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  • Review Article
  • OPEN ACCESS

Overview of Eurotium cristatum and its Fermentation Application in Tea

  • Qian Zhang1 and
  • Ju Zhang1,2,* 
 Author information
Future Integrative Medicine   2024;3(1):11-20

doi: 10.14218/FIM.2023.00077

Abstract

Eurotium cristatum (E. cristatum), commonly known as “golden flower”, is the dominant strain in the microbial fermentation process of Fu brick tea. E. cristatum has favorable biological characteristics, including enzyme production, antimicrobial properties, immune regulation, antitumor properties, fat reduction capabilities, and weight loss benefits. With its probiotic characteristics, E. cristatum can be combined with different varieties of tea substrates to make a variety of fermented teas. More importantly, in the process of tea fermentation, E. cristatum can secrete a variety of extracellular enzymes, including some hydrolytic enzymes and oxidoreductases. They metabolize and transform various chemical components in tea through a series of reactions such as oxidation, degradation, and condensation, which significantly affect the quality of tea. In this review, by summarizing its basic functional characteristics as well as its application in fermented tea, an in-depth analysis of the key problems existing in the fermentation application of E. cristatum is described and some beneficial suggestions are presented in order to provide a rich theoretical basis for the development and utilization of E. cristatum to a greater extent.

Keywords

Eurotium cristatum, Fermentation, Application, Tea

Introduction

Under specific temperature and humidity conditions, Eurotium cristatum (E. cristatum) has the ability to form golden cleistocyst shells on the surface of Fu brick tea, earning it the nickname “golden flower”.1E. cristatum is present during the flowering process of Fu brick tea and contributes to the fermentation process. The tea fermented by E. cristatum possesses various health benefits, including the ability to decrease blood glucose and blood lipid levels as well as to exhibit anti-oxidant properties.2E. cristatum has low nutritional requirements and can thrive in potato dextrose agar and rose bengal medium, showcasing its strong adaptability.3 It is also crucial in enhancing the quality of tea. By obtaining nutrients from tea, E. cristatum undergoes a series of metabolic transformations. In addition, it produces a range of extracellular enzymes that catalyze and transform various substances in tea, which have a significant effect on the color, aroma, and taste of tea.4 Researches have demonstrated that E. cristatum can significantly improve the biological activity of tea components.5 Moreover, it can enhance the content of aromatic compounds in tea and reduce the content of bitter and astringent compounds, thus greatly improving the overall taste and quality of tea. The development and utilization of E. cristatum have revealed some promising effects. Studies have shown that E. cristatum possesses numerous functions, including weight loss, lipid reduction, anti-oxidation, inhibition of harmful bacteria, antitumor effects, and regulation of intestinal microflora.6 In the future, these potential effects of E. cristatum are expected to drive the development of new fermentation products and offer vast application prospects. This review provides a comprehensive overview of the fundamental functional characteristics of E. cristatum and its applications in tea. It also describes directions for further research and development, aiming to facilitate the extensive and in-depth exploration and utilization of E. cristatum.

Overview of E. cristatum

Identification and nomenclature of E. cristatum

E. cristatum is one of the strains present during the flowering process of Fu brick tea that plays a vital role in determining the quality of tea. Therefore, numerous studies have been conducted to identify and name E. cristatum. Figure 1 shows the morphological characteristics of E. cristatum at different culture times. Initially, it was isolated from the “golden flower” found in dark teas such as Fu brick tea; hence, its common name is golden flower fungus. However, with the improvement of the isolation and identification technology of microorganisms, it was found that golden flower fungus contains many species and that it has a very rich microbial flora, including E. cristatum, Aspergillus chevalieri (A. chevalieri), Aspergillus cristatus (A. cristatus), etc. The process of identification and naming of E. cristatum has taken a long time. At first, in 1983, Professor Wen Qiongying named it A. chevalieri, but because one feature did not meet the requirements of the identification monograph, its name was changed to A. cristatus. Later, because it contains both sexual and asexual types, the name A. cristatus conflicted with Article 59 of the International Botanical Nomenclature, so it was finally named E. cristatum. Previous research reports have shown various identification results for the golden flower fungus,7 which led to differences in its nomenclature. Table 1 shows the identification and naming of E. cristatum in tea from different sources.8-13 In a study conducted by Yang et al.,8 golden flower fungi were isolated from eight commercially available golden flower teas. Through isolation and identification, the fungi obtained from the golden flower included Aspergillus coronatus and E. cristatum. Additionally, A. chevalieri and Aspergillus amstelodami were also isolated and identified from these tea samples. It is worth noting that the microbial species present in these tea samples varied significantly due to the different tea types and processing techniques. However, microbial species within the same category exhibited greater similarity.

