Foreword
Solid state fermentation refers to the process in which a microorganism grows on a solid substance in the absence or absence of free water. The water required to maintain microbial activity during the process is mainly a state of binding water or binding to a solid matrix. Most researchers believe that solid-state fermentation and solid substrate fermentation (Solidsubstratesfermentation) are the same concept, but Pandey et al. [1] believe that solid-base fermentation is a fermentation process in which solid matrix is ​​used as a carbon or nitrogen source without free water. Fermentation is a fermentation process that uses natural or inert substrates (such as synthetic foam) as a support in the absence of free water. This is collectively referred to herein as solid state fermentation.
In recent years, with the worldwide energy crisis and awareness of environmental protection, solid-state fermentation has been re-emphasized, mainly due to the large application of agricultural and industrial waste in solid-state fermentation, such as soil remediation, biotransformation and biology. Fuel, etc., is an ideal technology for industrial applications.
1 Factors affecting solid state fermentation
There are many factors affecting the solid-state fermentation process, mainly depending on the type of substrate, microbial selection and production scale, which can be roughly divided into biochemistry, physical chemistry and environmental factors. All factors are closely related and cannot be viewed independently. In a particular solid-state fermentation process, individual factors need to be distinguished as biochemical or physical factors. A factor can be seen as independent in a biochemical reaction, but it interacts in a physicochemical reaction, and vice versa. Therefore, it is necessary to analyze the influence of various factors in the solid state fermentation process.
1.1 Solid-state fermentation microorganisms
Fungi and bacteria are microorganisms that are used in solid fermentation, and fungi are ideal (as shown, fungal hyphae pass through the hull of the matrix to reach the starch granules). Inoculation of fungal spores has certain advantages over vegetative cells: inoculation is convenient, flexible and easy to store for a long time and high activity, but also has certain disadvantages, such as longer lag period, larger spore inoculation; induction before spore germination Spores enter metabolic activities and enzyme synthesis to prevent spores from sleeping. Some fermentation processes require mycelial inoculation, such as the insertion of Chaetomium hyphae into wheat straw for solid state fermentation. Inoculation density (pieces per gram of material) is also an important factor in solid state fermentation.
1.2 Moisture and water activity
The change in the water content of the substrate has an important influence on the growth and metabolism of the microorganism. Low moisture will reduce nutrient transport, microbial growth, enzyme stability and matrix swelling; high moisture will cause particle agglomeration, poor ventilation and bacterial infection. The moisture content range during solid-state fermentation should be controlled at 30% to 85%. Different microbial fermentation moisture should be different.
Whether the microorganism can grow on the substrate depends on the water activity Aw of the substrate. In addition to the influence of the matrix itself, the water activity is also related to the type and amount of the solute. Different microorganisms have different Aw requirements. In general, bacteria require Aw between 0.90 and 0.99; most yeasts require Aw between 0.80 and 0.90; fungi and a few yeasts require Aw between 0.60 and 0.70. Therefore, the main reason for the fungus used in solid-state fermentation is that it has low requirements on water activity and can reduce the contamination of bacteria. In the solid state fermentation process, due to the hydrolysis of the matrix, the dissolution of the substance, the Aw is lowered, and the lag period of the microorganism is prolonged, resulting in a decrease in biomass. Aw can be increased by adding sterile water, humidified air, and installing a sprayer to ensure normal growth of the cells.
1.3 Matrix and particle size
Solid-state fermentation substrates are often agricultural by-products, natural cellulose, solid waste, and the like. The inert structure of the raw material with macromolecular structure tightly encloses the nitrogen source and the carbon source material, which is not conducive to fermentation. Therefore, the pretreatment of the raw material is very important, and the packaged or granular particle size is mainly reduced by physical, chemical or enzymatic hydrolysis. Improve matrix availability. Solid-state fermentation is carried out using a natural matrix. As the microorganism grows, part of the carbon source material as a matrix structure is consumed, which affects mass transfer and heat transfer, and is usually improved by adding an appropriate amount of a support having a stable structure during the fermentation.
