A luffa extract having antioxidant activity and a preparation method thereof

Abstract

This invention discloses a loofah extract with antioxidant activity and its preparation method, belonging to the technical field of plant extracts and natural antioxidants. The method uses loofah leaves, loofah peel, or a combination thereof as raw materials. After pretreatment, crushing, and pulping, the materials undergo acidic hypoxia enzyme-inhibiting pretreatment to reduce the activity of polyphenol oxidase and / or peroxidase. Cell wall degrading enzymes are then added for multi-stage enzymatic hydrolysis to release bound polyphenols. Subsequently, an enzyme source with β-glucosidase, β-glucuronidase, and esterase activity is used to perform directional aglycone conversion of the bound polyphenols, converting bound flavonoids and / or bound phenolic acids into free flavonoid aglycones and / or free phenolic acids. Finally, the extract is obtained through alcohol-water extraction, macroporous resin enrichment, microencapsulation, and drying. The extract obtained by this invention has a high proportion of free active ingredients, good antioxidant activity, and good storage stability.

Description

Technical Field

[0001] This invention relates to the field of plant extract preparation technology, specifically to a loofah extract with antioxidant activity and its preparation method. Background Technology

[0002] Luffa is a common plant in the Cucurbitaceae family. Its leaves, peels, and other parts contain polyphenols, flavonoids, phenolic acids, and other components, which have the potential to be developed into plant-derived antioxidant raw materials. Currently, the utilization of luffa resources mainly focuses on consuming the fruit, luffa fiber, or luffa juice, while the utilization rate of by-products such as luffa leaves and peels is relatively low.

[0003] Existing methods for extracting loofah extracts mainly involve water extraction, alcohol extraction, ultrasonic extraction, or reflux extraction, with the extraction rate primarily improved by adjusting solvent concentration, extraction temperature, extraction time, and the solid-liquid ratio. While these methods can yield a certain amount of extract, the following problems remain: First, loofah leaves and peels are prone to enzymatic browning after crushing, and polyphenol oxidase and / or peroxidase can cause the loss of some active ingredients. Second, some flavonoids and phenolic acids in loofah raw materials exist in bound form, making it difficult to fully increase the proportion of free flavonoid aglycones and free phenolic acids through conventional extraction. Third, free active ingredients are sensitive to light, heat, oxygen, and moisture, resulting in insufficient storage stability after direct drying.

[0004] Therefore, there is a need for a method for preparing loofah extract that can reduce oxidation loss, promote the conversion of bound active ingredients, and improve the stability of the extract. Summary of the Invention

[0005] The purpose of this invention is to provide a loofah extract with antioxidant activity and its preparation method, so as to solve the problems of enzymatic browning, insufficient conversion of bound active ingredients, low proportion of free active ingredients, and insufficient storage stability of the extract in the existing loofah extract preparation process.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a method for preparing a loofah extract with antioxidant activity, comprising the following steps: Loofah leaves, loofah peel, or a combination thereof are pretreated and crushed into pulp to obtain loofah raw material pulp; The loofah raw material slurry was subjected to acidic hypoxia enzyme-inhibiting pretreatment to make the pH of the system 3.5-5.5 and to make the system in a hypoxia state, so as to inhibit the activity of polyphenol oxidase and / or peroxidase in the loofah raw material. Cell wall degrading enzymes were added to the loofah raw material slurry that had been pretreated with acidic hypoxia and enzyme inhibitors to carry out multi-stage enzymatic hydrolysis of the cell wall, and the enzymatic hydrolysis products were obtained. The enzymatic hydrolysis product is subjected to a directional conversion of bound polyphenols into aglycones. The directional conversion of bound polyphenols into aglycones uses an enzyme source with β-glucosidase activity, β-glucuronidase activity and / or esterase activity, so as to convert the bound flavonoids and / or bound phenolic acids in the loofah raw material into free flavonoid aglycones and / or free phenolic acids. The converted liquid was subjected to alcohol-water extraction and macroporous resin enrichment to obtain an enriched solution. The enriched solution was microencapsulated and dried to obtain a loofah extract with antioxidant activity.