Morphological characteristics of <italic>E. cristatum</italic> at different culture times.
Fig. 1  Morphological characteristics of E. cristatum at different culture times.

From (a)–(e), E. cristatum was cultured for 3 days, 4 days, 5 days, 6 days, and 7 days, respectively.

Table 1

Separation and identification of E. cristatum in different sources of tea

Tea classificationTea varietyFermentation degreeIdentification methodMicroorganisms in different tea samplesReference
Jingyang Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, and molecular identificationAspergillus cristatus8
Niu Jinhua Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, and molecular identificationAspergillus cristatus; Eurotium cristatum8
Jingwei Fu teaFully fermented tea, the fermentation degree is 100%.Morphology, and molecular identificationAspergillus cristatus8
Anhua dark teaFully fermented tea, the fermentation degree is 100%.Morphology, and molecular identificationAspergillus cristatus; Eurotium cristatum8
Anhua dark tea, yellow slicesFully fermented tea, the fermentation degree is 100%.Morphology, and molecular identificationAspergillus cristatus8
Hunan Anhua, Golden Fu teaFully fermented tea, the fermentation degree is 100%.Morphology, ITSAspergillus amstelodami9
Dark teaGuang xi black brick teaFully fermented tea, the fermentation degree is 100%.Morphology, Multigene phylogenetic analysis (β-tubulin gene, calcium regulator protein gene and RNA polymerase II gene)Aspergillus pseudoglaucus10
Hunan black brick teaFully fermented tea, the fermentation degree is 100%.Morphology, Multigene phylogenetic analysis (β-tubulin gene, calcium regulator protein gene and RNA polymerase II gene)Aspergillus cristatus10
Shaanxi Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, ITS, β-tubulin Phylogenetic analysisAspergillus cristatus11
Hunan Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, ITS, β-tubulin Phylogenetic analysisAspergillus cristatus11
Zhejiang Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, ITS, β-tubulin Phylogenetic analysisAspergillus chevalieri11
Sichuan Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, Molecular identificationAspergillus cristatus; Aspergillus chevalieri; Aspergillus niger12
Guizhou Fu brick teaFully fermented tea, the fermentation degree is 100%.Morphology, Phylogenetic analysis of multiple genesAspergillus cristatus13
Fuding golden flower old white teaLightly fermented tea, the fermentation degree is less than 10%.Morphology, and molecular identificationEurotium cristatum8
Pu’er raw teaLightly fermented tea, the fermentation degree is less than 10%.Morphology, and molecular identificationAspergillus cristatus; Eurotium cristatum8
White teaTianfu teaLightly fermented tea, the fermentation degree is less than 10%.Morphology, ITSAspergillus amstelodami9
Tianfu white teaLightly fermented tea, the fermentation degree is less than 10%.Morphology, ITSAspergillus pali9
Tianfu Nan waxy shan cooked cakeLightly fermented tea, the fermentation degree is less than 10%.Morphology, ITSAspergillus niger9
Black teaPu’er cooked teaFully fermented tea, the fermentation degree is less than 90%.Morphology, and molecular identificationEurotium cristatum8
Keemun Black TeaFully fermented tea, the fermentation degree is less than 90%.Morphology, ITSAspergillus pali9

ITS, internal transcribed spacer.