The size of the matrix is ​​related to the growth of microorganisms and the effect of mass transfer and heat transfer, which will directly affect the size of the reaction surface area that can be provided by the unit volume of particles, and also affect whether the cells easily enter the interior of the matrix particles and the supply rate of oxygen and metabolites. Removal rate, etc. [9]. Small particles can provide a larger microbial attack surface area and increase the rate of solid-state fermentation reaction. It is an ideal choice, but in many cases too small particles tend to cause substrate accumulation, and inter-particle void ratio also decreases, resulting in increased resistance. It has adverse effects on heat transfer and mass transfer, resulting in poor growth of microorganisms; large particles are beneficial to improve mass transfer and heat transfer efficiency due to the presence of larger gaps, and provide better breathing and aeration conditions, but the surface area of ​​microbial attack is better. small.
1.4 O 2 and CO 2 concentrations
The gaseous environment of the solid-state fermentation system directly affects the size of the biomass and the degree of enzyme synthesis, and it is necessary to control the air flow to adjust the gaseous environment. The theoretical respiratory entropy (RQ) of aerobic microorganisms is 1.0. Below 1.0 will affect the oxygen transport, and microbial growth will be hindered. By measuring the O 2 absorption rate and the CO 2 synthesis rate (on-line real-time determination by fermentation tail gas analyzer), it can be judged. The degree of growth of the microorganisms (change in the biomass of the reaction), by changing the partial pressure of O 2 and CO 2 , can control the growth and metabolism of the microorganisms, thereby regulating the solid state fermentation process.
1.5 Temperature and pH
Temperature control is important due to microbial growth, protein synthesis, enzyme and cell activity, and sensitivity of temperature synthesis to temperature synthesis. Most fungi have a growth temperature range of 20 to 55 °C [5] and a lethal temperature of 50 to 60 °C. During the fermentation process, microbial metabolism produces a large amount of heat, causing the temperature of the product to rise rapidly (sometimes up to 2/h). If the generated heat cannot be dissipated in time, it will affect spore germination, growth and product yield. In addition, the temperature of the materials in different solid layer fermentations is different (more than 3/cm in the logarithmic phase of microbial growth), resulting in uneven fermentation. Therefore, in the design of solid-state fermentation reactors, the main focus is on how to transfer heat. So far, the best solution is ventilation.
During the solid-state fermentation process, the pH value will change due to metabolic activities. The most common is the formation of organic acids, which causes the pH to drop. The optimum growth pH of different microorganisms is different. The pH range of fungi growth is 2.0~9.0, the optimum range is 3.8~6.0; the optimal range of yeast is 4.0~5.0. Low pH can effectively inhibit the growth of contaminating bacteria. It is difficult to carry out on-line measurement and control of the pH value by using a suitable technique, and a buffering substance (no effect on the reaction process) can be added to the fermentation raw material to buffer the pH change.
1.6 Ventilation and agitation
The demand for oxygen in the aerobic fermentation process and the need for mass transfer and heat transfer in the system, ventilation and agitation operations have an important impact. Increased air velocity provides oxygen for microbial growth and removes CO 2 , volatile metabolites, and heat of reaction, but many factors affect O 2 transport, such as air pressure, aeration, matrix voids, layer thickness, and culture. Base water, reactor geometry and the speed of the mechanical stirring device. Airflow intensity can be used as a criterion for judging ventilation strength, and ventilation quality is also important (especially gas humidity, which can change water activity). Proper ventilation strength and quality increase temperature control.
Due to the heterogeneity of the matrix, the ventilation process is likely to cause changes in cell metabolism, and it is necessary to increase the uniformity of material fermentation, moisture, temperature and gaseous environment by stirring. When selecting the matrix, the matrix properties should be considered to avoid agglomeration during the agitation process, but excessive tumbling may damage the mycelium and inhibit the growth of the cells. Intermittent agitation has a better effect than continuous agitation, and is more advantageous for the growth of mycelium and its attachment to the substrate.
