CN111349183B

CN111349183B - Pectin and its extraction method and preparation method

Info

Publication number: CN111349183B
Application number: CN202010322481.3A
Authority: CN
Inventors: 金丽梅; 高宇; 左锋
Original assignee: Heilongjiang Bayi Agricultural University
Current assignee: Heilongjiang Bayi Agricultural University
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2022-04-01
Anticipated expiration: 2040-04-22
Also published as: CN111349183A

Abstract

The invention discloses pectin and an extraction method and a preparation method thereof, wherein the extraction method comprises the following steps: adding organic weak acid salt into the acidic pectin extract, mixing, performing membrane filtration on the extract treated by the organic weak acid salt, performing alcohol precipitation or salting out on the extract, and separating to obtain the pectin. Wherein the addition amount of the organic weak acid salt in the mixed solution is 0.2-1 mg/mL. Because a proper amount of organic weak acid salt is added before membrane filtration, the pectin colloid is further stabilized, so that the pectin is not easy to precipitate on the surface of the membrane during membrane concentration, and meanwhile, pigments, saccharides, calcium, magnesium and other salts in the pectin can be removed, the membrane flux can be improved, and the membrane pollution can be effectively slowed down. In addition, in the salting-out process, low-valence salt can be used as a precipitating agent, so that complexation of high-valence cations in a salt solution and the pectin is avoided, the pectin can be easily desalted, the problems of low extraction rate, heavy color and low purity of the pectin are solved, and the ash content of the pectin is not high.

Description

Pectin and its extraction method and preparation method

Technical Field

The invention relates to the technical field of pectin preparation, and particularly relates to pectin and an extraction method and a preparation method thereof.

Background

Pectin is a natural, high-molecular substance composed of galacturonic acid, and is used as a gelling agent, a thickener, and the like in the food industry and the pharmaceutical industry, and the world demand is increasing year by year. For example, citrus peel contains a large amount of pectin (about 25%), and China generates a large amount of waste citrus peel every year, so extraction of pectin from citrus peel waste is an effective method for reasonable resource utilization.

At present, the traditional acid method is still used for extraction in the pectin extraction process, but the pectin extraction yield is generally low in the traditional acid method, and a large amount of phosphorus-containing sewage generated in the pectin production process is difficult to treat due to the sodium hexametaphosphate added in the acid extraction process. The pectin precipitation method mainly comprises an alcohol precipitation method and a salting-out method. The salting-out method generally adopts an aluminum salt precipitation method, an iron salt precipitation method, a calcium salt precipitation method, a mixed salting-out method and the like, but in the actual production, although high-valence positive salt can precipitate pectin, the obtained pectin finished product has heavier color, high ash content and poorer pectin quality, and is generally less adopted. Therefore, the extraction of pectin by alcohol precipitation is more common. However, the alcohol precipitation method consumes a large amount of alcohol (2-3 tons of alcohol are consumed per ton of finished pectin on average), because the pectin extract has a large volume and cannot be effectively concentrated, so that the ethanol solvent is used in large amount during alcohol precipitation. At present, the concentration of the pectin extracting solution is often improved by adopting a vacuum concentration method, and although some researches adopt an ultrafiltration membrane technology for concentration, the membrane pollution is serious, the membrane flux is small, the effect of effectively concentrating the acidic pectin extracting solution cannot be achieved, and the application value is not realized.

Therefore, the prior art has the technical problems that the pectin extracting solution can not be effectively subjected to membrane concentration, so that a large amount of alcohol and salt are consumed for alcohol precipitation or salting out, and the obtained pectin has poor quality.

In view of this, the invention is particularly proposed.

Disclosure of Invention

The invention aims to provide pectin and an extraction method and a preparation method thereof, so as to improve the problems.

The invention is realized by the following steps:

the embodiment of the invention provides a pectin extraction method, which comprises the following steps: adding organic weak acid salt into acidic pectin extract obtained by separation after extraction of pectin raw material, uniformly mixing, and then carrying out membrane filtration on the extract treated by the organic weak acid salt. Wherein the addition amount of the organic weak acid salt in the mixed solution is 0.2-1 mg/mL, optionally 0.2-0.25 mg/mL.

The embodiment of the invention also provides a preparation method of pectin, which comprises the step of carrying out alcohol precipitation or salting out on the membrane concentrated extract obtained by the extraction method of the pectin in the embodiment, and preferably, carrying out salting out on the membrane concentrated extract.

The embodiment of the invention also provides pectin prepared by the preparation method of the pectin in the embodiment.

Optionally, the pectin has a galacturonic acid content of greater than 89 wt%, a degree of esterification of greater than 77%, an ash content of less than 1.3 wt%, and an acid-insoluble ash content of less than 0.2 wt%.

The technical scheme of the embodiment of the invention has the following beneficial effects: by adding proper amount of pectin into acidic pectin extractive solutionOrganic weak acid salt for further separating calcium ions in macromolecular pectin dissolved in water from H⁺Replacing, changing macromolecular pectin into micromolecular pectin, further stabilizing pectin colloid in the extracting solution, ensuring that pectin solid is not easy to precipitate in the processed acidic extracting solution in the membrane filtering process, ensuring that the pectin solid is not easy to appear in the pectin extracting solution and is deposited on the surface of a membrane to cause membrane pollution, simultaneously complexing organic weak acid with impurities such as pigment, saccharide, calcium, magnesium and the like to form a complex with good water solubility, dissolving the complex in water, entering a penetrating fluid through membrane pores, reducing the adsorption of the substances on the surface of the membrane, effectively relieving the membrane pollution, and inevitably taking away a part of water molecules when the water-soluble complexes pass through the membrane pores, increasing the water flux, improving the flux in the ultrafiltration process and reducing the membrane pollution, furthermore, the ultrafiltration concentrated solution can be further concentrated in a forward osmosis system, so that the aim of effectively concentrating the concentration of the pectin extracting solution is fulfilled. After the acidic pectin extract is effectively concentrated, the subsequent salting-out or alcohol precipitation is carried out on the pectin extract due to Ca²⁺And impurities such as pigments and the like are removed, a small amount of low-valent salt or alcohol can be adopted to precipitate or precipitate the pectin, and the pectin is easily desalted after salting out, so that the problem of heavy color of the pectin is solved, the ash content of the pectin is not high, and the quality of the pectin is improved.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained according to the drawings without inventive efforts.

