Design of Solid-State Fermentation Systems for Polymer Hydrolytic Extracellular Enzyme Production by Filamentous Fungi

Summary

This protocol utilizes wheat bran in a rotary solid-state fermentation system to enhance enzyme production. The substrate, supplemented with inducers such as chitin, supports fungal growth under controlled conditions. Results demonstrate enzyme yields 4-6 times higher compared to submerged fermentation, showcasing the method's adaptability and effectiveness for diverse biotechnological applications.

Abstract

Solid-state fermentation (SSF) is a bioconversion process that utilizes a solid substrate that does not dissolve in an aqueous medium. Microorganisms grow on the surface of the substrate and penetrate its solid matrix to extract essential nutrients for their development. SSF is characterized by minimal free water, with a substrate moisture content maintained above 70%, and involves three interconnected phases -- gaseous, liquid, and solid. This protocol describes the use of wheat bran, an agro-industrial byproduct, as the base substrate for enzyme production in a rotary system. The substrate is supplemented with an inducer, such as chitin, chitosan, starch, or cellulose, to promote the synthesis of hydrolytic proteins. The system is highly adaptable, allowing the use of different fungal forms, including mycelium, spores, or pellets. In the methodology described, the inducer and substrate are mixed at a ratio of 1:100 (w/w), sterilized via autoclaving, and adjusted to the desired moisture level with sterile water. The fungal inoculum is then added, and the rotary system operates at 10 rpm to ensure adequate mixing and oxygenation. The system is incubated for 6-8 days under optimal growth conditions for mesophilic or thermophilic/thermotolerant fungi, enhancing its versatility. Following incubation, the enzyme is easily extracted using an appropriate cold buffer (e.g., acetate, citrate, or phosphate), depending on the type of enzyme. The extract is clarified through centrifugation and filtration to obtain a cell-free supernatant. The enzyme can then be further concentrated or purified as needed. The results demonstrated a 4-6-fold increase in enzyme activity compared to submerged fermentation (SmF), highlighting the effectiveness of the system. Its adaptability to different substrates, inducers, and fungal species makes it a valuable tool for various biotechnological applications.

Introduction

Solid-state fermentation (SSF) has emerged as a promising and sustainable bioconversion technology for producing high-value enzymes, bioactive compounds, and secondary metabolites. This technique involves the growth of microorganisms on solid substrates with minimal free water, simulating their natural environment and enabling efficient metabolic activity¹. The primary goal of this protocol is to optimize enzyme production through a rotary SSF system that ensures enhanced substrate utilization, oxygen diffusion, and process scalability. Employing wheat bran, an abundant agro-industrial byproduct, as the base substrate, contributes to the valorization of agricultural residues and promotes circular bioeconomy practices².

SSF has significant advantages over submerged fermentation (SmF), including lower energy and water consumption, higher product concentration, and compatibility with a wide range of inexpensive agricultural residues such as wheat bran, rice husks, and sugarcane bagasse³. Unlike SmF, which requires large volumes of water and expensive nutrient media, SSF systems leverage solid matrices that not only serve as microbial growth surfaces but also provide nutrients essential for microbial activity. Additionally, the limited free water in SSF minimizes contamination risks, making it a more robust option for enzyme production in industrial settings⁴. In addition to its operational advantages, SSF presents significant environmental and economic benefits compared to submerged fermentation (SmF). Studies have reported that SSF reduces water consumption by 50%-70% and lowers energy costs by more than 30% due to the absence of large water volumes requiring constant agitation and aeration. Moreover, the use of agro-industrial residues as substrates minimizes raw material costs and promotes circular economy practices by repurposing agricultural byproducts^2,4.

SSF has been extensively validated for its efficiency and scalability. For example, studies have reported a 4-6-fold increase in enzyme activity using SSF compared to SmF, highlighting the economic and environmental advantages of this technique^2,5. Additionally, the downstream process is simplified, as enzyme extraction typically requires less water and fewer purification steps. This makes SSF particularly attractive for industries aiming to reduce operational costs and environmental impact⁶.

The rotary SSF system described in this protocol offers several improvements over traditional static SSF methods. While static systems often face challenges such as uneven substrate colonization and oxygen limitation, the rotary configuration ensures thorough mixing and aeration, promoting uniform microbial growth^7,8,9. For instance, this system has been successfully employed to produce hydrolytic enzymes such as chitinases, amylases, and proteases using fungal species like Aspergillus and Trichoderma².

A key feature of this SSF system is its adaptability. The use of wheat bran as a base substrate demonstrates the potential of agro-industrial residues for cost-effective bioconversion³. Moreover, the supplementation of the substrate with inducers such as chitin, chitosan, and starch further enhances enzyme synthesis by stimulating specific metabolic pathways^2,10. The system is also compatible with different fungal forms, including spores, mycelium, and pellets, allowing users to tailor the process to their specific requirements².