[0007] Furthermore, the pretreatment includes one or more of the following: washing, impurity removal, low-temperature temporary storage, freeze-thaw treatment, and low-temperature crushing; when the loofah leaves and loofah peels are used in combination, the mass ratio of the two on a dry basis is 1:0.2 to 1:3.

[0008] Furthermore, in the acidic hypoxia enzyme inhibition pretreatment, an acidic color-protecting solution is used to adjust the pH of the system. The acidic color-protecting solution includes one or more of citric acid, ascorbic acid, sodium isoascorbate, phytic acid, malic acid, and lactic acid. The hypoxia state is achieved by one or more of the following methods: nitrogen gas introduction, carbon dioxide introduction, vacuum degassing, deoxygenated water, and closed treatment, ensuring that the dissolved oxygen in the system does not exceed 3.0 mg / L and the oxidation-reduction potential is -180 to +80 mV.

[0009] Furthermore, the cell wall degrading enzyme includes one or more of cellulase, hemicellulase, and pectinase; the multi-stage enzymatic hydrolysis of the cell wall is carried out at a temperature of 35–55°C for 0.5–5 hours and at a pH of 4.0–5.8.

[0010] Furthermore, the directional conversion of bound polyphenols to aglycones includes a first-stage lactic acid bacteria acidification protection treatment and a second-stage glycosidic bond hydrolysis conversion treatment; the first-stage lactic acid bacteria acidification protection treatment uses one or more of Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus brevis, and Lactobacillus casei to lower the pH of the system to 4.0-5.0; the second-stage glycosidic bond hydrolysis conversion treatment uses the enzyme source to convert for 1-12 hours at 30-55℃ and pH 4.0-6.0.

[0011] Furthermore, the enzyme source is selected from one or more of the following: microbial cells, microbial fermentation broth, fermentation supernatant, cell lysate, crude enzyme solution, immobilized enzyme, and enzyme preparation; the enzyme source has a β-glucosidase activity of 5-200 U / g dry basis, a β-glucuronidase activity of 1-100 U / g dry basis, and an esterase activity of 1-80 U / g dry basis.

[0012] Furthermore, the alcohol-water extraction uses an ethanol-water solution with a volume fraction of 30-80%; the macroporous resin enrichment uses one or more macroporous adsorption resins selected from AB-8, D101, HPD-100, HPD-400, and HP-20; the microencapsulation uses one or more of maltodextrin, gum arabic, β-cyclodextrin, hydroxypropyl-β-cyclodextrin, modified starch, sodium alginate, and chitosan as the wall material; and the drying is spray drying or freeze drying.

[0013] Furthermore, after the directional aglycone conversion of the bound polyphenols, the mass ratio of free flavonoid aglycones in the feed solution to total flavonoids increases by more than 50% compared to before the conversion; and / or, the total content of free apigenin, luteolin and quercetin increases by more than 80% compared to before the conversion.

[0014] The present invention also discloses a loofah extract with antioxidant activity prepared by the above preparation method, wherein the free flavonoid aglycones in the loofah extract account for 25-45% of the total flavonoids by mass.

[0015] This invention also discloses the application of the above-mentioned loofah extract with antioxidant activity in the preparation of antioxidants, food additives, cosmetic raw materials, daily chemical care products, cleaning and care compositions or plant-derived functional raw materials.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention employs an acidic, low-oxygen pretreatment to place the loofah pulp in an acidic and low-oxygen environment, thereby reducing the activity of polyphenol oxidase and / or peroxidase, and minimizing the oxidative loss of polyphenols and browning of the pulp during the crushing and pulping process.

[0017] This invention disrupts part of the cell wall structure in loofah leaves and pericarps through multi-level enzymatic hydrolysis of the cell wall, thereby increasing the release of bound polyphenols, bound flavonoids and bound phenolic acids, and providing a substrate basis for subsequent transformation treatment.

[0018] This invention utilizes an enzyme source possessing β-glucosidase, β-glucuronidase, and esterase activities to perform directional aglycone conversion of bound polyphenols. This hydrolyzes some of the bound structures of flavonoid glucosides, flavonoid glucuronides, phenolic acid glycoesters, or phenolic acid esters, thereby converting some bound flavonoids and / or bound phenolic acids into free flavonoid aglycones and / or free phenolic acids, thus increasing the proportion of free active ingredients in loofah extract.