FIG. 1 is a graph showing the effect of ultrafiltration membranes of different molecular weight cut-offs on color and clarity of a pectin retentate;

FIG. 2 is a graph showing the effect of different salts of weak organic acids on the flux of ultrafiltration membrane membranes;

FIG. 3 is a graph showing the effect of various salts of weak organic acids on color and clarity of a pectin retentate;

FIG. 4 is a graph showing the effect of sodium citrate on the Zeta potential of pectin extract;

FIG. 5 is a graph showing the effect of different amounts of sodium citrate on membrane flux;

FIG. 6 is a comparison of color and clarity of ultrafiltration retentate with varying amounts of sodium citrate added;

FIG. 7 shows the average particle size and particle size distribution of pectin prepared with different amounts of sodium citrate;

FIG. 8 is a photograph of pectins prepared by different processes.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The pectin provided by the invention and the extraction method and preparation method thereof are specifically explained below. Some embodiments of the present invention provide a method for extracting pectin, comprising: adding organic weak acid salt into acidic pectin extract obtained by separation after extraction of pectin raw material, uniformly mixing, and then carrying out membrane filtration on the extract treated by the organic weak acid salt. Wherein the addition amount of the organic weak acid salt in the mixed solution is 0.2-1 mg/mL, optionally 0.2-0.25 mg/mL.

Pectin has a large number of carboxyl groups, typically through Mg in the peel²⁺、Ca²⁺Linking pectin into macromolecules, insoluble in water, and hydrolyzing with acidic solution such as hydrochloric acid to convert carboxylic acid groups on pectin into H⁺Form i.e. the true carboxylic acid, which is soluble in water. While CaCl₂The solubility in water is particularly high, and calcium ions originally combined with carboxylate radicals on pectin are replaced by H⁺Ion replacementAfter the pectin is dissolved in water, the inventor finds that some macromolecular pectins are dissolved in water in the pectin extraction process through an acidic solution, so that when the acidic extracting solution is further concentrated through membrane filtration, the macromolecular pectins are easily precipitated and separated from the surface of a membrane, and the pectin concentration is increased during concentration, so that the pectin solution system is unstable, part of pectin is separated out, membrane pores are blocked, and the application effect of the membrane filtration concentrated pectin extracting solution is poor.

On the basis, through a great deal of research and realization, the inventor creatively adds a proper amount (0.2-1 mg/mL) of organic weak acid salt into the acidic extracting solution for treatment and then performs membrane filtration, thereby solving the problems that the acidic pectin extracting solution is difficult to be effectively concentrated to cause large usage amount of alcohol or salt and low pectin quality. The reason for this may be that, under the action of strong acid, the organic weak acid salt is quickly and completely converted into weak acid (organic acid), and the organic acid further acts on the pectin colloid, which degrades the macromolecular pectin colloid into smaller pectin colloid, so that the particle size of the pectin molecules in water is smaller and more uniform, and the deposition of pectin on the surface of the film is not easily caused. Meanwhile, the addition of the organic weak acid salt can also increase the Zeta potential of pectin particles in a solution system and increase the stability of a pectin solution system, namely, the phenomenon that pectin is separated out to block membrane pores due to the fact that the pectin system is unstable caused by the increase of the concentration of a concentrated solution during membrane filtration is avoided. In addition, the water-soluble complex formed by the organic weak acid and the small molecular substance is in a hydrated state in water, and the water-soluble complex inevitably carries away a part of water molecules when passing through the membrane pores, so that the water flux is increased. During hydrochloric acid hydrolysis, no organic weak acid salt is added, and pigments, saccharides, calcium, magnesium and other salts are likely to be adsorbed on the membrane to pollute the membrane, so that the water permeability resistance of the membrane is increased, and the water flux is reduced. After the organic weak acid salt is added, the impurities are removed, and the membrane flux is increased. Further, the organic acid can be mixed with pigment, saccharide, CaCl₂The small molecules of impurities such as salts are complexed, the molecular weight of pectin is about 7-14 ten thousand, so that an ultrafiltration membrane with the cutting molecular weight of 2-100 kDa is adoptedAfter filtration, the small molecules can enter the ultrafiltration permeating liquid to be removed, and the purposes of concentrating the pectin extracting solution and removing impurities are synchronously realized.

Therefore, the membrane pollution can be prevented in the membrane filtration in the aspects, so that the acidic pectin extraction can be effectively concentrated, the use amount of alcohol or salt can be greatly reduced in the subsequent pectin separation process, and the pectin quality is improved.

In addition, compared with the existing method of directly salting out the acidic extract, the pectin extract is greatly concentrated to facilitate pectin precipitation, so that the salt used in salting out can be high-priced Al³⁺Or Fe³⁺The purpose of pectin precipitation can be realized by using a small amount of low-valence salt, so that the complexation of high-valence cations in a salt solution and the pectin can be avoided, the pectin desalination is easy to carry out, and the problems of heavy color and high ash content of the pectin are solved. Meanwhile, the pectin with good color value and low ash content can be obtained by salting out by adding a small amount of salt, so the pectin preparation method capable of reducing the consumption of ethanol or salt is also provided, and the pectin can be extracted and prepared even without using ethanol.

In particular, in some embodiments, the salt of the weak organic acid comprises at least one of a sodium salt of a weak organic acid and a potassium salt of a weak organic acid. For example, the weak organic acid salt includes, but is not limited to, at least one of sodium citrate, potassium citrate, sodium oxalate, potassium oxalate, sodium alginate, potassium alginate, sodium caseinate and calcium caseinate, and preferably, the weak organic acid salt includes at least one of sodium citrate and calcium caseinate. Further, the weak organic acid salt may be sodium citrate.

In order to adapt to the tolerance conditions of membrane filtration, for example, in ultrafiltration, the pH of the extract after the treatment with the weak organic acid salt is adjusted to 3.0 to 3.5, for example, 3, before the membrane filtration.

