Journal of Chemistry
Volume 2019 (2019), Article ID 8562384, 7 pages
https://doi.org/10.1155/2019/8562384

Research Article

Production of Coconut Aroma in Solid-State Cultivation: Screening and Identification of Trichoderma Strains for 6-Pentyl-Alpha-Pyrone and Conidia Production

Rayhane Hamrouni,1,2,3 Josiane Molinet,¹ Lucie Miché,¹ Quentin Carboué,¹ Nathalie Dupuy,¹ Ahmed Masmoudi,² and Sevastianos Roussos¹

¹Aix Marseille Univ, Avignon Université, CNRS, IRD, IMBE, Marseille, France
²Univ. Manouba, ISBST, BVBGR-LR11ES31, Biotechpole Sidi Thabet, 2020 Ariana, Tunisia
³Univ. Tunis El Manar, FST, Campus Universitaire, Tunis, Tunisia

Correspondence should be addressed to Rayhane Hamrouni; rayhan.hamrouni@gmail.com

Received 30 January 2019; Accepted 17 April 2019; Published 9 June 2019

Academic Editor: Yiannis Kourkoutas

Copyright © 2019 Rayhane Hamrouni et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The polyketide, 6-pentyl-alpha-pyrone (6-PP), has a characteristic coconut aroma and is produced by many Trichoderma species. In this study, we screened formerly isolated Trichoderma strains for 6-PP production grown using solid-state fermentation (SSF). Sugarcane bagasse supplemented with a nutrient solution was used as the support material for the culture. We detected outstandingly strong 6-PP production in the case of two Trichoderma strains in this experiment. The strains were identified based on internal transcribed spacer (ITS) sequence analysis as Trichoderma asperellum (G7 and G18). We also investigated the production of 6-PP in the case of the best Trichoderma strain selected (T. asperellum G7) from two types of cultivation (SSF and liquid cultivation (LC)). The same liquid medium was used to impregnate sugarcane bagasse. Results led to a 6-PP production 80 times higher (85.11 μg·g·DM⁻¹) in SSF than that found in LC (10.00 μg·L⁻¹) after eight days of cultivation. For this, the SSF is the best option to produce 6-PP compared to liquid cultures.

1. Introduction

Solid-state fermentation (SSF) is an ancestral process, initially used for human alimentation [1]. Numerous studies have shown advantages of SSF over its liquid homologues, advantages related to quantitative and qualitative aspects in the productions of enzymes and secondary metabolites by filamentous fungi [2, 3]. Moreover, one of the main driving forces in the gain of popularity of SSF over the last decades is the growing concern about sustainability in bioprocess development linked to the possibility of using agroindustrial wastes as cheap culture media to perform fermentations [4, 5]. Agroindustrial residues are generally considered the best substrates for solid-state fermentation processes, mainly due to their low cost [6, 7]. In SSF, the solid medium provides support and nutrients for the microorganism’s development. A large number of microorganisms including bacteria, yeasts, and fungi have been used for cultivation on different solid substrates such as sugarcane bagasse, coffee husk, jatropha cake, sorghum pulp, olive pomace, and fruit seeds [8, 9]. However, filamentous fungi especially Hypocreacea are the best choice for secondary metabolites production [4]. The fungal genus Trichoderma, known since 1887 for its antagonistic properties, has been used as a biological control organism against several plant pathogens [10]. During their development, Trichoderma produce biomass, primary metabolites (enzymes and organic acids), conidia, and secondary metabolites like 6-pentyl-alpha-pyrone (6-PP), a fungicidal compound [11]. This lactone is also called “coconut lactone” because of its characteristic smell. Trichoderma has been well investigated because of its commercial importance used in food and pharmaceutical products [12]. On the contrary, 6-PP has attracted considerable attention as a control agent due to its powerful antifungal activity. Hence, the ability of 6-PP to control the growth of Botrytis cinerea has been highlighted by Walter et al. [13]. Several workers have also reported that 6-PP production is apparently related to the antagonism response of some Trichoderma species to disadvantageous culture conditions and that the solid-state culture can give higher yields of 6-PP than submerged fermentation [14–16]. Furthermore, chemical synthesis of 6-PP requires high temperature (400°C) and seven reaction steps [17]. For this reason, production of this compound by a biotechnological way can be an interesting alternative to chemical synthesis.