Basic functional characteristics of E. cristatum

E. cristatum possesses good biological efficacy, including enzyme production, bacteriostasis, immune regulation, antitumor effects, and lipid reduction. The production of extracellular enzymes has an important impact on the transformation of tea substrates, while the other characteristics have beneficial regulatory and improvement effects on the organism. Table 2 summarizes the important impact of E. cristatum fermentation on substrates and the organism.14–25

Table 2

Basic characteristics of Eurotium cristatum and the effects on substrates or the organism

CharacteristicsEffects on substrate or organismReferences
Enzyme productionProducing a variety of extracellular enzymes such as cellulase, protease, and polyphenol oxidase, polyphenol oxidase can reduce the bitter and astringent taste of tea, and improve the sensory quality of tea.14,15
AntibacterialThe inhibitory effect on bacteria is significant, but it is insensitive to mold and yeast.16–19
Regulate immunity, anti-tumorThe fungal polysaccharides and metabolic products produced by Eurotium cristatum itself have a good anti-inflammatory effect and can inhibit the growth of cancer cells.20–22
Lose weight and reduce cholesterolSignificantly reduced the content of triglycerides, low-density lipoprotein, and total cholesterol in the body.23–25

Enzymes produced by E. cristatum

In addition to meeting its own growth requirements, E. cristatum also plays a crucial role in the transformation, degradation, polymerization, and oxidation of various substances in tea. It possesses the ability to produce a range of extracellular enzymes, including polyphenol oxidase, cellulase, and protease, which facilitate the conversion of macromolecular substances such as pectin, polysaccharides, and cellulose into smaller molecular compounds. This enzymatic activity contributes to improve the unique qualities and flavors in tea.14 For instance, the polyphenol oxidase produced by E. cristatum can oxidize polyphenols in tea, resulting in the reduction of phenolic compounds, which leads to a reduction in the bitterness of the fermented tea. Li et al.15 discovered that during the solid fermentation of green tea, the PE-1 strain of E. cristatum produced a variety of extracellular enzymes, including cellulase, xylanase, amylase, protease, and tannase. These findings highlight the superior enzyme production capacity of E. cristatum and its close association with the quality of tea.

Bacteriostatic effects of E. cristatum

During its growth and metabolic processes, E. cristatum produces metabolites that exhibit antagonistic effects on the growth and reproduction of other microorganisms, effectively inhibiting the proliferation of harmful microbes.26 For example, Zhang et al.27 conducted experiments extracting a total of six active components from the fermentation products of E. cristatum. Through analysis and detection, it was found that two of these active components, FS-IV and FS-VI, have significant bacteriostatic effects and display excellent stability at temperatures of −20°C to 100°C and pH 2–13. The remaining four components did not exhibit noticeable inhibitory effects. In addition, Li et al.16 studied the fermentation broth of E. cristatum and discovered that inhibition on molds and yeasts was not prominent, while its inhibitory effect on bacteria was enhanced. The reason for this is that antimicrobial substances have different mechanisms of action on bacteria and fungi. For bacteria, antimicrobial substances mainly inhibit the growth and reproduction of bacteria by inhibiting the synthesis of their cell walls; while for fungi, antimicrobial substances mainly inhibit the function of the fungal cell membrane, that is, the permeability of the cell membrane is changed to inhibit the growth and reproduction of fungi. Therefore, the antimicrobial substances have different inhibitory effects on bacteria and fungi. A large number of studies have revealed that the fermentation broth of E. cristatum has a remarkable antimicrobial effect. Experimental results have indicated that certain substances produced during the growth of E. cristatum can hinder the growth and reproduction of other microorganisms. Moreover, Zhang et al.17 employed various commonly used organic solvents to extract and identify active substances in the fermentation broth. Their results showed the presence of antimicrobial active components in the ethyl acetate extract, which exhibited significant inhibitory effects on actinomycetes and Gram-positive bacteria. Furthermore, it was observed that these antimicrobial active components display good stability under acidic and thermal conditions. Du et al.18 isolated several aromatic glycosides from EN-220, which demonstrated inhibitory activity against Escherichia coli. Xiao et al.19 investigated extracts from the propagules of E. cristatum and found that certain extracts exhibited potent inhibitory effects on actinomycetes, bacteria, and fungi. Notably, the strongest inhibitory effects were observed against Escherichia coli and Staphylococcus aureus. Additionally, these antimicrobial extracts display excellent thermal stability at 25–80°C and pH 2–6 as well as are unaffected by acidic conditions, demonstrating good overall stability.

