Effect of pH on microbial fermentation and control

The effect of , pH on fermentationthe pH of the fermentation medium has a very obvious effect on microbial growth, and it is also an important factor that affects various enzyme activities in the fermentation processThe effect of pH on the growth and reproduction of microorganisms and product synthesis has the following aspects: (1) affecttheary activity of enzymes, when pH inhibits the activity of certain enzymes in the bacteria, it hinders the metabolism of the bacteria; Permeability, affecting the absorption of nutrients and excretion of metabolites by microorganisms, (3) affecting the dissocing of certain components in the medium, and then the absorption of these components by microorganisms, and (4) different pH values often cause different bacterial metabolic processes, so that the quality and proportion of metabolites changethe metabolism of nutrients in the medium is the main cause of the change of pH, the change of the pH of the fermentation fluid is the comprehensive effect of the metabolism of the bacteriaDue to improper pH, it may seriously affect the growth of bacteria and the synthesis of products, so for microbial fermentation has their own most suitable growth pH and the most suitable production pHThe pH requirements vary for different microorganismsMost microbial growth has the most suitable pH range and its upper and lower limits: the upper limit is around 8.5, beyond this limit, microorganisms will be unbearable and self-soluble; However, the pH in the bacteria is generally considered to be near neutralPH has a significant effect on the synthesis of products, because the growth of bacteria and product synthesis are the result of enzyme reaction, only the type of enzyme is different, so the synthesis of metabolites also has its own most suitable pH range, such as penicillin the most suitable pH range of 6.5 to 6.8The range of these two pHs is an important parameter for fermentation controlIn addition, pH can affect the form of certain moldsit is generally believed that the h-plus or OH-in-the-cell protein can affect the dissocability and charge of the enzyme protein, change the structure and function of the enzyme, and cause the change of enzyme activityHowever, the medium's H-plus or OH-does not act directly on intracellular enzyme proteins, but first acts on the weak acid (or weak alkali) outside the cell, making it a weak acid (or weak alkali) that is easy to pass through the molecular state of the cell membrane, which enters the cell and then dissoconnections, producing H-plus or OH-, changing the neutral state of the cell, which in turn affects the structure and activity of the enzymeSo the medium of H-plus or OH-is influenced by indirect effectsPH also affects the rate of use of bacteria to the matrix and the structure of cells, and affects the growth of bacteria and the synthesis of productsCollnig et alfound that the thickness of the cell wall producing aflatoxin decreased with the increase of pH: the diameter of the mycelium at pH6.0 was 2 to 3 m, the pH7.4 was 2 to 18 m, and the form of the mycelium returned to normal after the pH decreasedPH also affects the electrical charge status of the cell membrane of the bacteria body, causing the membrane permeability to change, thus affecting the absorption of nutrients and the formation of metabolites just as temperature affects fermentation, pH also affects product stability If in the fermentation of the beta-lactam antibiotic thienamycin (thienamycin), when examining the effects of pH on the biosynthesis of the product, it was found that between pH6.7 and 7.5, the yield of antibiotics was similar, higher or lower than this range, and the synthesis was inhibited In this pH range, the stability of sanamycin was not seriously affected, and the half-life was not significantly changed, but at pH-gt;7.5, the stability decreased, the half-life was shortened, and the fermentation unit decreased Penicillin has low fermentation units under alkaline conditions and is also related to the stability of penicillin because the pH has a significant effect on the growth of bacteria and the synthesis of products, so in industrial fermentation, maintaining the most suitable pH has become one of the key factors in the success or failure of production changes in pH in the fermentation process in the fermentation process, the change of pH is determined by the composition of the bacteria used, the composition of the medium and the conditions of culture In the metabolic process of germ production, the bacteria itself has a certain ability to adjust the pH of the surrounding environment and construct the most suitable pH Fermentation research was carried out with The Mediterranean Noca bacteria producing livermycin SV, using pH values of 6.0, 6.8, 7.5 three departure values, the results showed that the pH value of 6.8, 7.5, the final fermentation pH value reached about 7.5, mycelium growth and fermentation units have reached normal levels; This shows that the bacteria only have a certain self-adjustment ability the metabolism of nutrients in the medium, but also an important cause of pH changes, fermentation uses different types of carbon sources, pH changes are not the same Such as the pH change of gray jaundice fermentation, it is closely related to the type of carbon source used, such as lactose as a carbon source, lactose is slowly used, acetone acid accumulation is very small, pH is maintained between 6 to 7; the change of pH of the fermentation solution is a comprehensive result of the metabolic reaction of the bacteria From the change of pH of the metabolic curve, it is possible to speculate on the progress of various biochemical reactions in the fermentation tank and the possible causes of abnormal changes in pH In the fermentation process, it is necessary to select the composition of the fermentation medium and its ratio, and control the fermentation process conditions, in order to ensure that the pH will not produce obvious fluctuations, maintain in the best range, get good results Practice has proved that maintaining a stable pH is beneficial to the formation of the product , fermentation pH determination and control 1, fermentation pH the most suitable pH range of microbial fermentation is generally between 5 to 8, such as glutamate fermentation of the most suitable pH of 7.5 