A previous work has already been published for the production of 6-PP using SSF and Trichoderma strains [18].

This work aimed to screen Trichoderma strains derived from different natural sources on solid media for 6-PP production and to characterize the best strains able to produce a large quantity of 6-PP and conidia. Moreover, solid and submerged conditions were tested for the best Trichoderma strain selected to compare the production of 6-PP using the same composition of liquid culture medium.

2. Materials and Methods

2.1. Fungal Strains

Seventeen Trichoderma strains of the present study derived from various fungi collection were investigated: (i) Biotechnologie Environnementale et Chimiométrie, IRD/IMBE (Marseille, France); (ii) the Department of Agricultural Parasitology of UAAAN (Universidad Autónoma Agraria Antonio Narro) (Coahuila, Mexico); (iii) the Laboratory for Improvement and Protection of Olive Genetic Resources, Olive Tree Institute (Sfax, Tunisia); and (iv) the Tissue Culture Laboratory, Fitotechniki (Filothei-Arta, Greece).

The fungal conidia were stored at 4°C in a 5 ml bottle on potato dextrose agar (PDA) culture medium.

2.2. Inoculum Preparation

The preculture medium had the following composition: glucose: 30.0 g·L⁻¹; KH₂PO₄: 7.0 g·L⁻¹; Na₂HPO₄: 2.0 g·L⁻¹; NaNO₃: 2.0 g·L⁻¹; MgSO₄ × 7H₂O: 1.5 g·L⁻¹; CaCl₂ × 6H₂O: 0.1 g·L⁻¹; FeCl₃ × 6H₂O: 8.0 mg·L⁻¹; ZnSO₄ × 7H₂O: 1.0 mg·L⁻¹. The pH was adjusted to 5.6 before sterilization. Bottles (100 ml) containing 50 ml of this medium were inoculated with 5-day-old strains (1.0 cm² from PDA medium) and incubated under agitation (150 rpm, 5 cm diameter stroke) at 25°C for 4 days. The mycelium was homogenized in an Ultra-Turrax homogenizer, and this mycelial homogenate was used as the inoculum.

2.3. Culture Conditions for LC

Cultures of Trichoderma strains were grown in the following culture medium: glucose: 30.0 g·L⁻¹; NaNO₃: 0.6 g·L⁻¹; KH₂PO₄: 1.0 g·L⁻¹; KCl: 0.5 g·L⁻¹; MgSO₄ × 7H₂O: 0.5 g·L⁻¹; extract yeast: 1.0 g·L⁻¹; and 2 ml·L⁻¹ of trace element solution containing (in mg·L⁻¹in distilled water) CaCl₂ × 6H₂O (8.0), ZnSO₄ × 7H₂O (1.0), and FeSO₄ × 7H₂O (0.1). The pH was adjusted to 6.0 before sterilization. Bottles (250 ml) containing 100 ml of this medium were inoculated with 3.0 ml of the above mycelial homogenate and incubated under agitation (150 rpm) at 27°C. Each experiment has been performed in triplicate.

2.4. Culture Media and Conditions for SSC

The solid substrate comprising 4.0 g sugarcane bagasse was placed in 250 ml bottles supplemented with 10 ml of the same nutriment solution used for liquid culture. The humidity was set to 66% before sterilization. Sugarcane bagasse and nutriment solutions were individually autoclaved at 120°C during 1 hour. Each bottle was then inoculated with 3.0 mL of mycelial cell suspension. The incubation temperature was 27°C. Culture conditions such as incubation temperature, moisture level, inoculum concentration, and pH are fixed according to previous studies [18].

2.5. Production Kinetics

Kinetic profiles of 6-PP, conidia (for SSF), biomass, pH, and glucose were studied at the second, fourth, sixth, eighth, and tenth day of culture.

2.6. Conidia and 6-PP Analysis

All the results being expressed in gram of dry matter, the water content of each sample was measured as follows: 1.0 g of fermented material was introduced into a lab oven at 105°C to analyze the relative humidity of the sample.