Immune-modulating and antitumor effects of E. cristatum

Exopolysaccharides derived from E. cristatum have been shown to enhance the immune capacity and physiological function of the human body. Moreover, these polysaccharides exhibit strong antitumor activity and effectively inhibit the growth of cancer cells.20 In addition to the immunomodulatory and antitumor activities of fungal polysaccharides, E. cristatum also produces other bioactive components with outstanding anti-inflammatory properties. Almeida et al.21 have demonstrated that Compound 1, whose chemical name is 2-(2′,3-epoxy-1′,3′-heptadienyl)-6-hydroxy-5-(3-methyl-2-butenyl)benzaldehyde (Fig. 2),21 is a secondary metabolite produced by E. cristatum that displays inhibitory effects on cell growth, making it a valuable candidate for anticancer and antitumor applications. Xu et al.22 conducted a study on the anti-inflammatory effect and mechanism of benzaldehyde derivatives (2-6-H-5-B) derived from E. cristatum fermentation broth using lipopolysaccharide-induced RAW 264.7 cells. The results revealed that the 2-6-H-5-B compounds significantly inhibited the production of nitric oxide, tumor necrosis factor-alpha, interleukin 6, and monocyte chemoattractant protein 1 in activated macrophages, indicating strong anti-inflammatory activity. This experiment confirmed the potent anti-inflammatory activity of the benzaldehyde derivative 2-6-H-5-B derived from E. cristatum culture medium. In a study by Peng et al.,28E. cristatum fermentation broth was administered to immunosuppressed mice via gastric gavage for 15 days. The results demonstrated that the fermentation broth significantly enhanced the immune function of mouse cells. Further research revealed that the fermentation broth of E. cristatum could activate T lymphocytes and stimulate the production of the corresponding antibodies by B cells, thereby significantly improving immune function in mouse cells.

The chemical structure of compound 1.
Fig. 2  The chemical structure of compound 1.21

Weight loss and fat reduction effects of E. cristatum

In a study conducted by Kou et al.,23 the effects of E. cristatum fermentation broth on lipid metabolism in hyperlipidemia rats were investigated using sweet potato tea as the substrate. Male rats were used as the experimental animals and were fed under specific temperature as well as humidity conditions. The results showed that fermented sweet potato tea significantly reduced the weight and Lee’s index of obese rats as well as effectively decreased the degree of obesity. Furthermore, it promoted liver lipid metabolism in rats fed with a high-fat diet and reduced fat accumulation in their bodies. The levels of triglycerides, low-density lipoprotein, and total cholesterol in the serum of rats on a high-fat diet were significantly reduced. Additionally, Liu et al.24 examined the lipid-lowering effect of E. cristatum in Jingyang tea. They studied the impact of black tea extract after being infused with E. cristatum on obese model mice. The experimental results demonstrated that fermented tea effectively inhibited body mass growth, decreased serum total cholesterol levels, and suppressed the increase in adipocyte volume while reducing the diameter of subcutaneous adipocytes. These findings provide valuable guidance and reference for the development of functional tea beverages aimed at lowering blood lipid levels. Moreover, Xiao et al.25 investigated the effects of fermented orange peel black tea on the metabolism of mice induced by a high-fat diet. They found that the intervention of fermented orange peel black tea significantly improved the steatosis induced by a high-fat diet in mice. This improvement was mainly manifested by a reduction in vacuoles in mouse stem cells. The concentration of total cholesterol in mouse liver tissue was also significantly decreased.

Application of E. cristatum in fermented tea

E. cristatum is extensively utilized in diverse types of fermented tea, including mulberry leaf black tea, mulberry leaf green tea, litchi grass tea, etc. Table 3 shows the fermentation application of E. cristatum in different varieties of tea.29–46 In addition, many researches have been conducted on the fermented tea extracts, which were derived from the fermentation of E. cristatum. During the fermentation process of E. cristatum, the bioactive components in different varieties of tea undergo some changes, leading to significant impacts on the taste, aroma, and color of the fermented tea. The sensory quality of tea has been demonstrated to be enhanced through fermentation with E. cristatum, as evidenced by the data presented in column 5 of Table 3. The relationship between this change and the content of the main chemical components was also analyzed.

