 30 s heating followed by a 30 s pause, turning them regularly during the process, until the moisture content is approximately 6%~7%.

[0024] The dried hibiscus flowers were pulverized and passed through a 60-mesh sieve to obtain hibiscus pollen. The hibiscus pollen was mixed with a 60% ethanol solution at a ratio of 1:15 g / mL, stirred for 20 min, filtered, and the process was repeated once. The filter residue was dried at 50℃ to constant weight.

[0025] Pretreated hibiscus pollen with ethanol was mixed with citric acid aqueous solution at a ratio of 1:25 g / mL. The pH was adjusted to 2.0 with citric acid, and the mixture was extracted at 95℃ for 65 min. After extraction, the mixture was centrifuged at 4000 r / min for 10 min, and the supernatant was collected. The supernatant was concentrated under reduced pressure to half its original volume, and four times its volume of anhydrous ethanol was added. The mixture was then allowed to stand overnight at 4℃. The precipitate was collected by centrifugation, washed 2-3 times with anhydrous ethanol, and then freeze-dried under vacuum to obtain hibiscus low-methoxyl pectin.

[0026] The yield of the low-methoxyl pectin from hibiscus obtained in this example was determined to be 24%–26%, with a degree of esterification of 45%–50%, a galacturonic acid content of 61.00±1.30%, an HG domain content of 56.60±1.32%, an R1 value of 1.9458±0.0667, a weight-average molecular weight of 567.57±8.30 kDa, and a polydispersity index of 1.977±0.099. Among these, the R1 value is the molar ratio of (galactose + arabinose) / rhamnose, which is used to characterize the degree of branching of the RG-I domain.

[0027] Example 2

[0028] Select hibiscus flowers that are in full bloom, free from mold, browning, and pests, remove flower stalks and impurities, and process them within 2 hours of harvesting. Pre-freeze the fresh hibiscus flowers at -80℃ for 6 hours, then transfer them to a vacuum freeze dryer with a cold trap temperature of -80℃, a chamber vacuum degree of <5 Pa, and a partition temperature of 10℃, and dry them to constant weight.

[0029] The freeze-dried hibiscus flowers were pulverized and passed through a 60-mesh sieve to obtain hibiscus pollen. The hibiscus pollen was mixed with an 80% ethanol solution at a ratio of 1:15 g / mL, and the mixture was sonicated or stirred for 1 h. The mixture was then filtered, and the process was repeated 4 times. The filter residue was dried at 50°C to constant weight.

[0030] Pretreated hibiscus pollen with ethanol was mixed with citric acid aqueous solution at a ratio of 1:25 g / mL. The pH was adjusted to 2.0 with citric acid, and the mixture was extracted at 95℃ for 65 min. After extraction, the mixture was centrifuged at 4000 r / min for 10 min, and the supernatant was collected. The supernatant was concentrated under reduced pressure to half its original volume, and four times its volume of anhydrous ethanol was added. The mixture was then allowed to stand overnight at 4℃. The precipitate was collected by centrifugation, washed 2-3 times with anhydrous ethanol, and then freeze-dried under vacuum to obtain hibiscus low-methoxyl pectin.

[0031] The yield of hibiscus low-methoxyl pectin obtained in this example was determined to be 25%~27%, with a degree of esterification <50%, a galacturonic acid content of 66.50±1.40%, an HG domain content of 62.50±1.41%, an R1 value of 2.4359±0.0828, a weight-average molecular weight of 667.23±9.76 kDa, a number-average molecular weight of 429.26±19.84 kDa, and a polydispersity index of 1.554±0.078.

[0032] Example 3

[0033] Select hibiscus flowers that are in full bloom, free from mold, browning, and pests. Remove flower stems and impurities, and process them within 4 hours of harvesting. Place the fresh hibiscus flowers in a single layer on a microwave drying tray, set the microwave power to 250 W, and use an intermittent heating program of 40 seconds followed by a 40-second pause, turning them regularly during the process, until the moisture content is ≤8%.

[0034] The dried hibiscus flowers were pulverized and passed through a 40-mesh sieve to obtain hibiscus pollen. The hibiscus pollen was mixed with a 50% ethanol solution at a ratio of 1:10 g / mL, ultrasonicated for 30 min, filtered, and the process was repeated once. The filter residue was dried at 50℃ to constant weight.