Conidia suspension was prepared by mixing 1.0 g of the fermented material and 100 mL of distilled water containing tween 80 (0.01% v : v). The conidia were counted using a traditional hemocytometer as described by Roussos et al. [19].

Sugar concentration was determined after each enzyme reaction according to the amount of resulting reducing sugar using 3,5-dinitro salicylic acid (DNS) following the assay method described by Miller [20].

The extraction of aroma produced by fungi in SSF was performed by the Soxhlet system from 10 g of wet solid fermented material using pure heptane (99.7%, Sigma-Aldrich, USA). For the extraction of 6-PP produced in the liquid medium, a 10 ml aliquot of the filtered fermented broth was extracted with 100 ml of heptane.

Samples were then stirred for 20 minutes and concentrated under a rotatory evaporator at 40°C. 1 ml of the obtained heptanoic extract was then filtered with a 0.2 μm Millipore filter into vials and then analyzed with a gas chromatograph 7890A (GC) (Agilent Technology, USA) equipped with a split/splitless injector () and a flame ionization detector (). 6-PP was separated using a Supelcowax capillary column (internal diameter: 0.25 mm; length: 60 m; film thickness: 0.25 μm). The carrier gas was dihydrogen (column flow 1 mL·min⁻¹), and the split ratio was 2 : 1. The oven temperature was set as follows: 30 min at 180°C, from 180 to 230°C at 10°C·min⁻¹, and 10 min at 230°C. Quantitative analysis of 6-PP was carried out using the internal calibration method, using γ -undecanolactone (99%, Sigma-Aldrich) as an internal standard.

2.7. Molecular Identification

Total DNA was extracted from 50–200 mg of mycelium (fresh weight) using the NucleoSpin® Plant II Genomic DNA Purification Kit (Macherey-Nagel) according to the manufacturer’s procedure and stored at −20°C. Amplification of the ITS rDNA was performed by using the universal primers ITS1 (5′TCC GTA GGT GAA CCT GCG G 3′) and ITS4 (5′TCC TCC GCT TAT TGA TAT GC 3′), respectively. The PCR amplification was performed in 20 μL reaction mixtures containing 4 μl reaction buffer (5X), 2.6 μl MgCl₂ (25 mM), 3.5 μl dNTPs (1.25 mM), 0.65 μl of each primer (10 μM), 0.2 units of GoTaq® DNA polymerase (Promega), and 2 μl (20 ng) of template DNA. The PCR program consisted of an initial step at 94°C for 3 min, then 35 cycles of denaturation at 94°C for 30 s, annealing for 30 s at 50°C, and elongation for 30 s at 72°C, followed by a final elongation step at 72°C for 5 min. The ITS-PCR amplification patterns were migrated on standard 1% agarose gels in 0.5X Tris-acetate-EDTA (TAE). The amplified ITS fragments were sequenced (GATC Biotech). Phylogenetic analyses were performed using MEGA7 [21].

3. Results

3.1. Screening and Identification of 6-PP Producing Fungi

The 6-PP production by a total of 17 strains of Trichoderma strains isolated from different natural sources was examined. These experiments were performed on solid media with sugarcane bagasse impregnated with a nutritive solution. All strains were cultivated at 27°C and harvested after 8 days of total incubation. Only seven produced 6-PP (Table 1).

Table 1: Comparison of 17 strains of Trichoderma for their ability to produce 6-PP and conidia after 8 days of cultivation and molecular identification by ITS1 rDNA regions sequencing.