Table 3

Effect of Eurotium cristatum fermentation on quality of different kinds of tea

Tea speciesFermentation conditionsChanges in the major compoundsChanges in the biological activityChanges in sensory qualityReference
Mulberry leaf black teasolid stateThe contents of flavonoids and polysaccharides increased; polyphenol content decreased.Hydroxyl radical scavenging capacity increased; DPPH radical clearance rate decreased slightly.The tea soup exhibits a vibrant color and exudes a rich aroma, while the reduction in polyphenol content contributes to the alleviation of bitterness and astringency in its taste.29
Litchi grass teasolid stateTheabrownine, thearubin and theaflavin increased 1.29 times, 1.73 times and 3.02 times respectively; tea polyphenols, amino acids and soluble sugars were reduced by 31. 9%, 36. 3% and 25. 2%, respectively.The bitterness and astringency are significantly reduced as a result of the decrease in polyphenol levels. The increased presence of theabrownine, thearubin, and theaflavin imparts a vibrant orange-red color to the tea.30
Raw dark tealiquid stateTheabrownine, free amino acids and total flavonoids were increased by 5. 08%, 76.75% and 14.40%, respectively; a total of 12 kinds of aromatic esters and alcohols were added; tea extract, total protein and tea polyphenols were reduced by 21. 95%, 53. 80% and 36. 36%, respectively.The aroma is enhanced, characterized by a distinctive fungal fragrance attributed to the increase of aromatic esters and alcohols.31
Golden flower loose teasolid stateIncreased aromatic esters and aldehydes; the contents of free amino acids, flavonoids, soluble sugars, tea polyphenols, myricetin, catechins, kaempferol and quercetin decreased.The reduction in polyphenol levels contributes to a decrease in tea bitterness and astringency, while the increase in aromatic esters and aldehydes content results in an elevation of aroma components.32
Cyclocarya paliurus teasolid stateAromatic alcohols increased and flavonoids increased by 25.66%; the content of triterpenes, soluble sugars, free amino acids and polyphenols decreased by 27.91%, 13.46%, 10.74% and 53.04%, respectively.The increase in the concentration of aromatic alcohols contributes to a robust tea aroma, characterized by distinct herbal and fungal floral notes. Simultaneously, the reduction in polyphenol levels significantly diminishes the bitter and astringent taste of tea.33
Berry tea compound raw dark teasolid stateThe contents of gallic acid and GCG increased; alkaloids and total catechin decreased.The mellowness of tea is enhanced and the bitterness is mitigated as a result of the reduction in tea polyphenol content.34
Golden flower white teasolid stateTheabrownine, gallic acid, total flavonoids, theanine, caffeine and tea extract increased by 77.31%, 10.92%, 20.46%, 8.63%, 7.70% and 2.89%, respectively; thearubin, theaflavin, soluble sugars, free amino acids, catechin and tea polyphenols decreased by 30.96%, 48.83%, 9.38%, 27.44%, 52.89% and 24.26%, respectively.The interconversion of theabrownine, thearubin, and theaflavin imparts a vibrant color to fermented tea, while a significant reduction in tea polyphenol content effectively mitigates its bitter and astringent taste.35
Mulberry leaves green teasolid stateTea extract and DNJ increased by 3 times; polyphenols, flavonoids and polysaccharide decreased by 7.1%, 12% and 76.1%, respectively.The tea soup exhibits a vibrant orange color and exudes a rich aroma, accompanied by golden flower granules. The reduction in polyphenol content effectively mitigates the bitterness and astringency of the tea soup.36
Acanthopa nax senticosus teasolid stateAmino acids and tea polyphenols were reduced by 2.99% and 6.94%, respectively.Antioxidant activity was enhanced.The optimal ratio of polyphenols to amino acids contributes to the sweet and mellow taste of fermented tea, while a decrease in tea polyphenol content results in a reduction of its bitter and astringent flavors.37
Autumn green teasolid stateThe contents of gallic acid and theabrownine were increased by 3.68mg/g and 30. 49mg/g respectively; and the aromatic alcohols were increased; the contents of purine alkaloids, theaflavin, thearubin, polyphenols, flavonoids were reduced; the water-soluble carbohydrate compounds were reduced by 42. 63 mg/g; EGCG, ECG, EC were reduced by 0. 37 mg/g, 4. 99 mg/g, 0.40 mg/g, respectively.The interconversion of theabrownine, thearubin, and theaflavin gives fermented tea a bright color, the increase in aromatic alcohol content gives unique aroma of fermented tea; and the appropriate ratio of polyphenols to amino acids gives sweet taste and mellow taste of fermented tea.38
Docynia indica teasolid statePhloretin content increased.Enhanced antioxidant and antimicrobial activities.Safety and health improvement.39
Green teasolid stateIncreased gallic acid content; tea polyphenols, flavonoids, ester catechin EGCG, and ECG were decreased.The ability to lower blood pressure is improved.The taste is mellow and rich in flavor.40
Instant dark tealiquid stateThe content of non-ester catechins, alkaloids, thearubin and theabrownine increased; the content of ester catechins and free amino acids decreased.DPPH radical Scavenging capacity is improved.The bitterness is reduced due to the decrease in tea polyphenol content, the increase in the content of thearubin and theabrownine gives the tea soup a bright orange-red color.41
Fu brick teasolid stateIncreased aromatic alcohols; the total amino acid content decreased; EGCG and ECG were reduced by 79. 03% and 69. 88% , respectively.The increase in the content of aromatic alcohols enhances the aroma of fermented tea, while the decrease in polyphenol content reduces the bitter taste and astringent taste of the tea soup.42
Dandelion teasolid stateThe content of polyphenols was reduced.The tea soup has a bright color, full golden grains and a mellow taste.43
Tartary buckwheat teasolid stateSweet amino acids, water extract, theabrownine content increased; aroma compounds increased; the contents of bitter amino acids, flavonoids, and tea polyphenols were reduced.Enhanced total Antioxidant capacity and increased DPPH and free radical Scavenging capacity of ABTS.The increase in the content of sweet amino acids gives sweet taste of fermented tea, while the decrease in the content of bitter amino acids and tea polyphenols reduces the bitter taste and astringent taste of the tea, the increase in the content of aromatic alcohols and esters enhances the aroma of fermented tea.44
Eucommia ulmoides Teasolid stateThe content of chlorogenic acid increased 0.478 mg/g.Golden flowers have full granules and rich aroma.45
Tieguanyin Teasolid stateThe content of thearubin, theabrownine, total flavonoids, glucose, triterpenoids, total sugar, catechin, EC, EGC and C increased; the content of free amino acids, theaflavin, fructose, maltose, sucrose and polyphenols decreased; ECG and EGCG decreased by 88. 1% and 92.2%, respectively.DPPH, hydroxyl radical, ABTS radical scavenging capacity, Fe3+ reduction capacity, total reducing power increased, and antioxidant activity improved.The increase in the content of theabrownine and thearubin gives fermented tea a bright orange-red color, while the decrease in tea polyphenols significantly reduces the bitter taste and astringent taste of the tea.46