Further, in order to enable the organic weak acid salt to sufficiently act on the extracting solution, in some embodiments, the organic weak acid salt may be added and mixed uniformly by stirring for 1 to 10min after the organic weak acid salt is added, and the stirring speed may be 3000 to 4000r/min, for example 3500 r/min.

In some embodiments, the pectin material of the acid extract of pectin includes, but is not limited to, any of citrus peel, pineapple peel, sunflower, pea shell, passion fruit peel residue, pumpkin, apple peel, potato residue, aloe vera; for example, the pectin material can be citrus peel, and preferably, the pectin material is particulate citrus peel.

Further, the extraction of the pectin material with an acidic solution comprises:

mixing pectin raw material with an acidic solution, and extracting the pectin raw material with the acidic solution at 70-90 ℃, preferably 80-90 ℃, for at least 50min, preferably 60-80 min, preferably, the acidic solution is hydrochloric acid, the concentration of the acidic solution is 0.15-0.25M, for example, the concentration of the hydrochloric acid solution is 0.2M, and the solid-to-liquid ratio of the pectin raw material to the acidic solution is 1: 15-35, for example, the solid-to-liquid ratio is 1: 15. 1: 20. 1: 25. 1: 30 or 1: 35, preferably 1:15 to 25.

In the extraction process, the extracting solution needs to be heated to enable the extraction to be more sufficient, the condition that pectin is easy to extract is met, in order to heat more uniformly, in some embodiments, the extraction process is carried out in a water bath, and in the water bath process, the mixed material of the pectin raw material and the acidic solution is stirred, wherein the stirring speed can be 3000-4000 r/min, for example, the stirring speed can be 3500 r/min.

In some embodiments, where the pectin material is citrus peel in particulate form, the pectin material is prepared by a process comprising: cleaning fresh citrus peel, removing surface impurities, keeping the temperature of 90-100 ℃ for 5-10 min to inactivate pectinase in order to prevent pectin from being decomposed by pectinase in the extraction process, drying at 50-65 ℃ to constant weight, and crushing.

In some embodiments, when the acidic pectin extract is obtained by separation, solid-liquid separation can be achieved by centrifugal separation.

Further, in some embodiments, the membrane filtration comprises ultrafiltration using an ultrafiltration membrane, and in some embodiments, the ultrafiltration membrane comprises, but is not limited to, any one of a PSF membrane, a PES membrane, and a PAN membrane, preferablyThe size of the cut molecular weight of the ultrafiltration membrane is 2-100 kDa. In some embodiments, the ultrafiltration is performed at an operating pressure of 0.1 to 0.5MPa, preferably 0.3MPa, and a concentration ratio of 1.5 to 2.5, preferably 2. The extract treated by the organic weak acid salt is easy to react with pigment, sugar and CaCl after the organic weak acid salt is converted into weak acid under the acidic regulation₂And (3) complexing the small molecules of impurities such as salts, and the like, removing the small molecules under the ultrafiltration condition, synchronously realizing the purposes of concentrating the pectin extracting solution and removing impurities, and greatly reducing the dosage of alcohol or salt in the subsequent pectin separation process. In addition, the pectin extractive solution is greatly concentrated to facilitate pectin precipitation, so that the salt for salting out can be high-priced Al³⁺Or Fe³⁺The purpose of pectin precipitation can be realized by using a small amount of low-valence salt, so that the complexation of high-valence cations in a salt solution and the pectin is avoided, the pectin desalination is easy to carry out, and the problems of heavy color and high ash content of the pectin are solved.

Specifically, the ultrafiltration process may be: injecting the pectin extracting solution treated by the organic weak acid salt into a storage tank, enabling the pectin extracting solution to flow through ultrafiltration membrane assemblies with different cutting molecular weights (2-100 kDa), starting the test under the condition that the filtering pressure is 0.3MPa, stopping the test when the concentration ratio is 2 times, and taking trapped fluid.

Further, in the process of preparing pectin, the extracting solution contains more pectin colloid impurities, the pectin colloid is larger, the uniformity is poorer, and if direct forward osmosis concentration is carried out, a forward osmosis membrane is blocked, so that the aim of concentration cannot be achieved. In some embodiments of the present invention, the organic weak acid salt treatment, ultrafiltration and forward osmosis membrane concentration are combined, and the organic weak acid salt treatment and ultrafiltration process create conditions for the forward osmosis membrane concentration, so that the extract can be further concentrated by the forward osmosis membrane to further remove water in the extract, so that the volume of the pectin extract is greatly reduced, and the amount of ethanol or salt for precipitation is further reduced. That is, in some embodiments, the membrane filtration further comprises concentrating the retentate after ultrafiltration through a forward osmosis membrane, wherein the ultrafiltration mainly serves the purpose of primary concentration and removal of small molecule substances.In some embodiments, the forward osmosis membrane comprises any one of a cellulose acetate membrane, a polyamide membrane and a membrane material containing MOFs, and the membrane has a membrane flux of 20-400L/M when 1M NaCl is used as a draw solution and deionized water is used as a raw material solution²h, preferably, the concentration ratio is 2-5.

Specifically, the forward osmosis membrane concentration process may be: the method comprises the steps of placing a forward osmosis membrane with large water flux and good interception performance in a forward osmosis membrane component, placing a pectin extracting solution subjected to ultrafiltration concentration as a raw material solution in a feed solution barrel, and placing a drawing solution comprising any one or a mixture of several of sodium chloride, calcium chloride, sodium polyacrylate, polyacrylic acid, sodium citrate, glucose and sucrose. Respectively placing the raw material liquid and the drawing liquid in a material liquid barrel, simultaneously starting a raw material pump and a drawing liquid pump, wherein the flow ranges of the raw material liquid and the drawing liquid are respectively 100-500L/h, and the forward osmosis water flux ranges from 30-100L/m²h, the reverse solute permeation flux is 0.05-0.1 mol/m²h is between. And (4) stopping concentrating until the concentration is 2-5 times, and recycling the draw solution after nanofiltration or reverse osmosis treatment.