Strains
6-pentyl-α-pyrone (μg·g·DM⁻¹)
Conidia (×10⁸ conidia·g·DM⁻¹)
Best BLAST hit: species
Strain
GenBank accession number
Maximum similarity (%)

G7
86.69
0.49
T. asperellum
TV104
KP263611
100

G18
80.79
3.95
T. asperellum
TF1
KU341007
99

G3
39.83
1.12
T. asperellum
QT22046
KY225608
99

G11
34.45
2.31
T. asperellum
DWG3
KM268676
99

G19
32.68
2.21
Not identified
—
—
—

G9
16.30
5.78
T. asperellum
E-117
KU059966
99

G14
5.30
0.92
T. longibrachiatum
QT22111
KY225659
99

G2
Nondetected
2.32
Not identified
—
—
—

G4
Nondetected
2.50
Not identified
—
—
—

G5
Nondetected
1.11
Not identified
—
—
—

G13
Nondetected
2.70
T. longibrachiatum
T22
JN108920
99

G15
Nondetected
0.31
Not identified
—
—
—

G16
Nondetected
0.93
Not identified
—
—
—

G17
Nondetected
2.81
T. longibrachiatum
QT22040
KY209918
99

G21
Nondetected
2.07
Not identified
—
—
—

G22
Nondetected
1.14
Not identified
—
—
—

G23
Nondetected
1.22
Not identified
—
—
—

Characterization by ITS sequences analyses (Table 1) and morphological observations (Table 2) of the selected strains were conducted on the strains that produced the highest amount of 6-PP. Although they did not produce 6-PP, the strains G13 and G17 were also identified in order to compare the relationship between species and 6-PP and conidia production. Morphological characterization was performed on pure cultures grown for 3–6 days on PDA and was based on the ten characters (Table 2). Most discriminating ones were conidial shape, mycelial form, colony color, and culture smell.

Table 2: Morphological characters of Trichoderma selected strains grown on PDA Petri dish and incubated at 27°C for 4-5 days.

Isolate/characters
Appearance
Mycelium
Conidia

Colony growth rate (cm)
Colony color
Colony edge
Reverse colony color
Culture smell
Mycelial form
Mycelial color
Conidial color
Conidial wall
Conidial shape

T. asperellum G7
7-8 in 5 days
White to transparent
Smooth
Colorless
Cocount
Arachnoid
Watery white
White
Smooth
3-4 µm

T. asperellum G18
8-9 in 5 days
Green to dark green
Wavy
Creamish
Cocount
Floccose to arachnoid
White
Dark green
Rough
3–5 µm

T. asperellum G3
8-9 in 5 days
Olive green to dark green
Wavy
Creamish
Cocount
Arachnoid
White
Green
Smooth
3.6–4.5 µm

T. asperellum G11
8-9 in 3 days
Green
Wavy
Creamish
Cocount
Floccose
White
Green
Rough
3–5 µm

T. asperellum G9
8-9 in 3 days
Dark green
Wavy
Creamish
Cocount
Floccose
White
Dark green
Rough
3–5 µm

T. longibrachiatum G14
7-8 in 3 days
Yellow to orange
Smooth
Light yellow
Malt
Floccose
White
Transparent
Smooth
2.5–3.5 µm

T. longibrachiatum G13
8-9 in 3 days
Yellow to green
Smooth
Light yellow
Cocount
Floccose
White
Green
Rough
2.5–3.5 µm

T. longibrachiatum G17
7-8 in 3 days
Yellow to green
Smooth
Deep yellow
Malt
Floccose
White
Green to yellow
Smooth
2.5–3.5 µm

The molecular identification of our fungal strains was performed by sequencing the ITS 1 regions of rDNA and compared NCBI GenBank nucleotide database. The BLAST analyses showed that the ITS sequences of all sequenced strains were at least 99% similar to the corresponding GenBank sequences. All belonged to Trichoderma, with only two species identified: five strains were affiliated to the species Trichoderma asperellum (G7, G18, G3, G11, and G9) while three proved to be Trichoderma longibrachiatum (G13, G14, and G17). One of the T. asperellum strains (G7) showed the best 6-PP production with an average of 86.69 μg·g·DM⁻¹ of 6-PP (Table 1). Two other strains, T. asperellum G18 and T. asperellum G3, produced 80.79 μg·g·DM⁻¹ and 39.83 μg·g·DM⁻¹, respectively. The highest value of conidia production was shown by T. asperellum G9 (5.78 × 10⁸ conidia·g·DM⁻¹) followed by T. asperellum G18 and T. longibrachiatum G17 with 3.95 and 2.81 × 10⁸ conidia·g·DM⁻¹, respectively. Comparing the production of conidia and 6-PP, the maximum conidia production was showed by T. asperellum G9; however, this strain also showed the most limited amount of 6-PP (16.30 μg·g·DM⁻¹). Currently, Trichoderma strains are able to produce a large quantity of conidia and a green-colored fermented product characteristic due to conidia color. In this experiment, T. asperellum G7, presenting a limited conidia production and white to transparent color, is defined the best strain to produce 6-PP under the conditions tested. Based on these results, in order to evaluate the effect of conidia on 6-PP production and due to the interesting 6-PP properties, we selected T. asperellum G7 strain for further 6-PP production survey.