ABTS, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid); DNJ, 1-Deoxynojirimycin; DPPH, 2,2-diphenyl-1-picrylhydrazyl; EC, Epicatechin; ECG, Epicatechin gallate; EGC, Epigallocatechin; EGCG, Epigallocatechin gallate.

Effect of E. cristatum on tea taste

The amino acid content and polyphenol content of tea are crucial indicators that reflect tea quality and are closely related to the formation of the tea taste. During the process of fermentation, E. cristatum utilizes tea amino acids as a nitrogen source for its growth and synthesizes its own substances. Consequently, as the fermentation degree of E. cristatum deepens, the content of amino acids is decreased, which has a significant impact on the sensory quality of tea.47 During the growth and metabolism of E. cristatum, various extracellular enzymes such as polyphenol oxidase, cellulase, pectinase, and protease are produced. These enzymes play a role in transforming tea components.5 For example, in a study conducted by Ma et al.,29 mulberry leaf black tea was inoculated with E. cristatum. The results showed that fermentation by E. cristatum led to an increased production of flavonoids and polysaccharides in mulberry leaf black tea. However, the content of polyphenols in fermented tea decreased. Similarly, Yang et al.30 fermented lychee grass tea with E. cristatum. They observed a significant improvement in the flavor quality of the fermented lychee grass tea. Further investigation revealed that E. cristatum produced active metabolites during fermentation, which effectively enhanced the sensory quality of the fermented tea. The researchers also concluded that certain extracellular enzymes were produced by E. cristatum in the fermentation process, which facilitated the transformation, oxidation, and degradation of various components in tea. Polyphenol oxidase, for example, oxidizes phenolic compounds into quinones, thereby reducing the polyphenol content and significantly decreasing the bitter taste of fermented tea. Moreover, Zou et al.31 discovered that fermented tea exhibits a reduced content of tea polyphenols, which leads to a decrease in the bitterness of tea soup as well as significantly enhances the sensory quality of tea.

