Figure 4. The purification process of polysaccharides.
Polysaccharides have been widely accepted as nutraceuticals, which improve the immune function of the body. However, many polysaccharides remain as health products and fail to be developed into drugs. The main reason for this is that the separation and purification of polysaccharides is difficult, and the current technology level does not meet the requirements for this. Generally speaking, polysaccharides are hydrophilic macromolecules. The separation and purification methods of polysaccharides are different from those of small molecules. In addition, different polysaccharides have different properties, so different separation and purification methods must be used. This work requires not only a theoretical knowledge of polysaccharides, but also accumulated working experience in the separation and purification of polysaccharides.
3.1. Concentration Grading Method
This method mainly takes advantage of the fact that the solubility of different polysaccharides in different concentrations of organic solvents is different. The solubility of polysaccharides with larger molecular weights in ethanol or acetone are less than it is of those with a smaller molecular weight
[46]. Therefore, the molecular weight of the product can be controlled by adjusting the concentration of the organic solvent. This is usually done in the following manner:
While stirring, a high concentration of anhydrous ethanol is slowly added to the solution of the polysaccharide mixture to reach a final concentration of 25% ethanol (
v/
v). After the addition of ethanol, the solution is left for 2 h and then centrifuged to obtain the supernatant and precipitate (which may be referred to as the “first precipitate”). The precipitate is of a high MW polysaccharide grade. While stirring, ethanol is slowly added to the supernatant to reach a final concentration of 35% ethanol (
v/
v). The solution is left for 2 h and then centrifuged to obtain the supernatant and precipitate (which may be referred to as the “second precipitate”). The second precipitate is also a polysaccharide fraction, but its MW is lower than the first precipitate. The step-down process can be carried out further, depending on circumstances. The key to graded precipitation is to avoid co-precipitation as much as possible. The concentration of the polysaccharide mixture should not be too high, the ethanol should not be added too fast, and the pH of the solution should be near neutral. The lower the concentration of polysaccharide solution, the weaker the co-precipitation effect and the better the purification effect. However, if the polysaccharide concentration is too low, the recovery of the polysaccharide will be reduced and the consumption of ethanol will be greatly increased. Usually, the concentration of polysaccharide in the mixture is adjusted from 0.25% (
w/
v) to 3% (
w/
v) before using this method
[47,48,49,50][47][48][49][50]. The concentration grading method is commonly used in the research and development of polysaccharide nutraceuticals because it is much easier than column chromatography.
3.2. Column Chromatography Method
Column chromatography is the most widely used method for the purification of polysaccharides. Several methods of column chromatography are described, as follows:
3.2.1. Macroporous Resin
Macroporous adsorption resin is used to selectively adsorb organic substances from the solution by physical adsorption so as to achieve separation and purification. Its physical and chemical properties are stable; it is insoluble in acids, bases, and organic solvents, has good selectivity for organic substances, and it is unaffected by the presence of inorganic salts and strong ions, low molecular compounds, and swelling in water and organic solvents by the adsorption of solvents. The adsorption of macroporous resin relies on the van der Waals gravitational force between it and the adsorbed molecules (adsorbent), and works through its huge specific surface for physical adsorption, so that organic compounds can be separated by a certain solvent elution according to the adsorption force and its molecular weight size to achieve different purposes such as separation, purification, de-hybridization, and concentration. Macroporous resin can remove proteins, flavonoids, and pigments from polysaccharide solutions
[51,52,53,54][51][52][53][54].
3.2.2. Cellulose Column Chromatography
Cellulose is a common filling material in columns. First, after waiting for swelling, activation is performed using 0.5 mol/L NaOH with 0.5 mol/L HCL solution; the cellulose in the column is equilibrated with NaCL solution, and then the polysaccharide is loaded onto the cellulose column for purification. Afterwards, the cellulose columns are eluted separately using an eluent so that different polysaccharide levels can be continuously eluted
[46]. Polysaccharides can be separated according to different molecular weights or acid-based groups. In the elution process, the various polysaccharide fractions undergo several dissolution and precipitation processes in the cellulose column, and can eventually be separated from each other. This method can be called the “graded dissolution method”, which is basically the opposite of the graded precipitation method. Due to the high number of theoretical plates in the cellulose column chromatography, the purity of the eluate is higher
[55]. However, the disadvantage of this method is the low flow rate and the long period of time required. The flow rate seems to be too low, especially for highly viscous acidic polysaccharides.
3.2.3. Gel Column Chromatography
Gel column chromatography is based on the size and shape of polysaccharide molecules, i.e., the molecular sieve principle, to separate polysaccharides. This chromatographic method is widely used for the separation and purification of polysaccharides. In general, the crude polysaccharides obtained are first purified using macroporous resin and cellulose chromatography, and are then further purified using gel column chromatography. Commonly used gels are various types of Sephadex, Sepharose, Bio gels, and later Sephacryl, Superdex, and Superose. The eluents are salt solutions and buffers of various concentrations.
4. Physiological Activity and Product Development of Polysaccharides of Auricularia auricula
4.1. Physiological Activity of Auricularia auricula
4.1.1. Regulation of Intestinal Flora
The intestinal flora is essential for maintaining host health by regulating cellular activity and the immune system. According to previous studies, it is associated with leukemia infection
[56], small bowel colitis
[57], ischemic stroke
[58], obesity
[59], and a number of other harmful conditions.