Glucose concentration, biomass, and 6-PP production in culture medium (Figure 1) were determined for 12 days in a stirred liquid culture of T. asperellum G7. As can be seen from Figure 1, the biomass increased sharply to a level of 5.84 g·L⁻¹ at the end of 4 days, and thereafter, the increase was gradual, reaching a final value of 14.57 g·L⁻¹.

Figure 1: Evolution of 6-PP, glucose, and biomass in the liquid medium with Trichoderma asperellum G7.

The residual sugar decreased sharply to a level of 5.66 g·L⁻¹ at the end of fermentation, indicating rapid utilization of sugar. 6-PP production was started after 6 days of incubation, the maximum value of 6-PP production (10.00 μg·L⁻¹) was observed after the exponential phase, which is a common characteristic of secondary metabolites, and then decreased rapidly to attain values of 1 μg·L⁻¹ after 10 days. There was a wide pH variation in the liquid culture, and the pH decreased from 6.0 to 4.7, after 8 days.

3.2. Conidia Production by T. asperellum G7 Cultivated in SSF

In order to compare the production of 6-PP by T. asperellum G7 in liquid and in SSF, the same liquid medium tested in both cultures was used to impregnate sugarcane bagasse. 6-PP and conidia production was determined during 10 days of culture (Figure 2). Under SSF conditions, conidia production was initiated after 1 day of incubation. The conidia production increased sharply to a level of 3.7 × 10⁷ conidia·g·DM⁻¹ at the end of the second day, and thereafter, the increase was gradual, reaching a final value of 5.1 × 10⁷ conidia·g·DM⁻¹. After 2 days, 6-PP production started. The maximum value of 6-PP production was observed after 8 days (85.11 μg·g·DM⁻¹). After that, the 6-PP production decreased progressively to attain the value of 58.14 μg g·DM⁻¹ at 10 days. As seen in Figure 2, conidiation and 6-PP production follow a similar trend at the beginning of cultivation, and the production of 6-PP was started after one day of conidiation.

Figure 2: Evolution of conidia and 6-PP production in the solid medium, during the culture of T. asperellum G7 on solid medium.

4. Discussion

The production of secondary metabolites is an actual topic that is investigated following several ways, including bioreactors design, medium composition, and fungal strains [22, 23]. Concerning medium, substrates for SSF are usually homogeneous agroindustrial by-products which offer an economic advantage in this fermentation process [24]. Even so, to be fully both economically competitive and ecofriendly, the SSF must, to a larger extent, be carried out on local by-products.

It is well known that sugarcane bagasse is particularly suitable for SSF because of its porosity allowing good water absorption, indispensable to carry out the microbial metabolism [25]. Moreover, in terms of volume, it is the major solid agroindustrial by-product generated [26].

In this study, the use of sugarcane bagasse as support for fungal growth for conidia and secondary metabolites production using the solid-state culture was evaluated. Our main objective was to screen and identify Trichoderma strains producing an considerable amount of 6-PP. Among the 17 strains tested, 7 Trichoderma strains demonstrated the ability to produce 6-PP. The characterization of these strains by morphological and ITS gene analysis indicated that they were related to T. asperellum and T. longibranchiatum. Interestingly, while 6-PP is detected in traces in all the cultures of the tested T. longibrachiatum strains, the species asperellum is much capable of producing 6-PP under these conditions.

Among the strains that produce 6-PP, our results indicated that T. asperellum G7 was the best strain with an average 6-PP production of 85.11 μg·g·DM⁻¹. With a maximum value of 86.69 μg·g·DM ⁻¹, our value was significantly higher than those reported in literature studies for other species (T. harzianum IM206040, T. atroviride IM206040, and T. harzianum 4040) cultivated under the same conditions; [12, 15, 27–29].