1.7 Solid State Fermentation Reactor
Solid-state fermentation reactors are currently an important factor limiting solid-state fermentation for modern bioreactor engineering. There are several issues to consider when designing a reactor: sterilization, inoculation, mass transfer heat transfer, sampling, gas supply, parameter measurement and control. Many types of solid-state fermentation reactors have been available to date (including laboratory, pilot, and industrial production), and some are used for edible fungi, enzyme preparations, animal feed, and soil remediation.
1.7.1 Shallow fermentation reactor
The shallow-plate fermentation reactor is the simplest fermentation equipment of all reactor types. The structure is shown in the figure. It is the reaction device used in the production of traditional fermented food. However, the heat is mainly transmitted through the tray, even if it is cooled by electricity, it is not enough to remove the metabolic heat. . In addition, there are disadvantages such as high pollution risk caused by low mass transfer heat transfer rate and low utilization rate of the tray.
1.7.2 Fluidized bed reactor
The reactor mainly lays a powdery granular substrate on a metal mesh or a porous plate, and blows air from the bottom upward to form a fluidized bed state, and two different forms of fluidized bed structures are shown in the figure. The main parameters of the reactor are particle size and particle distribution. The narrower the particle size distribution, the easier it is to maintain the fluidized state. The closed system of the reactor can better maintain the sterility, and the fermentation can improve the air temperature and directly dry the product. This type of reactor has a low volume ratio.
1.7.3 Drum reactor
The basic form is to mount a cylindrical container on a rotating system. The rotating system mainly supports and provides power. The structure is shown in the figure. The rotation rate of the drum type fermenter is generally 1~16r/min, and some can reach higher rotation speed. The degree of damage of the fungal mycelium is sensitive to the rotation speed. This type of reactor focuses on solving the problem of agglomeration and sticking of the material, followed by a low reactor volume ratio. Adding a crushing plate (net) can solve the caking problem.
1.7.4 Disc reactor
The bottom of the disc reactor is usually made of two layers of metal mesh, and the sterile air is uniformly introduced from the bottom into a fermentation substrate of about 1 m thick. Several side-by-side spiral mixers rotate at a certain speed while rotating at an appropriate speed. There are also 2~3 nozzles on the agitator for hydration, and the structure is shown in the figure. The reactor is easy to scale up for industrial production, but cannot be aseptically operated and can only be used in natural fermentation and mixed fermentation processes.
1.7.5 Other
There are also different solid-state fermentation reactors, such as gas-phase dual-dynamic solid-state fermentation technology and equipment (shown in the figure. This technology has been successfully scaled from 2, 50, 800L in the laboratory to 25, 50, 70m 3 ). Industrial-scale production scale, which covers antibiotics, enzyme preparations, organic acids, food additives, bio-pesticide and bio-fertilizers.
2 Application of modern solid state fermentation technology
Solid-state fermentation technology has been widely used in traditional functional foods and wine brewing, such as soy sauce, rice wine, soybean meal, rice wine and white wine. From traditional solid-state fermentation to modern solid-state fermentation, this technology has played a major role in the production of antibiotics, enzyme preparations, concentrates, organic acids, bioactive substances, etc., and further expanded to biotransformation, biofuels, biological control, and waste treatment. In the field of bioremediation, solid-state fermentation has attracted close attention as a potential technology.
2.1 Biotransformation
The use of solid-state fermentation technology to bio-transform crops and crop residues to enhance their nutritional value has great economic value. The food and feed industry is now increasingly used. For example, the cassava and cassava residues are fermented by Rhizopus to improve their nutritional value; the white rot fungus or P. chrysosporium is used to degrade lignocellulose; the Trichoderma fermented palm is used to improve its utilization rate in the feed industry.
