Solid-state fermentation (SSF) refers to one or more microbial fermentation processes carried out in a state where the medium is solid, although it is rich in water, but there is no or almost no free-flowing water, and the substrate (matrix) is a polymer that is insoluble in water, which can not only provide carbon sources, nitrogen sources, inorganic salts, water and other nutrients required by microorganisms, but also a place for microbial growth, and is one of the technologies with the longest history of human use of microorganisms to produce products.
In recent years, there have been some significant developments in the field of solid-state fermentation engineering, which are mainly limited to the study of substrates (substrate characterization, substrate pretreatment and substrate sterilization), aseptic operation relativity and process control parameters (water activity, ventilation and mass transfer, temperature and pH).
Matrix (substrate) research:
In solid-state fermentation, the solid substrate not only provides the nutrients required by the microorganisms, but also acts as a fixed substance for the cells, and the substrate that can provide all the nutrients required by the microorganisms is considered the ideal substrate. Substrate has a unique role in solid-state fermentation, which affects the mass transfer, heat transfer and metabolic function of microorganisms in the microbial fermentation process. Therefore, the study of substrates is relatively thorough, mainly focusing on the characteristics of the matrix, pretreatment and sterilization.
01Matrix characteristics.
In order for microorganisms to grow on substrates and produce metabolites, they must be affected by the physical factors of the substrate itself (substrate particle size, shape, porosity, fiber content, viscosity, inter-particle diffusivity, etc.) and chemical factors (polymerization, hydrophobicity, crystallinity, electrochemical properties, etc.).
However, substrate particle size and humidity or water activity (see Process Control Parameters) are most important for microbial growth and activity in solid-state fermentation.
The size of the substrate particles directly affects the reaction surface area per unit volume, and also affects the growth of bacteria between particles, the oxygen supply rate and the carbon dioxide removal rate.
gamila.Using sugar beet as substrate to produce ethanol by solid state fermentation of monas, the effect of sugar beet particle size on the ethanol product was studied, and the results showed that small particles were obviously beneficial to the increase of ethanol yield. raopvetal.When rice bran was used for solid production of lipase, it was found that the particles decreased from 500 lm to 177 lm, and the lipase activity increased from 86 to 18upg. Pandey used Aspergillus niger solid fermentation to investigate the effect of substrate bran particle size on glucosidase, the optimal size was 425) 600lm, but if the use was less than 180lm, greater than 14mm mixed particles as substrate can achieve the same effect as 425)600lm particles.
Substrate pretreatment and absorption:
Most substrates are polymers, which are not only insoluble in water and less susceptible to attack at the beginning of microbial fermentation, but also that the substrates often lack nutrients required by certain microorganisms, which need to be added externally. To make the substrate more easily utilized by microorganisms, the substrate is often chemically or mechanically pretreated.
There are many methods of substrate pretreatment, including: steam explosion, leaching, crushing, cracking, grinding and other mechanical treatment and alkali chemical treatment. The growth of microorganisms should be attributed to the degree to which their secreted enzymes effectively break down the substrate. However, both physical and chemical factors of the substrate can promote or limit the growth of microorganisms.
The following factors determine the hydrolysis and utilization of substrates by microorganisms: (1) the types of microbial secreted enzymes;(2) the utilization and growth status of enzymatic hydrolysate by microorganisms;(3) Consumption of enzymes, substrates and hydrolysates;(3) diffusion rate of enzymes;(4) the area of substrate to be attacked;(5) Substrate inhomogeneity;(6) feedback inhibition of the final product;(7) The requirements of microorganisms for easily degradable carbon sources;(8) Enzyme kinetics, etc.
Medium sterilization:
We know that with the scale-up of reactors, large-scale sterilization of culture media will bring many problems, such as: changes in the physical and chemical properties of the medium, the formation of toxic compounds, and the loss of nutrients.
The sterilization heating and cooling time varies with the size of the medium, and the sterilization process is affected by different factors, such as the rate of killing microorganisms (proportional to the living cells of microorganisms), and the sensitivity of different microorganisms to temperature. For large-scale sterilization, high-temperature and short-time methods are generally used, so that the probability of nutrient destruction due to heat is reduced.
Another thorny problem encountered in the sterilization of culture media in industrial production is that some organic components of the medium are easily decomposed by heat, and even interact with each other at higher temperatures to form substances that are toxic to microorganisms.
Sometimes media sterilization should consider the effect of temperature on the physicochemical properties of the substrate in addition to killing microorganisms. For example, the physicochemical properties of bran (a substance commonly used in culture media) change at higher temperatures, which is generally conducive to the absorption of microorganisms, so the sterilization time is longer.