Some embodiments of the present invention also provide a method of preparing pectin, comprising:

s1, adding organic weak acid salt into the acidic pectin extract obtained by separation after extraction of the pectin raw material, uniformly mixing, and performing membrane filtration on the extract treated by the organic weak acid salt. Wherein the addition amount of the organic weak acid salt in the mixed solution is 0.2-1 mg/mL, optionally 0.2-0.25 mg/mL.

The extraction process of the pectin raw material, the addition treatment process of the organic weak acid salt, the process parameters of ultrafiltration, the selection of the filter membrane and the like by the acidic solution are referred to the above embodiment, and are not described again.

S2, precipitating the concentrated extractive solution with ethanol or salting out, preferably salting out.

Specifically, during alcohol precipitation, the pH value of the pectin hydrochloric acid extract is adjusted to 3.0 by using 6M ammonia water, the addition amount of alcohol is 1.0 times of that of the concentrated solution, the technological conditions are that the pectin is slowly added under stirring, the mixture is kept stand for 20min, and the pectin is obtained through filtration, washing and drying, wherein preferably, the alcohol comprises at least one of ethanol and propanol. When salting out is carried out, the addition amount of the salt is as follows: taking a salt solution of 100mg/mL as an additive, wherein the ratio of the salt solution to the membrane concentrated extracting solution is 2-20: 1, preferably 2 to 5:1, salting out pH of 4.5-7.0, preferably 4.5-5.5, salting out temperature of 40-90 ℃, preferably 50-70 ℃; preferably, the salt comprises at least one of sodium chloride, potassium chloride, magnesium sulfate, calcium chloride, ferric chloride, aluminum sulfate, sodium citrate, sodium alginate and sodium caseinate.

And S2, concentrating the extracting solution after the ultrafiltration membrane concentration through a forward osmosis membrane.

In some embodiments, the forward osmosis membrane comprises any one of a cellulose acetate membrane, a polyamide membrane and a membrane material containing inorganic MOFs, and preferably, the concentration ratio is 2 to 5. The specific process of forward osmosis membrane concentration is described in the previous embodiments and will not be described herein.

S3, and precipitating with ethanol or salt, preferably salt.

Wherein, the process of alcohol precipitation or salting out refers to the above embodiments, and is not described herein again.

S3, and precipitating with ethanol or salt, preferably salt.

And S4, carrying out nanofiltration treatment on the waste liquid after alcohol precipitation or salting-out.

The nanofiltration membrane used in the nanofiltration treatment comprises any one of nanofiltration membranes with the cut molecular weight of 100-1000 Da, for example, the nanofiltration membrane with the cut molecular weight of 200Da, 300Da, 400Da, 500Da or 700 Da.

Specifically, the nanofiltration treatment process may be: and (3) placing the salted pectin extraction waste liquid in the last step into a liquid feed barrel, pressurizing by a pump, introducing into a nanofiltration membrane component, and operating at the pressure of 0.3-1.5 MPa, wherein the operation is stopped when the concentration ratio is controlled to be 2-5.

S5, precipitating the waste liquid concentrated solution obtained by nanofiltration.

Specifically, the precipitating agent added for precipitation comprises at least one of ethanol, propanol, sodium chloride, sodium alginate, sodium citrate and sodium sulfate, and preferably, the precipitating agent is salt comprising at least one of sodium chloride, sodium alginate, sodium citrate, ferric chloride and calcium chloride.

Preferably, the addition amount of the precipitating agent is 1/2-1/5 of the volume of the concentrated solution, the process conditions are salting-out pH of 4.5-5.5 and salting-out temperature of 50-70 ℃.

Some embodiments of the invention also provide pectin prepared by the preparation method of the above embodiments. Preferably, the pectin has a galacturonic acid content of greater than 89 wt%, a degree of esterification of greater than 77%, an ash content of less than 1.3 wt%, and an acid-insoluble ash content of less than 0.2 wt%.

The features and properties of the present invention are described in further detail below with reference to examples.

Example 1

(1) Citrus peel pretreatment

Cleaning fresh mandarin orange peel, removing impurities on the surface of the peel, and keeping the temperature in 90 deg.C water for 8min to inactivate pectinase. And drying the enzyme-inactivated citrus peels to constant weight in a drying oven at 60 ℃, and crushing to 100-200 meshes for later use.

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.2, performing water bath at 85 deg.C for 80min, continuously stirring during water bath to uniformly heat, performing solid-liquid centrifugal separation at centrifugal speed of 3500r/min, and collecting supernatant as acidic pectin extractive solution.

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic pectin extract, mixing, and stirring at 3500r/min for 4 min. The amount of sodium citrate added to the mixed solution was 0.2 mg/mL.

(4) Ultrafiltration

Injecting the pectin crude extract treated by sodium citrate into a storage tank, allowing the pectin crude extract to flow through a PES ultrafiltration membrane module with a cut molecular weight of 3kDa, adjusting the pH of the solution to 3.0, starting the test under the condition of a filtration pressure of 0.3MPa, stopping the test when the concentration ratio is 2 times, and taking trapped fluid.

Examples 2 to 4

Examples 2 to 4 differ from example 1 only in that PES 10kDa, PES 30kDa and PES 100kDa are selected as the ultrafiltration membranes in this order.

Test example 1

The color value and clarity of the pectin extract before and after ultrafiltration in examples 1-4 were examined.

The detection results are shown in FIG. 1, and it can be seen from the results in FIG. 1 that the selection of the polyethersulfone ultrafiltration membrane and the ultrafiltration membrane with the molecular weight cutoff of 100kDa has the best treatment effect.

Examples 5 to 6

Examples 5 to 6 differ from example 1 only in that sodium citrate in example 1 was replaced with sodium caseinate and calcium caseinate in this order.

Test example 2

Recording the test time and the quality change of ultrafiltration of the embodiment 1 and the embodiments 5 to 6, and researching the attenuation rule of the membrane flux of the pectic acid extracting solution added with the salt solution and the membrane pollution condition; the change in flux of the membranes of example 1 and examples 5 to 6 was measured, and an example in which no organic weak acid salt was added was set as a control, and the results are shown in FIG. 2. And measuring the color value and clarity change of the pectic acid extract before and after ultrafiltration, and setting a contrast, wherein the organic weak acid salt 1 is sodium citrate, the organic weak acid salt 2 is sodium caseinate, and the organic weak acid salt 3 is calcium caseinate. The results are shown in FIG. 3.