Effect of E. cristatum on tea aroma

Fermentation of tea leads to the production of various ketones, alcohols, and aldehydes, causing changes in the composition and content of aromatic compounds. In a study conducted by Luo et al.,32 the volatile components of black tea raw materials and Jinhua loose tea were analyzed, and the research results showed that the fermentation of E. cristatum contributed to a significant increase in the content of nine volatile components. These components included (E)-linalool 3,7-oxide, (E)-furan linalyl oxide, acetophenone, (E)-2-nonenal, (E)-2-octenal, (E,E)-3,5-undecadien-2-one, tridecane, (E,E)-2,4-heptadienal, and methyl salicylate. These nine volatile components contributed to the unique fungal fragrance of Jinhua loose tea. Additionally, Zheng et al.48 fermented green tea extract with E. cristatum and detected a total of 38 aromatic compounds in the fermented extract, with 10 of them having a high aroma. Furthermore, several new active components with volatile aromas were discovered in the fermentation extract. Zou et al.49 fermented ginkgo biloba leaves with E. cristatum and observed a decrease in alcohols and an increase in aldehydes as well as ketones after fermentation. They suggested that the transformation of these aromatic components may be related to oxidation during the metabolic process of E. cristatum. Moreover, Chao et al.33 carried out solid-state fermentation of Cyclocarya paliurus (C. paliurus) tea using E. cristatum and found that the content of polyphenols and total free amino acids decreased by 53.04% and 10.74%, respectively. Through sensory evaluation and analysis of the main active substances of fermented C. paliurus tea, they identified 67 differential metabolites, with 31 of them being terpene compounds with a molecular formula of C15H24. These terpene compounds are the main contributors to the fruity and floral aroma of C. paliurus tea. Xiao et al.50 pointed out that E. cristatum enables the physiological metabolism of terpenic alcohol. In addition, a study conducted by Peng et al.34 inoculated E. cristatum on a series of berry tea-dark tea composites, which were obtained by combining berry tea and dark tea in varying proportions, and then the fermentation experiments were carried out by E. cristatum. Through sensory evaluation, they found that the flowering process resulted in an obvious fungal aroma in the black tea samples as well as in the Golden flower berry tea complex black tea samples. Likewise, Tu et al.51 fermented liquid green tea beverage using E. cristatum and analyzed the aromatic components of the fermented tea beverage. They discovered a significant increase in the content of more than ten alcohol compounds with aromatic components, which endowed the fermented tea beverage with a unique aroma.

Effect of E. cristatum on tea color

During the “blooming” process, E. cristatum produces various extracellular enzymes that promote the formation of theabrownine. Theabrownine has high biological activity, and its increased content gives fermented tea a brown and red color. It may also be the main flavor substance in fermented tea.50,52 Bo et al.35 investigated the dynamic changes of the main taste substances during the processing of Jinhua white tea and found that the sensory quality of Jinhua white tea changed significantly after the “blooming” process. The fermented tea exhibited excellent sensory attributes. The tea soup displayed a vibrant orange-yellow color, possessed a mellow flavor with decreased bitterness, and emitted a distinctive aroma. After the accumulation of Shoumei loose tea, the color changed to yellow-brown, and the golden flowers inside the tea brick began to grow vigorously on the eighth day of flowering, achieving a good flower effect. The soup color of Shoumei loose tea changed from yellow to orange-yellow, and as microorganisms propagated and polyphenols oxidized, the overall soup color gradually deepened to a bright orange-red. Furthermore, Zheng et al.36 fermented mulberry leaf green tea with E. cristatum. Sensory evaluation revealed that after fermentation by E. cristatum, a large number of golden flower fungi attached to the surface of mulberry leaf green tea, accompanied by a unique fungal aroma. The fermented tea exhibited a bright yellow-brown color, and the surface color of the fermented tea was pure.

Conclusions and perspectives

In summary, E. cristatum exhibits multifaceted biological effects including enzymatic production, bacteriostatic activity, immunomodulation, antitumor properties, as well as weight and lipid reduction. As a fermentation starter, it facilitates the transformation and metabolism of the tea constituents through processes such as oxidation, degradation, and condensation, thereby enhancing the sensory attributes and biological functionality of tea. Consequently, it represents a significant probiotic agent with substantial developmental potential. However, the current application of E. cristatum in fermentation still faces certain limitations, which are described below.

