Edible fungi are popular among the masses because of their rich nutrition and delicious taste.
With the vigorous development of the economy, people’s quality of life continues to improve, green and healthy diet is gradually becoming the mainstream.
Food demand is changing from meeting food and clothing to focusing on nutrition and health, modern people gradually realize the life concept of “balanced diet, reasonable nutrition”, and the demand for health food is increasing.
The nutritional value of edible fungi is only between plant source food and animal source food, and is superior to both in quality, while containing nutrition, flavor, activity and other substances.
Among them, nucleosides, flavonoids, polysaccharides, sterols and terpenes are the most widely studied functional active substances of edible fungi. These active substances have pharmacological effects such as immune regulation, antioxidant, antiviral, hypoglycemic and lipid-lowering, and anti-tumor.
Microbial fermentation technology is an effective way to improve the nutritional and health care value of food, among which solid fermentation refers to the microbial fermentation process using solid substrate as raw material (bran, rice bran, etc.).
The culture substrate exists in solid form and is not easily soluble in water, providing the required nutrients and growth environment for microorganisms.
Liquid fermentation technology, also known as deep culture, is a process in which nutrients and growth environment required for mycelium growth are prepared into liquid medium and then connected to bacteria for culture.
In recent years, the fermentation technology of edible (medicinal) bacteria has developed rapidly due to its advantages of simple fermentation process and short growth cycle.
The fermentation technology can be used to isolate bioactive substances efficiently, stably and quickly, and can effectively improve the yield and production efficiency of functional components in fermentation products. If it is widely used in the edible fungi industry, industrial production can be finally realized.
In this paper, a comprehensive evaluation was made on the directed fermentation of polysaccharides, sterols, nucleosides, triterpenoids, proteins, alkaloids, flavonoids and other functional components in edible fungi, aiming to provide reference for the fermentation of edible fungi.
Solid state fermentation technology of edible fungi
Solid fermentation technology has a very long history in our country, especially in liquor, vinegar and so on.
In recent years, with the development of modern biotechnology, fermentation technology has been applied in all aspects of human life, and the research on solid fermentation of edible fungi has gradually increased, which has become a new research hotspot in exploring the potential value of fermented products of edible fungi.
He Bin used the indicators of strain adaptability and safety as the screening basis for fermentation process optimization.
Ganoderma lucidum was used as the starting strain and Salvia miltiorrhiza as the fermentation substrate. The fermentation process was optimized by two-way solid fermentation technology.
The results showed that the content of ganodermic acid triterpene in mycelium was significantly increased.
Zhong Xue et al. treated corn flour with solid fermentation of Hericium erinaceus, Lentinus edodes, Pleurotus ferulae, Griflola frondosa and morel mushroom.
The research results showed that, The mycelium growth of hericius ericius and morchella obtained by fermentation in corn meal matrix was the best and particularly significant.
The DPPH free radical scavenging ability, ABTS+ free radical and iron ion reduction ability of hericius ericius fermentation products were increased by 8.08, 1.61 and 0.72 times, respectively.
The DPPH free radical scavenging ability, ABTS+ free radical and iron ion reduction ability of the fermented products were increased by 4.91, 1.12 and 1.59 times, respectively.
Liquid fermentation technology of edible fungi
Liquid fermentation technology refers to the fermentation process of making liquid medium by mixing organic or inorganic nitrogen source, inorganic salt, sugar and other essential nutrients required for microbial growth in aqueous solution.
Sterile air is injected into the medium, stirring or oscillating, and suitable culture environment and fermentation parameters are selected to produce a large number of mycelia and metabolites.
1. protein
Using Auricularia auricula as the fermentation strain, Liang Yunxia conducted experimental research on liquid fermentation of auricularia auricula mycelium into soybean residue, and summarized the transformation rules of enzymes, nutrients and metabolites of auricularia auricula during liquid fermentation of soybean residue.
The mycelial protein of auriculus auriculatus obtained by liquid fermentation of soybean residue was analyzed. The results showed that there were 17 kinds of amino acids in mycelial protein, which was very rich.
In addition, the mycelial protein cultured with soybean residue as culture medium has good support, stability and thermal stability.
2. polysaccharide
With Agaricus blazei as the research object, Wang used response surface analysis to optimize the liquid fermentation medium, and the research results showed that The maximum predicted values of exopolysaccharide content and mycelial biomass under the final optimized conditions were 0.367g·100-1·mL-1 and 1.047g·100-1·mL-1, respectively.