Examples 7 to 10

Examples 7 to 10 differ from example 1 only in that in examples 7 to 10, the amounts of sodium citrate added to the pectin extract solution were 0.25mg/mL, 0.50mg/mL, 0.75mg/mL, and 1mg/mL, respectively.

Test example 3

The effect of the addition amount of sodium citrate in example 1 and examples 7 to 10 on the Zeta potential of the pectin extract was determined by the following criteria:

as shown in FIG. 4, it can be seen from FIG. 4 that the organic weak acid salt in FIG. 4 is sodium citrate, and the Zeta potential value of the pectin extract is first greatly reduced after the sodium citrate is added, and then rapidly increased to about-3 mV when the concentration of the added (sodium citrate) in the extract exceeds 0.20-0.25 mg/ml. Therefore, the concentration of the organic weak acid salt is preferably 0.20-0.25 mg/ml.

Test example 4

The results of examining the membrane flux of the pectin extracts of examples 1 and 7 to 10 on an ultrafiltration membrane and setting the comparative example without adding sodium citrate are shown in FIG. 5, in which the membrane contamination degree is the highest and the stable flux is about 3.0L/m²h, as the concentration of sodium citrate increased to (0.25mg/L), the steady-state membrane flux value was maximized at about 9.1L/m²h, the above data show that the membrane pollution effect of the pectin solution added with the sodium citrate solution is obviously weakened, and the membrane flux is increased, which indicates that the sodium citrate solution has the effects of relieving membrane pollution and increasing the membrane flux on the ultrafiltration of the pectin extracting solution, and the organic weak acid salt is sodium citrate in fig. 5.

The color values and the clarity before and after ultrafiltration in examples 1, 7, 8 and 10 were measured, and the results are shown in FIG. 6, in which FIG. 6 the organic weak acid salt is sodium citrate.

Pectin was obtained from the ultrafiltered pectin extracts of examples 1 and 7-10 and the comparative example without sodium citrate by the following salting out procedure: the addition amount of the salt is as follows: taking a salt solution of 100mg/mL as an additive, wherein the volume ratio of the salt solution to the membrane concentrated extracting solution is 5:1, the salting-out pH is 7.0, the salting-out temperature is 70 ℃, the salting-out time is 80min, and the desalting time is 45 min. Filtering the salted mixture to obtain crude pectin, and washing the crude pectin, wherein the salt is ferric chloride.

The average particle size of pectin was determined, and the results are shown in fig. 7. fig. 7 shows that the average particle size of pectin can be significantly reduced by treating the acidic pectin extract with an organic weak acid salt. Wherein, the organic weak acid salt in figure 7 is sodium citrate.

Example 11

(1) Citrus peel pretreatment

(2) Extraction of citrus pectin

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic pectin extract, mixing, and stirring at 3500r/min for 4 min. The amount of sodium citrate added to the mixed solution was 0.25 mg/mL.

(4) Ultrafiltration

(5) Alcohol precipitation

Adjusting pH of pectin hydrochloric acid extractive solution to 3.0 with 6M ammonia water, adding 95% ethanol solution with alcohol amount 1.0 times of the concentrated solution, standing for 20min, filtering to obtain crude pectin, washing with anhydrous ethanol for 2 times, drying, and grinding.

Example 12

(1) Citrus peel pretreatment

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.2, performing water bath at 85 deg.C for 80min, continuously stirring during water bath to uniformly heat, performing solid-liquid centrifugal separation, performing centrifugal separation at rotation speed of 3500r/min to obtain acidic pectin extractive solution.

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic pectin extract, mixing, and stirring at 3500r/min for 10 min. The amount of sodium citrate added to the mixed solution was 0.25 mg/mL.

(4) Ultrafiltration

Injecting the pectin crude extract treated by sodium citrate into a storage tank, allowing the pectin crude extract to flow through a PES ultrafiltration membrane module with a cut molecular weight of 3kDa, adjusting the pH of the solution to 3.0, starting the test under the condition of a filtration pressure of 0.5MPa, stopping the test when the concentration ratio is 2 times, and taking trapped fluid.

(5) Salting out

The addition amount of the salt is as follows: taking a salt solution of 100mg/mL as an additive, wherein the volume ratio of the salt solution to the membrane concentrated extracting solution is 5:1, the salting-out pH is 7.0, the salting-out temperature is 70 ℃, the salting-out time is 80min, and the desalting time is 45 min. Filtering the salted mixture to obtain crude pectin, and washing the crude pectin, wherein the salt is ferric chloride.

Example 13

(1) Citrus peel pretreatment

Cleaning fresh orange peel, removing impurities on the surface of the peel, and keeping the temperature in water at 90 ℃ for 5-10 min to inactivate pectinase. And drying the enzyme-inactivated citrus peels to constant weight in a drying oven at 60 ℃, and crushing to 100-200 meshes for later use.

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.4, performing water bath at 85 deg.C for 80min, stirring during water bath to uniformly heat, and centrifuging to obtain acidic pectin extractive solution.

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic pectin extract, mixing, and stirring at 3500r/min for 8 min. The amount of sodium citrate added to the mixed solution was 1 mg/mL.

(4) Ultrafiltration

(5) Forward osmosis membrane concentration

The membrane flux is 100L/m²And h, placing the cellulose acetate forward osmosis membrane into a forward osmosis membrane component, using the pectin extracting solution subjected to ultrafiltration concentration as a raw material solution, using an extraction solution as a sodium citrate solution with the concentration of 1M, wherein the flow ranges of the raw material solution and the extraction solution are respectively 200L/h, and the concentration ratio is 5.

(6) Salting out

The addition amount of the salt is as follows: taking a salt solution of 100mg/mL as an additive, wherein the ratio of the salt solution to the membrane concentrated extract is 5:1, salting out pH of 7.0, salting out temperature of 70 deg.C, salting out time of 60min, and salt selected from ferric chloride. The salted-out mixture is filtered to obtain crude pectin, and the crude pectin is washed.