[0019] This invention employs a segmented approach, combining lactic acid bacteria acidification protection treatment with glycosidic bond hydrolysis and transformation treatment. This allows for controlled enzyme inhibition protection and component transformation, reducing potential issues such as browning, off-flavors, and loss of active ingredients that may result from direct fermentation.

[0020] This invention removes some impurities and improves the stability of active ingredients by using alcohol-water extraction, macroporous resin enrichment, and microencapsulation and drying, resulting in a loofah extract with good antioxidant activity and storage stability.

[0021] This invention uses loofah leaves, loofah peel, or a combination thereof as raw materials, which is beneficial to improving the utilization rate of loofah processing by-products. The acidic color-protecting agent, enzyme preparation, ethanol aqueous solution, macroporous resin, and microencapsulated wall material used are all commonly used raw materials or process materials, which are convenient for engineering implementation. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the preparation method of the present invention; Figure 2 HPLC chromatogram of loofah extract without glycosidic bond hydrolysis conversion treatment; Figure 3 This is the HPLC chromatogram of the loofah extract after glycosidic bond hydrolysis. Detailed Implementation

[0023] To make the technical solution, implementation path and beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] Combination Figure 1 The preparation method provided in this embodiment includes: loofah raw material preparation S101, acidic hypoxia enzyme inhibition pretreatment S102, multi-stage enzymatic hydrolysis of cell walls S103, directional conversion of bound polyphenols to aglycones S104, alcohol-water extraction and macroporous resin enrichment S105, microencapsulation and spray drying S106, and obtaining loofah extract with antioxidant activity S107. The steps are sequential processes. S102 is used to reduce the oxidation of polyphenols after crushing; S103 is used to increase the contact opportunity between bound polyphenols and subsequent enzyme sources; S104 is used to convert some bound flavonoids and bound phenolic acids into free flavonoid aglycones and free phenolic acids; and S105 and S106 are used to improve the content of active ingredients and product stability.

[0025] Example 1 In S101, fresh loofah leaves and mature loofah peels are taken, and rotten, insect-damaged, and muddy parts are removed. They are then washed with running water and drained. The loofah leaves and peels are mixed at a dry weight ratio of 1:1 and frozen at -20℃ for 6 hours, then thawed at 4℃, completing one freeze-thaw treatment. This freeze-thaw treatment loosens some of the cell structures in the raw material due to ice crystal formation, facilitating subsequent crushing and enzymatic hydrolysis. The frozen-thawed raw material is added to a crushing tank at a raw material to deionized water weight ratio of 1:5. Before crushing, nitrogen gas is introduced into the crushing tank for 5 minutes, and boiled and cooled deoxygenated water is used. During crushing, the temperature of the liquid is controlled to not exceed 10℃ to obtain loofah raw material slurry.

[0026] In step S102, citric acid and ascorbic acid were added to the loofah slurry to achieve a final concentration of 0.20% for citric acid and 0.08% for ascorbic acid, and the pH of the system was adjusted to 4.5. The mixture was treated at 8°C for 20 minutes under nitrogen protection and with closed stirring. During the treatment, the low-oxygen state was monitored using dissolved oxygen and redox potential electrodes, maintaining dissolved oxygen at approximately 1.1 mg / L and the redox potential at approximately +12 mV. In this step, the acidic conditions reduced the activity of polyphenol oxidase and peroxidase, ascorbic acid provided reductive protection for the small amount of oxidized intermediates already formed, and the low-oxygen environment reduced the oxygen source required for the polyphenol oxidation reaction, thereby reducing browning of the slurry.

[0027]

[0028] As shown in the table above, when acidic color protection and low-oxygen treatment were applied simultaneously, the residual PPO activity, residual POD activity, and browning index were all lower than those in the untreated group. This result indicates that S102 can reduce the oxidative loss of polyphenols in the feedstock before subsequent conversion.

[0029] In step S103, the liquid treated in step S102 is heated to 45°C, and a composite cell wall degrading enzyme is added for enzymatic hydrolysis. This composite cell wall degrading enzyme consists of cellulase, hemicellulase, and pectinase in a mass ratio of 1:1:1, with a total addition amount of 0.8% of the dried loofah raw material. The enzymatic hydrolysis pH is controlled at 4.8, and the hydrolysis time is 2 hours, with continuous stirring during the process. Cellulase degrades some of the cellulose structure, hemicellulase loosens the hemicellulose network, and pectinase disrupts the pectin-binding regions. The combined effect of these three enzymes increases the release of bound polyphenols, bound flavonoids, and bound phenolic acids from the cell wall-embedded regions.