SSF offers broad potential for application in various fields such as food biotechnology, biofuel production, and environmental remediation¹¹. Its integration of cost-effective substrates, exceptional enzyme yields, and high process flexibility establishes SSF as an essential approach for industrial-scale biotechnological innovations.

Protocol

The reagents and the equipment used in this study are listed in the Table of Materials.

1. Substrate preparation

NOTE: Use a commercial brand of wheat bran to minimize significant variations in substrate characteristics. Each batch of wheat bran varies due to multiple factors, making it a heterogeneous material that is difficult to standardize, leading to fluctuations in its constituent content. If a standardized material is required, choose an alternative matrix or perform a proximate chemical analysis of each batch of wheat bran to adjust it according to the needs.

1. Wash the wheat bran three times with sterile distilled water to remove organic matter residues, debris, and dust. This also removes simple sugars that could interfere with fermentation.
2. Spread the washed bran on an aluminum tray and dry it in an oven at 60 °C for 24 h.
3. Once dried, place the wheat bran into a sterile 50 mL conical tube.

2. Preparation of inoculum

NOTE: This protocol describes three methods for inoculum preparation: spore suspension, direct inoculation with mycelium disks, and cellular suspension. Establish the initial inoculum concentration and quantify protein levels for accurate yield calculations.

1. Preparation of spore suspension
   1. Transfer a 5 mm diameter agar disk saturated with mycelium onto a fresh potato dextrose agar plate. Incubate the plate at 28 °C for 5-7 days, or until the mycelium saturates the medium. Some fungi may require a longer incubation time.
   2. Add 5 mL of sterile distilled water to the plate and mechanically detach the spores using a sterile loop.
   3. Prepare a 1:100 dilution of the spore suspension. Place 10 µL in the center of a Neubauer chamber and count spores under a microscope. Calculate the spore concentration (spores/mL) based on the chamber's factor and dilution.
2. Cultivation of mycelium in liquid medium
   1. Transfer a 5 mm agar disk saturated with mycelium onto a fresh potato-dextrose-agar plate and incubate at 28 °C until saturation.
   2. Prepare 25 mL of potato-dextrose broth in a sterile 125 mL flask and autoclave.
   3. Transfer a 5 mm mycelium disk from the saturated plate into the sterile broth.
   4. Incubate the flask on a shaker at 125 rpm for 24-48 h, depending on the fungal strain. Extend the incubation time for slow-growing fungi.
   5. Collect 2 mL for the inoculum and 2 mL for dry weight determination.
3. Direct inoculation of mycelium disks
   1. Place a 5 mm agar disk saturated with mycelium on a fresh potato-dextrose-agar plate. Incubate at 28 °C until saturation.
   2. Use one mycelium disk as inoculum and another to determine the dry weight.

3. Preparation of the SSF system

NOTE: Inducers can be natural or commercial. Purified commercial inducers are preferred to minimize impurities that could alter fermentation efficiency. Adjust water additions to maintain a relative humidity of at least 90%.

1. Combine the following components in a sterile 50 mL conical tube: 5 g of dry wheat bran; 0.2 g of the inducer (e.g., commercial chitin); 5.5 mL of water (adjust based on the inducer's water absorption capacity); 5 mL of a sterile salt solution containing 16 g/L monobasic potassium phosphate, 4 g/L sodium sulfate, 2 g/L potassium chloride, 1 g/L calcium chloride, 400 mg/L zinc chloride, 60 mg/L boric acid, 40 mg/L sodium molybdate, 150 mg/L magnesium chloride, 100 mg/L ferric chloride, and 400 mg/L copper sulfate.
2. Measure the relative humidity using an electrode-based hygrometer, ensuring a minimum of 90% humidity. Insert the electrode probe directly into the reactor at varying depths to obtain a representative measurement of moisture distribution.
   1. Follow the steps to adjust the humidity if below 90%:
      1. Gradually add sterile distilled water in 1 mL increments per 10 g of substrate. After each addition, mix thoroughly to ensure uniform distribution of moisture.
      2. Allow the substrate to equilibrate for 10-15 min. Remeasure the humidity level.
      3. Repeat the above steps until the target humidity of 90% is reached. Avoid over-wetting the substrate throughout the process.
   2. Follow the steps to adjust the humidity if above 90%:
      1. Spread the substrate thinly in a sterile environment. Remove excess moisture by (1) exposing the substrate to laminar airflow, or (2) placing it in a drying chamber at 30 °C for 10-15 min.
      2. Alternatively, gently mix the substrate to promote even moisture redistribution. After treatment, reassess the humidity level.
      3. Repeat the drying or mixing step as needed until the humidity reaches 90%. Proceed with fermentation only once the target humidity is achieved.
3. Autoclave the tube at 15 psi for 15 min.
4. After cooling, inoculate the substrate with one of the following: 1 mL of spore suspension (1 x 10⁶-1 x 10⁷ spores/mL), 2 mL of cellular suspension, or one 5 mm mycelium disk.