Zhao Na optimized the liquid fermentation medium of Pleurotus eryngii by orthogonal test, and determined its components. The results showed that the fermentation products of Pleurotus eryngii contained polyphenols, flavonoids, proteins, crude polysaccharides and reducing sugars.
The contents of crude polysaccharide and reducing sugar were 0.688mg·mL-1 and 3.17mg·mL-1, which were twice and 2.6 times that of mycelium extract.
Using corn stalk as the base material, Wang Huan carried out liquid fermentation culture of three edible fungi, “Guangwen oyster mushroom”, “Golden Mushroom Yuhuang-1” and “Xianggu Wuxiang-1”, and determined the contents of reducing sugar, polysaccharide and total sugar in the mycelium obtained by liquid fermentation culture.
The results showed that Pleurotus ostreatus had the highest content of all components, with 2.82% reducing sugar, 12.84% polysaccharide and 19.01% total sugar content.
The reducing sugar content of mushroom Wuxiang-1 was 2.30%, the polysaccharide was 11.08% and the total sugar content was 17.59%. Pleurotus citrinopileatus Yuhuang-1 contained 2.10% reducing sugar, 10.67% polysaccharide and 17.81% total sugar.
Hu Wenji carried out liquid fermentation optimization culture of hericium hericium strains, and established the liquid fermentation optimization process of hericium hericium with mycelium and its intracellular polysaccharide as the screening index.
The optimal medium formula was yeast extract powder 19.83g·L-1, sucrose 30g·L-1, tryptone 5g·L-1, (NH4) 2SO45g·L-1, KH2PO44g·L-1, ZnSO40.02g·L-1, MgSO42.58g·L-1, CaCl20.01g·L-1, survival Prime B10.03g·L-1.
The optimal fermentation parameters were 5% inoculation amount, initial pH6.5, rotating speed of shaker 200r·min-1, fermentation temperature 26℃ and continuous fermentation for 6 days.
The mycelium and intracellular polysaccharide detected by a combination of various detection methods. The results showed that the main active components of the fermented mycelium of Hericium hericium were similar to those of wild hericium hericium.
3. Nucleoside components
Zhu Zixuan et al. optimized the liquid fermentation process of Sanghuangporus vaninii by single factor and orthogonal experiments.
The effects of different precursors (hypoxanthine, DL-aspartate, adenine, glycine, L-glutamine, L-glutamate) and regulatory factors (quercetin, vitamin B7, vitamin B12, sodium pyruvate, sodium citrate, sodium fluoride, sodium nitroprusside) on the production of nucleosides investigated.
The optimal fermentation conditions were initially pH7, culture temperature 32℃, inoculation amount 10%, shaking speed 140r·min-1, culture time 5d.
The optimal formula of liquid medium was peptone 5.00g·L-1, glucose 15.00g·L-1, sucrose 20.00g·L-1, CaCl21.50g·L-1, MgSO41.00g·L-1, MnSO40.50g·L-1, ZnSO41.00g·L-1, beef paste 2.50g·L -1;
Under the optimal liquid fermentation condition, the nucleoside yield of the fermentation product of Vanisan yellow was (291.16±6.21) mg·L-1, which was 32.3 times of that before optimization.
The optimal addition amounts of precursors and regulatory factors were hypoxanthine 1.50g·L-1, adenine 2.50g·L-1, quercetin 0.25g·L-1 and sodium pyruvate 3.50g·L-1.
4. Umami substance
Zhou Jingying studied the umami substances in the mycelium of Pleurotus geesterani by using biological enzymatic hydrolysis technology, fungal co-culture technology, and liquid fermentation technology. Six different proteases selected to study the enzymolysis effect.
The results showed that among the six different proteases, the flavor protease had the best enzymolysis effect. When the amount of enzyme was 1000U·g-1, the umami equivalent concentration was 1.94 times of that before optimization and 4.58 times of that before enzymatic hydrolysis.
Due to the good enzymatic hydrolysis effect of flavor protease, Aspergillus oryzae with better flavor protease production selected to co-culture and ferment with Aspergillus oryzae to promote the co-production of fresh food and optimize the co-culture fermentation conditions.
When the optimized protease activity reached 45.72U·mL-1, the maximum content of Asp+Glu reached 6.18mg·g-1, the maximum content of fresh nucleotide reached 3.00mg·g-1, and the maximum concentration of umami equivalent reached 147.45gMSG·100-1·g-1.
They 1.06, 1.14 and 1.60 times higher than before Aspergillus oryzae added, and increased by 5.64%, 13.64% and 17.03%, respectively.