[0030] In S104, lactic acid bacteria are first subjected to acidification and protection treatment, followed by glycosidic bond hydrolysis and transformation treatment. The solution obtained from S103 is cooled to 30°C and inoculated with Lactobacillus plantarum seed culture to achieve an initial viable count of approximately 1 × 10⁻⁶. 7 Fermentation at CFU / mL under hypoxic conditions for 12 hours lowered the system pH from 4.8 to 4.45. The lactic acid bacteria acidification treatment primarily serves to further reduce the system pH and redox potential, and to minimize the impact of endogenous oxidases on polyphenols. Fermentation time should not be too long, as prolonged fermentation increases acidity and may lead to the consumption of some polyphenols.

[0031]

[0032] After acidification and protection treatment, a glycosidic bond hydrolytic enzyme source is added to the feed solution. This enzyme source contains β-glucosidase, β-glucuronidase, and esterase. Based on dried loofah as the raw material, the β-glucosidase activity is 50 U / g, the β-glucuronidase activity is 25 U / g, and the esterase activity is 15 U / g. The pH of the system is adjusted to 5.0, and the conversion is carried out at 42℃ for 5 hours. β-glucosidase hydrolyzes some flavonoid glucosides, β-glucuronidase hydrolyzes some flavonoid glucuronides, and esterase promotes the cleavage of some phenolic acid glycoester or phenolic acid ester binding structures. Through this step, a portion of the bound flavonoids and bound phenolic acids are converted into free flavonoid aglycones and free phenolic acids. After the conversion is completed, the feed solution is heated to 85℃ and held for 5 minutes to terminate the enzyme reaction, and then cooled to 35℃.

[0033] The β-glucosidase activity was determined using p-nitrophenyl-β-D-glucosidase as a substrate, the β-glucuronidase activity was determined using p-nitrophenyl-β-D-glucuronide as a substrate, and the esterase activity was determined using p-nitrophenyl acetate or p-nitrophenyl butyrate as a substrate. All enzyme activities were defined as 1 U, defined as the amount of enzyme required to release 1 μmol of p-nitrophenol per minute. When using other substrates for detection, the conversion was based on the p-nitrophenol release method.

[0034] In S105, a 60% ethanol-water solution was added to the converted feed solution, making the mass-to-volume ratio of feed solution to ethanol-water solution 1:4. Ultrasonic-assisted extraction was performed at a power of 250W, a frequency of 40kHz, and an extraction time of 25 minutes. After extraction, the solution was centrifuged at 5000 r / min for 10 minutes, and the supernatant was collected. The supernatant was enriched using AB-8 macroporous adsorption resin at a flow rate of 1.5 BV / h. The resin was first washed with deionized water until no obvious sugar reaction was observed in the eluent, then eluted with 60% ethanol. The ethanol eluent was collected and concentrated under reduced pressure at 40℃ to a solids content of approximately 20%. The ethanol-water solution was used to extract flavonoids, phenolic acids, and other components, while the macroporous resin was used to remove some sugars, proteins, organic acids, and other water-soluble impurities, and to enrich polyphenols and flavonoids.

[0035] In S106, maltodextrin and gum arabic were added to the enrichment solution as wall materials at a mass ratio of 3:1, with the total amount of wall materials accounting for 25% of the dry matter of the extract. After homogenization, spray drying was performed at an inlet air temperature of 160°C and an outlet air temperature of 78°C, yielding a light yellowish-brown powder. Maltodextrin was used to form a drying carrier and reduce hygroscopicity, while gum arabic was used to improve encapsulation and dispersion. Through microencapsulation, the direct contact between free flavonoids and external oxygen, moisture, and light was reduced, thus slowing the rate of activity decline during storage.