4. Solid-state fermentation (SSF) procedure

NOTE: For kinetic studies or parameter evaluations at different times, prepare separate tubes for each time point to ensure representativity.

1. Avoid substrate clumping by vortexing the tubes at maximum speed for 5 min in 1-min cycles.
2. Place the tubes in a rotary mixer with a horizontal axis. Ensure the substrate moves freely inside the tubes. Set the mixer to operate at 10 rpm.
3. Incubate the mixer in an incubator at the microorganism's optimal growth temperature. Maintain the temperature reported for optimal enzymatic activity when using heat-sensitive inducers.

5. Extraction of enzymes

NOTE: The extraction fundamentals are based on the solubility and pH-maximum activity of the extracellular enzyme. As SSF avoids the water medium, the extracellular enzyme is involved in the water surrounding the solid matrix, which means the concentration is higher than in SmF. In this context, the selection of the best extraction buffer depends on the knowledge of the desired activity. The optimization of the extractions depends on the final enzyme concentration and the type of extraction buffer used.

1. After the desired fermentation period, resuspend the substrate in 20 mL of pre-chilled buffer. Examples include: 0.1 M acetate buffer, pH 5.6, for chitinase extraction; 0.02 M phosphate buffer, pH 6.9, for amylase extraction.
2. Vortex the tubes in cycles: 1 min at maximum speed, followed by 1 min on ice. Repeat 10 times.
3. Filter the suspension using paper filters and mechanically extract the supernatant by pressing.
4. Clarify the supernatant by centrifuging at 3000 x g for 15 min at 4 °C.
5. Use the crude extract directly or further purify the enzyme via column chromatography or centrifugal filters. Kinetic studies are also recommended to determine the Michaelis constant (Km) and the maximum rate of enzymatic transformation (Vmax)¹².

6. Optimization process

NOTE: Optimize this protocol by evaluating and adjusting the quality and concentration of inducers, as well as the type and concentration of the inoculum.

1. Determine the ideal fermentation time and refine the extraction steps to improve efficiency.
2. Control and fine-tune environmental conditions, including temperature, pH, and aeration.
3. Test various buffers and extraction conditions to enhance enzyme yield and stability.
4. Perform statistical analyses, such as response surface methodology, to identify the most influential variables and achieve optimal enzyme production.

Results

Figure 1A presents the schematic representation of the rotary mixer used in this system, which has a capacity for six conical tubes of 50 mL. Figure 2B illustrates the changes that occur in the wheat bran during conditioning before entering the solid-state fermentation process. As seen, no significant structural changes were observed.

Figure 2 shows the saturation of wheat bran after 6 days of solid-state...

Discussion

This study outlines a relevant protocol for optimizing enzyme production through solid-state fermentation (SSF) systems, specifically designed for filamentous fungi. Below, critical aspects of the methodology are discussed, alongside its significance, limitations, and potential applications.

The success of the protocol is highly dependent on key steps such as the preparation of the substrate and inoculum. Proper washing and drying of the wheat bran are essential for eliminating impurities that...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
125 mL Erlenmeyer flask	Sigma-Aldrich	CLS431684	For culturing mycelium in liquid medium.
50 mL conical tube	Sigma-Aldrich	CLS430921	For storing and preparing substrates and inoculum.
Acetate buffer, pH 5.6	Sigma-Aldrich	320866	For chitinase extraction.
Centricon filters	Millipore	UFC905024	For further purification of enzymes.
Counting cells chamber	Sigma-Aldrich	Z359629	Used to count spores under a microscope.
Filter paper	Whatman	1001-110	For filtering the enzyme extract.
Hygrometer	Todomicro	-	To measure relative humidity of the substrate.
Inducer (e.g., commercial chitin)	Sigma-Aldrich	C9752	Used to enhance enzyme production during fermentation.
Phosphate buffer, pH 6.9	Sigma-Aldrich	P5379	For amylase extraction.
Potato-dextrose agar	Sigma-Aldrich	P2182	Culture medium for growing fungal mycelium.
Potato-dextrose broth	Sigma-Aldrich	P6685	Liquid culture medium for growing fungal mycelium.
Rotary mixer	Thermo-Fisher Scientific	88-861-051	To keep substrate moving during fermentation.
Salt solution components (e.g., KH2PO4, Na2SO4, KCl, etc.)	Sigma-Aldrich	Multiple	For preparing sterile salt solution, see detailed recipe in the protocol.
Wheat bran	Comercial market	-	Substrate for solid-state fermentation.