1. Multiple Abnormalities During Fermentation and Their Adverse Effects
Many abnormal things can happen during fermentation. They hurt fermentation results and product quality.
Poor Mycelial Growth
- Poor seed quality: Seeds may not be active enough. For example, they are stored too long or kept in bad conditions. This makes seed cells less active. They grow and sprout slowly in the fermentation medium. Or, the seeds are not pure. They have other bacteria or dirt. These compete with the target bacteria for food and space at the start of it. This stops the target bacteria from growing normally.
- Poor fermentation performance of the strain: The strain itself may have problems in its genes. It may change. So it cannot use food well for growth and metabolism during fermentation. Or, the strain does not fit the fermentation environment well. For example, it can only stand a small range of temperature, pH, dissolved oxygen and other conditions. If these conditions change a little, the strain cannot grow normally.
- Unfavorable environmental conditions: The temperature during fermentation is not controlled well. Too high or too low temperature affects the activity of microbial enzymes. This slows down the strain’s metabolism and growth. The pH is not right. This breaks the acid-base balance inside microbial cells. It stops various chemical reactions inside cells from working normally. There is not enough dissolved oxygen. This limits the breathing of aerobic microorganisms. It also stops the strain from growing.
- Insufficient inoculum size: The amount of seeds added is too small. So the strain grows slowly at the start of fermentation. At first, there are not many active cells. They need a long time to get used to the new environment. They also need time to multiply to a sufficient number for normal growth. During this time, the culture is easy to be contaminated by other microorganisms. It may also be affected by environmental factors.
Abnormal pH (Overly High or Low)
- Imbalanced metabolism of nutrients: During zymosis, microorganisms break down carbon sources, nitrogen sources and other nutrients. This process produces acidic or alkaline substances. For example, microorganisms use a lot of nitrogen-containing compounds. They may produce alkaline substances like ammonia. This makes the pH go up. When microorganisms break down carbon sources, organic acids (like lactic acid, acetic acid) build up. This makes the pH go down. If this metabolic imbalance continues, the pH will become abnormal.
- Influence of ventilation rate: Too much or too little ventilation can affect the pH. If there is too much ventilation, a lot of carbon dioxide is pushed out of the fermentation broth. This reduces the carbonic acid in the broth. So the pH goes up. If there is not enough ventilation, microorganisms may breathe without oxygen. They produce a lot of acidic substances. This makes the pH go down.
2. Abnormal Dissolved Oxygen Level (Taking Glutamic Acid Fermentation as an Example)
- Stable dissolved oxygen in the early fermentation stage: After adding the strain to the fermentation tank, the strain is getting used to the new environment. Its metabolic activity is not strong. It does not need much oxygen. So the dissolved oxygen concentration stays almost the same.
- Declining and stable dissolved oxygen during the logarithmic growth phase: The strain gets used to the environment. It enters the fast growth phase. It multiplies quickly. Its metabolic activity is strong. It uses a lot more oxygen. So the dissolved oxygen concentration drops fast. When the oxygen supply of the fermentation system matches the oxygen used by the strain, the dissolved oxygen concentration stays stable.
- Rising dissolved oxygen again in the late fermentation stage: In the later part of fermentation, the strain starts to age. Its cells are less active. Its metabolism slows down. It needs less oxygen. The oxygen supply of the system stays the same. So the dissolved oxygen concentration goes up again.
3. Excessive Foam
- Poor mycelial growth and slow metabolic rate: The strain does not grow well. Its metabolism is slow. This may cause some substances to build up in the fermentation broth. Or, the metabolism is unbalanced. This changes the physical and chemical properties of the broth. So the broth is more likely to produce foam. For example, if the strain grows poorly, the amount and properties of colloidal substances (like proteins) in the broth change. This makes the foam more stable.
- High content of protein-based colloidal substances: The fermentation broth has a lot of colloidal substances. These include proteins and polypeptides. These substances can reduce the surface tension of the liquid. So foam is easy to form during fermentation. Also, these colloids can stick to the surface of bubbles. They form a stable foam layer. So the foam does not break easily.
- Improper medium sterilization: When sterilizing the culture medium, the temperature is too high. Or, the sterilization time is too long. This causes chemical reactions in components like glucose. Substances like aminosugars are produced. Aminosugars can reduce the surface tension of the liquid. They make the fermentation broth more likely to foam. They may also stop the strain from growing. This further disrupts the normal fermentation process.
About Ferbio
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