Example 14

(1) Citrus peel pretreatment

Cleaning fresh orange peel, removing impurities on the surface of the peel, and keeping the temperature in water at 90 ℃ for 5-10 min to inactivate pectinase. And drying the enzyme-inactivated citrus peels to constant weight in a drying oven at 60 ℃, and crushing to a certain size of 100-200 meshes for later use.

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution according to the material-liquid ratio of 20, adjusting the pH to 2.1, carrying out water bath for 50-90 min at the temperature of 80-90 ℃, continuously stirring in the water bath process to uniformly heat the mixture, and carrying out centrifugal separation to obtain an acidic extracting solution.

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic extract, mixing, and stirring at 3500r/min for 7 min. The amount of sodium citrate added to the mixed solution was 0.25 mg/mL.

(4) Ultrafiltration

Injecting the crude extract of the gel after the treatment of sodium citrate into a storage tank, making the crude extract flow through a PES ultrafiltration membrane component with the cut molecular weight of 3kDa, adjusting the pH value of the solution to 3.0, starting the test under the condition of the filtration pressure of 0.3MPa, stopping the test when the concentration ratio is 2 times, and taking trapped fluid.

(5) Forward osmosis membrane concentration

The membrane flux is 50L/m²And h, placing the forward osmosis membrane in a forward osmosis membrane component, using the pectin extracting solution subjected to ultrafiltration concentration as a raw material solution, using an extraction solution as a mixture of 0.5M glucose and 0.5M sodium chloride, wherein the flow ranges of the raw material solution and the extraction solution are respectively 200L/h, and the concentration ratio is 3.

(6) Salting out

The addition amount of the salt is as follows: taking a salt solution of 100mg/mL as an additive, wherein the ratio of the salt solution to the membrane concentrated extract is 5:1, the salting-out pH is 7.0, the salting-out temperature is 70 ℃, and the salting-out time is 60 min. Wherein the salt is sodium caseinate. The salted-out mixture is filtered to obtain crude pectin, and the crude pectin is washed.

(7) Nanofiltration

Specifically, the nanofiltration treatment process may be: and (3) placing the salted pectin extraction waste liquid in the last step into a liquid feed barrel, pressurizing by a pump, introducing into a nanofiltration membrane component, and operating at the pressure of 0.3-1.5 MPa, wherein the operation is stopped when the concentration ratio is 3.

(8) Salting out again

The addition amount of the salt is as follows: using 100mg/mL salt solution as additive, wherein the ratio of salt solution to membrane concentrated extractive solution is 5:1, the salting-out pH is 5.0, the salting-out temperature is 60 deg.C, and the salting-out time is 60 min. Wherein the salt is calcium chloride. The mixture after the re-salting out is filtered to obtain pectin, and the pectin is washed.

Examples 15 to 17

Examples 15 to 17 differ from example 14 in that the salts used for the two salting-out operations were replaced by aluminium sulphate, ferric chloride, sodium chloride, in that order.

Example 18

(1) Citrus peel pretreatment

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a ratio of 20 materials to liquid, adjusting pH to 2.2, performing water bath at 80 deg.C for 70min, stirring to uniformly heat, and centrifuging to obtain acidic extractive solution.

(3) Treatment of organic weak acid salts

Adding sodium citrate into the acidic extract, mixing, and stirring at 3500r/min for 8 min. The amount of sodium citrate added to the mixed solution was 0.25 mg/mL.

(4) Ultrafiltration

Injecting pectin crude extract into storage tank, allowing it to flow through PES ultrafiltration membrane module with cut molecular weight of 100kDa, adjusting pH of the solution to 3.0, starting test under filtration pressure of 0.3MPa, stopping test when concentration ratio is 2 times, and collecting retentate.

(5) Forward osmosis membrane concentration

The membrane flux is 200L/m²And h, placing the MOFs forward osmosis membrane in a forward osmosis membrane component, using the pectin extracting solution subjected to ultrafiltration concentration as a raw material solution, using an extraction solution as a mixture of a sodium citrate solution with the concentration of 1M and sodium chloride with the concentration of 1M, wherein the flow ranges of the raw material solution and the extraction solution are 300L/h respectively, and the concentration ratio is 3.

(6) Salting out

The addition amount of the salt is as follows: taking 100mg/mL salt solution as an additive, wherein the volume of the salt solution and the membrane concentrated extract is 5:1, salting out pH of 5.0, salting out temperature of 60 deg.C, salting out time of 60min, and salt selected from sodium citrate. The salted-out mixture is filtered to obtain crude pectin, and the crude pectin is washed.

Example 19

This case differs from example 18 only in that "the amount of salt added is: taking saturated salt solution as an additive, wherein the ratio of the salt solution to the membrane concentrated extract is 20:1, salting out pH of 7.0, salting out temperature of 70 ℃, salting out time of 60min, desalting for 45min, wherein the salt is selected from aluminum sulfate. The salted-out and desalted mixture is filtered to obtain crude pectin, and the crude pectin is washed. "replace step (6).

Comparative example 1

Commercial pectin was purchased from Baizhuo pectin Biotech Ltd, Henan.

Comparative example 2

(1) Citrus peel pretreatment

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.2, performing water bath at 85 deg.C for 80min, stirring to uniformly heat the solution, centrifuging the solid-liquid mixture at 3500r/min to obtain pectin hydrochloric acid extractive solution.

(3) Alcohol precipitation

Adjusting pH of pectin hydrochloric acid extractive solution to 3.0 with 6M ammonia water, adding 95% ethanol solution at volume ratio of 1:1.5, standing for 20min, filtering to obtain crude pectin, washing with anhydrous ethanol for 2 times, drying, and grinding.

Comparative example 3

(1) Citrus peel pretreatment

(2) Extraction of citrus pectin

Washing the treated citrus peel powder for three times by using deionized water at 70 ℃, and mixing the washed citrus peel powder with deionized water according to the weight ratio of 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.2, performing water bath at 85 deg.C for 80min, stirring to uniformly heat, and centrifuging to obtain pectin hydrochloric acid extractive solution.