[0036] In S107, a loofah extract with antioxidant activity was obtained. The extract was a light yellow to yellowish-brown powder, dispersible in water, and had no obvious burnt or rancid odor. Analysis showed that its total polyphenol content was 181.6 mg GAE / g, total flavonoid content was 96.3 mg RE / g, and the total amount of free apigenin, luteolin, and quercetin was 27.63 mg / g. The free flavonoid aglycones accounted for 28.7% of the total flavonoids by mass.

[0037] Example 2 The main difference between this embodiment and Example 1 is that the enzyme source used in the second stage of glycosidic bond hydrolysis in S104 is a combination of Aspergillus niger fermentation supernatant and Lactobacillus plantarum cell lysate. Measurements showed that, converted to dried loofah as the raw material, this enzyme source exhibited β-glucosidase activity of 40 U / g, β-glucuronidase activity of 18 U / g, and esterase activity of 10 U / g. The glycosidic bond hydrolysis temperature was 45℃, and the conversion time was 6 hours; all other treatment conditions were the same as in Example 1. This embodiment illustrates that glycosidic bond hydrolysis can utilize commercial enzyme preparations, as well as enzyme sources such as microbial fermentation supernatant and cell lysate, as long as they possess the corresponding glycosidic bond or ester bond hydrolysis activity, achieving the conversion of bound components to free components.

[0038] Example 3 The main difference between this embodiment and Example 1 is that in S104, the β-glucosidase activity is 80 U / g dry basis raw material, the β-glucuronidase activity is 45 U / g dry basis raw material, and the esterase activity is 20 U / g dry basis raw material; the conversion temperature is 45℃, and the conversion time is 6 hours; in S105, the ethanol concentration eluted from the macroporous resin is 70%. All other treatment conditions are the same as in Example 1. Compared with Example 1, the amount of free flavonoid aglycones generated in this embodiment is further increased, but the amount of enzyme preparation and process cost also increase accordingly. Therefore, appropriate conditions can be selected according to the product target.

[0039] Comparative Example To illustrate the effect of each step on the obtained loofah extract, the following comparative examples were set up. Comparative Example 1 used the same raw material and was directly extracted with 60% ethanol at 45°C with stirring for 60 minutes, without acidic hypoxia-induced enzyme inhibition pretreatment, multi-stage cell wall enzymatic hydrolysis, or directional conversion of bound polyphenols to aglycones. Comparative Example 2 was directly extracted with 60% ethanol after acidic color protection. Comparative Example 3 was directly extracted with ethanol after acidic hypoxia-induced enzyme inhibition pretreatment and multi-stage cell wall enzymatic hydrolysis, without glycosidic bond hydrolysis. Comparative Example 4 was fermented with *Lactobacillus plantarum* for 24 hours after acidic hypoxia-induced enzyme inhibition pretreatment and multi-stage cell wall enzymatic hydrolysis, without the addition of glycosidic bond hydrolytic enzyme source. Comparative Example 5 was not subjected to acidic hypoxia-induced enzyme inhibition pretreatment; the loofah was directly crushed, enzymatically hydrolyzed, and subjected to glycosidic bond hydrolysis. Comparative Example 6 was basically the same as Example 1, but the glycosidic bond hydrolytic enzyme source contained only β-glucosidase and esterase, and did not contain β-glucuronidase activity. Comparative Example 7: The concentrated liquid from Example 1 after resin enrichment was directly spray-dried without the addition of maltodextrin and gum arabic for microencapsulation.

[0040] Detection methods Total polyphenol content was determined using the Folin-Ciocalteu method with gallic acid as the standard, and the results are expressed as mgGAE / g. Total flavonoid content was determined using the sodium nitrite-aluminum nitrate-sodium hydroxide colorimetric method with rutin as the standard, and the results are expressed as mgRE / g. Free flavonoid aglycones were detected by HPLC, including apigenin, luteolin, and quercetin. HPLC used a C18 reversed-phase column with a gradient elution of acetonitrile-0.1% formic acid aqueous solution, and the detection wavelength was 330–370 nm. DPPH radical scavenging rate was determined using a 0.10 mg / mL sample solution, ABTS radical scavenging rate was determined using the ABTS radical cation method, and FRAP iron ion reducing power was calculated using the FeSO4 standard curve. PPO activity was detected at 420 nm using catechol as a substrate, and POD activity was detected at 470 nm using a guaiacol and hydrogen peroxide system.

