(1) Factors affecting fermentation yield
Substrate concentration in batch fermentation, often because the pre-base mass concentration is too high, inhibits (or inhibits) biosynthetic enzymes or inhibits the growth of mycelium (such as the inhibition or inhibition of glucose and money, phenylacetic acid growth inhibition ), And the low substrate concentration in the later period limits the mycelial growth and product synthesis.To avoid this phenomenon, the fed-batch operation method is usually used in penicillin fermentation, that is, slow flow of the substrate that is easy to produce inhibition, inhibition and restriction To maintain a certain optimal concentration. Special attention must be paid here to the addition of glucose, because even small fluctuations beyond the optimal concentration range will cause severe suppression or limitation, slowing down or stopping the rate of biosynthesis. At present, the detection of sugar concentration is still difficult to perform online, so the feeding of glucose is not controlled according to sugar concentration, but is adjusted indirectly according to pH value, dissolved oxygen or CO2 release rate.
(2) Temperature The optimum temperature for penicillin fermentation may vary slightly depending on the strain used, but it is generally believed that it should be around 25 ° C. Excessive temperature will obviously reduce the fermentation yield, at the same time increase the maintenance consumption of glucose and reduce the conversion rate of glucose to penicillin. For mycelium growth and penicillin synthesis, the optimum temperature is not the same, generally the former is slightly higher than the latter, so some fermentation processes use higher temperatures during the mycelium growth stage to shorten the growth time, after reaching the production stage It is appropriate to lower the temperature to facilitate the synthesis of penicillin.
(3) pH value The optimal pH value for penicillin fermentation is generally considered to be around 6.5, sometimes it may be slightly higher or lower, but the pH value should be avoided as much as possible, because penicillin is unstable under alkaline conditions, easy Accelerate its hydrolysis. In a medium with weak buffering capacity, the change in pH is a reflection of the acceleration of glucose flow. An excessively high feed rate causes the accumulation of acidic intermediate products to lower the pH value; an excessively low sugar addition rate is insufficient to neutralize ammonia produced by protein metabolism or alkaline compounds produced by the metabolism of other physiological alkaline substances and cause the pH value to rise. #z! w; X # B (X
(4) Dissolved oxygen For aerobic penicillin fermentation, dissolved oxygen concentration is an important factor affecting the fermentation process. When the dissolved oxygen concentration drops below 30% saturation, the penicillin yield drops sharply, and below 10% saturation, it causes irreversible damage. If the dissolved oxygen concentration is too high, it means that the mycelium is not growing well or the sugar addition rate is too low, which causes a decrease in respiratory intensity and also affects the production capacity. Dissolved oxygen concentration is a dynamic equilibrium point between oxygen transfer and oxygen consumption, and oxygen consumption is directly proportional to carbon energy consumption, so dissolved oxygen concentration can also be used as a reference index for glucose flow control.
(5) Mycelium concentration During fermentation, the mycelium concentration must be controlled so as not to exceed the critical cell concentration, so that the oxygen transfer rate and the oxygen consumption rate are balanced at a certain dissolved oxygen level. The critical cell concentration of penicillin fermentation varies with the respiratory intensity of the strain (depending on the size of the maintenance factor, the greater the maintenance factor, the higher the respiratory intensity), fermentation ventilation and agitation capacity, and the rheological properties of the fermentation. Strains with low breathing intensity reduce the rate of oxygen consumption in fermentation, while fermentors with strong aeration and agitation capabilities and low-viscosity fermentation broth increase the rate of oxygen transfer during fermentation, thereby increasing the critical cell concentration. ! R9L; Q-y9 |
(6) Mycelium growth rate Fermentation experiments conducted with a chemostat prove that under the condition of glucose limited growth, the specific production rate of penicillin has a certain relationship with the specific growth rate of mycelium. When the specific growth rate is lower than 0.015h-1, the specific production rate is directly proportional to the specific growth rate, when the specific growth rate is higher than 0.015h-1, the specific production rate is independent of the specific growth rate. Therefore, the fermentation process To achieve and maintain the maximum specific production rate, the specific growth rate must not be lower than 0.015h-1. This specific growth rate is called the critical specific growth rate. For the production stage of batch fed fermentation, maintaining a critical specific growth rate of 0.015h hopper means that the mycelium concentration or fermentation broth volume should be doubled every 46h, which is difficult to achieve in actual industrial production. In fact, the specific growth rate controlled by the penicillin industrial fermentation production stage is much lower than this theoretical threshold, but it can still achieve a high specific production rate. This is due to the fact that part of the mycelial autolysis in the fed-batch fermentation process adopted by the industry constantly offsets part of the growth, so although the apparent specific growth rate is lower, the specific growth rate is actually higher. Zhiti House, Zhiti Space, China Zhiti Forum, Zhiti Forum, Zhiti Net, O% M5v (t7s3A # m.v5`I
(7) Mycelium morphology In the long-term strain improvement, the penicillin-producing bacteria differentiated into two main forms of filamentous growth and nodular growth in submerged culture. The former is generally higher than the production rate because all mycelium can fully contact the substrate and oxygen in the fermentation broth; the latter has a significantly reduced viscosity of the fermentation broth, which greatly increases the oxygen transfer rate between the gas-liquid two phases. This allows more mycelium to grow (ie, the critical cell concentration is higher), and the volumetric productivity of the fermentor is even higher than the former.
In the fermentation of filamentous fungi, controlling the mycelium morphology to maintain proper branching and length, and avoiding ball formation, is one of the key factors for high yield. In the fermentation of globular fungi, keeping the mycelium balls at an appropriate size and tightness, and minimizing the content of free mycelium, is also one of the key factors to give full play to its production capacity. The control of this morphology is closely related to the feeding conditions and rate of sugar and nitrogen sources, the agitation shear strength and specific growth rate.
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