When comparing the two types of cultures (solid and liquid culture), a significant difference on 6-PP biosynthesis was observed. The maximum 6-PP concentration in SSF was 80 times higher than the one obtained in LC. These results confirm those previously obtained by Sarhy-Bagnon et al. [15] and Galindo et al. [30]; these authors explain that the inhibitory effect of 6-PP and the acidification observed in LC appeared to be responsible for the low concentrations obtained in the liquid medium. These results underline the differences between SSF and LC already observed in the literature. Indeed, for many secondary metabolites, SSF exhibits increased production when compared to its liquid counterpart [31].

On the contrary, a very marked pattern was observed on the relationship between conidiogenesis and 6-PP production: growth on the solid medium showed that conidia and 6-PP production followed a similar trend at the beginning of cultivation for T. asperellum G7. This does mean that the production of conidia and secondary metabolites from the genus Trichoderma is more related to the culture conditions which cause an increase in the concentration of 6-PP and conidia. This effect has a direct impact on the technical feasibility of conidia production because under these culture conditions, it is possible to use reactors such as trays, bags, or bioreactors to scale up the fermentation process for 6-PP and conidia production.

T. asperellum G7 selected by this screening is a promising strain, thanks to its ability to produce considerable quantities of 6-PP. The behavior of this strain should be investigated against phytopathogenic fungi such as, for example, Fusarium oxysporum, Botrytis cinerea, Crinipellis perniciosa, and Rhizoctonia solani [12, 13, 32, 33]. For this reason, it is important to produce 6-PP, conidia, or biomass in large amounts to meet field requirements. In addition, the opportunity for local agroindustrial by-products of low commercial value to be used in a biotechnological process makes this process an important valorization approach.

5. Conclusion

Trichoderma strains isolated from different natural sources and derived from different laboratories (Mexico, Tunisia, Greece, and France) were screened for 6-PP production. 6-PP production could be detected on solid media in the case of 7 Trichoderma strains. Among those, five belonged to the species T. asperellum (G7, G18, G3, G11, and G9). However, strains belonging to longibrachiatum species were unable to significantly produce 6-PP. The present study shows for the first time a new strain “T. asperellum G7” able to produce large amount of coconut aroma. Comparison between SSF and LC revealed that, for this strain, 6-PP production was continuous during 10 days of incubation on the solid medium, whereas in LC, 6-PP production decrease was observed in addition to being 80 times inferior compared to SSF. This indicates a clear advantage of SSF over LC for this process.

Abbreviations

SSF: | Solid-state fermentation
LC: | Liquid cultivation
PDA: | Potato dextrose agar
6-PP: | 6-pentyl-alpha-pyrone.

Data Availability

The 6-PP analysis data used to support the findings of this study are currently under embargo while the research findings are commercialized. Requests for data, 12 months after publication of this article, will be considered by the corresponding author.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors thank EURASMUS+ mobility for the financial support during the stay in France. R. H. also thanks Tunisian Republic “Tunis El Manar University”, Institut de Recherche pour le Development (IRD), the Institut Méditerranéen de Biodiversité et d’Ecologie Marine et Continentale (IMBE), and the planning director office of Aix-Marseille University for the support and technical facilitiy.

References

S. R. Couto and M. Á. Sanromán, “Application of solid-state fermentation to food industry-a review,” Journal of Food Engineering, vol. 76, no. 3, pp. 291–302, 2006.

R. De la Cruz Quiroz, S. Roussos, D. Hernández, R. Rodríguez, F. Castillo, and C. N. Aguilar, “Challenges and opportunities of the bio-pesticides production by solid-state fermentation: filamentous fungi as a model,” Critical Reviews in Biotechnology, vol. 35, no. 3, pp. 326–333, 2015.

J. C. Mateos Diaz, J. A. Rodríguez, S. Roussos et al., “Lipase from the thermotolerant fungus Rhizopus homothallicus is more thermostable when produced using solid state fermentation than liquid fermentation procedures,” Enzyme and Microbial Technology, vol. 39, no. 5, pp. 1042–1050, 2006.