to 8.0 However, the pH of fermentation varies with the bacteria and the product Since fermentation is a multienzyme complex reaction system, the most suitable pH of each enzyme is not the same, therefore, the most suitable pH growth of the same species may be different from the most suitable pH value of product synthesis For example, the synthetic citric acid is synthesized at pH2 to 3, and oxalic acid accumulates when the pH is close to neutral Glutamate production bacteria accumulate glutamate under neutral and slightly alkaline conditions, forming glutamine under acidic conditions Glutamate fermentation in different stages of the pH value requirements are different, fermentation pre-control pH7.5, fermentation mid-stage pH7.2, fermentation late pH7.0, in near-release tank, in order to extract glutamate after the process, pH6.5 to 6.8 for good Such as the primary metabolites acetone butanol clostridium spores fermentation, pH in neutral, the growth of the bacteria is good, but the yield of the product is very low, the actual fermentation of the most suitable pH of 5 to 6 This is especially true in the fermentation of secondary metabolites antibiotics, the optimum pH of streptomycin-producing bacteria growth is 6.2 to 7.0, while the optimal pH for synthetic streptomycin is 6.8 to 7.3 Therefore, different pH ranges should be controlled according to different stages of fermentation process, so that the output of the product can be maximized the most appropriate pH for is determined based on experimental results The fermentation medium is adjusted into different starting pH values for fermentation, and during fermentation, the pH is timed to measure and adjust to maintain the starting pH, or the buffer is formulated to maintain it Regularly observe the growth of bacteria, with the highest pH value of bacterial growth to the optimal pH of bacterial growth The optimum pH of product synthesis can be measured in the same way However, the most suitable pH of the same product is also related to the bacteria used, the composition of the medium and the culture conditions Such as the most suitable pH of synthetic penicillin, has been reported 7.2 to 7.5, 7.0 and 6.5 to 6.6 and other different values, resulting in such a difference, may be used by strains, media composition and fermentation process caused by different In determining the most suitable pH for fermentation, the influence of culture temperature should be considered from time to time, and if the temperature increases or decreases, the most appropriate pH value may also change 2, pH control in various types of fermentation process, the experimental results of the most suitable pH, the experimental growth rate of the bacteria and the product ratio generation rate (Qp) and other three parameters of the correlation of four cases (see Figure 7-3) :(1) The first case is that the most suitable pH values of s and Qp are in a similar wide range (a), this fermentation process control; The pH range is very narrow, and the range of the s (or Qp) wider (b) ;(3) The third case is that the pH value s and Qp are sensitive to the pH, their most suitable pH value is the same (c), the fermentation pH of the second and third cases should be strictly controlled; after understanding the requirements of the most pH in the fermentation process, a variety of methods should be used to control First of all, we need to consider and test the basic formula of fermentation media so that they have a suitable ratio so that the pH change in the fermentation process is within the appropriate range Because the medium contains metabolic acids (such as glucose production ketoacid, (NH4) 2SO4) and alkali -producing substances (such as NaNO3, urea) and buffers (such as CaCO3) and other components, they in the fermentation process to affect the change of pH, especially CaCO3 and ketoacid and other reactions, and play a buffer ingenuity, so its dosage is more important In batch fermentation, this method is often used to control the change of pH the ability to adjust pH using the above method is limited, if not enough, can be used in the fermentation process directly to add acid or alkali and supplement way to control, especially the method of supplementing, the effect is more obvious It used to be controlled by direct addition of acids (e.g H2SO4) or alkali (NaOH), but is now commonly used to control them with physiological lysis (NH4) 2SO4) and alkaline substances (e.g ammonia, urea) They not only adjust the pH, but also replenish the nitrogen source When the fermentation of pH and ammonia nitrogen content are low, the addition of ammonia water, can achieve the purpose of regulating pH and ammonia nitrogen supplement; In the case of additional defoamers (e.g soybean oil), it is also possible to increase air flow to speed up the metabolism of fatty acids to regulate pH Transamin is generally to make compressed ammonia gas or industrial ammonia (concentration of about 20%), the use of a small amount of intermittent addition or continuous automatic flow plus, to avoid adding too much at a time causing local partial alkali Ammonia is very easy to react with copper to produce toxic substances, the fermentation has an impact, so it is necessary to avoid the use of copper-made ammonia equipment at present, it has been more successful to use the method of supplementation to adjust the pH, such as amino acid fermentation using fluid-replenished urea (NH2) 2CO method, especially secondary metabolites antibiotic fermentation, more commonly used this method This method can not only achieve the purpose of stabilizing the pH, but also can continuously supplement nutrients, especially substances that can produce a deterrent effect A small number of multiple additions can also relieve the deterrent effect of product synthesis and increase the yield of the product In other words, the method of supplementation can simultaneously achieve the purpose of supplementary nutrition, prolonging fermentation cycle, regulating pH and the characteristics of the culture liquid (e.g bacteria concentration, etc.) The most successful example is the refeeding process of penicillin, which controls the range of changes in pH (now automated) by controlling the supplementation rate of glucose, and whose penicillin production is 25% higher than the production of pH with a constant added sugar rate and acid or alkali