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 valori....

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.

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.