A. Pandey, C. R. Soccol, and D. Mitchell, “New developments in solid state fermentation: I-bioprocesses and products,” Process Biochemistry, vol. 35, no. 10, pp. 1153–1169, 2000.

Q. Carboué, I. Perraud-Gaime, M. S. Tranier, and S. Roussos, “Production of microbial enzymes by solid state fermentation for food applications,” in Microbial Enzyme Technology for Food Applications, R. C. Ray and C. M. Rosell, Eds., pp. 437–451, CRC Press, Boca Raton, FL, USA, 2017.

L. P. Ooijkaas, F. J. Weber, R. M. Buitelaar, J. Tramper, and A. Rinzema, “Defined media and inert supports: their potential as solid-state fermentation production systems,” Trends in Biotechnology, vol. 18, no. 8, pp. 356–360, 2000.

K. Chundakkadu, “Solid state fermentation. An overview,” Critical Reviews in Biotechnology, vol. 25, no. 1-2, pp. 1–30, 2005.

M. Raimbault and D. Alazard, “Culture method to study fungal growth in solid fermentation,” European Journal of Applied Microbiology and Biotechnology, vol. 9, no. 3, pp. 199–209, 1980.

T. Robinson, D. Singh, and P. Nigam, “Solid-state fermentation: a promising microbial technology for secondary metabolite production,” Applied Microbiology and Biotechnology, vol. 55, no. 3, pp. 284–289, 2001.

S. Lanjewar, R. Ulla, and K. Sharma, “Survey of aeromycoflora present in the indoor environment of FCI godown Tilda,” Journal of Phytology, vol. 4, no. 6, pp. 7-8, 2012.

S. C. Dubey, M. Suresh, and B. Singh, “Evaluation of Trichoderma species against Fusarium oxysporum f. sp. ciceris for integrated management of chickpea wilt,” Biological Control, vol. 40, no. 1, pp. 118–127, 2007.

L. Serrano-Carreón, C. Flores, B. Rodríguez, and E. Galindo, “Rhizoctonia solani, an elicitor of 6-pentyl-α-pyrone production by Trichoderma harzianum in a two liquid phases, extractive fermentation system,” Biotechnology Letters, vol. 26, no. 18, pp. 1403–1406, 2004.

M. Walter, K. S. H. Boyd-Wilson, J. H. Perry, and R. A. Hill, “Botrytis tolerance to 6-pentyl-alpha-pyrone and massoialactone,” New Zealand Plant Protection, vol. 53, pp. 375–381, 2000.

A. Kalyani, S. G. Prapulla, and N. G. Karanth, “Study on the production of 6-pentyl-α-pyrone using two methods of fermentation,” Applied Microbiology and Biotechnology, vol. 53, no. 5, pp. 610–612, 2000.

V. Sarhy-Bagnon, P. Lozano, G. Saucedo Castañeda, and S. Roussos, “Production of 6-pentyl-α-pyrone by Trichoderma harzianum in liquid and solid state cultures,” Process Biochemistry, vol. 36, no. 1-2, pp. 103–109, 2000.

P. Bonnarme, A. Djian, A. Latrasse et al., “Production of 6-pentyl-α-pyrone by Trichoderma sp. from vegetable oils,” Journal of Biotechnology, vol. 56, no. 2, pp. 143–150, 1997.

R. G. Berger, “Aroma compounds from microbial de novo synthesis,” in Aroma Biotechnology, R. G. Berger, Ed., pp. 51–77, Springer, New York, NY, USA, 1995.

R. Hamrouni, J. Molinet, N. Dupuy, A. Masmoudi, and S. Roussos, “Trichoderma spores and 6-pentyl-alpha-pyrone production in solid state culture for biological control,” in Proceedings of the 15th International Conference on Environmental Science and Technology, pp. 1–127, Rhodes, Greece, August-September 2017.

S. Roussos, A. Olmos, M. Raimbault, G. Saucedo-Castañeda, and B. K. Lonsane, “Strategies for large scale inoculum development for solid state fermentation system: conidiospores of Trichoderma harzianum,” Biotechnology Techniques, vol. 5, no. 6, pp. 415–420, 1991.

