Foam is a common issue in industrial fermentation, and if not properly controlled, it can disrupt the entire process. Understanding how foam forms, what problems it can cause, and how to manage it is essential for smooth and efficient fermentation.
Types of Foam in Fermentation
There are generally two types of foam that appear during fermentation. The first type forms on the surface of the liquid. This surface foam is light, filled with gas, and clearly separated from the liquid underneath. It often appears in the early stages of fermentation or in thin seed cultures.
The second type is more complex and occurs in thicker fermentation liquids, especially those containing mycelium. This foam is finely dispersed, stable, and blends smoothly with the surrounding liquid. As you move from the bottom to the top of the tank, the gas concentration in the liquid gradually increases, making the foam denser near the surface.
Why Foam Forms During Fermentation
Foam forms for several reasons:
- Ventilation and Stirring: High levels of air and agitation can create more foam, especially early in the process when the culture medium is still rich and reactive. Reducing the air flow and stirring speed early on, then increasing it gradually, can help minimize foam. Defoamers can also be added during this phase if needed.
- Medium Composition and Viscosity: Nutrient-rich, thick culture media are more likely to foam and are harder to stir. These conditions promote stable, persistent foam.
- Strain Type and Inoculation Size: Fast-growing strains quickly use up nutrients and produce less foam. Slower-growing strains may need a higher inoculation volume to prevent excessive foam.
- Sterilization Quality: Poor sterilization can damage nutrients, stunt microbial growth, and lead to cell breakdown, which increases foam production. In such cases, even adding defoamers may not help.
The Impact of Foam
Uncontrolled foam can cause several problems during fermentation:
- Reduces the effective volume of the tank, lowering equipment utilization.
- Creates an uneven microbial environment.
- Increases the risk of contamination.
- Can lead to product loss.
- Traps gases, affecting oxygen transfer and metabolic processes.
- May cause cells to break down prematurely.
- Makes downstream extraction more difficult if defoamers are not carefully chosen.
Detecting and Controlling Foam
The simplest way to detect foam is by observing the fermentation tank’s sight glass. If foam continues to rise, defoamer can be added manually.
For more advanced control, sensors can be installed near the top of the tank. When foam reaches the sensor, it sends a signal to automatically add defoamer from a storage tank. Once the foam level drops, the signal stops, and defoamer addition is halted.
Foam can be controlled through physical or chemical methods:
Physical Defoaming
This method relies on mechanical force to break down foam. It can be done inside the tank using devices like foam-breaking paddles or outside the tank by removing foam and separating the liquid, then returning it.
Pros: No chemical contamination, environmentally friendly, low cost.
Cons: Doesn’t address the root causes of foam formation.
Chemical Defoaming
This involves adding substances that destabilize the foam structure. These chemicals can neutralize electric charges or reduce surface tension in the foam, causing it to collapse.
Pros: Highly effective and fast-acting.
Cons: Some defoamers can interfere with oxygen transfer or make downstream processing more difficult if not carefully selected.
Types of Defoamers
- Natural Oils: Such as corn, rice bran, soybean, cottonseed, fish oil, and lard. These are biodegradable and safe for most processes.
- Polyether-Based: Common in production, including polyoxypropylene glycerol and polyoxyethylene propylene glycol. Suitable for bacterial fermentation.
- Higher Alcohols: Like octadecanol, sometimes used with a carrier. Cold-pressed lard works well in penicillin production. Polyethylene glycol provides long-lasting foam control.
- Silicone-Based: Such as polydimethylsiloxane. These are very effective, especially in low-alkalinity bacterial fermentations.
Choosing the Right Defoamer
When selecting a defoamer, several factors should be considered:
- It must be safe for the fermentation process and final product.
- It should withstand sterilization without breaking down or corroding equipment.
- It must not interfere with sensors (e.g., pH or oxygen sensors).
- It should be affordable and readily available.
- It should act quickly and last long enough to prevent foam reformation.
- It should not affect oxygen transfer or cause issues during product extraction.
Foam is a natural part of fermentation but can become a serious issue if left unmanaged. By understanding the causes and using the right combination of monitoring techniques, physical controls, and chemical defoamers, it’s possible to keep foam under control and ensure a smooth, efficient fermentation process.