[0041] Experimental results

[0042] As shown in the table above, the content of free aglycones was low when only ethanol extraction was performed; when only cell wall enzymatic hydrolysis was performed, the total polyphenols and total flavonoids increased, but the increase in the proportion of free aglycones was limited; when only lactic acid bacteria fermentation was performed, the proportion of free aglycones increased, but was still lower than that of the complete process. Comparative Example 5 shows that without acidic hypoxia enzyme inhibitor pretreatment, even with glycosidic bond hydrolysis, the total polyphenols and antioxidant activity were affected. Comparative Example 6 shows that without β-glucuronidase activity, the amount of free aglycones generated was lower than that in Example 1. Examples 1 to 3 all showed a high proportion of free aglycones and DPPH scavenging rate.

[0043] Combination Figure 2 and Figure 3 As can be seen from the comparison, in the sample without glycosidic bond hydrolysis, the characteristic peak intensities of bound flavonoids such as P1 and P2 are higher, while the characteristic peak intensities of free flavonoid aglycones such as P4 and P5 are lower. After treatment with S104, the peak intensities of P1 and P2 decrease, while the peak intensities of P4 and P5 increase. This change is consistent with the results of increased free aglycone content in the table above, indicating that some of the bound flavonoids and bound phenolic acids are converted into free components after glycosidic bond hydrolysis. Figure 2 and Figure 3 The weak response peaks not marked in the text are non-target accompanying peaks and are not used as evaluation peaks for the hydrolysis and transformation of glycosidic bonds.

[0044]

[0045] In the table above, P1, P2, and P3 are calculated with the relative peak area before conversion set to 100, while P4 and P5 are normalized with the response value before conversion as a reference. As the conversion time increases, the characteristic peaks of the bound state gradually decrease, while the characteristic peaks of the free flavonoid aglycone gradually increase. After 5 hours of conversion, the peak areas of P4 and P5 approach a plateau, and further extending the conversion to 8 hours does not significantly improve their performance. Therefore, in Example 1, 5 hours was selected as a more suitable conversion time.

[0046]

[0047] The antioxidant activity results showed that the DPPH scavenging rate, ABTS scavenging rate, and FRAP value of Examples 1 and 3 were all higher than those of the comparative example. This indicates that the improved antioxidant activity is not only due to the increased release of total polyphenols, but also related to the increased proportion of free flavonoids and free phenolic acids.

[0048]

[0049] Storage stability results showed that after 90 days of storage at 40°C in the dark, the retention rates of free aglycones and DPPH in Example 1 were both higher than those in the unmicroencapsulated Comparative Example 7. This indicates that microencapsulation and drying in S106 can reduce the loss of free active ingredients during storage.

[0050] In summary, a continuous process is formed between S101 and S107: S101 provides suitable loofah pulp for enzymatic hydrolysis and conversion; S102 reduces oxidative loss; S103 releases bound substrates; S104 increases the proportion of free active ingredients; S105 enriches the target components; S106 improves powder stability; and S107 yields a loofah extract with antioxidant activity. The results of the above examples and comparative examples demonstrate that this method can increase the proportion of free flavonoid aglycones while reducing browning, and improve the antioxidant activity and storage stability of the obtained extract.

[0051] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Those skilled in the art can adjust the raw material ratio, enzyme source type, enzyme activity range, extraction conditions, resin type, or wall material combination based on the disclosure of the present invention. As long as the technical route of acidic hypoxia enzyme inhibition pretreatment, multi-stage enzymatic hydrolysis of cell walls, directional aglycone conversion of bound polyphenols, alcohol-water extraction and macroporous resin enrichment, microencapsulation and drying is still adopted, all of these are reasonable modifications of the technical solutions of the present invention.