(3) Salting out

Preparing saturated salt solution from aluminum sulfate, adding the solution according to the volume ratio of 20:1 of the extracting solution to aluminum salt, salting out at a certain salting-out temperature of 70 ℃, at a salting-out pH value of 7.0, and desalting for 45 min; filtering to obtain crude pectin, storing the filtrate in refrigerator at 4 deg.C, and washing with crude pectin.

Comparative example 4

This comparative example differs from example 14 only in that the treated citrus peel powder was washed three times with deionized water at 70 c, in a 1: adding deionized water and 0.2M hydrochloric acid solution at a material-to-liquid ratio of 20, adjusting pH to 2.2, performing water bath at 85 deg.C for 80min, continuously stirring in the water bath process, adding sodium citrate into the acidic extractive solution, mixing well, stirring for 6min at 3500 r/min. The amount of sodium citrate added to the mixed solution was 0.25mg/mL, and the mixture was centrifuged to obtain an acidic extract. "alternative embodiment Steps (2), (3) in example 14.

Comparative example 5

This comparative example differs from example 14 only in that sodium citrate is replaced by sodium hexametaphosphate.

The extraction rates of the pectins obtained in examples 11 to 19 and those of the pectins in comparative examples 2 to 5 were compared, and the results are shown in Table 1.

TABLE 1 comparison of extraction rates of pectin by different extraction methods

The pectin obtained in examples 11 to 17 was compared with the pectin obtained in comparative examples 1 to 5 in terms of quality, and the results are shown in Table 2.

TABLE 2 Effect of different extraction methods on pectin quality

In the table, the primary salting-out refers to salting-out after ultrafiltration or forward osmosis membrane concentration, and the secondary salting-out refers to salting-out after nanofiltration.

Comparing the commercial pectin in comparative example 1 with the pectin obtained in comparative example 2 and example 14, as shown in fig. 8, it can be seen that the pectin prepared by the embodiment of the present invention has better quality.

It should be noted that, the above detection process refers to the following criteria:

a. the flux (J) of the membrane and the rejection (R) of the solute during the test were calculated according to the following equations:

(1) membrane flux (J)

J＝V/AΔt (1)

In the formula: j-membrane flux, L/(m)²·h)；

V-water permeation volume, L;

a-effective filtration area, m²；

Δ t-filtration time, h.

(2) Membrane retention rate (R)

R＝(C_f-C_P)/C_f×100％ (2)

In the formula: r-retention,%;

C_f-concentration before permeation (mg/L) or conductivity value (μ S/cm);

C_pconcentration after permeation (mg/L) or conductivity value (. mu.S/cm).

b. Pectin galacturonic acid content determination

The standard curve is drawn by adopting a carbazole sulfate method, taking the content of galacturonic acid as an abscissa and absorbance at 530nm as an ordinate.

And (3) measuring the pectin sample: the sample determination method is the same as the literature "DischeZ.A.A. New specific color reaction of hexuronic acids [ J ] 1947,167(1): 189-; the pectin extract is diluted by a certain multiple by 1mL when being measured, and the subsequent measurement is the same as that of a standard curve diluent. Comparing the light absorption value of the sample with a standard curve, calculating the concentration of the galacturonic acid in the sample, calculating the content of the galacturonic acid according to the concentration, and calculating the content of the galacturonic acid according to the formula (3).

In the formula: mass fraction of X1-galacturonic acid, Gala,%;

c-the concentration of galacturonic acid, μ g/mL, found from the standard curve;

v-total volume of pectin extract, mL;

k-dilution times of the extracting solution;

m-sample mass, g.

c. Determination of color value and clarity of pectin extract

The test uses spectrophotometry to determine the values of A420 and T625 of the solution samples, which are the color value and clarity of the solution.

d. Zeta potential measurement of pectin extract

In the test, a Malvern Zeta potentiometer is adopted to test the Zeta potential values of different pectin extracting solutions.

In conclusion, the scheme of the embodiment of the invention develops a pectin extraction method for reducing the consumption of ethanol or salt, even a method for extracting pectin without using ethanol, and also provides a method for reducing membrane pollution in the ultrafiltration process of pectin extract. And the impurities in the pectin can be removed, the purity and the color of the pectin are improved, the indexes of the pectin such as ash content are reduced, and the extraction rate of the pectin is also improved. In addition, the recycling of the micromolecule pectin in the wastewater is enhanced, and the method is environment-friendly and suitable for large-scale popularization and production.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A pectin extraction method is characterized by comprising the following steps:

adding organic weak acid salt into acidic pectin extract obtained by separation after extraction of pectin raw materials, uniformly mixing, and performing membrane filtration on the extract subjected to organic weak acid salt treatment;

the addition amount of the organic weak acid salt in the mixed solution is 0.2-1 mg/mL, and the acidic pectin extracting solution is obtained by performing solid-liquid separation on a pectin raw material after acidic solution extraction;

the method for extracting pectin raw material by using acidic solution to obtain acidic extracting solution comprises the following steps:

mixing the pectin raw material with an acidic solution, and extracting the pectin raw material for at least 50min by using the acidic solution at the temperature of 70-90 ℃, wherein the acidic solution is hydrochloric acid, the concentration of the acidic solution is 0.15-0.25M, and the solid-to-liquid ratio of the pectin raw material to the acidic solution is 1: 15-35.

2. The extraction method according to claim 1, wherein the amount of the weak organic acid salt added to the mixed solution is 0.2 to 0.25 mg/mL.

3. The extraction process of claim 1, wherein the salt of the weak organic acid comprises at least one of a sodium salt of a weak organic acid and a potassium salt of a weak organic acid.

4. The extraction method according to claim 3, wherein the weak organic acid salt comprises: at least one of sodium citrate, potassium citrate, sodium oxalate, potassium oxalate, sodium alginate, potassium alginate, sodium caseinate and calcium caseinate.

5. The extraction process according to claim 3, wherein the weak organic acid salt comprises at least one of sodium citrate and calcium caseinate.

6. The extraction method according to claim 1, wherein the pH of the extract solution treated with the weak organic acid salt is adjusted to 3.0 to 3.5 before the membrane filtration.