[0035] Pretreated hibiscus pollen with ethanol was mixed with citric acid aqueous solution at a ratio of 1:20 g / mL. The pH was adjusted to 2.2 with citric acid, and the mixture was extracted at 92℃ for 70 min. After extraction, the mixture was centrifuged at 5000 r / min for 8 min, and the supernatant was collected. The supernatant was concentrated under reduced pressure to 2 / 3 of its original volume, and 3 times its volume of anhydrous ethanol was added. The mixture was then allowed to stand at 6℃ for 10 h. The precipitate was collected by centrifugation, washed 2-3 times with anhydrous ethanol, and dried under low temperature and vacuum to obtain hibiscus low-methoxyl pectin.

[0036] Example 4

[0037] Select hibiscus flowers that are in full bloom, free from mold, browning, and pests, remove flower stalks and impurities, and process them within 1 hour of harvesting. Pre-freeze the fresh hibiscus flowers at -70℃ for 8 hours, then transfer them to a vacuum freeze dryer with a cold trap temperature of -75℃, a chamber vacuum degree of <8 Pa, and a partition temperature of 8℃, and dry them to constant weight.

[0038] The freeze-dried hibiscus flowers were pulverized and passed through a 60-mesh sieve to obtain hibiscus pollen. The hibiscus pollen was mixed with a 70% ethanol solution at a ratio of 1:20 g / mL, stirred for 1.5 h, filtered, and the mixture was repeated 3 times. The filter residue was dried at 50℃ to constant weight.

[0039] Pretreated hibiscus pollen with ethanol was mixed with citric acid aqueous solution at a ratio of 1:30 g / mL. The pH was adjusted to 1.8 with citric acid, and the mixture was extracted at 98℃ for 60 min. After extraction, the mixture was centrifuged at 3000 r / min for 15 min, and the supernatant was collected. The supernatant was concentrated under reduced pressure to 1 / 3 of its original volume, and 5 times its volume of anhydrous ethanol was added. The mixture was then allowed to stand at 2℃ for 24 h. The precipitate was collected by centrifugation, washed 2-3 times with anhydrous ethanol, and then freeze-dried under vacuum to obtain hibiscus low-methoxyl pectin.

[0040] Comparative Example 1 Hot air drying is used instead of microwave drying or vacuum freeze drying, and the remaining steps are the same as in Example 1.

[0041] Fresh hibiscus flowers were placed in a hot air drying oven and dried at 60°C to constant weight. After pulverizing and sieving, they were subjected to ethanol pretreatment, citric acid extraction, alcohol precipitation, and drying under the same conditions as in Example 1.

[0042] The results showed that the total phenolic and total flavonoid contents of the hibiscus flower raw materials dried in the hot air-drying group were significantly lower than those in the microwave-drying group. The resulting pectin had a galacturonic acid content of 57.00±1.25%, an HG domain content of 53.80±1.26%, a weight-average molecular weight of 609.89±8.92 kDa, and a polydispersity index of 1.599±0.080. These results indicate that hot air drying is not conducive to the preservation of phenolic / flavonoid active components, and is not as beneficial as vacuum freeze-drying for obtaining pectin with high GalA / HG structure retention.

[0043] Comparative Example 2 Natural air drying was used instead of microwave drying or vacuum freeze drying, and the remaining steps were the same as in Example 1.

[0044] Fresh hibiscus flowers were placed in a well-ventilated, shady place to air dry naturally until constant weight. After pulverizing and sieving, they were subjected to ethanol pretreatment, citric acid extraction, alcohol precipitation, and drying under the same conditions as in Example 1.

[0045] The pectin obtained from the naturally air-dried group had a galacturonic acid content of 56.50±1.20%, an HG domain content of 50.70±1.24%, and an R1 value of 1.5565±0.0537, indicating a high degree of branching. The results suggest that long-term environmental exposure during natural air drying can lead to an increased proportion of RG-I and neutral sugar side chains, which is detrimental to obtaining pectin with a high HG linear structure. A detailed comparison is shown in the table below.