G. L. Miller, “Use of dinitrosalicylic acid reagent for determination of reducing sugar,” Analytical Chemistry, vol. 31, no. 3, pp. 426–428, 1959.

S. Kumar, G. Stecher, and K. Tamura, “MEGA7: molecular evolutionary genetics analysis version 7.0 for bigger datasets,” Molecular Biology and Evolution, vol. 33, no. 7, pp. 1870–1874, 2016.

G. Viniegra-Gonzále z, R. Guevara-Gonzalez, and I. Torres-Pacheco, “New horizons for the production of industrial enzymes by solid-state fermentation,” in Biosystems Engineering: Biofactories for Food Production in the Century XXI, pp. 319–340, Springer International Publishing, Berlin, Germany, 2014.

L. Chen, X. Yang, W. Raza, J. Luo, F. Zhang, and Q. Shen, “Solid-state fermentation of agro-industrial wastes to produce bioorganic fertilizer for the biocontrol of Fusarium wilt of cucumber in continuously cropped soil,” Bioresource Technology, vol. 102, no. 4, pp. 3900–3910, 2011.

A. Pandey, C. R. Soccol, and C. Larroche, Current Developments in Solid State Fermentation, Springer, New York, NY, USA, 2008.

Y. Shi, X. Xu, and Y. Zhu, “Optimization of Verticillium lecanii spore production in solid-state fermentation on sugarcane bagasse,” Applied Microbiology and Biotechnology, vol. 82, no. 5, pp. 921–927, 2009.

H. H. M. Fadel, M. G. Mahmoud, M. M. S. Asker, and S. N. Lotfy, “Characterization and evaluation of coconut aroma produced by Trichoderma viride EMCC-107 in solid state fermentation on sugarcane bagasse,” Electronic Journal of Biotechnology, vol. 18, no. 1, pp. 5–9, 2015.

S. Oda, K. Isshiki, and S. Ohashi, “Production of 6-pentyl-α-pyrone with Trichoderma atroviride and its mutant in a novel extractive liquid-surface immobilization (Ext-LSI) system,” Process Biochemistry, vol. 44, no. 6, pp. 625–630, 2009.

N. C. Ladeira, V. Jordao Peixoto, M. Pessanha Penha, E. B. De Paula Barros, and S. G. Gomes Ferreira Leite, “Optimization of 6-pentyl-αa-pyrone production by solid state fermentation using sugarcane bagasse as residue,” Bioresources, vol. 5, no. 4, pp. 2297–2306, 2010.

M. M. W. Etschmann, I. Huth, R. Walisko et al., “Improving 2-phenylethanol and 6-pentyl-α-pyrone production with fungi by microparticle-enhanced cultivation (MPEC),” Yeast, vol. 32, no. 1, pp. 145–157, 2015.

E. Galindo, C. Flores, P. Larralde-Corona, G. Corkidi-Blanco, J. Antonio Rocha-Valadez, and L. Serrano-Carreón, “Production of 6-pentyl-α-pyrone by Trichoderma harzianum cultured in unbaffled and baffled shake flasks,” Biochemical Engineering Journal, vol. 18, no. 1, pp. 1–8, 2004.

U. Hölker, M. Höfer, and J. Lenz, “Biotechnological advantages of laboratory-scale solid-state fermentation with fungi,” Applied Microbiology and Biotechnology, vol. 64, no. 2, pp. 175–186, 2004.

R. Scarselletti and J. L. Faull, “In vitro activity of 6-pentyl-alpha-pyrone, a metabolite of Trichoderma harzianum, in the inhibition of Rhizoctonia solani and Fusarium oxysporum f. sp. Lycopersici,” Mycological Research, vol. 98, no. 10, pp. 1207–1209, 1994.

L. C. Ng, A. Ngadin, M. Azhari, and N. A. Zahari, “Potential of Trichoderma spp. as biological control agents against bakanae pathogen (Fusarium fujikuroi) in rice,” Asian Journal of Plant Pathology, vol. 9, no. 2, pp. 46–58, 2015.