7. The method according to claim 6, wherein the pH of the extract solution treated with the weak organic acid salt is adjusted to 3 before the membrane filtration.

8. The extraction method according to claim 1, wherein the organic weak acid salt is added and mixed uniformly, and the mixture is stirred for 1-10 min at a stirring speed of 3000-4000 r/min.

9. The extraction method according to claim 1, wherein the pectin material of the acid extraction solution of pectin is selected from any one of citrus peel, pineapple peel, sunflower, pea shell, passion fruit peel residue, pumpkin, apple peel, potato residue, and aloe.

10. The extraction process according to claim 9, wherein the pectin starting material is citrus peel.

11. The extraction process of claim 10, wherein the pectin material is a particulate citrus peel.

12. The extraction method according to claim 1, wherein the step of extracting the pectin material with an acidic solution to obtain an acidic extract comprises:

mixing the pectin raw material with an acidic solution, and extracting the pectin raw material for 60-80 min by the acidic solution at the temperature of 80-90 ℃, wherein the acidic solution is hydrochloric acid, the concentration of the acidic solution is 0.2M, and the solid-to-liquid ratio of the pectin raw material to the acidic solution is 1: 15-25.

13. The extraction method according to claim 1, wherein the extraction process is carried out in a water bath and the mixture of the pectin material and the acidic solution is stirred.

14. The extraction method according to claim 13, wherein the stirring speed is 3000 to 4000 r/min.

15. The extraction process according to claim 1, wherein, when the pectin material is a granular citrus peel, the pectin material is prepared by a method comprising: cleaning fresh citrus peel, preserving heat in water at 90-100 ℃ for 5-10 min, drying at 50-65 ℃, and crushing.

16. The extraction process according to any one of claims 1 to 15, wherein the membrane filtration comprises ultrafiltration using an ultrafiltration membrane.

17. The extraction method according to claim 16, wherein the ultrafiltration membrane comprises any one of a PSF membrane, a PES membrane, and a PAN membrane.

18. The extraction method according to claim 16, wherein the cut molecular weight of the ultrafiltration membrane is 2-100 kDa.

19. The extraction method according to claim 16, wherein the ultrafiltration is performed at an operating pressure of 0.10 to 0.50 MPa and a concentration ratio of 1.5 to 2.5.

20. The extraction process according to claim 16, characterized in that the operating pressure of the ultrafiltration is 0.3MPa and the concentration ratio is 2.

21. The extraction process of claim 16, wherein the membrane filtration further comprises concentrating the ultrafiltered retentate through a forward osmosis membrane.

22. The extraction method as claimed in claim 21, wherein the forward osmosis membrane comprises any one of cellulose acetate membrane, polyamide membrane and membrane material containing Metal Organic Frameworks (MOFs), and when 1M NaCl is used as draw solution and deionized water is used as raw material solution, the membrane flux is 20-400L/M²h is between.

23. The extraction method according to claim 21, wherein the concentration ratio is 2 to 5.

24. The extraction process according to claim 21, wherein the draw solution in the forward osmosis process comprises any one of sodium chloride, calcium chloride, sodium polyacrylate, polyacrylic acid, sodium citrate, glucose, sucrose, or a mixture thereof.

25. A method for producing pectin, characterized by subjecting a membrane-concentrated extract obtained by the method for extracting pectin according to any one of claims 1 to 24 to alcohol precipitation or salting out.

26. The method according to claim 25, wherein the membrane-concentrated extract is salted out.

27. The method for preparing pectin according to claim 25, wherein the alcohol is added in an amount of 0.8-1.2 times of the extract during alcohol precipitation under the process conditions of adding the alcohol solution under slow stirring, standing for 20min after adding, filtering, washing and drying to obtain pectin.

28. The method of claim 25, wherein the alcohol comprises at least one of ethanol and propanol.

29. The method for producing pectin according to claim 25, wherein the salt is added in an amount of: taking a salt solution of 100mg/mL as an additive, wherein the ratio of the salt solution to the membrane concentrated extracting solution is 2-20: 1, the salting-out pH is 4.5-7.0, and the salting-out temperature is 40-90 ℃.

30. The method for producing pectin according to claim 29, wherein the salt is added in an amount of: taking a salt solution of 100mg/mL as an additive, wherein the ratio of the salt solution to the membrane concentrated extracting solution is 2-5: 1, the salting-out pH is 4.5-5.5, and the salting-out temperature is 50-70 ℃.

31. The method of claim 29, wherein the salt comprises at least one of magnesium sulfate, calcium chloride, ferric chloride, aluminum sulfate, sodium citrate, sodium alginate and sodium caseinate.

32. The method for preparing pectin according to any one of claims 25 to 31, wherein the waste solution after alcohol precipitation or salting out is subjected to nanofiltration, and a concentrated solution of the waste solution obtained by nanofiltration is subjected to precipitation.

33. The method according to claim 32, wherein the precipitating agent added for precipitation comprises at least one of ethanol, propanol, ferric chloride, aluminum sulfate, sodium alginate, sodium citrate, and magnesium sulfate.

34. The method of claim 33, wherein the precipitating agent is an alcohol comprising at least one of ethanol and propanol.

35. The method according to claim 33, wherein the precipitating agent is added in an amount of 1.0 times the volume of the concentrated solution under conditions such that the precipitating agent comprises at least one of ethanol and propanol, and is added under slow stirring for 20min, filtered, washed, and dried to obtain the pectin.

36. The method for preparing pectin according to claim 32, wherein the nanofiltration membrane used in the nanofiltration treatment comprises any one of nanofiltration membranes with a cut molecular weight of 100-1000 Da, the nanofiltration membrane comprises a nanofiltration membrane with a cut molecular weight of 200Da, 300Da, 400Da, 500Da or 700Da, and the operating pressure of the nanofiltration treatment is 0.3-1.5 MPa.

37. The method of preparing pectin according to claim 36, wherein the operating pressure of the nanofiltration treatment is 0.5 MPa.

38. The method for preparing pectin according to claim 32, wherein the concentration ratio of nanofiltration is 1.5-3.0.