1. Carbon Balance
Carbon in the culture medium = Carbon in the bacteria + Carbon in the product + Carbon in the CO₂ released
Carbon is the primary energy source for cell growth and product synthesis, making accurate estimation of CO₂ emissions crucial. While exhaust gas analyzers are commonly used for measurement, it should be noted that this method, which uses cooling and water removal from the exhaust gas, can result in significant errors. Since CO₂ is highly soluble in water, this method may result in lower readings. Without instrumentation, experience suggests that the total carbon content of the culture medium (including feed) is approximately 2.3–2.6 times the carbon content in the cells and product, or 10–15 times the total nitrogen content of the culture medium (including feed).
2. Nitrogen, phosphorus and sulfur balance
N, P, S in the culture medium = N, P, S in the bacteria + N, P, S in the product + N, P, S remaining in the culture medium.
These elements form the basis of cell structure, enzymes, and metabolites, and their balance directly affects bacterial growth and biosynthesis efficiency. Nitrogen, phosphorus, and sulfur are all present in excess in the culture medium, so subsequent optimization is primarily based on experimental verification..
To achieve efficient and stable operation of the fermentation process, material balance alone is not enough. The following three key balances must be achieved in the dynamic process:
1. Balance between oxygen consumption rate and oxygen supply rate
Oxygen is a crucial factor in aerobic fermentation. Microorganisms consume oxygen during growth and metabolism, and insufficient oxygen supply can limit bacterial growth and reduce product synthesis efficiency.
How to achieve balance: By real-time monitoring of dissolved oxygen (DO) changes, the stirring rate, ventilation volume or tank pressure are adjusted to ensure that the oxygen supply rate meets the oxygen consumption needs of the bacteria.
Optimization significance: Maintaining an appropriate dissolved oxygen level can avoid both hypoxic inhibition and excessive oxygen supply causing energy waste or cellular oxidative damage.
2. Balance between dilution rate and growth rate
In fed-batch fermentation, the flow rate (dilution rate) of fresh medium must match the specific growth rate of the bacteria. The dilution rate here is the ratio of the rate at which fresh medium flows into the fermenter to the volume of culture medium in the fermenter during continuous or fed-batch fermentation.
How to achieve balance:With the help of online biomass monitoring or indirect parameters (such as pH and DO inflection point), the critical dilution rate is determined to keep the bacteria at the optimal specific growth rate.
Optimization significance:It can prevent the bacteria from growing too fast, which may lead to nutrient depletion or accumulation of metabolic byproducts, and it can also prevent the reactor from being inefficient due to slow growth.
3. Balance between substrate flow acceleration rate and substrate consumption rate
The addition of nutrients (such as carbon sources and nitrogen sources) must precisely match the actual consumption rate of the bacteria, otherwise there will be an excess or shortage of substrate.
How to achieve balance:This is divided into two main phases. In the research phase, we use sophisticated, complex, and expensive methods to find the optimal balance point. We dynamically adjust the feeding strategy by monitoring the residual substrate concentration (such as glucose and ammonium ions) in real time or using soft sensing technology.
During the production phase, we use simple, robust, and reliable methods to reproduce the optimal equilibrium points found during the research phase, such as using pH and DO inflection points, and adjusting the feed rate based on the corresponding OD conditions.