5. triterpene
Zhu Yongle et al. optimized and screened various induction factors affecting triterpene content in liquid fermentation of Morchellaspp., and determined the enzyme activity of two key enzymes required in triterpene synthesis pathway.
The results showed that, taking the content of triterpenoids in morchella as the response value, a single factor test carried out on the magnetic field intensity, gibber, salicylic acid, 2, 4-dichlorophenoxyacetic acid and CaCl2 induction factors, and then the treatment conditions of different induction factors optimized by response surface method, and the optimal induction conditions obtained:
The magnetic field intensity was 2.81mT, the concentration of salicylic acid was 214.54μmol·L-1, and the mass concentration of gibberellin was 0.56g·L-1.
Under this condition, the maximum accumulation of triterpenes was 60.26mg·g-1, which was 160.72% of that before the optimization of liquid fermentation culture.
Therefore, using gibberellin, salicylic acid and magnetic field to treat morchella liquid fermentation medium can significantly increase the content of triterpenes in morchella.
With Antrodia cinnamomea as the object to study the liquid fermentation technology, researchers cinnamomea cinnamomea and Cinnamomea cinnamomea cinnamomea were cinnamomea cinnamomea and cinnamomea cinnamomea cinnamomea and cinnamomea cinnamomea cinnamomea were cinnamomea cinnamomea and cinnamomea cinnamomea and cinnamomea cinnamomea were cinnamomea cinnamomea and cinnamomea cinnamomea and cinnamomea cinnamomea were cinnamomea cinnamomea and cinnamomea cinnamomea. The results showed that extreme low temperature had significant effects on the total biomass and triterpene content in the mycelium of Antrodia antrodia.
The stimulation of light cycle had significant effect on the triterpene content, but had little effect on the biomass. The total triterpenes increased to 1.54 times of the control group after light treatment for 12h.
The optimal temperature treatment 12℃ for 2 hours a day (28℃ for the rest of the day), and the total triterpene content in the mycelia of Antrodia camphor increased to 2.42 times that of the control group.
The triterpene content and biomass of mycelium obtained by adding 1% exogenous vegetable oil to the liquid fermentation medium significantly increased.
Adding corn oil had the best effect, and when the addition amount was 1%, the inoculation amount was 10%.
After 14 days of fermentation, the triterpenoid content and biomass of the mycelium of Antrodia cinnamomea in the fermentation solution increased by 4.11 times and 1.56 times, respectively, compared with the control group.
6. flavone
Sun Yuhan et al. screened excellent morchella strains with high flavonoid yield.
Extracellular flavonoid content used as the screening evaluation index.
NaNO2-Al (NO3) 3 colorimetric method used to preliminary screen extracellular flavonoid yield of 7 Morchella strains.
Single factor and orthogonal test used to optimize the liquid fermentation conditions of the selected strains.
The results showed that Y-3 had the highest yield of flavonoids, which reached 37.524mg·L-1.
The strain cultured in the medium of 20.0g·L-1 glucose, 4.0g·L-1KH2PO4, 6.0g·L-1 urea, 0.01g·L-1 vitamin B6, 0.5g·L-1MgSO4 at 17℃, 170r·min-1 and pH7 for 8 days.
The flavonoid yield increased to 107.105mg·L-1, which 285.4% higher than that before optimization.
7. polyphenol
Li Yanting et al. studied the liquid fermentation conditions of Leucopaxillus giganteus with a single factor test to obtain a higher content of polyphenols in Leucopaxillus giganteus.
The optimal nitrogen and carbon sources for polyphenols accumulation in liquid fermentation were peptone and glucose, the optimal carbon to nitrogen ratio was 10:1, and the optimal pH was 5.
The optimal formula of liquid fermentation medium was 6.12g·L-1 peptone, 22.40g·L-1 glucose, 0.5g·L-1 potassium dihydrogen phosphate, 0.25g·L-1 magnesium sulfate, 10mg·L-1 vitamin B1, pH5.
Under the optimal culture conditions, the mass concentration of total polyphenols in mycelium of mycelium was 59.98mg·L-1.
8. cordycepin
Zou Yanan et al. optimized the liquid fermentation medium for Cordyceps militaris strain by single factor test and response surface test, so as to obtain the best culture conditions for Cordyceps militaris strain CY1909 with high content of cordyceps.
The components of yeast peptone (Xinhe) and yeast powder (Angel) are similar, and the content of glutamate is rich.