According to the research, as shown in
Figure 5, there are as many as 51 metabolites regulated by Auricularia auricula polysaccharide, which are mainly concentrated in the arginine biosynthesis pathway, followed by the arginine and proline, glycine, serine, and threonine, glycerophospholipids, and sphingolipid metabolic pathways
[60]. These pathways can lower total and LDL cholesterol levels and alter the composition of the intestinal flora. The relative abundance levels of
Lactobacillus johnsonii,
Weissella cibaria, Kosakonia covanii,
Enterococcus faecalis,
Bifidobacterium animalis, and
Bacteroides uniformis are significantly upregulated, while
Firmicutes bacteria m10-2 are downregulated. The biological activity of AAP may be related to the regulation of the endogenous metabolism and intestinal flora composition. Zhang et al.
[8]. found that A. auricular upregulated the high-abundance SCFA-producing genus
Bacteroides and Paraprevotella in a dietary fiber-rich diet, while AAP could better enrich several lower-abundance SCFA-producing bacteria, such as
Flavonifractor and
Clostridium IV.
Figure 5. Mechanism of Auricularia auricula polysaccharides.
4.1.2. Anti-High Cholesterol
Auricularia auricula polysaccharides have multiple regulatory effects on high cholesterol. AAP significantly reduces body lipid and triglyceride levels in
Cryptobacterium hidrad, and has a significant protective effect against intracellular free radical generator-induced damage and increases the activity of antioxidant enzymes, including superoxide dismutase (SOD) and catalase (CAT)
[13]. Using mice with hyperlipidemia as a model, AAP significantly reduced serum and liver TC, TG, and serum LDH-c levels in mice
[61]. It can also protect the liver by enhancing antioxidant effects as a blood lipid-lowering agent
[62].
4.1.3. Hypoglycemic Effect
Hyperglycemia is also common globally.
Auricularia auricula polysaccharide, as a botanical heteropolysaccharide, can reduce blood sugar, especially for streptozotocin-induced type 2 diabetes (T2DM)
[63]. Its hypoglycemic activity is regulated by metabolic pathways. AAP can activate oxidative stress and NF-κB signaling and proinflammatory cytokine production
[64], and regulates the akt/ampk signaling pathway
[65]. AAP can control the blood sugar balance in the human body from multiple angles.
4.1.4. Anti-Cancer
Auricularia auricula polysaccharides also play an important role in cancer treatment because of their safety and efficacy. For patients with gastrointestinal cancer (GIC), they significantly improved the treatment response rate and survival rate (0.5 years, 1 year, and 2 years), and improved immune function without increasing the incidence of adverse reactions. This treatment also has a good adjuvant effect on enhancing platinum (L-OHP and DDP) and adriamycin (ADM)
[66]. In this context, therapies based on biopolymer prodrug systems represent promising alternatives to improve the pharmacokinetic and pharmacological properties of drugs and reduce their toxicity.
4.1.5. Anti-Oxidation and Anti-Aging
Polysaccharides have a great effect on antioxidation because of their unique spatial structure, especially the
Auricularia auricula polysaccharide.
The ABTS+ clearance of AAPs reached 37.95 ± 0.53% in Hidradenia
[16]. Under acidic conditions, the clearance rate reached 97.94 ± 0.87%
[67]. Different monosaccharide compositions and molecular weights also have different antioxidant effects. Therefore, in order to research the antioxidant mechanism and application in depth, polysaccharide raw materials should be divided and analyzed in greater detail.
4.1.6. Anti-Viral
Polysaccharides have good antiviral activity. As an effective and low-toxic antiviral component, the polysaccharide has broad medical prospects and is worth further study.
The results showed that AAPS significantly inhibited the cell infectivity of NDV in the chicken embryo fibroblast (CEF) culture system
[14].
4.2. Practical Application of Auricularia auricula Polysaccharide
4.2.1. Biological Anticorrosive Film
Antibacterial and biocompatible films have attracted much attention due to their wide potential for application. Although a lot of work has been done in this area, the research in this field is still very active and is accompanied by the continuous development of new materials
[68]. However, the research into polysaccharides in the field of biofilms is still in its early stages, and finding more suitable materials and process methods is the next step.
4.2.2. Edible Products
Auricularia auricula has a history of thousands of years as a food and medicinal material, but its edible products are few and its audience is small, so it has no great commercial value. Currently, research on the consumption of
Auricularia auricula is focused on its use as an ingredient in different food products. the complexation behaviour of AAP and whey protein isolates can be applied to the beverage industry, as exemplified by this
[69].
5. Conclusions and Future Outlook
There are still some shortcomings and limitations in the pretreatment, extraction, separation, purification, and classification of
Auricularia auricula polysaccharides. First, the current extraction, separation, purification, raw material pretreatment, and crude polysaccharide purification methods of
Auricularia auricula polysaccharides remain in the laboratory stage, there is no standardized processing method, and commercial large-scale processing technology has yet to be established. The methods for raw material pretreatment and crude polysaccharide purification extracted from
Auricularia auricula polysaccharides are under-researched. However, there is no doubt about the benefits of
Auricularia auricula polysaccharides to human health. They can not only treat chronic diseases such as hyperglycemia and hyperlipidemia, but also have a good therapeutic effect in the treatment of serious diseases such as cancer.