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Causes of Foam in Fermentation Broth Filtration


Causes of foam Formation in Fermentation Broth Filtration 

In the process of aerobic fermentation, the formation of foam has certain rules. The amount of foam is not only related to the amount of ventilation and the intensity of agitation, but also related to the nature of raw materials used in the culture medium. The protein raw material in the fermentation medium is the main foaming material, and the more the content, the easier the foaming. The incomplete hydrolysis of polysaccharide and high dextrin content are also easy to cause foam. The sterilization method and operating conditions of the culture medium will affect the change of the composition of the culture medium and the production of foam during fermentation. The cell itself also has the function of stabilizing foam. The higher the cell concentration of the fermentation broth, the easier it is to bubble. 

The harm of foam 

For fermentation, too much foam will bring many negative effects to fermentation. If too much foam is not eliminated in time, it will lead to liquid escape and loss of fermentation liquid. At the same time, foam reduces the charging coefficient of the fermentation tank, increases the heterogeneity of the bacteria, and increases the chance of contaminating the mixed bacteria.

Detection and Control of foam in Fermentation Broth Filtration 

According to the causes and laws of foam formation, the excessive formation of foam can be prevented from the characteristics of the production strain itself, the composition and proportion of the culture medium, sterilization conditions and fermentation conditions. The defoaming methods commonly used in the fermentation industry are divided into mechanical defoaming and chemical defoaming. 
Mechanical defoaming 
Mechanical defoaming is a method of eliminating foam by physical action. The principle is to use strong mechanical vibration or pressure changes to cause foam to break. Its advantage is that it does not need to add other substances in the fermentation broth, which reduces the pollution opportunities caused by adding defoamers. The disadvantage is that the factors causing foam stability cannot be eliminated fundamentally. There are two methods of mechanical defoaming, namely, in-tank defoaming and out-of-tank defoaming. 
The defoaming in the tank includes rake type defoaming paddle, rotary round plate type, air suction type, impact reflection plate type, disc type and ultrasonic mechanical defoaming. The mechanical defoaming of the rake defoamer is to install the rake defoamer on the mixing shaft in the fermentation tank, and the tooth surface is slightly higher than the liquid level. When a small amount of foam is generated, the rake tooth will break the foam at any time. The impact reflection plate defoaming is a method of blowing gas into the upper part of the liquid surface, and then blowing back to the liquid surface through the impact reflection of the impact plate set on the upper part of the liquid surface to break the foam generated on the liquid surface. 

Defoaming outside the tank includes rotary vane type, spray type, centrifugal force type and steering plate type mechanical defoaming. Spray defoaming refers to defoaming water, culture liquid and other liquids through appropriate spray spray truck. Defoaming outside the rotating blade tank is to lead the foam out of the tank, use the impact force and shear force generated by the rotating blade to defoaming, and then the liquid returns to the fermentation tank after defoaming. 

Chemical defoaming 
Chemical defoaming is a method of using chemical defoamers to eliminate foam, which is the most widely used defoaming method in fermentation industry at present. Its advantage is that the defoaming effect is fast and reliable, but the excessive amount of defoamer will increase the production cost, and may affect the growth and metabolism of bacteria. 

In general, chemical defoamer is a kind of surfactant. When the defoamer contacts the film surface of the bubble, it can reduce the local surface tension of the film surface of the bubble. Because the balance of force is destroyed, the bubble will break. When polar surfactant exists on the surface of foam to form a double electric layer, adding strong polar defoamer with opposite charge can compete with the foaming agent for space on the liquid film, and reduce the mechanical strength of the bubble film surface, thus causing the foam to burst. When the liquid film of foam has a large surface viscosity, the addition of some substances with weak molecular cohesion can reduce the surface viscosity of the film, promote the liquid loss of the liquid film and break the foam.