[0046] HAD 57.00±1.25 53.80±1.26 1.6789±0.0584 609.89±8.92 381.37±17.62 1.599±0.080 The molecular weight is acceptable, but the activity is poorly maintained. MD 61.00±1.30 56.60±1.32 1.9458±0.0667 567.57±8.30 287.07±13.27 1.977±0.099 Active ingredients have significant advantages; pectin is suitable for compound formulations. ND 56.50±1.20 50.70±1.24 1.5565±0.0537 591.48±8.65 343.89±15.89 1.720±0.086 It has a high degree of branching and is suitable for comparative studies. SD 62.30±1.32 57.35±1.34 2.0426±0.0685 460.40±6.74 233.28±10.78 1.974±0.099 Photo-oxidation exposure leads to low molecular weight VFD 66.50±1.40 62.50±1.41 2.4359±0.0828 667.23±9.76 429.26±19.84 1.554±0.078 Maintain optimal structure, with primary protection as the focus. The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing low-methoxyl pectin from hibiscus flowers, characterized in that: Includes the following steps: S1: Pre-dry fresh hibiscus flowers to obtain dried hibiscus flowers; S2: The dried hibiscus flowers obtained in S1 are crushed and sieved to obtain hibiscus pollen; S3: Mix the hibiscus pollen obtained in S2 with a citric acid aqueous solution, adjust the pH to 1.5~2.5, and extract at 90~100℃ for 50~80 min to obtain the extract; S4: Perform solid-liquid separation on the extract obtained in S3 and collect the liquid phase components; S5: The liquid phase component obtained in S4 is concentrated and mixed with ethanol to precipitate. The precipitate is collected, dried, and hibiscus low-methoxyl pectin is obtained.

2. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 1, characterized in that: The drying pretreatment described in S1 is microwave drying. The microwave drying adopts an intermittent heating program with a microwave power of 150~600 W. Each heating lasts for 10~60 s, followed by a pause of 10~90 s, until the moisture content is ≤10%.

3. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 2, characterized in that: The microwave drying process uses a microwave power of 250~350 W, with each heating cycle lasting 20~40 s followed by a 20~40 s pause, drying until the moisture content is ≤8%.

4. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 1, characterized in that: The drying pretreatment described in S1 is vacuum freeze drying, which includes: pre-freezing fresh hibiscus flowers at -60~-80℃ for 4~12h, then transferring them to a vacuum freeze dryer and drying them to constant weight under the conditions of cold trap temperature -50~-90℃, vacuum degree ≤100 Pa, and partition temperature -10~30℃.

5. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 4, characterized in that: The cold trap temperature for the vacuum freeze-drying process is -70 to -80°C, the vacuum degree is ≤5 Pa, and the partition temperature is 5 to 15°C.

6. A method for preparing low-methoxyl pectin from hibiscus flowers according to any one of claims 1-5, characterized in that: In step S3, before mixing the hibiscus pollen with the citric acid aqueous solution, an ethanol pretreatment step is also included: the ethanol pretreatment involves mixing the hibiscus pollen with an ethanol solution of 40% to 90% by volume, stirring or sonicating for 0.5 to 2 hours, separating the solid and liquid, collecting the filter residue, drying it, and using it for subsequent extraction.

7. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 6, characterized in that: When the drying pretreatment of S1 is microwave drying, the ethanol pretreatment uses an ethanol solution with a volume concentration of 40%~70%, the treatment time is 10~40 min, and it is repeated 1~2 times; when the drying pretreatment of S1 is vacuum freeze drying, the ethanol pretreatment uses an ethanol solution with a volume concentration of 60%~90%, the treatment time is 0.5~2 h, and it is repeated 2~5 times.

8. A method for preparing low-methoxyl pectin from hibiscus flowers according to any one of claims 1-5, characterized in that: In step S3, the citric acid aqueous solution has a material-to-liquid ratio of 1:20 to 1:30 g / mL. The extraction pH is 1.8 to 2.2, the temperature is 92 to 98°C, and the extraction time is 60 to 70 min. In step S5, the concentration is carried out under reduced pressure to 1 / 3 to 2 / 3 of the original volume. The ethanol is anhydrous ethanol or ethanol with a volume concentration ≥95%. The volume ratio of the liquid phase component to ethanol is 1:3 to 1:

5. The precipitation is carried out at 2 to 8°C for 8 to 24 h.

9. The method for preparing low-methoxyl pectin from hibiscus flowers according to claim 1, characterized in that: The hibiscus flower mentioned in S1 refers to Hibiscus syriacus, belonging to the Malvaceae family and the Hibiscus genus. Hibiscus syriacus The floral organs of the hibiscus (L.) are dried within 0.5 to 6 hours after harvesting.

10. A hibiscus low-methoxyl pectin prepared by the method for preparing hibiscus low-methoxyl pectin according to claim 4 or 5, characterized in that: The hibiscus low-methoxyl pectin has an esterification degree of <50%, a galacturonic acid content of ≥60%, an HG domain content of ≥58%, an R1 value of ≥2.30, a weight-average molecular weight of ≥620 kDa, and a polydispersity index of ≤1.70.

Images