Claims

1. A method for preparing a loofah extract with antioxidant activity, characterized in that, Includes the following steps: Loofah leaves, loofah peel, or a combination thereof are pretreated and crushed into pulp to obtain loofah raw material pulp; The loofah raw material slurry was subjected to acidic hypoxia enzyme-inhibiting pretreatment to make the pH of the system 3.5-5.5 and to make the system in a hypoxia state, so as to inhibit the activity of polyphenol oxidase and / or peroxidase in the loofah raw material. Cell wall degrading enzymes were added to the loofah raw material slurry that had been pretreated with acidic hypoxia and enzyme inhibitors to carry out multi-stage enzymatic hydrolysis of the cell wall, and the enzymatic hydrolysis products were obtained. The enzymatic hydrolysis product is subjected to a directional conversion of bound polyphenols into aglycones. The directional conversion of bound polyphenols into aglycones uses an enzyme source with β-glucosidase activity, β-glucuronidase activity and / or esterase activity, so as to convert the bound flavonoids and / or bound phenolic acids in the loofah raw material into free flavonoid aglycones and / or free phenolic acids. The converted liquid was subjected to alcohol-water extraction and macroporous resin enrichment to obtain an enriched solution. The enriched solution was microencapsulated and dried to obtain a loofah extract with antioxidant activity.

2. The preparation method according to claim 1, characterized in that, The pretreatment includes one or more of the following: washing, impurity removal, low-temperature temporary storage, freeze-thaw treatment, and low-temperature crushing; when the loofah leaves and loofah peels are used in combination, the mass ratio of the two on a dry basis is 1:0.2 to 1:

3.

3. The preparation method according to claim 1, characterized in that, The acidic hypoxia enzyme inhibition pretreatment uses an acidic color-protecting solution to adjust the pH of the system. The acidic color-protecting solution includes one or more of citric acid, ascorbic acid, sodium isoascorbate, phytic acid, malic acid, and lactic acid. The hypoxia state is achieved by one or more of the following methods: nitrogen gas introduction, carbon dioxide introduction, vacuum degassing, deoxygenated water, and closed treatment. The dissolved oxygen in the system is kept below 3.0 mg / L, and the oxidation-reduction potential is between -180 and +80 mV.

4. The preparation method according to claim 1, characterized in that, The cell wall degrading enzymes include one or more of cellulase, hemicellulase, and pectinase; the multi-stage enzymatic hydrolysis of the cell wall is carried out at a temperature of 35–55°C for 0.5–5 hours and at a pH of 4.0–5.

8.

5. The preparation method according to claim 1, characterized in that, The directional conversion of bound polyphenols to aglycones includes a first-stage lactic acid bacteria acidification protection treatment and a second-stage glycosidic bond hydrolysis conversion treatment. The first-stage lactic acid bacteria acidification protection treatment uses one or more of Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacillus brevis, and Lactobacillus casei to lower the pH of the system to 4.0–5.

0. The second-stage glycosidic bond hydrolysis conversion treatment uses the enzyme source to convert the enzyme at 30–55°C and pH 4.0–6.0 for 1–12 hours.

6. The preparation method according to claim 1 or 5, characterized in that, The enzyme source is selected from one or more of the following: microbial cells, microbial fermentation broth, fermentation supernatant, cell lysate, crude enzyme solution, immobilized enzyme, and enzyme preparation; the enzyme source has β-glucosidase activity of 5-200 U / g dry basis, β-glucuronidase activity of 1-100 U / g dry basis, and esterase activity of 1-80 U / g dry basis.

7. The preparation method according to claim 1, characterized in that, The alcohol-water extraction uses an ethanol-water solution with a volume fraction of 30-80%; the macroporous resin enrichment uses one or more macroporous adsorption resins selected from AB-8, D101, HPD-100, HPD-400, and HP-20; the microencapsulation uses one or more of maltodextrin, gum arabic, β-cyclodextrin, hydroxypropyl-β-cyclodextrin, modified starch, sodium alginate, and chitosan as the wall material; and the drying is performed by spray drying or freeze drying.

8. The preparation method according to claim 1, characterized in that, After the directional aglycone conversion of the bound polyphenols, the proportion of free flavonoid aglycones in the total flavonoids in the feed solution increases by more than 50% compared with that before the conversion; and / or, the total content of free apigenin, luteolin and quercetin increases by more than 80% compared with that before the conversion.

9. A loofah extract with antioxidant activity prepared by the method according to any one of claims 1 to 8, characterized in that, The free flavonoid aglycones in the loofah extract account for 25-45% of the total flavonoids by mass.

10. The use of the loofah extract with antioxidant activity as described in claim 9 in the preparation of antioxidants, food additives, cosmetic raw materials, daily chemical care products, cleaning and care compositions or plant-derived functional raw materials.

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