Some studies have shown that glutamic acid is the precursor of glutamine, and glutamine can greatly increase the output of extracellular cordycepin in liquid fermentation of Cordyceps pupa.
Glycine can affect the synthesis of cordycepin precursors (since adenine and other precursors not added in the single factor test, the cordycepin yield in mycelium significantly increased after the addition of appropriate amount of glycine);
Adenine is the precursor of adenosine, and adenosine is the direct precursor of cordycepin, so adenine has obvious positive regulation on the synthesis of cordycepin.
The optimal medium formula for producing cordycepin was 52.25g·L-1 yeast peptone, 8.43g·L-1 yeast powder (Angel), 3g·L-1 adenine, 0.06g·L-1 ferrous sulfate and 12g·L-1 glycine.
The content of cordycepin in mycelium was 6.35g·L-1 under the optimal culture conditions.
9. ergosterol
Guo Gengxin et al. selected several Chinese medicinal materials suitable for morchella fermentation to increase ergosterol content through single-factor test.
Combined with response surface analysis, the changes in ergosterol yield after addition of several Chinese medicinal materials analyzed and studied, and whether the addition of astragalus extract in the liquid fermentation process of morchella affected the yield of ergosterol focused on.
The results showed that Radix Bupleurum (Lonicera japonica), Radix Astragalus yunnanensis and Radix Codonopsis clematidea had significant promoting effects on morchella growth and ergosterol production.
The effect of Astragalus was the most significant, and the mycin biomass (dry mass) and ergosterol yield of morchella reached the maximum when the supplemental level of Astragalus was 100g·L-1, which were 10.57g·L-1 and 47.69mg·L-1, respectively, which were 2.83 times and 2.50 times that of the control group.
The addition of 50g·L-1 Angelica sinensis, Glycyrrhiza uralensis and Isatis tinctoria can promote the growth of morchella mycelium to a certain extent, but when the dosage exceeds the dosage, It will have a strong inhibitory effect on the growth of morchella mycelia.
According to the results of response surface analysis, the fermentation temperature was the most important factor affecting the content of ergosterol, followed by the fermentation time and dosage of Astragalus.
The optimal process parameters required for liquid fermentation medium were 8% inoculation amount, fermentation temperature 26℃, Astragalus dosage 97.60g·L-1, rotational speed 180r·min-1 and fermentation time 11d. The ergosterol content in the mycelia of morchella was as high as 59.16mg·L-1 under these conditions. Compared with the previous process optimization, the increase is 21.95%.
Zhang Zhong et al. analyzed and studied the factors affecting ergosterol content by using response surface method, and finally determined the optimal process of ergosterol production in the liquid fermentation medium of hericium simerii after optimization and screening:
Compound carbon source 14g·L-1 (mass ratio of malt extract powder to glucose was 6.6:7.4), yeast autolysate powder 18g·L-1, inorganic salt 3.9g·L-1 (mass ratio of KH2PO4 to K2S2O8 was 2.6:1.3);
The required inoculation amount was 10% and the culture time was 7 days.
Under this culture condition, the mycelial biomass (dry mass) reached 11.24g·L-1, and the yield of ergosterol reached 69.79mg·L-1, which increased by 71.08% and 81.56%, respectively.
Discussion and summary
With the increasing demand for edible fungi market, edible fungi industry is facing great challenges, but also bring more development opportunities for edible fungi industry.
In recent years, there have been more and more researches on the precision fermentation of edible fungi, and the potential value of edible fungi mycelium has gradually become a new research hotspot.
In addition to mycelium or spores, edible fungi also produce many physiological active substances such as nucleosides, polysaccharides, terpenes, flavonoids, alkaloids and sterols during fermentation.
The research on the oriented fermentation technology of edible fungi not only improves the added value of edible fungi, but also promotes the development of edible fungi industry, and also facilitates the development of edible fungi functional products, so as to promote the deep processing field of edible fungi.
At present, most of the researches on the fermentation technology of edible fungi are based on mycelial biomass, total sugar and total triterpene yield, and the exploration of functional components with biological activity is insufficient.
High value-added flavoring, health food and cosmetics and medicines with clear ingredients are important development directions for further development and application of edible fungi mycelium in the future.
Further research can conducted from the following aspects:
1) Increase the pharmacologic activity as the target to screen suitable edible fungi varieties, and explore the synthesis of their active substances;
2) Mining and developing functional ingredients of edible fungi, optimizing fermentation medium and separating fermentation products to better provide stable active substances;
3) Increase the yield of active substances of edible fungi by means of fermentation control.