2.2 Biofuels
The production of biofuels by solid-state fermentation of agricultural and industrial residues can be broadly classified into two broad categories: gas and liquid biofuels. Purification of traditional biogas can lead to new biofuels; biohydrogen production is a relatively new type of biofuel gas, combined with anaerobic fermentation of agro-industrial waste by hydrogen bacteria, acid-producing and methanogenic bacteria. Liquid biofuels have recently been classified as bioethanol and biodiesel. Biodiesel has continued to grow as a potential alternative to petroleum due to the inspiring importance of bioethanol in the world's energy crisis. The use of solid-state fermentation method to produce bioethanol can eliminate the sugar preparation process, save costs; reduce the fermenter volume, no waste water; reduce energy consumption, etc. The fermentation process is produced by yeast invertase and alcoholic enzymes on natural raw materials (such as sugar beet , apple pomace, sweet sorghum and cassava, etc.) for transformation. Unlike alcohol, biodiesel is an ester, and bio-fermented ethyl or methyl esters can be blended with conventional diesel or 100% as biodiesel. Amin and other microalgae treatment of industrial wastewater to produce algae oil has made breakthroughs, and algae oil can be used as biodiesel after simple treatment.
2.3 Biological control
Biological control is a method that neither pollutes the environment nor kills pests or germs. It is a control method that uses beneficial organisms or other organisms to inhibit or eliminate harmful organisms. Commonly used are fungi, bacteria, viruses and energy. Secretion of antibiotic-resistant antibiotics (no pollution to humans and the environment). By using solid-state fermentation to produce fungal insecticides, the virulence of drugs to pests is greatly improved. For example, the early Beauveria bassiana and Bacillus thuringiensis insecticide; and the combined use of Pseudomonas, Haciendron and Trichoderma viride can minimize the transformation of Fusarium oxysporum. The use of solid-state fermented microbial fertilizer can alleviate the continuous cropping obstacles of watermelon and cucumber.
2.4 Garbage disposal
At present, domestic and domestic urban waste treatment mainly adopts landfill, incineration, fermentation and other methods. The landfill technology covers a large area and is not easy to be degraded. The waste incineration technology has a high degree of reduction, but the investment is huge and is subject to smoke emission. The restriction of fermentation technology has the advantages of reduced volume, reduced volume, low degree of harmlessness and recyclability, which has become the first choice for waste treatment at home and abroad. The use of solid-state fermentation technology to process and treat domestic waste not only solves the problem of resource shortage, but also reduces waste discharge. Germany's Eggersmann uses the Horstmann tunnel silo fermentation system to process classified organic wastes and heavy metals-free industrial hazardous materials to produce high-grade organic fertilizers with a processing capacity of 73,000 tons/year. The ECOPARE Waste Treatment Plant in Barcelona, ​​Spain, processes municipal mixed waste and restaurant waste using pre-waste sorting, aerobic composting, anaerobic fermentation, and biogas power generation. The daily processing capacity is 1,050 tons. The Edmonton treatment plant in Canada uses the drum fermentation process technology to produce 125,000 tons of composted compost every year. Boluo County, Guangdong Province, China, uses a system integration technology that combines sorting, organic waste fermentation, fertilizer processing, combustible pyrolysis, gasification power generation, and inorganic landfill to treat domestic waste and produce organic compound fertilizer.
2.5 Bioremediation
Solid-state fermentation biotechnology is a useful tool for biodegradation of toxic compounds and environmental bioremediation. For example, the third institute of the State Oceanic Administration used the Rhodococcus sp. (TW53) to repair oil-contaminated seawater and lake water. The decomposition rate of petroleum reached 90%, and the collection of bacteria also obtained 59.18% of FA-rich fat. Bioremediation of caffeine-containing substances by Postreatus for bioremediation can achieve the purpose of caffeine degradation. Greek scholars use microbes to remove humic acid from landfills, with removal rates of more than 85%.
Transfer from Henan University of Technology (Natural Science), Vol. 32, No. 1 "solid-state fermentation technology of modern technology, equipment and application research progress" Author: Lang Li, Yang, Xue Yongliang
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