First, the fermentation variations among strains of E. cristatum remain unclear. The diversity of the source and the variety of E. cristatum strains have led to unclear studies on the composition differences and characteristics of the self-fermentation broth, and the ingredient markers for its biological activity have not been resolved, thus limiting the standardized management and precise application of E. cristatum.

Second, the fermentation and transformation mechanism of E. cristatum as a starter culture remains elusive. Currently, there is a wide range of fermented teas made with E. cristatum, but the fermentation characteristics, component transformations, and metabolic mechanisms of different tea substrates have not been comprehensively compared. The substrate specificity as well as advantages and disadvantages of various fermentation substrates remain unclear, which limits the application range of E. cristatum fermentation and the market share of related products.

Third, the active mechanism of E. cristatum and its fermentation products remains unclear, while the exploitation and utilization of its active functions are inadequate. Studies have demonstrated the immunomodulatory, antitumor, anti-oxidative, and intestinal microbiota-modulating activities of E. cristatum; however, the underlying mechanisms remain elusive. Moreover, while most fermented tea products rely on its sensory-enhancing properties, there is still a significant market gap for health foods and bioproducts that harness its active effects.

In light of the aforementioned issues, the following recommendations are proposed for future fermentation applications of E. cristatum:

  • Screening out superior strains of E. cristatum with high productivity. Selecting teas of diverse origins and grades, conducting separation, identification, and fermentation culture processes, as well as employing metabolomics technology, a comprehensive and in-depth investigation could be conducted on the metabolites present in fermented broth from different sources. The aim is to elucidate key quality markers and to identify the advantageous characteristics of high flowering efficiency of E. cristatum for quality control;

  • Optimize the fermentation conditions according to the inherent advantageous characteristics of the fermentation substrate. When utilizing E. cristatum for the fermentation of various substrates, the advantageous chemical composition of the fermentation substrate itself first needs to be accurately grasped. In the process of optimizing the fermentation substrate of E. cristatum by making use of modern microbial fermentation technology, the content of the target chemical component is utilized as an evaluation criterion to enhance the fermentation efficiency of E. cristatum to a greater extent;

  • Conduct further molecular mechanism exploration on the biological efficacy of E. cristatum and its fermentation products in order to clarify the target of its activity and to develop related biomedical preparations combined with genetic engineering technology. At the same time, it can be combined with the probiotic properties of E. cristatum to expand its application in the fermentation of nontea food, agricultural products, and medicinal and edible plants as well as other substrates.

Under the background of great health in today’s society, there has been an increasing focus on the relationship between the biological efficacy of E. cristatum fermentation products and human health. In the future, utilizing important techniques such as metabolomics technology, modern microbial fermentation technology, and genetic engineering technology to develop an increasing number of health-care fermented products derived from E. cristatum will become an innovative direction for future development.

Abbreviations

A. chevalieri: 

Aspergillus chevalieri

A. cristatus: 

Aspergillus cristatus

ABTS: 

2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)

C. paliurus: 

Cyclocarya paliurus

DNJ: 

1-Deoxynojirimycin

DPPH: 

2,2-diphenyl-1-picrylhydrazyl

E. cristatum: 

Eurotium cristatum

EC: 

epicatechin

ECG: 

epicatechin gallate

EGC: 

epigallocatechin

EGCG: 

epigallocatechin gallate

ITS: 

internal transcribed spacer

Declarations

Acknowledgement

None.

Funding

This research was funded by the Central Government Guides Local Science and Technology Development Fund Project of Hebei Province (No. 226Z2801G), a Postdoctoral research project of Hebei Province (No. B2022005014), Science and Technology Innovation Projects of the Hebei Academy of Agriculture and Forestry Sciences (No. C21R0803, C22R0802), and Research Fund of Hebei University of Economics and Business (No. 2021SGYB04).

Conflict of interest

The authors declare no conflict of interests relevant to this study.

Authors’ contributions

QZ summarized and organized this paper by searching and reading a large number of papers. JZ summarized the “Conclusions and perspectives” and mainly revised and edited the paper.

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Overview of Eurotium cristatum and its Fermentation Application in Tea

Qian Zhang, Ju Zhang
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